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Review

Spectroscopic and Microscopic Characterization of Inorganic and Polymer Thermoelectric Materials: A Review

by
Temesgen Atnafu Yemata
1,*,
Tessera Alemneh Wubieneh
2,*,
Yun Zheng
3,*,
Wee Shong Chin
4,
Messele Kassaw Tadsual
1 and
Tadisso Gesessee Beyene
1
1
Department of Chemical Engineering, Bahir Dar Institute of Technology, Bahir Dar University, Bahir Dar P.O. Box. 26, Ethiopia
2
Department of Materials Science and Engineering, Bahir Dar Institute of Technology, Bahir Dar University, Bahir Dar P.O. Box. 26, Ethiopia
3
Key Laboratory of Flexible Optoelectronic Materials and Technology, Ministry of Education, Jianghan University, Wuhan 430056, China
4
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
*
Authors to whom correspondence should be addressed.
Spectrosc. J. 2025, 3(4), 24; https://doi.org/10.3390/spectroscj3040024
Submission received: 16 June 2025 / Revised: 9 September 2025 / Accepted: 20 September 2025 / Published: 14 October 2025
(This article belongs to the Special Issue Advances in Spectroscopy Research)

Abstract

Thermoelectric (TE) materials represent a critical frontier in sustainable energy conversion technologies, providing direct thermal-to-electrical energy conversion with solid-state reliability. The optimizations of TE performance demand a nuanced comprehension of structure–property relationships across diverse length scales. This review summarizes established and emerging spectroscopic and microscopic techniques used to characterize inorganic and polymer TE materials, specifically poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). For inorganic TE, ultraviolet–visible (UV–Vis) spectroscopy, energy-dispersive X-ray (EDX) spectroscopy, and X-ray photoelectron spectroscopy (XPS) are widely applied for electronic structure characterization. For phase analysis of inorganic TE materials, Raman spectroscopy (RS), electron energy loss spectroscopy (EELS), and nuclear magnetic resonance (NMR) spectroscopy are utilized. For analyzing the surface morphology and crystalline structure, chemical scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) are commonly used. For polymer TE materials, ultraviolet−visible–near-infrared (UV−Vis−NIR) spectroscopy and ultraviolet photoelectron spectroscopy (UPS) are generally employed for determining electronic structure. For functional group analysis of polymer TE, attenuated total reflectance–Fourier-transform infrared (ATR−FTIR) spectroscopy and RS are broadly utilized. XPS is used for elemental composition analysis of polymer TE. For the surface morphology of polymer TE, atomic force microscopic (AFM) and SEM are applied. Grazing incidence wide-angle X-ray scattering (GIWAXS) and XRD are employed for analyzing the crystalline structures of polymer TE materials. These techniques elucidate electronic, structural, morphological, and chemical properties, aiding in optimizing TE properties like conductivity, thermal stability, and mechanical strength. This review also suggests future research directions, including in situ methods and machine learning-assisted multi-dimensional spectroscopy to enhance TE performance for applications in electronic devices, energy storage, and solar cells.

Graphical Abstract

1. Introduction

The increasing global energy demand and escalating evidence of environmental issues have spurred intensive research into alternative energy technologies. Over two-thirds of the energy consumed globally is dissipated or lost as heat energy released into the environment. This presents a noteworthy opportunity to harness this mostly unexploited waste heat energy for generating clean and renewable energy with no emissions, providing economic and environmental benefits for sustainable development.
Thermoelectric generators (TEGs) are solid-state instruments that directly transform thermal energy into electrical energy according to the principle of the Seebeck effect [1,2]. According to the Seebeck effect, TEGs can change the temperature difference across the hot and cold side values (ΔT) into electricity [3]. In contrast, when electrically powered, they can produce a ΔT value that enables active heating or cooling, with the capability of switching between hot and cold sides as required [4]. TEGs possess numerous advantages compared with other energy harvesting devices, including their long lifespan, scalability, high reliability, and lack of moving parts, providing compatibility with various systems [5,6]. Hence, they serve different purposes in industries like automotive, portable electronic devices, buildings, and aerospace [7,8,9,10,11,12]. Nevertheless, the extensive adoption of TE technology is restricted because of the lower efficacy of existing devices and materials. To overcome this restriction, extensive development efforts have been dedicated to finding more efficient TEGs [13].
All TE devices, regardless of type, can share a basic architecture encompassing many kinds of p-and n-type legs, which are connected electrically in series and thermally in parallel [14,15], as portrayed in Figure 1A. TE devices (specifically micro-TE devices) have received increasing commercial value in recent years, with a promising future, as illustrated in Figure 1B. As such, the development of TE devices with multifunctional, stable, and high performance remains a commercially promising direction with broad application potential.
The efficiency of current TE materials is technically determined by the dimensionless figure of merit ZT = (σS2T)/κ, where σ, S, κ, and T are the electrical conductivity, the Seebeck coefficient, the total thermal conductivity, and the absolute temperature [16,17,18,19], respectively, and high ZT values correspond to the high theoretical efficiency (η) of a TE material and device. In addition to improving ZT, enhancing η involves optimizing heat transfer, decreasing internal resistance, and using rational device designs to attain mechanical flexibility [20].
In recent years, remarkable progress has been attained in TE materials science and engineering [21]. Deep understanding of the mechanisms of charge and heat transport, together with advances in fabrication techniques and multiscale structural design, has led to substantial enhancement in TE material performance [22]. Thus far, TE materials including GeTe [23,24,25,26], PbTe [27], SnSe [28,29], Cu2Se [30], and AgSbTe2 [31] have exhibited ZT values greater than 2, with some approaching 3 through microstructural engineering and compositional tuning. On the TE device side, innovations in manufacturing technologies have realized TE devices with η values above 13% [27,32,33] under laboratory conditions.
Figure 1. (A). Illustration of a representative TE device made of many kinds of p-type and n-type TE materials. (B). Global markets for TE devices and micro-TE devices by millions of U.S. dollars during 2022–2028. Adapted with permission from ref. [34], Copyright 2025, American Chemical Society. (C). Variations in ZT, S2σ, σ, S, and κ (including lattice κl and electronic κe contributions) with n. Reproduced with permission from ref. [35], Copyright 2024, Wiley.
Figure 1. (A). Illustration of a representative TE device made of many kinds of p-type and n-type TE materials. (B). Global markets for TE devices and micro-TE devices by millions of U.S. dollars during 2022–2028. Adapted with permission from ref. [34], Copyright 2025, American Chemical Society. (C). Variations in ZT, S2σ, σ, S, and κ (including lattice κl and electronic κe contributions) with n. Reproduced with permission from ref. [35], Copyright 2024, Wiley.
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Nevertheless, most of these inorganic-based TE applications have the disadvantage of higher cost and possess comparatively low conversion η values (<6%) [36]. Also, the manufacturing process steps of these inorganic TE materials are complex, with multiple stages and techniques such as hot pressing and melting-spinning, which require significant time and advanced instruments [37]. Inorganic TE materials possess a rigid surface, which will limit the contact area when the surface is uneven. Another limitation is that most inorganic TE raw materials are less-abundant resources, which can make these TE applications more costly. Additionally, some elements are toxic, including Bi and Te; therefore, TE is not appropriate for applications requiring direct human body contact [38,39,40].
In the past several years, organic materials like conductive polymers, small molecules, and organic TE composite materials and devices have made substantial progress owing to their diverse advantages including largely adjustable molecular configuration/structure, lower cost, super-flexibility, abundant sources, tunable electrical properties, lightweight characteristics, high mechanical flexibility, environmentally benign nature, low-cost fabrication, and solution processability methods [38,41,42,43,44,45,46,47,48,49,50,51,52]. Due to their relatively unchanging and low k (generally ranging from 0.1 to 0.4 W/mK), even with increases in σ, their TE properties/performance are usually estimated by employing the power factor (PF, S2σ) instead of ZT. Various kinds of TE materials have been established in different forms, such as free-standing films, thin films, and bulk. Thin-film organic TE material fabrication, usually with a thickness varying from several nanometers to micrometers, can provide numerous benefits compared to bulk TE materials, such as versatile engineering design possibilities, owing to their dimensionality reduction, compatibility with microfabrication approaches, cost-effectiveness, scalability, and improved conversion efficiency [44,53,54,55,56,57]. These attributes can fabricate film-based TEGs, especially appropriate for integration into physical and chemical sensors, heat sources with curved surfaces, and flexible and wearable electronic devices. Thus far, several prudential approaches of both the synthesis of materials and the manufacture of super-flexible instruments have been established, and their corresponding TE properties/performances have been enhanced [58,59,60,61]. However, they are still far from practical applications. Hence, engineering of existing materials or material ingenuity, as well as understanding of the in-depth fundamental molecular mechanism for TE property improvement, is urgent.
Amongst all of the organic or polymer TE materials, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is probably the most extensively studied and the most successful conducting polymers (CPs) related to several practical applications, as PEDOT:PSS can be easily dispersed in water or some polar organic solvents, whereas most CPs in the conductive state are intractable and insoluble in any solvent. Moreover, it possesses easy film formation, superior transparency in the visible range, and superior thermal stability [62], as well as high S and high σ with superior TE behaviors that can be used for flexible TE generators. Therefore, PEDOT:PSS with excellent TE behaviors can be employed for flexible TEGs. It must have both superior σ and S. Nevertheless, σ and S are interdependent. To overcome these challenges, various materials and approaches have been developed to boost the TE performance/properties of PEDOT:PSS [62], including the manufacture of inorganic particles/PEDOT:PSS composites [63,64], secondary doping and dedoping engineering [43,65,66], doping, dedoping process and ionic energy filtering [65], sulfuric acid crystallization [67,68], solvent treatment [41,69,70,71,72,73], construction of polymer nanostructure [74,75,76], and various pre- or post-treatment techniques, as summarized in Table 1. Indeed, TE research of PEDOT:PSS has already established a breakthrough in achieving higher PF and ZT. For instance, extremely high PF and ZT values of 1285 ± 67 µW/mK2 and 0.80 ± 0.04, respectively [77], have been reported for acid (H2SO4), base (NaOH), dimethyl sulfoxide (DMSO), and a DMSO solution of tetrathiafulvalene (TTF) sequentially treated PEDOT:PSS thin film. In another study [65], the same authors have reported PF and ZT values of, respectively, 1285 ± 67 µW/mK2 and 1.05 (world record values) at room temperature for acid, base, and vitamin C (i.e., a reductant), and then coated with 1-ethyl-3-methylimidazolium dicyanamide (EMIM:DCA) (i.e., an ionic liquid)-treated PEDOT:PSS thin film in the organic TE field, which are almost similar to those state-of-the-art inorganic-based TE counterparts. With regard to the future time perspective, PEDOT:PSS films are a potential candidate for TE materials for practical functions, and there have been substantial progress and notable improvement in the TE of PEDOT:PSS in more recent years. However, PEDOT:PSS-based TE devices/materials are still in their initial stage of development as their TE properties/performance, with respect to ZT and PF values, are presently inadequate for practical purposes [65,77].
In order to further boost the PF of PEDO:PSS films, various efforts can be made to improve their S value by employing different methods like chemical doping, including charge transfer doping, protonic acid doping, and oxidative (or reductive) doping, and molecular engineering, as well as nanostructuring [65,78,79,80,81,82,83,84,85,86]. Nevertheless, the S of organic materials (including PEDOT:PSS) with higher TE performance/properties is substantially lower compared to their inorganic counterparts [87]. It should also be noted that S depends upon the free (mobile) charge carrier concentration, n, and charge carrier effective mass, m*, with a relationship as Sm*/n2/3 [88]. Figure 1C portrays the connection of n with ZT, PF (S2σ), σ, S, and κ2 [35]. As can be observed in Figure 1C, the bottleneck of TE materials lies in optimizing these often-correlated parameters simultaneously. The sensitivity of the materials themselves to impurities and dopant concentrations further complicates measurements. This is because of the strong dependence of σ and to a lesser extent of S on n. The development of TE materials with higher properties/performance requires a comprehensive understanding of structure–property relationships at atomic, nanoscale, and mesoscale levels [6,35,89,90]. As a result, the use of the energy filtering method is likely a remedy that can boost the S greatly and slightly decrease the σ through manipulation of the electronic transport behaviors [65,83,87].
The energy filtering (EF) effect is a technique wherein the lower-energy charge carriers are solely filtered out, only allowing higher-energy charge carriers to pass through, for the improvement of the S and hence the PF, which cannot be established in traditional bulk materials [91,92,93,94,95]. The mechanism is that hotter electrons can pass an additional barrier, while colder electrons get blocked at this barrier. The mechanism is that it allows the passing of only hot electrons (higher-energy carriers) and blocks the cold electrons (lower-energy carriers) by using barriers like nano-inclusions or nanocomposites, as presented in Figure 2A. As a result, the S is enhanced [96]. Such a filtering effect has been reported for ZnO-based materials [97,98,99], indium gallium arsenide superlattice films [100], nanocrystalline PbTe [101,102,103], bulk PbTe with Pb nanoparticles [104], and nanostructured SiGe [105]. An additional benefit which usually comes together with such an EF mechanism is an additional phonon scattering [100,102,103,106], which decreases the phonon part of the k and can thereby also enhance the ZT. One disadvantage of the filtering effect is often a decline in the σ [107].
Figure 2. (A). Diagrammatic representation of EF effect with various heights of energy barrier incorporating nanocomposites and nano-inclusions. The transport of holes with higher and lower energies is illustrated correspondingly with the red and blue straight arrows. The return arrows show the filtering of the lower-energy holes through the energy barrier. Adapted with permission from ref. [96] Copyright 2024, Elsevier. (B). Schematic of the EF of E/PABV films. Sketch of the hole and ion accumulations in an E/PABV heterostructure due to temperature gradient (left side); and schematic illustration of the EF of the holes of PABV through the potential barrier made through the cation accumulation of EMIM:DCA owing to temperature gradient (right side). Adapted with permission from ref. [65] Copyright 2025, Wiley.
Figure 2. (A). Diagrammatic representation of EF effect with various heights of energy barrier incorporating nanocomposites and nano-inclusions. The transport of holes with higher and lower energies is illustrated correspondingly with the red and blue straight arrows. The return arrows show the filtering of the lower-energy holes through the energy barrier. Adapted with permission from ref. [96] Copyright 2024, Elsevier. (B). Schematic of the EF of E/PABV films. Sketch of the hole and ion accumulations in an E/PABV heterostructure due to temperature gradient (left side); and schematic illustration of the EF of the holes of PABV through the potential barrier made through the cation accumulation of EMIM:DCA owing to temperature gradient (right side). Adapted with permission from ref. [65] Copyright 2025, Wiley.
Spectroscj 03 00024 g002
Fascinatingly, the incorporation of nanostructure interfaces into TE materials to filter lower-energy charge carriers through EF also functions effectively in polymer TE composites. For instance, boosts in the S and PF in polyaniline (PANI)/carbon nanotube (CNT) and tellurium (Te)/PEDOT:PSS composites have been reported by several researchers and ascribed to the EF effect [108,109,110,111,112,113]. Thus, the utilization of nanocomposite materials is an effective technique for boosting the TE behaviors of polymer-based composites. Amongst these materials, carbon-based materials such as graphene and CNTs have been employed owing to their mechanical behaviors and higher σ. Introduction of these fillers can enhance the σ by forming better network conductivity and potentially leading to enhanced S through lower-energy charge carrier filtering at the carbon filler and PEDOT junction. Other fillers, conversely, provide the merit of a higher value of S by integrating the merits of both inorganic and conductive polymers TE materials for an enhanced PF. When there is no appropriate energy barrier, the holes with lower and higher energy can be easily accumulated toward the cold end by the temperature gradient, resulting in a lower S. In the presence of any suitable energy barrier for the charge carriers within the material, the holes with lower energy can be filtered, which are near the Fermi level [114]. Consequently, the mean energy of the accumulated holes increases, giving rise to an enhancement in the S. Nevertheless, this energy barrier has not been very high, as an extremely high energy barrier can result in blocking of the holes of both lower and higher energies and thereby remarkably decrease the σ of the material [87]. These strategies open up new paths for the development of polymer TE materials with boosted S and elevated PF [87,115,116].
For instance, more recently, Du et al. [65] (2025) have established PEDOT:PSS films with a record-high S of 111 µ/VK and PF of 1285 µW/mK2, leading to a room-temperature ZT of 1.05 by using a combination of doping engineering, dedoping, and the ionic EF effect, as portrayed in Figure 2B. This value is the world record ZT for polymers and polymer composites, which is similar to that of the outstanding inorganic TE materials. They employed a sequential acid, base, vitamin C, and 1−ethyl−3−methylimidazolium dicyanamide (EMIM:DCA) treatment of PEDOT:PSS films (abbreviated as E/PABV) [65]. The anions and cations of EMIM:DCA collect at the hot and cold ends, correspondingly, the so-called Soret effect, as illustrated in Figure 2B (left side). The accumulated cations can create an electric field, which can become an energy barrier for the collection of the holes at the PEDOT:PSS (p-type) cold side, in accordance with Maxwell’s equation (Figure 2B, right side). Consequently, the lower-energy holes are blocked, resulting in a high mean energy (ET) of the holes collected at the cold end. Hence, the S is elevated as it is linearly related to the difference between ET and the Fermi energy (EF), as S ∝ |ET − EF|. This mechanism can account for the elevated S of E/PABV as compared to Na:DCA/PABV.
Recently, various characterization methods have been used for the investigation of typical behaviors of materials. Herein, we present a comparison between various characterization techniques in terms of sample preparation, information, advantages, and limitations, and details are summarized in Table 2. The basic principles of the different microscopy and spectroscopy techniques and general guidelines for interpreting the chemical composition, functional groups, morphology data, and crystal structure information offered by these characterization approaches can be obtained in the references [117,118] and references listed therein. The TE performance of a certain type of material strongly associates with its electronic, structural, and morphological as well as chemical properties; hence, a thorough characterization of these properties is essential to deeply understand the behaviors of TE materials as well as to design novel materials with required behaviors for property enhancement. Various spectroscopic and microscopic approaches have been applied for the characterization of TE materials nowadays to examine the mechanism for TE enhancement and study material features, including structure, composition, and various properties—such as chemical, physical, electrical, etc. More advanced methods can provide additional information on the chemical properties and molecular structure as well as interfacial surface properties. The development of highly effective TE materials has offered different novel approaches for boosting the recital of TE materials owing to chemical, physical, and structural behaviors. Spectroscopic and microscopic characterization techniques play a pivotal role in understanding these fundamental properties, providing crucial insights for rational design and optimization strategies [119,120].
Despite the many review articles reported in the field of TE materials [2,34,121,122,123,124,125,126,127,128,129,130,131,132], there remains a lack of reviews in the published literature on applied spectroscopic and microscopic techniques for inorganic and organic TE materials and how these modern approaches/techniques contribute to understanding the mechanism of TE behaviors of both inorganic and polymer TE materials in terms of electronic, structural, morphological, and chemical features. The objective of this review is to bridge the gap in the literature by systematically comparing spectroscopic and microscopic techniques for inorganic and organic TE materials. In this review, we will emphasize spectroscopic and microscopic techniques that have already been applied for TE characterization—with examples and general guidelines for interpreting observations/data derived from each characterization technique for both inorganic and polymer TE materials. Section 1 introduces basic TE principles and summarizes various recent strategies for boosting the TE recital of both inorganic and organic materials. Section 2 focuses on various spectroscopic and microscopic data interpretation derived from inorganic TE materials. Section 3 highlights the interpretation of various spectroscopic and microscopic techniques data generated for polymer-based TE materials, specifically PEDOT:PSS. Ultimately, this review offers a summary and recommends the future directions in TE research and strategies.
This review paper is intended to summarize the various spectroscopy and microscopy techniques that have already been applied for the characterization of inorganic and polymer TE materials, as illustrated in Figure 3. For inorganic TE, energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultraviolet–visible (UV-Vis) spectroscopy are widely applied for electronic structure characterization. For phase analysis of inorganic TE materials, Raman spectroscopy (RS) and nuclear magnetic resonance (NMR) spectroscopy, as well as electron energy loss spectroscopy (EELS), are utilized. For analyzing the surface morphology and crystalline structure, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were commonly used. For polymer TE materials, ultraviolet–visible–near-infrared (UV−Vis−NIR) spectroscopy and ultraviolet photoelectron spectroscopy (UPS) are generally employed for determining electronic structure. For functional group analysis of polymer TE materials, RS and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR−FTIR) are broadly utilized. XPS is used for elemental composition analysis of polymer TE. For the surface morphology of polymer TE, atomic force microscopy (AFM) and SEM are applied. Grazing incidence wide-angle X-ray scattering (GIWAXS) and XRD are employed for analyzing the crystalline structure of polymer TE materials. In this paper, each spectroscopic technique is discussed by providing examples of spectroscopic techniques that have already been employed for TE characterization. We believe the review will provide information about advanced spectroscopic and microscopic characterization techniques to predict the performance of composites and data interpretation guidelines for potential applications in various disciplines such as electronic devices, including memristors and neuromorphic devices, organic electrochemical transistors (OECTs), light-emitting diodes (LEDs), photodetectors, and a hole transport layer (HTL) for solar cells, as well as energy storage devices including batteries and supercapacitors.

2. Spectroscopies and Microscopy Techniques of Inorganic TE Materials

Inorganic TE materials, often semi-conductors, generally exhibit higher TE performance; however, they are more expensive and less flexible [130,132]. Inorganic materials—including chalcogenides (e.g., SnSe, PbTe, Bi2Te3) [133,134], Zintl phase compounds [135,136], skutterudites, clathrates [137], and oxides—have shown significant promise as TE materials [122,132].
The fundamental mechanisms that contribute to the boosted TE performance of inorganic materials are elucidated by using different spectroscopy characterizations, including EDX, UV−Vis spectroscopy, XPS, XRD, RS, NMR, and EELS. The general data interpretation guidelines for inorganic TE materials are explained in the sections below.

2.1. Electronic Structure Characterization

2.1.1. EDX Spectroscopy

Characterization of TE materials using EDX spectroscopy is pivotal for understanding their microstructural and electronic properties. EDX, often combined with scanning electron microscopy (SEM), allows for the analysis of local carrier concentration and band alignment, which are crucial for optimizing TE performance. This technique can effectively assess the band diagram and estimate band offsets in multi-phase materials—enhancing the filtering of undesired charge carriers [138]. As can be seen from Figure 4A, it also provides detailed elemental composition and distribution data essential for optimizing their performance. The paper utilizes SEM with EDX to characterize multi-phase TE materials, assessing local carrier concentration and band alignment, which are crucial for optimizing TE performance in magnesium silicide-based composites [139]. According to Cha et al. [140], the characteristic SEM picture and its corresponding elemental maps obtained by using EDS were recorded on the polished surface of the spark plasma sintering (SPS) processed specimen; the Sn precipitate was seemingly invisible in the SEM-EDS detection limit (Figure 4B), contrary to the result for the specimen that was subjected to further ball milling and annealing (Figure 4D). These results showed that isolated Sn would be better pulverized and homogenously distributed by the ball milling process.

2.1.2. UV−Vis Spectroscopy

UV−Vis spectroscopy serves as a fundamental instrument for probing optical properties and electronic band structure of TE semi-conductors, measuring absorption, reflection, or transmission of light in the ultraviolet and visible electromagnetic spectrum regions [141,142]. The absorption edge value in the UV-Vis spectrum directly correlates with the material’s optical bandgap, a critical parameter influencing carrier concentration and effective mass, thereby impacting σ and S [37]. For instance, according to Rashad et al. [143], the absorption spectra of Bi2Te3 samples were analyzed by using a UV−Vis spectrophotometer at room temperature, as indicated in Figure 5A (left side). As can be observed in Figure 5A (left side), the absorption spectrum for the compound nanostructures showed absorption peaks for all specimens—including STNaPv−Bi2Te3, STPv−Bi2Te3, ST NaEd−Bi2Te3, STEd−Bi2Te3, and ST−Bi2Te3 at 284, 279, 280, 278.8, and 278 nm, respectively. The peaks were shifted/moved from 278 nm to 284 nm, which showed the impact of the nanostructure morphology and size. In addition, this redshift showed a reduction in Eg, hence raising the size of the particle. Normally, the two absorption peaks between 900 nm and 1100 nm in the IR region showed that the nano-powder of Bi2Te3 possessed the ability of thermal energy absorption from IR, as illustrated in Figure 5A (left side). In addition, Tauc plots derived from UV−Vis data enable the determination of direct or indirect bandgaps with high precision [144,145]. The energy of the bandgap or optical bandgap was determined for all specimens by using Tauc’s equation, as described by αhν = A(hν − Eg)n [146,147], where A is an energy-independent constant, α is the absorption coefficient, and n = 1/2 and 2 for indirect and direct transitions, respectively. The (αhν)2 changes with respect to E (hν) for various Bi2Te3 specimens, and the extrapolation of a straight line to intersect through the E-axis is portrayed in Figure 5A (right side). It can be easily observed that the bandgap type (direct) for these materials resulted from transitions produced by electrons, indicating these materials include solely the electrons between the conduction and valence bands with no interaction in the lattice [143].
In another study, UV−Vis spectroscopy was employed to elucidate the impact of Ov caused by specific high-pressure synthesis (SHPs) on the ZnO band structure and was applied to reflect the bandgap change. As portrayed in Figure 5B (right side), the bandgap of 3.22 eV of the raw specimen (between the conduction band minimum (CBM) and the valence band maximum (VBM)) was estimated using Tauc sketch from the UV−Vis absorption spectrum, as presented in Figure 5B (left side); the high-pressure synthesis (HPs) and SHPs resulted in decreased bandgaps of, respectively, 2.99 eV and 2.98 eV. Moreover, for the SHPs specimen, there was another abrupt transformation in the absorption spectra at the longer wavelength, as depicted in Figure 5B (left side), resulting in a tail at 2.74 eV, as portrayed in Figure 5B (right side). Therefore, it is judicious that the tail at 2.74 eV can be attributed to the deep impurity states induced by Ovs, i.e., from the CBM level to the Ovs level [142].

2.1.3. XPS

XPS provides surface-sensitive analysis of elemental composition, chemical states, and electronic structure of topmost layers—crucial for understanding surface phenomena affecting TE device performance [148,149]. XPS was utilized to analyze the chemical states and composition of the AZO thin films. XPS measurements were carried out by a PHI−5400 X-ray photoemission spectrometer with a monochromatic Mg Kα (1253.6 eV) radiation source. Core-level binding energy (B.E) shifts reveal information about chemical bonding and oxidation states—particularly important for air-stable TE materials [131]. Recent developments in hard X-ray photoelectron spectroscopy (HAXPES) enable bulk-sensitive electronic structure analysis—bridging bulk and surface property relationships [150,151]. Angle-resolved XPS (ARXPS) provides depth-profiling capabilities essential for understanding interfacial effects and compositional gradients [152,153].
XPS chemical shift analysis enables quantitative assessment of dopant segregation and incorporation phenomena [154,155]. High-resolution XPS studies have revealed preferential dopant segregation at grain boundaries, significantly impacting electrical transport properties [156]. Ultraviolet photoelectron spectroscopy (UPS) complements XPS by probing valence band electronic structure and work function, critical parameters for understanding carrier injection and extraction in TE devices [157,158]. As illustrated in Figure 6, the combined XPS/UPS analysis provides comprehensive electronic structure characterization from core levels to valence band edge [158,159,160]. For instance, the oxidation states of each constituent element in BiCuSeO have thus far been established to be Se2−, Bi3+, O2, and Cu1+ according to the necessity of the overall level of charge neutrality. Presumably, BiCuSeO is not a characteristic ionic compound, and therefore, the oxidation states of the elements have not been clearly defined. As a result, the authors performed XPS tests to verify the oxidation states. Despite a small quantity of Bi2+ being found, most of the Bi was certainly in the 3+ state within the BiCuSeO (undoped), as portrayed in Figure 6(Ba), whereby a smaller peak at the low-B.E side for the Bi3+ peak was observed. Within the Ca-doped specimens, no Bi2+ was evidently seen. Instead, a new peak appeared on the high-B.E side of the Bi3+ peak (see Figure 6(Bb)), showing some Bi ions with a higher oxidation state, apparently Bi4+. On the other hand, calcium (Ca) possesses a stable 2+ oxidation state—which is certainly verified in Figure 6(Bc)—whereby the 2p1/2 and 2p3/2 doublets with the B.E spacing of ca. 3.66 eV and intensity ratio of ∼1:2 are always seen within multiple Ca compounds, including CaO, CaSO4, and CaCO3. Consequently, one of the mechanisms for charge compensation in the doping of Ca2+ was discovered through the oxidation of a certain Bi3+ into Bi4+.

2.2. Phase Analysis

2.2.1. RS

RS provides complementary vibrational analysis to IR, probing different selection rule-allowed phonon modes and offering unique insights into crystal symmetry, stress states, and defect structures [162,163]. RS provides characteristic phonon mode fingerprints, enabling material identification and quality assessment [164,165,166]. Peak positions, widths, and intensities reveal structural distortions, phase transitions, and doping effects on lattice vibrations [163,167]. Temperature-dependent Raman studies reveal anharmonic behavior through frequency shifts and peak broadening, directly correlating with k properties [168,169]. Recent investigations on SnSe single crystals have demonstrated exceptionally strong anharmonicity, contributing to record-low k [170,171]. Structural disorder and defects manifest as Raman peak broadening, frequency shifts, or the appearance of disorder-induced modes [172,173]. Quantitative analysis of peak parameters enables defect concentration estimation and its impact on transport properties [174]. Raman peak position shifts provide sensitive measures of local strain within TE materials [175], as displayed in Figure 7. Micro-Raman mapping enables spatial strain distribution analysis, particularly valuable for understanding stress effects in TE devices [175,176]. Resonance RS—achieved by matching excitation energy to electronic transitions—provides enhanced sensitivity to specific phonon modes and information about electron–phonon coupling [15]. This technique has proven particularly valuable for studying topological insulator TE materials [177].
For instance, to examine the lattice characteristics of Bi2[AE]2Co2Oy, Li et al. carried out the RS tests, as depicted in Figure 7A. For Bi2CaSrCo2Oy (BCSCO), Bi2Sr2Co2Oy (BSCO), and Bi2Ba2Co2Oy (BBCO)—there are two notable phonon peaks, i.e., the ~440 cm−1 (E1g) and ~615 cm−1 (A1g) modes—that denote the in-plane vibration and out-of-plane vibration of the oxygen atoms, respectively. The Raman spectrum of Bi2Ca2Co2Oy (BCCO) is different from the specimens, in which the peaks are observed at ~300 cm−1 and ~450 cm−1. It can be due to the relatively prominent Ca2+ and Ca−O layers in contrast to other specimens (BBCO, BSCO, and BCSCO) [178]. Raman spectra tests at room temperature were also performed in order to verify the incorporation of Ca within the Bi2Sr2Co2Oy (BSCO) lattice, as illustrated in Figure 7B. The Raman spectrum for Bi2Sr2−xCaxCo2Oy (for x = 0.5, 0.3, and 0) specimens from 500 cm−1 to 950 cm−1 at a temperature of 300 K was recorded (Figure 7B), showing two noticeable phonon peaks for all samples at ~618 and ~815 cm−1. The observed Raman peak at 618 cm−1 was allocated to A1g modes—signifying the out-of-plane vibrational mode for the oxygen atom. A small Raman peak position shift to the right was seen with the increase in the Ca content (Figure 7C). This observation can be elucidated by the rise in the Ca−O bond strength, with regard to the Sr−O bond [161].

2.2.2. NMR Spectroscopic

Solid-state NMR spectroscopy offers unique insights into dynamic processes, chemical bonding, and local atomic environments and is particularly valuable for understanding the incorporation of dopant and structural disorder effects [179,180]. Solid-state NMR can differentiate crystallographically distinct sites for specific nuclei (e.g., 207Pb, 119Sn, 77Se, and 125Te) within TE materials [181,182]. For instance, as can be seen in Figure 8A, the PbTe-rich part of the phase seen here comprises less sulfur compared to the nominal/theoretical composition; simulation studies yielded values of x Te-rich = 0.45 and 0.26 for nominal x = 0.5 and 0.3, respectively. In addition, excellent fits were discovered from observation of the model of spinodal decomposition, which was anticipated in this concentration area. This observation was in good agreement with the early stage of spinodal decomposition that was inferred from quenching the x = 0.5 specimen from the comprehensive scattering data analysis. A periodically modulated composition in space was generated by spinodal decomposition. This resulted in a local composition distribution; nevertheless, the major contributions were from the maximum and minimum values of x [182].
Quadrupolar coupling constants and chemical shifts offer detailed information about local symmetry, bond lengths, and coordination environments [183,184]. Recent multinuclear NMR observations on complex chalcogenides have shown subtle local structure variations influencing transport behaviors [185,186]. NMR chemical shift analysis enables estimation of dopant incorporation sites as well as mechanisms [187].
Recent studies on PbTe-based materials have employed 207Pb NMR to elucidate dopant-induced local structure modifications [188,189]. NMR spectroscopy can characterize and detect vacancies, point defects, and interstitial atoms—usually unresolvable by diffraction approaches [190,191]. Variable-temperature NMR offers insights into defect dynamics and their temperature dependence [191,192]. In skutterudite TE materials, NMR has been extensively employed to elucidate rattling dynamics and guest atom filling fractions within cage structures [193]. These tests directly correlate with phonon scattering boost and k reduction [194,195]. NMR relaxation rate analysis can give information about mobility and carrier dynamics, though challenging for excellent conductive TE materials [196,197]. Recent developments in high-field NMR have expanded applicability to more conductive systems [198,199,200].
Figure 8. (A). 125Te NMR spectra for quenched PbTe1x Sx (i.e., x = 0.5 and 0.3)—which were compared by bar graphs within the models by presuming incipient separation of phase. (a,b) Model supposing segregation of close to neat PbS—with insignificant NMR signal of 125Te. (c,d) Model of two components approaching the spinodal decomposition. The model indicated in (c,d) offers a more precise composition estimate—supposing that the spinodal decomposition during the early stages has generated inhomogeneous PbTe1−xSx alloying regions within the matrix. Adapted with permission from ref. [182] Copyright 2012, Wiley. (B). Dynamic in situ TE characterization for amorphous Ge (aGe)-based thin films: (a) radial profiles of selected area electron diffraction (SAED) patterns obtained at various stages through the experiment. Reflections that correspond to fcc Ge were indexed. (b) Low-loss electron energy loss (EEL) spectrum recorded at various stages indicated the Ge−M, aC, and Ge edge bulk plasmons. (c) Recorded voltage obtained by applying current for double sweeps as displayed by dashed lines (down) and solid lines (up) with a variation from blue color to red color through the rise in the value of current. The calculated values of resistance at the highest applied current were presented using black crosses for each sweep (secondary y-axis). Adapted with permission from ref. [201] Copyright 2025, Elsevier.
Figure 8. (A). 125Te NMR spectra for quenched PbTe1x Sx (i.e., x = 0.5 and 0.3)—which were compared by bar graphs within the models by presuming incipient separation of phase. (a,b) Model supposing segregation of close to neat PbS—with insignificant NMR signal of 125Te. (c,d) Model of two components approaching the spinodal decomposition. The model indicated in (c,d) offers a more precise composition estimate—supposing that the spinodal decomposition during the early stages has generated inhomogeneous PbTe1−xSx alloying regions within the matrix. Adapted with permission from ref. [182] Copyright 2012, Wiley. (B). Dynamic in situ TE characterization for amorphous Ge (aGe)-based thin films: (a) radial profiles of selected area electron diffraction (SAED) patterns obtained at various stages through the experiment. Reflections that correspond to fcc Ge were indexed. (b) Low-loss electron energy loss (EEL) spectrum recorded at various stages indicated the Ge−M, aC, and Ge edge bulk plasmons. (c) Recorded voltage obtained by applying current for double sweeps as displayed by dashed lines (down) and solid lines (up) with a variation from blue color to red color through the rise in the value of current. The calculated values of resistance at the highest applied current were presented using black crosses for each sweep (secondary y-axis). Adapted with permission from ref. [201] Copyright 2025, Elsevier.
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2.2.3. EELS

High-spatial-resolution EELS combined with TEM provides nanoscale elemental and electronic structure information crucial for understanding interfacial phenomena and nanostructure effects [202,203]. EELS enables determination of local bandgaps, plasmon excitations, and oxidation states at nanometer spatial resolution [204]. This capability proves invaluable for characterizing interfaces, grain boundaries, and nanostructures in TE materials [205,206]. EELS analysis for TE interfaces reveals electronic structure modifications, charge transfer phenomena, and local composition variations that affect transport properties [22,207]. Recent studies have demonstrated a correlation between interfacial electronic structure and enhanced S in nanostructured materials [131,208]—EELS compositional mapping provides detailed analysis of element distribution, segregation phenomena, and phase boundaries at nanoscale resolution [209,210]. This information can prove the crucial importance of understanding nanostructure stability and transport property optimization [211].
For instance, Hettler et al. [201] employed a dynamic in situ TE characterization by inducing thermovoltage in the crystallization engineering of an amorphous Ge thin film by the application of higher current densities. The SAED pattern radial profile is portrayed in Figure 8(Ba) with a blue line showing two broad peaks—which were centered close to 3 nm−1 and 5.4 nm−1. The lower-loss EEL spectra (i.e., blue line as presented in Figure 8(Bb)) verified the existence of both Ge, which was reflected through the bulk plasmon near the Ge−M edge and 16 eV, and amorphous C (aC), which was visible at 25 eV (i.e., in the bulk plasmon). After this unplanned initial crystallization, the authors employed a double current sweep in series by increasing the highest current up to 240 A, in order to continue the crystallization engineering process within a controlled condition. The sweeps are portrayed in Figure 8(Bc)—in which dashed and solid lines represent the down- and up-sweeps, respectively—and the colors were blue at first and changed to red with the rising of the highest current. The estimated value of resistance at the highest current for each sweep was sketched using black color crosses and was seen to remain constant (73 K) up to an employed current (I) of 13 A before decreasing owing to the increase in σ of the semi-conducting Ge and an induced Joule heating effect. Above an employed current of about 70 A, the down- and up-sweeps started to vary significantly, showing a sample configurational/structural modification that was induced by the electrical current. The authors mentioned that this alteration was connected to, initially, an entire size growth in the crystallization region and, secondly, a grain size growth in the central area.

2.3. Surface Morphology and Crystalline Structure Analysis

2.3.1. SEM

SEM is the fundamental characterization method in which the TE material surface is explored. For instance, Figure 9(Aa–Ac) displays field emission SEM (FESEM) pictures for the prepared Bi2Te3-Bi2Se3 composite fracture surface of specimens, which were hot-pressed at a temperature range of 623–673 K and 80 MPa within a vacuum. These pictures clearly showed that the grain size and density of the composite specimens were both enlarged with sintering temperature. It can be observed from Figure 9A that the grain size of specimens that were hot-pressed at 673 K (Figure 9(Ab,Ac)) is clearly larger compared to that of the specimen that was hot-pressed at 623 K (Figure 9(Aa)) [212]. Figure 9(Ba) shows an SEM picture of a drop-cast film for the crystalline Te nanorod passivated by PEDOT:PSS that was created into a smooth thin-film nanocomposite by using the solution-casting technique [109]. Figure 9(Bb) presents the SEM image of the cross-sections of the composite of CNT (5 wt.%) and polymer (poly(vinyl acetate) (PVAc) following the freeze-fracturing of the composites. The SEM picture with higher magnification is shown in Figure 9(Bc), which was a portion of the specimen in Figure 9(Bb) (i.e., portrayed with a yellow solid square) [213].

2.3.2. TEM

TEM offers higher resolution as compared to that of SEM for better observations of TE materials. For instance, Figure 10A,B show the TEM pictures of the prepared Bi2Te3 nanopowders and PTH, indicating the differences in shape and particle size. It can be observed from Figure 10A that the prepared Bi2Te3 powders largely comprised nanosheets, with approximate sizes of 50 to 200 nm. The synthesized PTH powders were spherical with a mean diameter of approximately 200 nm (Figure 10B) [214]. Figure 10C presents TEM pictures of the crystalline Te nanorod, which was passivated using PEDOT:PSS [109].

2.3.3. XRD

XRD remains indispensable for characterizing the crystal structure, phase purity, and microstructural features of inorganic TE materials [215,216]. XRD results/patterns provide unique crystallographic fingerprints, enabling identification of primary TE phases and secondary phase detection [217,218]. Phase purity critically influences TE performance, as secondary phases can severely degrade transport properties [219,220,221]. Advanced Rietveld refinement techniques enable quantitative phase analysis and precise structural parameter determination [222,223]. Recent developments in machine learning-assisted phase identification have accelerated the discovery of new TE phases [224,225,226]. Precise diffraction peak position analysis enables determination of lattice parameters and their evolution with composition, temperature, and doping level [227,228]. Vegard’s law analysis provides insights into solid solution formation limits and phase boundaries [229,230,231,232].
High-temperature XRD studies have revealed temperature-induced structural transitions, contributing to enhanced TE performance in materials like SnSe and GeTe [233,234,235]. Diffraction peak broadening analysis by the Scherrer equation provides crystallite size information, crucial for understanding nanostructuring effects on k reduction [236,237,238]. Warren–Averbach analysis enables the separation of size and strain broadening contributions [36,239]. Texture analysis through orientation of distribution function calculations reveals preferred crystallographic orientations impacting anisotropic transport properties [240,241]. While XRD cannot directly visualize point defects, systematic analysis of peak intensities, profile shapes, and diffuse scattering provides valuable defect information [207,242]. Advanced pair distribution function (PDF) analysis enables local structure characterization, complementing average structure information [243,244].
For instance, XRD results/patterns of (Sn1−xGex)Se for powders and SPS-pressed samples are presented in Figure 11(Aa–Ac) [216]. The authors showed that diffraction peaks can be well indexed to SnSe with the orthorhombic phase (Pnma space group, PDF #48-1224). The XRD results verified that all the (Sn1−xGex)Se samples were single-phase in the absence of any impurity. The anisotropic structures were seen in all samples sintered by SPS. The relative intensities of the two reflection peaks (111) and (400) were specifically diverse in the patterns of (b) and (c), indicating the inclination of grains growing along these planes. The SEM picture of Figure 11(Ad) indicates the freshly fractured surface morphologies of the specimens parallel to the direction of sintering for (Sn0.99 Ge0.01)Se. The pronounced layered structures were seen arranged along the parallel direction, indicating the grains were preferably reoriented into the disk plane through the sintering stage. Figure 11(Ba) shows the XRD results/patterns of the Sn0.99Na0.01Se-Ag8SnSe6 (Sn0.99Na0.01Se-STSe), SnSe-Ag8SnSe6 (SnSe-STSe), Sn0.98Ag0.01Na0.01Se, Sn0.99Na0.01Se, Sn0.99Ag0.01Se, and SnSe specimens [245]. The main peaks in the XRD results were indexed to the orthorhombic SnSe phase (JCPDF No. 014-0159) with a Pnma symmetry. Moreover, observable peaks pertinent to the orthorhombic β−Ag8SnSe6 (ICSD No. 95093, space group of Pmn21) were seen in the expanded XRD results/patterns of the SnSe−STSe and Sn0.99Na0.01Se−STSe specimens, as demonstrated in Figure 11(Bb).

3. Spectroscopic and Microscopic Approaches Applied to Polymer TE Materials

Conducting polymers—also known as synthetic metals because they exhibit properties of metals, or plastics, or semi-conductors—have been extensively studied due to their high σ, low k, low density, advanced portability, and flexibility since they were first discovered in 1977 [246,247]. Organic TE materials—including polycarbazole, polyazulene, polythiophenes, poly(phenylene vinylenes) (PPV), polyacetylene (PA), polypyrrole (PPy), polyaniline (PANI), and their composites with carbon nanostructures or inorganic materials—have been extensively investigated, and all these materials possess π-conjugated structures [248,249,250,251,252,253,254,255,256]. The S and σ parameters of organic TE materials are strongly interdependent. For instance, Russ et al. investigated the change in the S and PF for a variety of organic n-and p-type TE materials and composites with regard to their σ (Figure 12). The authors established that S reduces with the increase in σ via the relation Sσ−1/4, while PF rises with the increase in σ via the empirical relationship PF = σS2σ1/2 [257]. Amongst all those usually employed conducting polymers, PEDOT:PSS has attracted increasing interest from both academia and industry owing to its superior σ and S, low k, and solution processability, as well as stability in the air amongst all these organic TE materials. PEDOT is doped by PSS to form dispersion in water and is broadly employed in the field of solar cells and light-emitting diodes, as well as other optical devices [258]. One of the best characteristics of PEDOT:PSS is that n can be controlled without altering S too much through the doping process. Also, PEDOT:PSS is mainly commercially available—and it can be fabricated on a large scale—which makes it a good candidate material for TE applications [259]. Hence, organic-based TE materials, particularly PEDOT:PSS, are promising candidates for use in the field of energy conversion materials in the future. Various treatment approaches have been established to enhance the TE performance/properties of PEDOT:PSS solution or films, by employing various acids, solvents, reducing agents, and ionic liquids, as summarized in Table 1. The enhancement is mainly ascribed to the enhanced σ or S [2,62,65,72,260,261]. However, pre- or post-treatment processing, which involves toxic materials to attain such enhanced TE performance, with unsatisfactory/poorer mechanical flexibility, usually limits the scale-up to industrial production. Therefore, discovering facile methods for the pre- or post-treatment of PEDOT:PSS is crucial, with particular emphasis on achieving chemical/mechanical robustness. There has also been little agreement about the origin and main mechanism responsible for PEDOT:PSS TE performance improvement until now. Hence, further work/research is essential to fully comprehend the fundamental mechanisms of these pre- or post-treatment methods, in order to optimize the TE behaviors/recital of PEDOT:PSS. Herein, in the sections below, we discuss the recent TE characterization techniques that have already been applied to study the mechanism of the boosted TE performance/behaviors of PEDOT:PSS films treated by numerous pre- or post-treatment approaches (as summarized in Table 1) and the general data interpretation guidelines for PEDOT:PSS TE materials.

3.1. Electronic Structure

3.1.1. UV−Vis−NIR Spectroscopy

To understand the mechanism of the boosted TE behaviors of PEDOT:PSS films, the untreated and treated PEDOT:PSS films were characterized by employing UV-Vis-NIR spectroscopy. For instance, UV−Vis−NIR spectra for the pristine and sequential formamide (F), sodium formaldehyde sulfoxylate (SFS), and 1−butyl−3−methylimidazolium bis(trifluoromethanesulfonyl) amide (BMIM−TFSI)-treated PEDOT:PSS films are shown in Figure 13A. The absorption peak band detected at ca. 225 nm was mainly ascribed to the PSS. The reduced intensity of the absorption peak band at ca. 225 nm for PEDOT:PSS films indicates the removal of some PSSH chains for the treated PEDOT:PSS films. Compared with the F-PEDOT:PSS spectra—which indicated a notable intensity decrease at ca. 225 nm—the spectra of SFS−F−PEDOT:PSS as well as BMIM-TFSI-SFS-F-PEDOT:PSS almost did not change, like other treatments [262], showing that treatment with F efficiently detached PSSH, leading to a rise in σ compared to the untreated PEDOT:PSS film [263]. In another study, Luo et al. employed UV−Vis absorption spectra to examine the interaction of a mixture of DMSO and 1−ethyl−3−methylimidazolium tetrafluoroborate (EMIMBF4) with PEDOT:PSS thin films (Figure 13B). Nevertheless, the PEDOT/PSS ratio for films treated by DMSO/EMIMBF4 was not clearly defined owing to the signal convergence [264]. Figure 13C also displays the UV-Vis absorption spectrum of acid-treated PEDOT:PSS films (PA) and acid and then rhodamine 101 (R101)-treated PEDOT:PSS films (PRA). The UV−Vis absorbance spectrum indicates the transformation of electrons from R101 into PEDOT:PSS (Figure 13C). The absorption peak band close to 900 nm is assigned to the polaron, while the absorption close to 600 nm is ascribed to the neutral state in PEDOT. The polaron peak band becomes more notable for PRA compared to PA, which is not owing to the R101 absorption [83]. Figure 13D displays the UV−Vis−NIR spectrum of the untreated/pristine, benzenesulfonic acid (BSA)-doped, and DMSO-treated, as well as hydrazine (HZ)-treated PEDOT:PSS films. Typically, PEDOT chains possess three oxidation states—including neutral, polaron, and bipolaron states—that resemble the absorption peaks at 600 nm, 900 nm, and 1400 nm, respectively. As displayed in Figure 13D, there was no noticeable absorption peak band in the 450 nm-to-1100 nm range in pristine/untreated, BSA-doped, and DMSO-treated films. Nevertheless, following dedoping by solutions of HZ, stronger absorption peak bands were observed at ca. 600 nm and ca. 900 nm. Such a shift showed a transformation from the bipolaron into polaron and neutral states following treatment with HZ [82]. Figure 13E shows the UV−Vis spectrum within the range of 400 to 1100 nm. As presented in the spectra, the pristine PEDOT:PSS film showed a wide absorption within the near-infrared region, showing the major role of PEDOT2+. While referring to the deep eutectic solvents (DESs) and choline chloride (ChCl)-treated film spectrum, the existence of ChCl introduced a polaron absorption near 900 nm, indicating the PEDOT chains dedoping from bipolaron state to polaron state. Comparatively, no detectable conversion was observed in the spectra of PEDOT:PSS film with ethylene glycol (EG) post-treatment [265].
More recently, Chen et al. [77] reported an enhanced PEDOT:PSS TE by treatment with tetrathiafulvalene (TTF). Figure 13F shows the Vis−NIR spectra for PEDOT:PSS (PP), - DBAP (acid and then base−DMSO) and acid–base–DMSO–tetrathiafulvalene (TDBAP) films. The absorption bands at 193 nm and 225 nm were mainly attributed to the benzene ring regions of rich PSS or PSSH. The intensities of the two absorption peak bands dropped after the acid post-treatment as a result of the phase separation of the insulating PSSH shell from PEDOT:PSS. There was an apparent reduction in the intensity of the two absorption peak bands following post-treatment by TTF solution. This cannot be attributed to the TTF absorption. Apparently, TTF—as a weak reducing agent—may barely dedope DBAP. TTF comes to be TTF+ after the reduction, and then TTF+ and PSS are rinsed away from PEDOT:PSS. In addition, some TTF molecules stayed in PEDOT:PSS, and they can possess a specific interaction with the conjugated PEDOT chains. The three bands at 1200, 900, and 600 nm were assigned to the neutral, polaron, and bipolaron states, respectively. The polaron band of DBAP was slightly weaker compared to the BAP film. This can be attributed to the rise in crystallinity, which can delocalize the holes and hence lower the values of the polaron density. After post-treatment with a solution of TTF with DBAP, a shoulder appears at approximately 600 nm. This was not because of the absorption of TTF, as TTF possesses no absorption within this range. The polaron absorption band of TDBAP is also remarkably stronger compared to that of DBAP. These conversions can be attributed to the slight reduction in PEDOT by using TTF. Furthermore, the peak position of the polar band was blue-shifted after the post-treatment with the TTF solution. It appeared at 1004 nm for DBAP and was shifted to 956 nm for TDBAP.
Generally, the explanation of the UV-Vis-NIR spectrum of PEDOT:PSS has been amended. For instance, Zozoulenko et al. [266] have interpreted according to the density functional theory calculations that the absorption peak at ca. 700−1000 nm may be ascribed to both polaronic and bipolaronic states, and that the peak in the NIR range may be attributed to polarons as well as bipolaron states originating from a higher oxidation state of PEDOT. Nevertheless, the summaries made here remain valid in light of these new views/explanations, since the decreasing/increasing tendency in the range of Vis−NIR helps the change from a higher to a lower oxidation level/state following post-treatment by solution.

3.1.2. UPS

According to Mott’s theory, S is proportional to the slope of the electronic density of states (DOS) at the Fermi energy level [6,267]. The UPS study can be employed to scrutinize the transformation in slope. For instance, Wang et al. reported the UPS spectra of BSA and HZ-treated PEDOT:PSS films (Figure 14A). The authors showed that the HZ-treated film exhibited a somewhat lower work function (Φ) than that of the BSA-treated film. Based on band theory analysis, the Fermi level (EF) of PEDOT:PSS position was shifted deep into the interior of the valence band for the heavily doped PEDOT films. A lower value of Φ suggested the moving of EF to the top of the deep valence band, in which DOS varied abruptly; therefore, the S was boosted for HZ-treated PEDOT films [82].
Another study by Yemata and colleagues reported that, along with increasing the TE properties of the PEDOT:PSS films, the ionic liquid (IL) treatment of the films affected other behaviors of PEDOT:PSS films, which are relevant to the operation of the device, like Φ. They showed that UPS was a key method to measure the surface Φ by determining the secondary-electron cut-off (Ec). The impact of valence band and Φ on the pristine and treated PEDOT:PSS thin films was estimated by applying UPS tests (Figure 14B) [263]. In fact, based on the UPS tests, the Φ can be estimated using Φ = hv – Ec, where hv is the photon energy (hv ≤ 21.2 eV) of the UPS light source, and the Ec of the spectral width is found from the energy gap between the Fermi edge and inelastic secondary-electron emission (SEE) cut-off [268,269]. The authors discovered that treatment of the film by IL resulted in a reduction in the Φ from 4.7 eV to 4.4 eV (Figure 14B). They also indicated that Φ ranging from 4.7 eV to 5.4 eV was reported in the literature for PEDOT:PSS thin films [270,271], which is in good agreement with the results exhibited in Figure 14B. The variation in the value of Φ is suggested to be associated with the top-layer differences, which may contain an excess of the PSS shell [269,272]. The top layer with rich PSS chains can be altered using the inclusion of higher-boiling solvents [273] and other processing conditions [274].
In the more recent method, the Φ of the sequential acid (H2SO4), base (NaOH), DMSO, and tetrathiafulvalene (TTF)-treated PEDOT:PSS films was investigated by UPS (Figure 14C,D). As exhibited in Figure 14C,D, treatment with DMSO or TTF solution shifted the cut-off edge of the spectra imaged with the UPS. The ϕ was determined in terms of the binding cut-off energy (E cut-off) and the photon energy (21.2 eV) using a cut-off of ϕ = 21.2-E. The ϕ of PP was 4.78 eV, and it decreased to 4.47 eV for BAP. This can arise by the dedoping of PEDOT:PSS using NaOH, which shifted up the Fermi level. The ϕ of DBAP was also 4.57 eV for DBAP, and it reduced to 4.33 eV for TDBAP, suggesting the shifting up of the Fermi level by treatment with TTF solution [77].
Guan et al. also employed UPS to study the Φ for MXene (Ti3C2Tx) (two-dimensional (2D) n-type material), which was mixed with PEDOT:PSS. As displayed in Figure 14E, the Φ values of MXene and PEDOT:PSS were 4.61 and 4.84 eV, respectively. They showed that the PEDOT:PSS Fermi level was lower compared to that of MXene. Hence, the electrons close to the MXene Fermi level were transferred from MXene to PEDOT:PSS until the Fermi levels were aligned. As both PEDOT:PSS and MXene possess a metallic electronic configuration, the transfer of charge can occur only in a narrow range. The authors concluded that this cannot be observed through XPS, RS, or UV−Vis absorbance spectroscopy [275].
Kim et al. applied UPS spectra to investigate the effect of polar solvent vapor annealing (PSVA) treatment in the Φ values for PEDOT:PSS/Bi2Te3 NWs composite film by employing DMSO as a polar solvent (Figure 14F). The treatment by PSVA enables altering the Φ value of the PEDOT:PSS and tuning the barrier energy between Bi2Te3 NWs and PEDOT:PSS [276]. The untreated PEDOT:PSS film and the PSVA-treated PEDOT:PSS films (shorter duration) showed higher Φ compared to that of Bi2Te3 NWs (Φ = 4.83 eV), whereas all the other specimens—including PSVA-treated films (long duration)—showed lower Φ compared to that of Bi2Te3 NWs, confirming the successful tuning of the Φ of PDOT:PSS by simply changing the PSVA duration [276].
Figure 14. (A). UPS spectra for HZ (red) and BSA (black)-treated PEDOT:PSS films. Here, the inset sketch is the DOS for BSA- and HZ-treated PEDOT:PSS. Adapted with permission from ref. [82] Copyright 2018, Wiley. (B). UPS spectra of untreated, SFS−F−PEDOT:PSS and BMIM−TFSI−SFS−F−PEDOT:PSS films obtained by employing He I photon (21.22 eV): (#1) untreated, (#2) SFS-F-PEDOT:PSS, and (#3) BMIM−TFSI−SFS−F−PEDOT:PSS. Adapted with permission from ref. [263] Copyright 2020, Frontiers. (C). The cut-off edge and (D) Fermi level of the UPS spectra for PP, BAP, DBAP, and TDBAP films. Adapted with permission from ref. [77] Copyright 2025, Wiley. (E). UPS spectra of pure PEDOT:PSS and neat MXene. Adapted with permission from ref. [275] Copyright 2020, American Chemical Society. (F). UPS observations for pure Bi2Te3 NWs or powder and composite films based on short or long PSVA duration. Adapted with permission from ref. [276] Copyright 2020, Elsevier.
Figure 14. (A). UPS spectra for HZ (red) and BSA (black)-treated PEDOT:PSS films. Here, the inset sketch is the DOS for BSA- and HZ-treated PEDOT:PSS. Adapted with permission from ref. [82] Copyright 2018, Wiley. (B). UPS spectra of untreated, SFS−F−PEDOT:PSS and BMIM−TFSI−SFS−F−PEDOT:PSS films obtained by employing He I photon (21.22 eV): (#1) untreated, (#2) SFS-F-PEDOT:PSS, and (#3) BMIM−TFSI−SFS−F−PEDOT:PSS. Adapted with permission from ref. [263] Copyright 2020, Frontiers. (C). The cut-off edge and (D) Fermi level of the UPS spectra for PP, BAP, DBAP, and TDBAP films. Adapted with permission from ref. [77] Copyright 2025, Wiley. (E). UPS spectra of pure PEDOT:PSS and neat MXene. Adapted with permission from ref. [275] Copyright 2020, American Chemical Society. (F). UPS observations for pure Bi2Te3 NWs or powder and composite films based on short or long PSVA duration. Adapted with permission from ref. [276] Copyright 2020, Elsevier.
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3.2. Functional Groups Analysis

3.2.1. ATR-FTIR Spectroscopy

ATR-FTIR spectroscopy can be employed to examine the impact of the various treatments on the chemical structures of PEDOT:PSS films. For instance, Alemu et al. employed ATR-FTIR to confirm the washing away of PSS chains for methanol (MeOH)-treated PEDOT:PSS film. After PEDOT:PSS film treatment through the dipping technique, the solution of MeOH was vaporized to increase the concentration of the removed PSS, and the ATR-FTIR tests were performed by dropping the MeOH on the infrared (IR) cell. Figure 15A shows the IR spectrum for a solution of MeOH obtained after MeOH vaporization—and it exhibited a characteristic PSSH spectrum. The absorbance peaks at 1005, 1035, 1125, and 1165 cm−1 were assigned to the sulfonate (SO3) group vibration of the PSS. Moreover, the authors compared the PSSH spectrum of a commercial product of poly(sodium 4-styrenesulfonate) (PSSNa) (solid pellet)—and they observed a similar spectrum. This observation verified that the PSS chain was cleansed from the PEDOT:PSS film by treatment with MeOH without any chemical alteration. The authors also discussed that MeOH (with the highest dielectric constant (ε)) induced a charge screening effect between the PSS (negatively charged) and PEDOT (positively charged), resulting in Coulombic interaction reduction between them. This can lead to a significant phase segregation/separation of PEDOT and rich PSS on the nanometer scale, which can be depicted by segregation of the rich PSS phase. Further, the hydrophilic site of MeOH can easily dissolve and enhance the hydrophilic phase separation of PSS and then facilitate its wash away from the PREDOT:PSS film [277].
Dong et al. also used ATR-FTIR spectrum for the analysis of PEDOT:PSS/DMSO films after post-treatment with various bases, including lithium hydroxide (LiOH), sodium hydroxide (NaOH), and potassium hydroxide (KOH), as shown in Figure 15B. The peak (i.e., at ca. 1155 cm−1) was greatly diminished after treatment with base. This signal was ascribed to an asymmetric stretching of PSS (S=O groups) in the form of a proton, showing the conversion of SO3H into SO3. The peak at ca. 1524 cm−1 was assigned to the symmetrical stretching of (Cα=Cβ) thiophene ring, while the reduction in the peak ca. 1557 cm−1 (i.e., shifted to 1547 cm−1) was ascribed to the asymmetric stretching (Cα=Cβ) vibration, together indicating a conversion from a more quinoid structure to a more benzoid structure. In addition, a redshift from 1263 cm−1 to 1249 cm−1 (inter-ring Cα−Cα′ stretching) appeared—indicating that the Cα−Cα′ bond varied from a quinoid to benzoid structure. All these conversions can also support the dedoping process induced by base treatment. Notably, LiOH-treated PEDOT:PSS films showed a noticeable shoulder at ca. 1220 cm−1. This peak was assigned to the S=O groups that stretch from SO3Li+, showing a strong interaction of Li+ and PSS [278].

3.2.2. RS

In addition to UV−Vis−NIR spectra, RS can be employed to elucidate whether the various treatment techniques change the doping level of PEDOT:PSS films. Hence, to explain the influence of pre- or post-treatment on S, interest is directed to the structure PEDOT chains. It has already been verified that the oxidation level/state is vital for the S of PEDOT [279]. Acid/base chemistry, chemical dedoping engineering, and the electrochemical dedoping process can be employed for the transformation of the oxidation state/level [85,279]. It was also pointed out that upon the dedoping engineering, PEDOT can undergo three levels/states, as described in Scheme 1 [264]. The bipolaron level/state in PEDOT chains can be slowly dedoped into a polaron and then a neutral (doped) level. Through dedoping engineering, the σ is reduced whilst the S shows a boosting trend. These three PEDOT states exhibit absorbance at various wavelengths (PEDOT in the bipolaron level/state exhibits a broad absorbance in the near-infrared region (NIR, 700–900 nm) [280]; chains in the polaron state exhibit absorbance close to 900 nm [280]; and the neutral polymer PEDOT chains exhibit absorbance near 600 nm [280,281]).
For instance, Luo et al. employed Raman spectra to elucidate the PEDOT:PSS films treated with various DMSO concentrations and a mixture of DMSO and EMIMBF4, as illustrated in Figure 16A. The absorbance of the untreated PEDOT:PSS film indicated a broad absorbance in the NIR region. There was no clear noticeable transformation in the DMSO-treated film. In contrast, EMIMBF4 treatment introduced a polaron absorption band, indicating the dedoping of PEDOT. In comparison with the partially dedoped PEDOT to the neutral state (600 to 700 nm referencing absorbance), EMIMBF4 only dedoped PEDOT from the bipolaron state to the polaron state. The authors confirmed that along with UV−Vis spectra and XPS analysis, it was confirmed that the EMIM cations dedoped the PEDOT and raised the density of polaron, as it was verified through the enhancement of S and reduction in σ. As displayed in Figure 16A, the spectra of DMSO-treated films appear like that of the untreated PEDOT:PSS film. Nevertheless, EMIMBF4-treated PEDOT:PSS films reveal a narrower band near 1436 cm−1, which was shifted to a smaller wavenumber region in contrast to the untreated PEDOT:PSS film. An additional peak was also observed at ca. 1510 cm−1. The alterations in Raman spectra verified the dedoping of PEDOT chains upon EMIMBF4 treatment [264].
In another study, Kyaw et al. employed RS along with Vis−NIR spectra to explore whether formamide-treated PEDOT:PSS films change the doping states of PEDOT:PSS. As shown in Figure 16B, the Raman peak near 1447 cm−1, which emerged from the five-membered thiophene ring (symmetric Cα=Cβ groups stretching), often shifted to a smaller wavenumber region along with a narrower bandwidth as the doping state diminished from the bipolaron level to the polaron state or neutral level. An insignificant shift in wavenumber (approximately 1 cm−1) and the bandwidth variation in the symmetric Cα=Cβ group stretching indicated that the formamide post-treatment did not alter the doping state noticeably, indeed accounting for a small variation in S value [70].
Wang et al. [82] also applied Raman spectra to study the transformation in untreated camphorsulfonic acid (CSA), as well as BSA-doped PEDOT:PSS films, as illustrated in Figure 16C. The authors pointed out that strong Raman bands were observed between 1400 cm−1 and 1500 cm−1, which were ascribed to the symmetric Cα=Cβ group stretching of PEDOT thiophene rings. The Raman peak appeared at 1441 cm−1 for the pristine film. The redshift in the Raman peak was detected for both CSA- and BSA-doped films, showing a resonant structure transformation from a benzoid to a quinoid configuration, as portrayed in the inset of Figure 16C. This conformational transformation provides the facilitation of long conjugated length and offers mobility of charge carriers. Another study by Wang and colleagues [282] applied Raman spectra to scrutinize untreated, MeOH, DMSO, and trifluoromethanesulfonic acid (TFMS)−MeOH-treated PEDOT:PSS films, as presented in Figure 16D. They showed that the MeOH and DMSO treatment did not cause a noteworthy transformation in its Raman band at about 1421 cm−1, which was ascribed to the stretching vibrations of the Cα=Cβ groups. On the other hand, the Raman band at 1421 cm−1 was downshifted to 1387 cm−1 by TFMS−MeOH treatments—switching from the benzoid configuration to the quinoid configuration in PEDOT chains.

3.3. Elemental Composition Analysis

XPS

The S2p XPS can be employed to investigate the impact of pre- or post-treatment and the interaction of pre- or post-treatments with PEDOT:PSS thin films. The loss of PSS chains from the PEDOT:PSS thin film following treatment can also be verified by using the XPS spectra of the PEDOT:PSS films. The two XPS bands with B.E between 166 eV and 172 eV are due to the S2p band of the sulfur atoms from PSS, while the two doublet XPS bands with B.E between 162 eV and 166 eV originate from the S2p band of the sulfur atoms from the PEDOT chain [283,284,285,286,287]. The S2p XPS band increased for PEDOT compared to that of PSS for PEDOT:PSS films following various treatments, suggesting the removal of rich PSS chains from the PEDOT:PSS films by the treatment.
For instance, Yemata et al. employed XPS spectra for formic acid-, sequential formic acid-, and HZ-treated PEDOT:PSS films. They noticed that the S2p intensity of the PEDOT boosted as compared to that of PSS due to the loss of the rich PSS chains, as illustrated in Figure 17A (i.e., deconvoluted S2p XPS spectra). The B.E values of 165.3 and 164.1 eV, belonging to S2p1/2 and S2p3/2, in the S(2p) core level spectra for PEDOT treated by formic acid were shifted to the low B.E at 164.9 eV and 163.7 eV, belonging to S2p1/2 and S2p3/2 for PEDOT treated by 0.15 wt.% HZ, correspondingly, indicating that the reduction in doping state/level following HZ treatment resulted in a decrease in B.E. These sulfur atoms in the PEDOT were shifted to low energy after the HZ treatment, confirming the dedoping of PEDOT [69].
Li et al. [83] also employed the S2p XPS to investigate the impact of zwitterion treatment on PEDOT:PSS films and study the interaction of zwitterion with PEDOT:PSS thin films. The XPS spectra for PEDOT:PSS, PDOT:PSS/rhodamine 101 (R101) (PR), PEDOT:PSS/N−dodecyl−N,N−dimethyl−3−ammonio−1−propane-sulfonate (DDMAP) (PDd), and PEDOT:PSS/1−(N,N−dimethylcarbamoyl)−4−(2−sulfoethyl) pyridinium hydroxide (DMCSP) (PDm) thin films are portrayed in Figure 17(Ba). The zwitterions—especially R101—induced a remarkable redshift related to the S2p XPS band of PEDOT chains. For PEDOT:PSS, the S2p B.Es were 164.1 eV and 165 eV, and these BE values were shifted to 163.95 eV and 164.9 eV for PDm and 163.85 eV and 164.75 eV for PR. On the other hand, PDd did not cause the redshift related to the S2p B.Es. The redshifts in PDm and PR were ascribed to the electron transfer from DMCSP to PEDOT and R101 to PEDOT, respectively. These were also in good agreement with their π−π overlapping of PEDOT chains. The -R101 caused the redshift in the S2p XPS bands for PEDOT chains following acid treatment. As displayed in Figure 17(Bb), the XPS intensity of the S2p bands of PEDOT remarkably increased compared to that of PSS, owing to the loss of rich PSS chain following the acid post-treatment, whereas the B.Es for PA were almost similar to that of the PEDOT:PSS film. For PRA films, the B.Es were shifted to 163.85 eV and 164.9 eV. Also, the occurrence of R101 induced the redshift related to the S2p XPS bands of PEDOT following the sequential acid and base treatment. Similarly, as shown in Figure 17(Bc), the B.Es were assigned to be 163.85 eV and 164.90 eV for PAB—and these B.E values were shifted to 163.8 eV and 164.85 eV for PRAB. Figure 17(Bd) exhibits the N1s spectra of XPS for PRAB, PRA, PR, and R101. The N1s spectra/bands for R101 were deconvoluted to a broad band spectrum at ca. 399.66 eV (i.e., arises because of the neutral N atom) and a small shoulder at about 401.54 eV (i.e., emerged from NH+ because of the transformation of a proton from the carboxylic group and the nitrogen atom) [69]. The minor N1s band spectra of XPS because of NH+ become significant for both PR and PRA; nevertheless, they become unremarkable for PRAB. The observations showed that the protonation of the R101 N atom occurs through an aqueous solution of PEDOT:PSS (i.e., ca. pH = 2) and the deprotonation through base treatment. Moreover, the major B.E for N1s spectra was blue-shifted for PRAB, PRA, and PR in contrast to that of pure R101. This blueshift in N1s B.E indicated the transfer of an electron from R101 into PEDOT, which is in good agreement with the redshift for S2p B.Es within PEDOT.

3.4. Surface Morphology Analysis

As discussed in the above sections, the spectroscopic studies and XPS tests, together with the diminished thickness, indicate the extraction of rich PSS chains, leading to a boosted PEDOT/PSS ratio in support of rich PEDOT (conductive) grains. Nevertheless, examining the increased TE performance for various pre- or post-treatments by different methods—with almost no reduction in the thickness of the film being observable—shows that the extraction of rich PSS chains alone cannot elucidate the excellent increase in the TE performance of PEDOT:PSS films. Hence, further investigations of the relationship among the morphology, structure, and TE behaviors of the untreated and pre/post-treated PEDOT:PSS films by different methods have to be performed using surface morphological characterization techniques such as AFM and SEM, as well as crystalline structure analysis approaches including XRD and GIWAXS.

3.4.1. AFM

AFM pictures can be taken to characterize the surface morphology before and after various treatments of PEDOT:PSS films, as the change in the AFM pictures can suggest the PEDOT:PSS polymer chains’ conformational transformation. In addition, information about the change in the AFM images can be extracted to elucidate the likely transformation within the morphology, as well as the correlation between TE properties and morphology. For instance, the morphologies of BMIM−TFSI−SFS−F−PEDOT:PSS, SFS−F−PEDOT:PSS, F−PEDOT:PSS, and untreated PEDOT:PSS were studied using AFM (Figure 18A). The treated films had highly non-uniform surfaces, increased particle size, and resulted in a more readily charge carrier transport that yielded an enhanced σ compared to that of the untreated PEDOT:PSS film. As shown in Figure 18(Aa,Ae), the pristine film did not display any apparent grains, showing that the PEDOT-rich chains were well-intermixed with the PSS-rich chains, and the soft shells (PSS-rich domains) typically covered the film. It has already been observed earlier that the treatment can result in strong phase segregation/separation between the rigid core (PEDOT-rich core) and the soft PSS-rich shell and reduction in PSS chains following various treatments of PEDOT:PSS films, leading to improved PEDOT-rich interconnected grains (Figure 18(Ab–Ah)) [264,288]. Following dedoping by ILs, the PEDOT-rich interconnected grains were boosted, leading to an improved σ compared to the untreated PEDOT:PSS film. This may partially address the mechanism for why treatment with ILs can improve PF with a negligible reduction in σ [263].
Luo et al. also employed AFM to examine the possible morphology transformations and the correlation between the σ and morphology of the PEDOT:PSS film following various chemical treatments, as portrayed in Figure 18B. The DMSO-treated PEDOT:PSS film (P/DMSO) showed noticeable elongated grains, which can possibly be due to the loss of excess PSS chains from the film surface. The phase separation and depilation of excess PSS chains were also verified by the surface roughening of the film. The root-mean-square (rms) surface roughness was 1.36 nm, 2.00 nm, 1.28 nm, and 1.6 nm for the untreated, P/DMSO, EMIMBF4-treated PEDOT:PSS (P/IL), and sequential DMSO- and EMIMBF4-treated PEDOT:PSS (P/DMSO/IL) films, respectively. Correspondingly, phase segregation was also found for the P/IL film—and the grains were slightly elongated with short and circular PEDOT-rich grains. Similarly, elongated PEDOT-rich grains were obtained for P/DMSO films, and the subsequent EMIMBF4 treatment resulted in morphology transformation, and hence, circular and short PEDOT-rich grains were produced. The circular/short PEDOT-rich grains rendered the film with low surface roughness. In contrast to P/DMSO (with rms value of 2 nm), the decrease in surface roughness for P/DMSO/IL (with rms value of 1.64 nm) showed the likely reaction of EMIMBF4 with the PEDOT:PSS thin film that constrained the elongation of PEDOT-rich grains, resulting in circular and short PEDOT-rich grains [264].
Another study by Wang et al. reported the PEDOT:PSS film surface morphologies using various post-treatments (i.e., three specimens—including pristine, BSA-, and CSA-doped PEDOT:PSS thin films—were recorded and compared). As shown in Figure 18C (upper row), the untreated PEDOT:PSS film exhibited quite a smooth and uniform surface with an rms roughness value of 1.16 nm. Following BSA or CSA doping, the film becomes rougher with a larger domain size—and rms increased to 1.45 nm and 1.88 nm, respectively. Moreover, the authors recorded the film surface images by using a digital camera (Figure 18C, lower row). Hence, both the AFM pictures and the enhanced rms values suggested a phase segregation between PSS chains and PEDOT chains. These resulted in a PEDOT chain conformational transformation (i.e., from a coil-like configuration to a fiber structure), resulting in more contact points within PEDOT-rich chains for a better transport of charge carriers. It is fascinating to note that CSA-doped PEDOT:PSS films were rougher compared to BSA-doped PEDOT:PSS films, which could be connected to the CSA stereo configuration. These structures dominantly impede the electrical contact within the PEDOT-rich chains to some extent, which consequently influences carrier mobility [82].

3.4.2. SEM

SEM images can be employed to elucidate the surface morphologies [289], the cross-sectional morphologies [290], and the thicknesses (using an accelerating voltage of 20 kilo volt (kV)) [291] for the PEDOT:PSS films before or after various pre- and/or post-treatment techniques. For instance, SEM was applied to study the morphologies of the zwitterions such as rhodamine 101 (R101) and PEDOT:PSS polymer films. As shown by SEM images (Figure 19(Aa,Ab)), a rhodamine 101 (R101)-treated PEDOT:PSS(PR) thin film revealed an extremely smooth surface. There were no clear alterations that can be seen on the surface of the film following sequential/combination treatments of PR with acid and base (PRAB). The EDX pictures of sulfur (S) and nitrogen (N) illustrated that R101 was uniformly dispersed in/on PEDOT:PSS [83]. In another observation, as portrayed in Figure 19B, the cross-sectional SEM pictures of (I) untreated, (II) formamide (CH3NO), (III) CH3NO and then sulfuric acid (H2SO4) (CH3NO/H2SO4), and (IV) CH3NO/H2SO4 and then sodium borohydride (NaBH4) (CH3NO/H2SO4/NaBH4) post-treated PEDOT:PSS thin films revealed distinct morphological differences. The authors confirmed that there was film thickness reduction after CH3NO/H2SO4 treatment due to the depilation of excess-free PSS chains between their (100) planes. In addition, the H2SO4-treated specimen exhibited a microstructure with lamella stacking that spread along the whole cross-section. The formation of a well-ordered microstructure was attributed to the significant loss of rich PSS chains aggregating within metallic grains as insulating domains. In the absence of these insulator domains, the direct and efficient contact within metallic grains will allow charge carriers to move smoothly in the PEDOT:PSS thin film, resulting in high charge carrier mobility. Thereby, σ was significantly elevated without sacrifice of S. Further NaBH4 treatment did not create any noticeable alteration to the film microstructural composition/crystallinity [290].
SEM images were also employed to characterize the morphologies of PEDOT:PSS films after formic acid treatment. As portrayed in Figure 19(Cb,Cc), the SEM pictures showed a segregated darker line, which was created after formic acid treatment and in good agreement with the SEM images obtained after MeOH treatment (see Figure 19D) [277]. These lines were attributed to insulator-rich PSS chains, and they were easily washed away after rinsing with distilled (DI) water. The film thickness was also decreased by about 20 nm to 25 nm following formic acid treatment. The authors also confirmed that there was almost no alteration in σ following DI water rinsing, but they sometimes observed certain film damage due to the high hydrophilic behavior of water. They also noted that PSS chain was only physically present on the surface of the film; nevertheless, functionally, the PSS chain was separated [289]. In another study, the same authors applied SEM to investigate the morphological change for MeOH-treated PEDOT:PSS. Figure 19D displays the SEM pictures of untreated and treated PEDOT:PSS films by using various approaches. The authors indicated that the darker lines observed in the SEM pictures in Figure 19(Dc,Dd) were the separated rich PSS chains which can be easily washed away by dipping in MeOH solution [277].

3.5. Crystalline Structure Analysis

3.5.1. GIWAXS

GIWAXS and grazing incidence small-angle X-ray scattering (GISAXS) have been broadly utilized for the investigation of functional materials with a specific emphasis on thin-film behaviors. GIWAXS data are usually 2D diffractograms comprising diffraction rings of various crystal planes, while GISAXS has a longer detector distance compared to that of GIWAXS. Both GIWAXS and GISAXS can be employed for nanostructured morphological characterization for the orientation and packing of conjugated polymers or aromatic molecules, nanoparticle assemblies, block copolymers, or nanocomposites. The convenient GIWAXS/GISAXS geometric scattering enables attaining complete information regarding the structure of thin films and elucidating specimens in well-described conditions, which can be controlled by coating processes or exposure to temperature, solvent drying, or vapor. In addition, with appropriate detectors and X-ray sources, information regarding phase transitions and ordering kinetics can be achieved down to the millisecond time scale [292,293].
GIWAXS plays a substantial role in investigating polymer PEDOT:PSS TE materials. The PEDOT:PSS thin-film structure can be characterized by GIWAXS in order to obtain more complete information regarding the packing of the PEDOT chains, where it is a very well-proven written protocol to study/characterize the crystalline/molecular structure of polymers lying at the surface or beneath a smooth layer [294,295,296,297]. The 2D GIWAXS with a larger area of mapping is an effective approach to elucidate molecular orientation and stacking. Since AFM tests are restricted to a slight local region of sampling and offer only the surface specific information, the inner PEDOT:PSS thin-film morphologies can be probed by employing GISAXS. Because of the geometry of grazing incidence, outstanding morphological information can be accessed and extracted from GISAXS [298,299]. It has already been verified that it is a vital technique for understanding and quantifying multiple property−structure relationships of thin films, which basically restrict the performance of optoelectronics. The crystalline regions of the sample films concerning structure and orientation can be probed and quantified by GIWAXS. In addition, GIWAXS enables the extraction of information about the ratio of PEDOT/PSS in the crystalline parts. Contrary to XPS, in which the information about the depth of excited electrons is restricted to the topmost surfaces/layers of the film—therefore making it a surface/layer-sensitive technique—GIWAXS measurements can provide guidance for probing and understanding the inner film configuration/structure in conjunction with a film specimen with large volume. This offers comprehensive information/data from the inner film with excellent statistics over a large specimen volume while being non-destructive [48,278,297,300]. In addition, the scattering geometry of GIWAXS and time-resolved GIWAXS is inherently feature-compatible with both under operando and in situ studies/setups (comprising standard protocols, ISOS) [301]. Hence, GIWAXS measurements have to be performed to detect how the various pre-/post-treatments change the overall PEDOT:PSS thin-film structure.
For instance, GIWAXS was used to characterize the semicrystalline structure of the PEDOT:PSS thin film before and after treatment, as presented in Figure 20(Aa–Ad). Corresponding to the surface morphology, the bulk film crystal structure had a significant transformation after the alkali/base solution post-treatments. Figure 20(Ae) displays the line profiles that rely on the full integration extracted from the pictures to attain a better visualization of the structural changes. The lower-angle part featured two clear peaks, including a distance (100) reflection that was situated at the lowest scattering vector (q) values, and the second peak was situated close to 0.48 Å−1. It was clear that the distance (100) peak placed close to q = 0.24 Å−1 for the untreated PEDOT:PSS film decreased in intensity following alkali/base post-treatment (i.e., a reduction in 37%, 20%, and 4% for KOH, NaOH, and LiOH, respectively) and shifted to 0.36, 0.32, and 0.28 Å−1 for the KOH, NaOH, and LiOH-treated PEDOT:PSS films, respectively. This implies that the adjacent (100) plane spacing gradually decreased from the untreated PEDOT:PSS film to the KOH-treated film (i.e., from 26.2 Å for the untreated to 22.4 Å for KOH, 19.6 Å for NaOH, and 17.4 Å for NaOH). The highly compact packing arrangements along the direction of [100] are contemplated to aid the transport of charge carriers within the crystal [302]. Notably, the peak intensity situated near the q value of 0.48 Å−1 along the qz direction displays a strong rise going from the untreated PEDOT:PSS film to the KOH-treated PEDOT:PSS film. There was also a clear peak (100) position shift by the alkali/base metal atom employed, while the peak position at a q value of 0.48 Å−1 was almost unchanged. In addition, the tendency of these two peak intensities was the opposite. The results indicate that the peak at a q value of 0.48 Å−1 was not linked to the second-order peak of (100). Here, two kinds of packing were observed, each one described by a diverse position of (100) peaks, including type I and type II crystals of PEDOT. Type I indicated doped PEDOT surrounded by the PSS chain and showed a large d spacing in the (100) direction, where a peak of (100) was placed toward smaller values of (q). Conversely, type II crystals of PEDOT exhibited slight or even no observable PSS within the crystals of PEDOT, resulting in a smaller (100) d spacing, where a peak of (100) was situated at high values. In this condition, the packing of PEDOT differs from largely type I to a blend of type I and II, based on the behaviors of the employed solution (i.e., basic/alkaline nature) (the estimated ratio of type I/type II was changed from 1.80 for untreated PEDOT:PSS to 1.31 for LiOH, 1.10 for NaOH, and 1.05 for KOH-treated PEDOT:PSS films. These observations showed the gradual loss of PSS chains by the doping process from the crystallites of the PEDOT chain, while the base/alkali treatment caused dedoping. In addition to the transformation in the packing of the PEDOT structure, the excess chains of PSS not directly linked with PEDOT also underwent transformation after post-treatment by base or alkali. The loss of peak of PSS showed a noticeable change from near 1.32 Å−1 for the untreated PEDOT:PSS film to 1.24 Å−1, 1.28 Å−1, and 1.28 Å−1 for LiOH-, NaOH-, and KOH-treated films, respectively. This result implies that the solutions of alkali base had an interaction with the PSS chain within the films, leading to a larger mean distance amongst the chains of PSS (from 4.8 Å for untreated to 5.1 Å, 4.9 Å, and 4.9 Å for LiOH-, NaOH-, and KOH-treated PEDOT:PSS films, respectively). Conversely, the peak of (010), which was linked to the ordering along the direction of PEDOT π–π stacking, was maintained at a similar ca. 1.83 Å−1 peak position, implying that the alkali post-treatment did not influence the inner molecular packing distance for PEDOT along the direction of π-π stacking. Nevertheless, the peak of (010) width transformation was seen following alkali post-treatment, as portrayed in Figure 20(Ae). The broadening of the detected peak of (010) was small for LiOH-treated PEDOT:PSS; nevertheless, it was not slight for the remaining two specimens. Furthermore, the peak of (010) exhibited a stronger intensity improvement along the qy in-plane direction (horizontal) that implied edge-on orientation as well as a reduction along the qz near-out-of-plane direction (vertical), which showed face-on orientation during film treatment, as illustrated in Figure 20(Ae) for the profile of azimuthal intensities. The estimated edge-on and face-on crystal fractions for untreated DMSO/PEDOT:PSS films were 12.3 and 53.3%, respectively, whereas these fraction values were altered to 18.1 and 48% for the alkali-treated specimens, as estimated in previous work reported by the same authors [303]. This observation implied that the post-treatment with alkali/base resulted in a more stable edge-on orientation for the crystallites of PEDOT. This change in the orientation of the crystal could somewhat compensate for the σ loss, restricting the droplet in σ. In general, the GIWAXS observations, together with the AFM observation, noticeably showed that elongated grains present in the untreated PEDOT:PSS film were created using heavily doped PEDOT crystals with favored face-on orientation, whereas minor globular domains existing in the treated films were created by lesser doped (or even with neutral) minor crystals of PEDOT with a slight noticeable orientation, although they still mostly exhibited a face-on orientation [278].
As shown in Figure 20B, Park et al. also explored the distinctive effects of PEDOT:PSS film treatment by trifluoromethanesulfonic acid (TFSA) on intra-grain transport by examining GIWAXS results/patterns. In contrast to untreated PEDOT:PSS films, which have an amorphous structure, the MeOH and TFSA treatments caused the appearance of multiple strong diffraction peaks [304].
In another work, 2D GIWAXS information/data were further depicted for (i) sulfuric acid (H2SO4), (g) nitric acid (HNO3), (e) formic acid (HCOOH), (c) hydrochloric acid (HCl), (b) ethylene glycol (EG)-treated films, and (a) untreated PEDOT:PSS film, as portrayed in Figure 20(Ca–Ci); the obtained Figure 20(Cd,Cf,Ch,Cj) reveals additionally H2SO4-, HNO3-, HCOOH-, and HCl-treated films which were subjected to H2O rinsing. The spots creating the results/patterns in 2D space clearly display distinguishable Bragg peaks in the qz direction—originating from lamellar stacking of the alternating PEDOT chain and PSS chain—that were usually observed in both GIWAXS and XRD observations in PEDOT:PSS films [300,305,306]. The H2SO4-treated sample in Figure 20(Ci) exhibited the most particular Bragg peak positions in the qz direction, which were attributed to a much higher order and were accompanied by the highest σ values among the studied treatment approaches. This can support the link between well-improved order in the crystalline structure and higher σ. Nevertheless, this higher order appears to be destroyed by following H2O rinsing/washing. Moreover, as shown in Figure 20(Ca) for the pristine specimen, a higher-intensity non-oriented signal was observed at about q ≈ 1.3 Å−1, which can be ascribed to a π-π stacking distance of ca. 4.8 Å between PSS in the amorphous PSS chains [305]. Remarkably, the signal for amorphous PSS was diminished for the various treatments, showing the depilation of the rich PSS chains in the thin films. The slightest signal in amorphous PSS chains was observed for the H2SO4- and HNO3-treated specimens, which went together with the significant thickness reduction induced by these treatments (Figure 20(Ca)). Below the diminished amorphous chains of PSS signal, additional oriented Bragg peaks were observed, particularly for H2SO4- and HNO3-treated films (Figure 20(Cg,Ci). Particular Bragg peaks at about 1.25 Å−1 and 60° were observed, perhaps created from an oriented chain of PSS π−π stacking, which was created by a templating impact of crystallizing and neighboring PEDOT. Together with the depilated content of the PSS chain, an extra oriented Bragg peak at about a q value of 40° and 1.75 Å−1 was discovered—this might be assigned to an extra orientation in π−π stacking of PEDOT within the edge-on and face-on orientation. For all that, the above-mentioned scattering patterns/results were a further explanation for higher crystallinity. The change/shift in the orientation of stacking distance within edge-on and face-on was observed earlier; however, it is often enclosed with the signal originating from the profile of highly non-oriented π−π stacking of PEDOT. Nevertheless, in this situation, the peak was visibly detached from the π−π stacking of PEDOT between edge-on and face-on [300].

3.5.2. XRD

XRD approaches are powerful and non-destructive characterization instruments with minimal specimen preparation. XRD analysis can be applied to scrutinize both crystalline materials (i.e., materials having long-range order of atomic arrangement) and noncrystalline materials (i.e., materials revealing a short order of atomic arrangement). XRD offers the first information in terms of crystal defects, degree of crystallinity, texture coefficient, orientation parameter, micro and macro strain, average crystallite size, crystalline structure, phase of materials, and electron density. When radiation impinges on a solid, scattering of the coherent radiation through periodically spaced samples of atoms yields scattered beams that create both spot features from a single crystalline specimen and ring patterns from a polycrystalline specimen. The patterns/results and intensities of the diffraction maxima (lines or peaks), as well as their position (interplanar spacing dhkl or Bragg angle θ), can be correlated to a specific crystal structure. XRD characterization gives information about the bulk and polycrystalline thin films, as well as multilayer structures, which is vital in various scientific and material science/engineering disciplines [307,308,309]. While infrared spectroscopy (IR), mass spectrometry (MS), and nuclear magnetic resonance (NMR) are better suited for gases and liquids (and solids in relation to NMR), XRD can elucidate the crystal structure [307].
For instance, Figure 21A displays the XRD results of the PEDOT:PSS thin films treated by different concentrations of H2SO4. For the untreated PEDOT:PSS (i.e., for pristine without any treatment by H2SO4), the XRD results possessed four typical peaks—25.6° (d = 3.5 Å), 17.7° (d = 5.0 Å), 6.6° (d = 13.4 Å), and 2θ = 3.8° (d = 23 Å)—in which the lattice spacing (d) was determined by employing Bragg’s law. The high-angle reflections at 2θ = 25.6° and 17.7° were attributed to inter-chain interaction stacking structure distance (010) of PEDOT and the amorphous halo of PSS, respectively, while the two lower 2θ peaks at 6.6° and 3.8° were indexed to a lamellar stacked distance (100) for the two distinct alternate orderings of PSS and PEDOT. XRD results showed that the PEDOT:PSS films slowly changed with the increase in the concentration of H2SO4. Following 20% and 50% H2SO4 treatment, the 2θ peaks at 6.6° and 3.8° were shifted to a lower angle by approximately 0.4° with an increase in intensity in the XRD information, which shows enhanced crystallinity and a raised lamella stacking distance. Nevertheless, in accordance with the σdc tests, notable alterations were seen following treatment with 80% H2SO4, in which PEDOT:PSS displayed two stronger d(100) 2θ peaks at 6.2° and 4.4°, with second-order d(200) 2θ peaks at 13.3°and 9.2°. On the other hand, the PEDOT:PSS treated by the 100% H2SO4 sample displayed only a stronger 2θ peak at 6.2°, with a second-order reflection peak at ca. 13.3°. The observations showed that the PEDOT:PSS film preferred a specific type of lamellar stacking for the two separate alternating orderings of PSS and PEDOT with the increase in H2SO4 concentrations [67].
In addition, pristine, DMSO-treated, and TFMS−MeOH-treated PEDOT:PSS thin films were examined by XRD to elucidate the mechanism of the enhanced TE performance, as portrayed in Figure 21B [282]. The two peaks at 2θ = 25.6° and 3.5° in the untreated PEDOT:PSS film were assigned to the lattice d spacings of 3.5 Å and 25.2 Å, as estimated using 2d sin θ = λ (Bragg’s law). The d spacing of 25.2 Å, which was discovered at 2θ ≈ 3.5°, was ascribed to the lamellar stacking distance (100) because of the distinct alternate ordering distance in the PEDOT chain and PSS. This lamella stacking distance was in good agreement with the widths of the PEDOT chain and PSS chain, which were 7.5 Å and 15.5 Å, respectively, as calculated based on chemical structural simulation [310]. In contrast, the d spacing at ∼3.5 Å detected at about 2θ = 25.6° was ascribed to the π−π stacking distance (010) in the PEDOT. Following treatment with TFMS−MeOH, there was negligible alteration within the lamella stacking distance, ranging from 25.2 Å to 23.2 Å, whereas the π−π stacking distance gradually decreased within the range of 3.5 to 3.4 Å. The decreases in π−π stacking distance imply that both the PEDOT chain and PSS chain were changed from the benzoid configuration to the quinoid configuration and hence attained a more planar structure following treatment by TFMS−MeOH. Moreover, the peak linked with the distance (010) plane was reduced compared with that of the distance (100) plane following treatment with TFMS-MeOH, implying this treatment resulted in a shift in the PEDOT:PSS layer orientation along the perpendicular direction to the substrate. In contrast with untreated PEDOT:PSS XRD peaks, the TFMS−MeOH-treated thin film resulted in shaper diffraction characteristic peaks with a higher intensity within the lower-angle reflections at 2θ = 3.8° and 6.6°, assigning distinctly to the lamella stacking distance (100) of two separate alternate orderings for the PEDOT chain and PSS chain, indicating a high degree of crystallization in PEDOT:PSS thin films [282].
Furthermore, XRD analysis was used to inspect the crystallinity of the PEDOT:PSS thin films treated by a combination of HNO3 and 1−butyl−3−methylimidazolium trifluoromethanesulfonate ([bmim][OTf]), or 1−butyl−3−methylimidazolium tetrafluoroborate ([bmim][BF4]), as depicted in Figure 21C. The HNO3-treated PEDOT:PSS (denoted as N−PEDOT:PSS) film exhibited a huge transformation in the intensity spectra of d(100), along with a noticeable existence of the second-order diffraction peak of d(100) at about 2θ = 13.0°, whereas a negligible transformation within the distance of lamella stacking for the two separate alternative alignments of the PEDOT and PSS was discovered from 13.0 to 13.4 Å, and the distance of π−π stacking was also slightly raised from 3.4 Å to 3.5 Å. The increase in π-π stacking distance upon HNO3 treatment showed the conversion of PEDOT from a benzoid structure into a quinoid structure, leading to the creation of a more planar configuration. In addition, the diffraction peak distance (100) intensity was considerably elevated following HNO3 treatment. Compared with N-PEDOT:PSS, the diffractions of distance (100) at the lower angle for both ([bmim][OTf])-treated N−PEDOT:PSS and ([bmim][BF4])-treated N−PEDOT:PSS films were somewhat shifted to about 6.6°, assigning to the lattice a d spacing of ca. 13.2 Å, and a slight decrease in intensity of the diffraction signal was discovered, creating a decrease in crystallinity that is possibly pertinent to the reduction in σ following post-treatment by ILs. The distance of diffraction peak intensity (100) was notably enlarged, which was ascribed to the rise in thin-film crystallinity and an increase in several ordered aggregates within the interchain synergic π−π stacking in the PEDOT chains. Additionally, the inclusion of IL preferentially elevated the π−π coupling within PEDOT:PSS, boosting the crystallinity and charge carrier mobility. Hence, the PEDOT:PSS thin films treated by the combination of HNO3, [bmim][OTf], and [bmim][BF4] resulted in an enhanced PEDOT:PSS interchain coupling and densely packed PEDOT chains with lamella stacking within the two kinds of assemblies, resulting in elevated S of the PEDOT:PSS thin films by interface scattering. Importantly, interface scattering comprises grain boundary scattering and surface scattering. The interface scattering mechanism in the charge carrier transport of PEDOT:PSS thin films has an essential function in both the σ and S of PEDOT:PSS films. The post-treatment removed PSS along with the increase in crystallinity of PEDOT, resulting in transformations of the grain size and grain boundaries of PEDOT, therefore improving TE performance [311].
In another study, the XRD was employed to understand the mechanism of TE behavior improvement in PEDOT:PSS thin films treated with CH3NO, CH3NO−H2SO4, and CH3NO−H2SO4−NaBH4, as shown in Figure 21D. In addition to the depilation of excess/rich PSS chains, another cause for the largely elevated diffraction peak intensities was the change in the molecular configuration of PEDOT chains from a benzoid configuration to a quinoid structure, caused by the post-treatment with H2SO4. In the average molecular configuration change, a C−C single bond within two different monomers was substituted through a π bond. The quinoid behavior prefers the PEDOT chain extension, resulting in an improved crystallized molecular configuration. In CH3NO−H2SO4−NaBH4-treated PEDOT:PSS, the treatment by NaBH4 solution (a strong reducing agent) was aimed to boost the S and σ of the PEDOT:PSS specimen by modulating its level of oxidation. This reduction process was reversed into the oxidation process by H2SO4e post-treatment, causing a molecular configuration reversion from quinoid behavior to benzoid behavior. In this XRD result, reductions in the intensities of the first diffraction peak positions of the two doublet peaks and a minor shift/move to higher 2θ values were observed, originating from this reversed change/transformation. With the removal of quinoid behavior, more compact and coiled conformations of PEDOT chains were detected [290].
Figure 21. (A). XRD results for H2SO4-treated PEDOT:PSS thin films. Adapted with permission from ref. [67] Copyright 2014, Wiley. (B). XRD results of pristine PEDOT:PSS thin films with various solvent treatments. Adapted with permission from ref. [282] Copyright 2018, The Royal Society of Chemistry. (C). XRD results for (d) [bmim][BF4] −N−PEDOT:PSS; (c) [bmim][OTf] −N−PEDOT:PSS; (b) N−PEDOT:PSS; and (a) pristine films. Adapted with permission from ref. [311] Copyright 2020, The Royal Society of Chemistry. (D) XRD patterns for PEDOT:PSS thin films with various chemicals under different conditions. Adapted with permission from ref. [290] Copyright 2019, American Chemical Society.
Figure 21. (A). XRD results for H2SO4-treated PEDOT:PSS thin films. Adapted with permission from ref. [67] Copyright 2014, Wiley. (B). XRD results of pristine PEDOT:PSS thin films with various solvent treatments. Adapted with permission from ref. [282] Copyright 2018, The Royal Society of Chemistry. (C). XRD results for (d) [bmim][BF4] −N−PEDOT:PSS; (c) [bmim][OTf] −N−PEDOT:PSS; (b) N−PEDOT:PSS; and (a) pristine films. Adapted with permission from ref. [311] Copyright 2020, The Royal Society of Chemistry. (D) XRD patterns for PEDOT:PSS thin films with various chemicals under different conditions. Adapted with permission from ref. [290] Copyright 2019, American Chemical Society.
Spectroscj 03 00024 g021
Table 1. Summary of spectroscopic and microscopic techniques applied for characterization of TE properties of some typical PEDOT:PSS polymers reported in recent years.
Table 1. Summary of spectroscopic and microscopic techniques applied for characterization of TE properties of some typical PEDOT:PSS polymers reported in recent years.
PEDOT:PSS-Based TE Materials
Through Various Post-Treatments
CharacterizationAchievementsPF
(μW/mK2)
σ (S/cm)S
(μV/K)
Ref.
Pristine (untreated) PEDOT:PSS XPS, UPS, RS, AFM, XPS, CVs, RS, UV−Vis−NIR, and XRDThe k and ZT of pristine PEDOT:PSS were measured to be <1 W/mK and in the order of 10−6, respectively<0.04<115–18[69,70,83,282,312,313,314,315,316]
Triflic acidRS and UV-Vis-NIRTE properties were enhanced by both σ and S simultaneously94168623.6[317]
Tetrahedrite (TH) Cu12+xSb4S13SEM, RS, and AFMThe greatest TE performance/properties were attained for a pellet specimen with a ZT value of ca. 0.12 at 473 K0.1980120[318]
Cu0.98Zn0.02FeS2/PEDOT:PSSSEM, XRD, RS, and EDXNontoxic, inexpensive, and abundant Zn-doped chalcopyrite (Cu1−xZnxFeS2 with x = 0.03, 0.02, and 0.01) was combined with PEDOT:PSS19.118.2−61.3[319]
Cu0.98Zn0.02FeS2/PEDOT:PSS/grapheneSEM, XRD, RS, and EDXThe optimum film preserved above 80% of the σ after bending cycles of 2000, and a five-leg TE prototype built for optimum ternary films produced 4.8 mV at ΔT = 13 °C. ∼23.777.4−61.3[319]
DMSO/PEDOT:PSS/SWCNTs/NaBH4AFM, SEM, XPS, RS,A home-made TE device, manufactured employing eight pieces of PEDOT:PSS/SWCNTs films, exhibited a power output of 391 nW for ΔT = 20 K411171849[320]
HNO3/passing N2 gasXPS, UV−Vis−NIR, AFM,
CVs, XRD, and RS
The HNO3 treatment was responsible for the loss of insulating excess PSS while the N2 gas pressure was responsible for the PEDOT chain conformational change94.32693-[312]
Polyaniline, polypyrrole, and polythiopheneAFM, FE−SEM, FR−IR, UV−Vis, TEM, and XPSThe organic composite polymers offer high flexibility, PF, and efficient TE devices 51274483[321]
Formic acidSEM, AFM, XPS, and UV−VisFormic acid—possessing a higher dielectric constant—screened the charge between the PEDOT chain and PSS chain, leading to phase separation between them-2050-[289]
Formamide/SFSAFM, UV−Vis−NIR, RS, XPS, and XRDThe cross-plane k of the untreated/pristine film was decreased from 0.59 W/mK for the untreated/pristine film to 0.29 W/mK for the SFS-F-PEDOT:PSS film, resulting in a ZT value that ranges from ca. 0.07 to ca. 0.14 at 300 K185.869351.8[316]
H2SO4, NaOH(DMSO)/(N−DMBI)AFM, UV−Vis, RS, XPS, and UPS The highest PF for solid polymer films was established 765.1186464.1[43]
Femtosecond laser irradiationUV−Vis−NIR, RS, XRD, and TGAA simple, environmentally friendly strategy without any chemical dopants and treatments was established19803.1 -[322]
Femtosecond laser/ammonia vapor atmosphere UV−Vis−NIR, RS, XRD, and TGAThe prepared organic TE device shows an output voltage of 5.3 mV at a ΔT = 40.6 °C28.24413.1926.15[323]
Femtosecond laser irradiation/EMIM-DCA (IL)UV−Vis−NIR, RS, XRD, and TGAA TE device generated a voltage of 0.8 mV at ΔT = 11.7 K40.89960.720.63[324]
EMIM:TCMUV−Vis−NIR, RS, and XRSThe hybrid showed the best performance by optimizing both the σ and S175116338.8[297]
Bi2Te3/rGOXRD, RS, SEM, and XPSA 12-fold increase was exhibited as compared to the original PEDOT films by greatly enhancing TE performance93.161522.424.7[325]
H2SO4/NaOH/Vitamin C/
EMIM:DCA
XPS, RS, AFM, FESEM-EDS, and UV−Vis−NIR The greatest ZT value of 1.05 (world record) was recorded for polymer composites12851043111[65]
H2SO4 and TDAE solutionUV−Vis−NIR, RS, XPS, and AFMThe water treatment before the TDAE solution treatment is crucial for the high 526155258.2[84]
Anthracene, naphthalene, and pyreneUV−Vis, XPS, SEM, AFM, and UPS TE properties that were enhanced by the splitting of the lower polaron energy level of PEDOT and inducing the π−π overlapping between the conjugated PEDOT and aromatic compounds 289140445.5[86]
MeOHXPS and UPSMeOH treatment partly restored the electrical behaviors of the denatured PEDOT:PSS films0.6422.6916.9[150]
PEGL with MeOH8.66345.2015.8
Bi0.5Sb1.5Te3AFM, FE−SEM, XRD, SAXS, WAXS, and UV−Vis−NIR40 wt.% BST particles with a mean size of 1.389 mm at room temperature501050215[326]
Acid (H2SO4)/base (NaOH)/
(DMSO)/DMSO solution of
TTF
AFM, XPS, RS, and UV−Vis−NIRGreatly enhanced the S but did not affect σ too much, thus resulting in a huge improvement in the overall TE properties and obtained a ZT value of 0.80 ± 0.041285 ± 672554 ± 16171.0 ± 4.1[77]
MeOHFTIR, XPS, SEM, UV-Vis, XPS, and AFMEnhanced σ by four orders of magnitude-1362-[277]
Formic acidXPS, RS, and AFMZT of ca. 0.32 was recorded by employing the Harman technique at room temperature80.6190020.6[327]
H2SO4, water, and ethanol solution of TDAEAFMOutstanding TE behaviors/properties by the sequential treatments526155258.2[84]
DMSO AFM, UV−Vis, RS, and XPSDMSO post-treatment resulted in a boosted σ and S. 30.1930.4117.99[264]
FormamideXPS, AFM, RS, and UV−Vis The post-treatment decreased the cross-plane k from ca. 0.54 to ca. 0.19 W/mK, resulting in a ZT = 0.04 at room temperature88.7≈292917.4[70]
HNO3/imidazolium-based ILsAFM, UV−Vis, RS, XPS, and XRDThe κ correspondingly reduced from 0.6 to 0.3 W/mK for the pristine and treated PEDOT:PSS films. ZT = ca. 0.12 was attained at 300 K. 11.21260 ± 6134.8 ± 1.8[311]
Formic acid/HZAFM, UV−Vis, RS, and XPSThe substantial improvement in the S and the PF was ascribed to the depilation of the PSS chain—and more essentially—the decrease in the PEDOT doping level by HZ treatment93.551442.7[69]
DMSO/EMIMBF4AFM, UV−Vis, RS, and XPSThe corresponding ZT was obtained to be 0.068 by assuming a k = 0.17 Wm/K at 300 K38.46--[264]
Formamide/SFS/imidazolium-based ILsAFM, UV−Vis, RS, XPS, UPS, and XRDAn optimized PF of ~239.2 μW/K2m was obtained after sequential post-treatment ~239.264161.1[263]
CH3NO)/H2SO4/NaBH4AFM, UV−Vis, XPS, SEM, and XRDThe TE device resulted in a higher output power density = ca. 1 μW/cm2 using the human arm as a heating source141178628.1[290]
TFAAFM, UV−Vis, RS, and XPS More importantly, ~80% of the S and σ was retained after 20 days 97.1 ± 5.4374816.0 ± 1.1[313]
DMSO/H2SO4AFM, RS, SEM, and XPSThe TE device exhibited an output power = 2.25 nW using a ΔT of 25 K 80.84464-[328]
TFMS−MeOHAFM, UV−Vis, XPS, RS, and XRD At the optimized conditions, the ZT value was found to be 0.19. 142298021.9[282]
Sulfuric acid/PSSH In situ RSThe enhancement in the S by polyelectrolytes was ascribed to the EF resulting from the individual ion Soret effect 401212043.5[329]
Sulfuric acid/base/PSSNaIn situ RSThe EF resulting from the ion individual Soret effect by polyelectrolytes enhanced the S401173248.1[329]
H2SO4XRD, HAADF−STEM, and UV−Vis−NIRThe concentrated H2SO4 post-treatment resulted in highly ordered crystalline PEDOT:PSS nanofibrils-4380-[67]
HNO3AFM, UV−Vis−NIR, XPS, RS, and XRDHNO3 post-treatment significantly enhanced the σ value with a small drop in S76.0 ± 5.33200 ± 89 16 ± 1.2[311]
[bmim][OTf]/HNO3UV−Vis−NIR, AFM, XPS, RS, and XRDThe PEDOT:PSS films were stable, and more than 85% of their σ and S values were retained at 75% RH and 70 °C for 20 days∼152 ± 11.21260 ± 61 4.8 ± 1.8 [311]
[bmim][BF4]/HNO3AFM, UV−Vis−NIR, XPS, RS, and XRDMaintained excellent long-term stability at 75% RH and 70 °C for 20 days137 ± 12.51188 ± 4533.9 ± 1.9[311]
PSVA using DMSOFE-SEM, XPS, UPS, and GIWAXS The PSVA treatment approach increased the EF impact of PEDOT:PSS/Bi2Te3 NWs226102647[276]
DMSO/HZAFM, RS, XPS, and UV−Vis−NIR An optimized PF was obtained for HZ (0.0175 wt.%) in DMSO-treated film at room temperature. 1122142[66]
Alkali Base (KOH)UV−Vis−NIR, AFM, GIWAXS, and ATR−FTIR The post-treatment approaches employed were simple and green alkali–base solutions50.018451.9[278]
MeOH/TFSA AFM, GIWAXS, RS, and XPS A secondary vapor treatment method was employed104.2205322.5 [304]
MSARS, XPS, and UV−VisThis treatment approach induced secondary doping and significantly improved σ75299815.8[56]
Acid (MSA)/Base (NaOH)RS, XPS, and UV−VisThe enhancement in S and PF can be ascribed to the partial dedoping of the PEDOT chain by NaOH258183537.5[56]
Acid/base/R101 SEM, EDX, FS, RS, XPS, UV−Vis, and UPSThe k of pristine and PRAB were measured to be 0.204 and 0.234 W/mK, respectively, as recorded by the laser flash method. Hence, the optimal ZT = 0.46 achieved for PRAB. R101 (with a conjugated structure) provided the highest increase in S. The boost of S was ascribed to EF caused by the zwitterion dipole moment and the π−π overlapping between PEDOT:PSS and conjugated R101.546-61.6[56]
Acid/base/DMCSPRS, XPS, FS, UV−Vis, and UPSDMSCP (with a conjugated pyridine ring) offered modest S385-55.5[56]
Acid/base/DDMAPRS, XPS, UV−Vis, and UPSDDMAP (with a saturated structure) offered the slightest improvement in S323-46.8[56]
Ti3C2TxXRD, SEM, UPS, XPS,
and UV−Vis
Enhanced S for a p-type polymer/n-type filler TE composite has been discovered for the first time. The enhancement in S was ascribed to the EF effect. 140.1736.443.6[275]
H2SO4UV−Vis, RS, and AFMDetailed Raman map analysis clarified microstructural behaviors distributed in large areas caused by dopants-4358-[330]
Abbreviations: Thermoelectric properties: Electrical conductivity (σ); power factor (PF); thermal conductivity (k); Seebeck coefficient (S); and figure of merit (ZT). Various treatment materials: 1-ethyl-3-methylimidazolium dicyanamide (EMIM:DCA); polydimethylsiloxane (PDMS); 1-ethyl-3methylimidazolium: tricyanomethanide (EMIM:TCM); reduced graphene oxide (rGO); 1-ethyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]); lithium tetrafluoroborate (LiBF4); 1-butyl-3-methylimidazolium trifluoromethanesulfonate- [bmim][OTf]; sodium formaldehyde sulfoxylate (SFS); 1-Ethyl-3-MethylImidazolium DiCyAmide (EMIM-DCA); 5 wt.% glycerol loaded into the PEDOT:PSS solution (PEGL); 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4); Vf = volume fractions; 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4); rhodamine 101 (R101); 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide (BMIM-TFSI); N-dodecyl-N,N-dimethyl-3-ammonio-1-propane-sulfonate (DDMAP); tetrahedrite (TH) (Cu12+xSb4S13); 1-(N,N-dimethylcarbamoyl)-4-(2-sulfoethyl) pyridinium hydroxide (DMCSP); NanoMap-PS (NM-PS); Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS); nitric acid (HNO3); nitrogen (N2); sulfuric acid (H2SO4); dimethyl sulfoxide (DMSO); sodium hydroxide (NaOH); bismuth telluride (Bi2Te3), tetrakis(dimethylamino)ethylene (TDAE); methanol (MeOH); tetrathiafulvalene (TTF); hydrazine (HZ); 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4); sodium formaldehyde sulfoxylate (SFS); formamide (CH3NO); sodium borohydride (NaBH4); trifluoroacetic acid (TFA); trifluoromethanesulfonic acid (TFMS); Poly(4-styrenesulfonic acid) (PSSH); 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI); poly(sodium 4-styrenesulfonate) (PSSNa); nitric acid (HNO3); polar solvent vapor annealing (PSVA); potassium hydroxide (KOH); trifluoromethanesulfonic acid (TFSA); methanesulfonic acid (MSA); MXene (Ti3C2Tx); bismuth antimony telluride alloy (Bi0.5Sb1.5Te3); Single-walled carbon nanotubes (SWCNTs); Zn-doped chalcopyrite (Cu1−xZnxFeS2), and 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI). Characterization techniques: Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR); cyclic voltammograms (CVs); ultraviolet–visible–near-infrared (UV-Vis-NIR) spectroscopy; small-angle X-ray scattering (SAXS); Raman spectroscopy (RS); fluorescence spectroscopy (FS); ultraviolet photoelectron spectroscopy (UPS); scanning electron microscopy (SEM); atomic force microscopy AFM; field emission scanning electron microscopy (FE-SEM); X-ray photoelectron spectra (XPS); thermogravimetric analysis (TGA); grazing incidence wide-angle X-ray scattering (GIWAXS); and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).
Table 2. Comparison of various modern characterization approaches in terms of sample preparation, main information, advantages, and limitations for analysis of materials.
Table 2. Comparison of various modern characterization approaches in terms of sample preparation, main information, advantages, and limitations for analysis of materials.
TechniquesSample PreparationMain InformationAdvantagesLimitationsRef.
SEM
Conductive sample with an area not exceeding 10 by 4 cm—in solid form and free from moisture—if not with coating
Needs extensive sample preparation in high vacuum or low pressure
Surface topography, size, and elemental composition of specimens using EDS or WDS, surface morphology, structure/shape, and crystallography of materials
Offers 3-D images
Great resolution of 1 nm with magnification up to 300,000 times
Employed to determine the elemental composition of the specimen
Operates in a dry and high-vacuum environment
It is a real-time imaging technique of nanoparticles (NPs)
  • Not precise for tests < 10 nm
  • Restricted to solid and non-organic specimens that are not too large to be put into the vacuum pressure
  • All the specimens must be durable to lower pressure
  • Operates in high vacuum around 10−3 pa and changes the environment of the sample, which affects the accuracy
  • It does not provide any internal information owing to the restricted penetration depth and introduces artifacts and changes the original state of composites
  • Expensive and time-consuming
  • Have little but quite possible radiation risk exposure owing to the electrons scattered from the surface beneath the specimen, and researchers are recommended to take safety measures
[117,331,332,333,334,335,336,337,338,339]
FE-SEM
Specimen synthesis can take multiple forms, despite conductive or drying coating being a common approach
Surface morphology, elemental composition of the specimen by employing EDS or WDS, sample surface, size, and geometry
  • FE-SEM image capability is better than that of SPM and STM
  • FESEM delivers resolution at the nanoscale, usually between 1 nm and 5 nm
  • Costly, restricted to solid, and requires coating to avoid charging
[336,340]
SEM-EDSHas an energy of ca. 40 keV
Can be used for various samples
Chemical composition, external morphological, elemental analysis, and dispersion, nanostructure
Qualitative data about the specimen composition
Offers 2-D images of superior resolution and shows the geometry of a specimen and spatial variations
The SEM images showed prismatic crystals with a length and diameter of ca. 1000 and 150 nm, respectively
[338,339,341,342]
EDS The samples prepared for SEM can be analyzed/detected directly by EDS. Nevertheless, if there is specimen coating owing to poor conductivity, the coating material which can be detected/analyzed in the EDS spectra requires a careful selection of coating material so that it cannot overlap with the elements present in the specimen Chemical formula and elemental/chemical composition of the sample
Can be used for a bulk analysis with a higher depth of typically 1 µm with a larger area
Rapid analysis time
Provide the mass fraction of each element for the detecting sample
EDS is usually combined with SEM, as both SEM and EDS share a similar electron beam
  • Restriction in detecting/analyzing light elements
  • Specimens are required to be conductive
  • It cannot be employed for the quantification of ions
  • The typical limits of detection for EDS are 1000 ppm or 0.1 wt.%
  • The element identification (quantitative analysis) using EDS is more efficient for elements with superior atomic mass (greater than or equal to 20)
[117,343,344]
AFM
Specimens no greater than 10 mm by 10 mm with up to 2 mm thickness
Dry and liquid specimens with or without vacuum
Morphology, distribution, surface roughness, surface structure, mechanical and electrical properties,
size (10−20 nm lateral resolution, <1 vertical resolution), and 3−D shaping
Superior resolution image of thin materials
Non-destructive to the specimen
Can utilize automated specimen preparation
Generates topographical and 3−D photographs of the specimens in superior quality
Extremely compatible with various specimens and test environments
Quantify 3D information with extremely superior vertical resolution (~0.01 nm), saving money and time
Conduct non-destructive imaging and mechanical tests in any desired environment (liquid, vacuum, air) with atomic-scale spatial resolution
  • Sample preparation may require spin coating
  • Limited to surface analysis
  • Tapping mode and contact mode of analysis can result in specimen damage and tip contamination
  • Restricted to conducting and semi-conducting specimens
  • Expensive and time-consuming specimen analysis and synthesis
[117,331,332,333,334,335,336,337,342,345,346,347]
TEM
Specimen is required to be small and thin (≤150 μm thickness and <2.5 mm diameter)
Specimen synthesis/preparation is complex and can be performed only by a trained skilled expert
Conventional TEM sample synthesis comprises mechanical grinding and polishing, and dimpling, as well as ultimately ion thinning
Microstructure, shape, NP size, aggregation state, topology, elemental, crystal structure, conductivity or magnetics, growth kinetics of composites, defects in the specimen, and detect and quantify NPs in matrices
Offer internal configuration/structure with proper detectors
Can be employed to estimate the film thickness with superior resolution up to 50 pm
Superior for investigating the agglomeration state of the particles
Can be employed for the in situ analysis (i.e., can be applied to study the dynamical phenomenon on the nanoscale
It can be employed to determine the internal composition of materials
Operates on low voltage and cannot destroy the specimen as SEM can
  • Difficult and time-consuming process
  • Images do not indicate topography data for volatile or wet specimens
  • Require expensive and special maintenance
  • May not be suitable for thick specimens (greater than 100 µm)
[117,332,333,335,336,342,346]
XPS
Requires flat and clean specimen surfaces
Does not require special synthesis
Specimen preparation methods like cleaning, puttering, and polishing are usually necessary to avoid roughness, oxides, and surface contaminants
Composite area of 5−10 mm2 with specimen depth not exceeding 4 mm
Chemical states, elemental composition, oxidation states, specimen surface, electronic structure,
crystallographic, surface, degradation, and functional group
No sample damage
Chemical bonding
Excellent method for estimating the oxidation state of the elements in a specimen
Determine particle chemical states, topography, size, and shape
Identification and/or quantification of elements
To identify chemical states and elemental composition
Quantitatively show data in terms of the empirical formula, chemical and electronic levels/states of elements in the specimen, and their interactions with metal centers
Identify the photoelectric effect to elucidate the elemental composition, electronic structure, and oxidation states of elements within a material
Useful to estimate the oxidation state of the element
Sensitive surface analysis
Offers qualitative information about contamination and the nature of the nanocomposite
  • Surface-sensitive, surface charging, limited depth profiling, restricted sensitivity for light element detection (e.g., helium and hydrogen), and elements with a lower atomic number (e.g., nitrogen and carbon,)
  • Difficult to distinguish between subtle chemical variation and chemically analogous species
  • Poor lateral resolution
  • Difficult for functional groups analysis
  • The limit of detection is analogous to EDS at 1000 ppm
  • The nanoparticle size (ca. 10 nm depth profiles)
  • Prolonged processing time
  • Cannot detect helium and hydrogen
  • Solid specimens only
  • Specimens have to be protected from contamination
  • Specimen storage is substantial
[117,118,331,332,334,341,342,345,346,348,349]
XRD
No/minimal specimen preparation
Dry non-destructive measurement with minimum synthesis
Can be applied to analyze powders and solids
For powders of polymer, usually, 100–200 mg of powder is required
Solid specimens are generally cut into smaller parts/sections—typically 20 mm by 20 mm
Crystal structure, structure, orientation, size, phase, composition, crystalline size (1 nm to 100 nm), crystallographic, nature of the
phase, elemental composition, sample purity, crystalline grain size,
lattice parameters, and the chemical compounds
Non-destructive approach of analysis (i.e., the specimen can be further employed for other functions)
A quick and powerful method for identification of unknown material
Resolution in angstrom scale
Crystallinity and structure correlate with the microscopic results of the bulk sample
Rapid and offers information on the crystal structure
Can be employed for quantitative analysis
Detection of a broad range of crystalline compounds, indicating sharp, higher intensity peaks and signifying significant crystallinity
Usually performed on specimens of powder
It can be used to study the effect of incorporating the fillers into the polymer matrix
Distinguish glassy and amorphous structures
Data interpretation is straightforward
It does not require very elaborate training
  • Cannot offer information about the mechanism of interacting between metal organic frameworks (MOFs) and heavy metal ions, only single binding/confrontation state of the specimen is accessible, less accuracy for small crystals and less sensitive heavy elements
  • No particle size information and peaks overlap for some compounds
  • Not appropriate for amorphous specimens, and XRD peaks are very broad for particles with a size of < 3 nm.
  • No strong interaction of X-ray beam with lighter elements, and this can restrict the detection of elements
  • X-rays can be employed with an electromagnetic radiation with a wavelength in the order of 10−10 m
  • More precise for testing larger crystal structures than smaller ones and requires tenths of a gram of specimen which must be ground into fine powder
  • Standard reference files for inorganic compounds must be accessible for comparison of the XRD pattern
  • Single-phase and homogeneous material are best for the identification of an unknown
  • For a mixture of specimens, only the materials having a minimum of 2% amount can be estimated
  • Determination of unit cell—indexing of observations/patterns is complicated for non-isometric crystal systems
[117,331,334,335,337,341,342,346]
GIWAXSThin films
No extensive specimen preparation
Crystal information at steady state, and composition evolution It can be employed for the investigation of functional thin films, in situ observation, depth resolution, rich structural information, non-destructive and no-contact probing, sensitive structural resolution, and high signal-to-noise ratioInfluenced by energy range, size, flux, and 2D area detector behaviors focusing on optics, the incident X-ray beam shape, and the spatial and temporal resolution of the detector[292,293,301,350,351,352,353]
UPSSurfaces and thin-film specimens are required to be vacuum-compatible, and specimens are required to be free from surface adsorbates, oxides, or contaminants
Specimen preparation approaches such as handling, drying, and cleaning may alter the surface properties or introduce artifacts. Requires a specimen that can efficiently emit lower-energy electrons that limit the conductive or semi-conductive materials for their purposes
Electronic structure/properties, band structure, and energy levelsNon-destructive and provides interface and surface behaviors with atomic-scale resolution
Investigates molecule electronic structures, atoms, and solids. Estimates the Φ of a material.
Gives insights into interface phenomena, surface reactions, electronic behaviors of semi-conductor devices, and energy levels of inorganic and organic materials
  • Complex data interpretation, surface sensitivity, and does not offer comprehensive chemical information, which interferes with the analysis and is vacuum-compatible.
  • Instrumentation complexity, poor light element sensitivity, difference in cross-sections of photoionization, restricted depth profiling, surface charging effects, higher vacuum requirement, and limited to the range of photon energies, etc.
[118,354,355]
UV−Vis Non-destructive measuring method for inorganic, organic, and biological materialEnable monitoring chemical reactions in real time by estimating transformations in absorbance/transmission with respect to time, hints on the shape of the NP, molecular structure, electronic
transitions, size (structural properties), aggregation conditions, concentration, elemental composition, functional group, optical properties, and bandgap energy information
Wide area of applicability, superior sensitivity with a limit of detection in the range of mg/L, facile and lower-cost characterization approach, and higher selectivity
Analyze both qualitative and quantitative measurements
It can be employed to estimate the bandgap of a semi-conductor
Allowing the identification of unknown compounds depending on their absorption spectra
  • Time-intensive specimen synthesis
  • Interference between the matrix and solvent effects. Wide and overlapping absorption bands complicate the interpretation of the spectrum
  • Specimen compatibility
  • It offers limited chemical specificity, mainly when dealing with overlapping absorption bands or complex mixtures
  • Does not provide comprehensive structural information about stereochemistry, bond angles, or molecular geometry
  • May not provide comprehensive insights into conformational transformation or molecular structure
  • Limited sensitivity for specimens with weak absorbance or lower concentrations
  • Restricted wavelength range (mainly spanning from 190 nm to 800 nm)
[118,331,337,341,342,356,357]
RSMinimal specimen preparation
Specimen preparation stages such as mounting, grinding, and drying can bring artifacts or change specimen behaviors, possibly influencing Raman spectra
The Raman spectrometer specimen area is often <10 cm2 with a thickness <2.5 cm, put over a silicon or glass substrate
Chemical composition, bonding, molecular structure, functional groups, chain orientation properties of the material, and interfacial surface behaviors. It can detect diatomic molecules like nitrogen and oxygen. Non-destructive test and a comparatively simple method. Excellent for studying transformations in the crystal structure of polymer owing to variations in the mechanical and chemical behaviors
Offers information about composition, structure, and properties
Highly functional group sensitivity with better peak intensity
Can be employed for oxidation observations throughout a broad temperature range
Polar molecules possess smaller Raman signal
Limited depth penetration, specimen heating, and requires a longer time for the analysis
Fluorescent light release for certain specimens may result in background noise
Raman spectra can be challenging and complex to interpret, particularly for specimens with mixtures of compounds, multiphase materials, or overlapping peaks
[117,118,334,349]
NMRCareful specimen preparation, non-destructive.
Requires a
larger quantity of specimens, usually in the range of mg to µg.
The specimen preparation needs skilled hands and requires maintaining the specimen’s solubility, stability, and purity
It offers dynamics of organic molecules at the atomic level. It gives comprehensive information on the structure, composition, molecular conformation, degradation, and functional group purity
  • Provides information on the local magnetic field of nearby atomic nuclei to offer detailed information about molecular motion, chemical bonding, and molecular structure
  • Less background interference.
  • Detection of polar molecules
  • High sensitivity and non-destructive analysis
  • Study specimens in solid-state solution, or even for living organisms
  • Estimate the structure of organometallic or organic compounds and study the biological, chemical, and physical behaviors
  • Procedure takes a longer time for analysis
  • Paramagnetic substances like sodium and oxygen possess a weaker NMR signal.
  • Sensitivity, limited information depth resolution, magnetic field homogeneity, signal overlap
  • Not relevant to all nuclei, and quenching issues may exist in the presence of paramagnetic metal ions
  • Size ranges from 1 nm to 5 nm
[117,118,334,337,342,346,347,348,349,350,351,352,353,354,355,356,357,358]
TGA A small quantity of specimen (usually 2 mg - 20 mg) is placed into a crucible (pan) of appropriate size, which is often built of materials like platinum or aluminum
After that, the specimen is loaded into the TGA instrument
Thermal stability, composition and mass of stabilizers, and material decomposition kinetics
Kinetics of decomposition of the material
The thermal stability and chemical reaction
Composition of material
Study transformations in the mass of a specimen with respect to time or temperature under controlled atmospheric circumstances
The TGA data can be used to elucidate the type of reaction, including vaporization, sublimation, and decomposition
  • Does not offer certain insights into the extent of coverage influence of the surface
  • Limited kinetic information, interpretation of results, sensitivity to environmental conditions, calibration, atmospheric effects, temperature range, and sample size and homogeneity
  • Sample destruction and restricted quantity of elements to be analyzed
[118,334,341,342,346,359]
FTIR
Requires specimens to be manufactured as solutions, pellets, or thin films to obtain good spectra
Specimen preparation can add artifacts or change the specimen’s behavior, possibly impacting the precision and interpretation of observations
Gas, liquid, and solid samples
Powder of 2 to 5 mg of composites
Provides information for functional groups for liquid, solid, or gas specimens, surface composition, molecular structures and chemical compositions, molecular bonds between matter, compounds, molecules, compatibility, miscibility, and degradation and interaction of NPs
Fast analysis time (in seconds)
Superior sensitivity in parts per million (ppm), non-destructive and quick data acquisition, and can be used for characterizing a broad range of inorganic and organic materials
To study the surface chemistry of metal NPs
Functional groups offer the chemical reaction behaviors of the compound
Determine functional groups
Used for the identification of organic, inorganic, and polymeric materials utilizing infrared light for scanning the samples
  • Analysis of aqueous solutions by FTIR is difficult because of the strong water absorbance in infrared spectra.
  • It can be used only in the range of 100 µm–1 µm wavelength
  • FTIR cannot detect/distinguish molecules of two identical atoms like nitrogen and oxygen since they fail to absorb infrared radiation
  • Limited quantitative precision, chemical interference, overlapping bands, instrumental noise and drift, limited sensitivity for dilute specimens, water absorption, and surface sensitivity
[117,118,283,341,342,360]
EELSThin films (<100 nm), conductive
Thicker specimens may need thinning methods like focused ion beam milling or ion milling to decrease the effects of scattering
Electronic excitation, chemical bonding and functionality, collective atoms interaction with neighbors, elemental composition, atoms chemical state, collective
interactions of atoms with their neighbors, the resonance of bulk plasma, and the number and type of atoms
Structural and electronic behaviors of materials at the nanometer scale
Offers superior spatial resolution
Probe nanoscale interfaces and features within materials
Provides chemical and elemental specificity
Bonding structures/configurations within complex specimens
Provide details for specimen composition and behaviors
  • Costly and electron beam can destroy the specimen
  • Requires thin-film specimens (typically <100 nm)
  • Requires special expertise for spectral analysis and data interpretation
  • Surface sensitivity, sample thickness, signal-to-noise ratio, quantitative analysis challenge, and attaining superior spatial resolution can be challenging, mainly for heterogeneous or thick specimens
[118,342,346,361,362,363,364,365]
Abbreviations: scanning electron microscopy (SEM); Fourier-transform infrared spectroscopy (FT−IR); field emission scanning electron microscopy (FE−SEM); energy-dispersive X-ray spectroscopy (EDS); nuclear magnetic resonance (NMR); wavelength-dispersive X-ray spectroscopy (WDS); atomic force microscope (AFM); transmission electron microscopy (TEM); scanning electron microscopy–energy-dispersive X-ray spectrometry (SEM−EDS); X-ray photoelectron spectroscopy (XPS); electron; grazing incidence wide-angle X-ray scattering (GIWAXS); ultraviolet photoemission spectroscopy (UPS); Raman spectroscopy (RS); thermogravimetric analysis (TGA); X-ray diffraction (XRD); and energy loss spectroscopy (EELS).

4. Conclusions, Challenges and Future Perspective

4.1. Conclusions

In summary, in this review, we summarize both established and emerging spectroscopic and microscopic techniques that had already been applied for the characterization of inorganic and polymer TE materials, specifically PEDOT:PSS, with examples and how these modern approaches/techniques contribute to understanding the mechanism of the fundamental TE behaviors of inorganic and polymer TE materials that have been already reported in the literature with respect to electronic, structural, morphology and chemical features. This review offers an overview of the spectroscopic characterization for studying the mechanisms of TE enhancement in both inorganic and polymer TE materials, highlighting the recent developments in the field.
This review summarizes various spectroscopy and microscopy techniques that have already been applied for the characterization of inorganic and polymer TE materials. For inorganic TE, -EDX spectroscopy, UV−Vis spectroscopy, and XPS were widely applied for electronic structure characterization. For phase analysis of inorganic TE materials, RS, NMR spectroscopy, and EELS were utilized. For analyzing the surface morphology and crystalline structure, SEM, TEM, and XRD were commonly used. For polymer TE materials, UV−Vis−NIR spectroscopy and UPS were generally employed for determining electronic structure. For functional groups analysis of polymer TE, ATR-FTIR and RS were broadly utilized. XPS was used for elemental composition analysis of polymer TE. For the surface morphology of polymer TE, AFM and SEM were applied. GIWAXS and XRD were employed for analyzing the crystalline structure of polymer TE materials. In this paper, each spectroscopic technique is discussed by providing examples of spectroscopic techniques that have already been employed for TE characterization.
Overall, this review provided insight into the various characterization approaches applied for both inorganic and organic TE that were performed using a static approach—and it will provide the merits of understanding the properties and arrangement of TE composites and predict the performance of TE materials as well as optimize the manufacturing processes by boosting their specific properties—including σ, S, κ, thermal stability, environmental stability, mechanical strength, super-flexibility, and film formation, as well as their combined features. In addition, a thorough characterization of electronic, structural, morphological, and chemical features was provided to design novel approaches and materials with the required behaviors for boosting the TE properties.
Furthermore, the review looks ahead, recommending substantial future research areas. These comprise in situ spectroscopy and microscopy methods for tracking the dynamic evolution of a TE sample/specimen, as well as combining machine learning-assisted analysis with multi-dimensional and ultra-fast spectroscopy and microscopy approaches in order to boost TE performance.
Contemplating its importance today, we believe the review has the potential to be an important resource for researchers providing information about advanced spectroscopic and microscopic characterization techniques to predict the performance of TE materials/composites as well as data interpretation guidelines for potential applications in various disciplines such as electronic devices, including memristors and neuromorphic devices, organic electrochemical transistors (OECTs), light-emitting diodes (LEDs), photodetectors, and a hole transport layer (HTL) for solar cells, as well as energy storage devices including batteries and supercapacitors.

4.2. Challenges and Future Perspective

Despite the notable improvement in the performances of TE materials, they are still limited to the laboratory, and they are far from practical applications. Thus far, the TE characterization experiments reported were performed using a static approach without causing/inducing composition and structural transformation in the sample. These static analyses can offer substantial information on TE performance since they permit correlating the fundamental TE properties/behaviors with the composition and structure of the entire investigated sample at higher resolution, reaching down to the atomic-scale features. For instance, by manufacturing materials with typical defects for microstructural engineering, it would be attainable to estimate the contribution of a particular grain boundary engineering on the S. Nevertheless, an in situ method typically refers to the potential of tracking the dynamic evolution of a sample within in situ spectroscopy and microscopy. Hence, continued efforts are suggested to focus on the following issues to boost TE performance:
(1)
Advanced synchrotron-based analytical tools, including scattering and spectroscopy, as well as imaging capabilities combined with in situ and operando applications, have to be performed, as these approaches can give fundamental insights to understand the behaviors of TE materials, such as geometric and electronic structures under realistic operating conditions.
(2)
Emerging techniques such as machine learning-assisted analysis have to be combined with multi-dimensional and ultra-fast spectroscopy, and microscopy approaches have to be implemented, as they are promising approaches to further advance our understanding of basic relationships in TE materials and accelerate the discovery of materials.
(3)
Fundamental electronic band structure characterization by UV-Vis-NIR and photoelectron spectroscopy, as well as sophisticated lattice dynamics analysis via RS, has to be performed, as these techniques can give a deeper understanding of structure–property relationships for TE materials.
(4)
The combination of multiple spectroscopic techniques must be established, as this enables multiscale characterization spanning atomic to mesoscale phenomena, revealing the complex interplay between electronic transport, phonon scattering, and nanostructural features.
(5)
The synergy between material synthesis, advanced spectroscopic characterization, and property optimization has to be harnessed, as this can drive innovations in TE materials science. As we move toward more sophisticated materials with hierarchical structures and coupled functionalities, the role of comprehensive spectroscopic analysis becomes increasingly critical for achieving breakthrough performance levels.
(6)
The correlation between crystalline microstructure engineering and TE performance/properties is still unclear, and the mechanism of crystallization from a molecular engineering point of view, as well as structure–property correlations at atomic, nanoscale, and mesoscale levels, has to be investigated for the development of higher-performance organic polymers including PEDOT:PSS and organic/inorganic composites.
(7)
New methods and new post-treatment procedures, along with the optimization of fabrication approaches, filler materials, and interfacial engineering, as well as the development of theoretical models, have to be pursued to attain superior TE properties/performance for PEDOT:PSS- and PEDOT:PSS-based TE composites.
(8)
The EF technique has to be applied to boost S while slightly reducing σ and elevating PF for higher-performance polymer TE materials, including PEDOT:PSS.

Author Contributions

T.A.Y.: Conceptualization, Writing—Original Draft, Methodology, Supervision, Reviewing, and Editing; T.A.W.: Investigation, Data Curation, Writing—Original draft, Reviewing, and Editing; Y.Z. and W.S.C.: Conceptualization, Methodology, Supervision, Reviewing, and Editing. M.K.T. and T.G.B.: Data Curation, Reviewing, and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 3. Various characterization approaches for inorganic and polymer (specifically PEDOT:PSS) TE materials.
Figure 3. Various characterization approaches for inorganic and polymer (specifically PEDOT:PSS) TE materials.
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Figure 4. (A). Backscattered electron image and (B) corresponding EDX mapping for (0.5)Mg2Sn/(0.5)Mg2Si0.98Bi0.02 composite. Adapted with permission from ref. [139] Copyright 2023, Elsevier. (C). SEM image and EDX elemental mapping of Sn and Se for Sn0.90Pb0.15Se0.95Cl0.05 melt-synthesized sample, showing Sn precipitate. (D). SEM image and EDX elemental mapping of Sn Se subject to melting, further ball milling, and annealing, showing uniform element distribution. Adapted with permission from ref. [140] Copyright 2019, American Chemical Society.
Figure 4. (A). Backscattered electron image and (B) corresponding EDX mapping for (0.5)Mg2Sn/(0.5)Mg2Si0.98Bi0.02 composite. Adapted with permission from ref. [139] Copyright 2023, Elsevier. (C). SEM image and EDX elemental mapping of Sn and Se for Sn0.90Pb0.15Se0.95Cl0.05 melt-synthesized sample, showing Sn precipitate. (D). SEM image and EDX elemental mapping of Sn Se subject to melting, further ball milling, and annealing, showing uniform element distribution. Adapted with permission from ref. [140] Copyright 2019, American Chemical Society.
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Figure 5. (A). UV−Vis spectra for Bi2Te3 with various sample syntheses (left side) and corresponding Tauc sketches for the determination of bandgap (right side). Adapted with permission from ref. [143] Copyright 2018, Taylor & Francis. (B). UV−Vis absorption spectra (left side) and Tauc plot for determination of bandgap for sample ZnO-based TE materials (right side). Adapted with permission from ref. [142] Copyright 2024, Wiley.
Figure 5. (A). UV−Vis spectra for Bi2Te3 with various sample syntheses (left side) and corresponding Tauc sketches for the determination of bandgap (right side). Adapted with permission from ref. [143] Copyright 2018, Taylor & Francis. (B). UV−Vis absorption spectra (left side) and Tauc plot for determination of bandgap for sample ZnO-based TE materials (right side). Adapted with permission from ref. [142] Copyright 2024, Wiley.
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Figure 6. (A). (a) XPS spectra for Bi2Sr2−xCaxCo2Oy, where x = 0.5, 0.3, and 0 for red, blue, and black lines, respectively. (b) High-resolution XPS spectrum of the Ca-2p core level for x = 0.5, 0.3, and 0 for red, blue, and black lines, respectively. (c) High-resolution XPS of the Co-2p core level for x = 0.5, 0.3, and 0 for red, blue, and black lines, respectively. (d) XPS valence band spectra of Bi2Sr2−xCaxCo2Oy for x = 0.5 (red line) and 0.3 (black line) samples. Adapted with permission from ref. [161] Copyright 2023, MDPI. (B). (a,b) XPS spectra for BiCuSeO at the B.E range in Bi 4f. The smaller open circles denote the estimated experimental points; the solid line represents the total of all the peak fittings portrayed by the dashed lines. (c) XPS spectrum of the Ca (2p). The smaller open circles denote the estimated experimental points; the solid line represents the total of all the peak fittings portrayed by the dashed lines. Adapted with permission from ref. [148] Copyright 2016, Elsevier.
Figure 6. (A). (a) XPS spectra for Bi2Sr2−xCaxCo2Oy, where x = 0.5, 0.3, and 0 for red, blue, and black lines, respectively. (b) High-resolution XPS spectrum of the Ca-2p core level for x = 0.5, 0.3, and 0 for red, blue, and black lines, respectively. (c) High-resolution XPS of the Co-2p core level for x = 0.5, 0.3, and 0 for red, blue, and black lines, respectively. (d) XPS valence band spectra of Bi2Sr2−xCaxCo2Oy for x = 0.5 (red line) and 0.3 (black line) samples. Adapted with permission from ref. [161] Copyright 2023, MDPI. (B). (a,b) XPS spectra for BiCuSeO at the B.E range in Bi 4f. The smaller open circles denote the estimated experimental points; the solid line represents the total of all the peak fittings portrayed by the dashed lines. (c) XPS spectrum of the Ca (2p). The smaller open circles denote the estimated experimental points; the solid line represents the total of all the peak fittings portrayed by the dashed lines. Adapted with permission from ref. [148] Copyright 2016, Elsevier.
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Figure 7. (A). Raman spectra of the corresponding samples at 300 K. Adapted with permission from ref. [178] Copyright 2017, AIP Publishing. (B). Raman spectra of Bi2Sr2-xCaxCo2Oy with x = 0.5, 0.3, and 0 specimens at room temperature. (C). Expanded peak positions for the Raman band, which was marked by arrows for (B). Adapted with permission from ref. [161] Copyright 2023, MDPI.
Figure 7. (A). Raman spectra of the corresponding samples at 300 K. Adapted with permission from ref. [178] Copyright 2017, AIP Publishing. (B). Raman spectra of Bi2Sr2-xCaxCo2Oy with x = 0.5, 0.3, and 0 specimens at room temperature. (C). Expanded peak positions for the Raman band, which was marked by arrows for (B). Adapted with permission from ref. [161] Copyright 2023, MDPI.
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Figure 9. (A). FESEM pictures of the fracture surface of the Bi2Te3-Bi2Se3 composite specimens that were hot-pressed at a temperature of (a) 623 K, and (b,c) 673 K and 80 MPa. Adapted with permission from ref. [212] Copyright 2011, Springer Nature (B). (a) SEM picture of a drop-cast PEDOT:PSS passivated crystalline Te nanorods thin film composite. Adapted with permission from ref. [109] Copyright 2010, American Chemical Society. (b) SEM pictures of the cross-sections of CNT (5 wt.%)—PVAc composites are indicated in panels b and c for freeze-fractured composites. The SEM image with higher magnification indicated in panel (c) is a part of the specimen in (b), as shown by a solid square with yellow color. It obviously indicated that CNTs (as shown with arrows), which are wrapped around particles at the emulsion (as shown with yellow dotted lines) instead of being homogenously blended. Adapted with permission from ref. [213] Copyright 2008, American Chemical Society.
Figure 9. (A). FESEM pictures of the fracture surface of the Bi2Te3-Bi2Se3 composite specimens that were hot-pressed at a temperature of (a) 623 K, and (b,c) 673 K and 80 MPa. Adapted with permission from ref. [212] Copyright 2011, Springer Nature (B). (a) SEM picture of a drop-cast PEDOT:PSS passivated crystalline Te nanorods thin film composite. Adapted with permission from ref. [109] Copyright 2010, American Chemical Society. (b) SEM pictures of the cross-sections of CNT (5 wt.%)—PVAc composites are indicated in panels b and c for freeze-fractured composites. The SEM image with higher magnification indicated in panel (c) is a part of the specimen in (b), as shown by a solid square with yellow color. It obviously indicated that CNTs (as shown with arrows), which are wrapped around particles at the emulsion (as shown with yellow dotted lines) instead of being homogenously blended. Adapted with permission from ref. [213] Copyright 2008, American Chemical Society.
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Figure 10. (A). TEM pictures of the Bi2Te3 nanopowders manufactured through the hydrothermal synthesis approach; (B). TEM images of PTH synthesized through the chemical oxidative polymerization method. Adapted with permission from ref. [214] Copyright 2012, Springer. (C). TEM picture indicating the PEDOT:PSS passivated crystalline Te nanorod. Adapted with permission from ref. [109] Copyright 2010, American Chemical Society.
Figure 10. (A). TEM pictures of the Bi2Te3 nanopowders manufactured through the hydrothermal synthesis approach; (B). TEM images of PTH synthesized through the chemical oxidative polymerization method. Adapted with permission from ref. [214] Copyright 2012, Springer. (C). TEM picture indicating the PEDOT:PSS passivated crystalline Te nanorod. Adapted with permission from ref. [109] Copyright 2010, American Chemical Society.
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Figure 11. (A). The XRD results/patterns of (Sn1−xGex)Se: (a) powders, (b) planes perpendicular, (c) planes parallel to the direction of SPS pressing, and (d) SEM pictures of the SPS-sintered fractured surfaces of (Sn0.99Ge0.01)Se bulk specimen parallel to the direction of the press. Adapted with permission from ref. [216] Copyright 2016, The Royal Society of Chemistry. (B). (a) Powder XRD patterns of SnSe (1), Sn0.99Ag0.01Se (2), Sn0.99Na0.01Se (3), Sn0.98Ag0.01Na0.01Se (4), SnSeAg8SnSe6 (SnSe-STSe) (5), and Sn0.99Na0.01Se-Ag8SnSe6 (Sn0.99Na0.01Se-STSe) (6) specimens. (b) Expanded XRD results/patterns of SnSe, SnSe-STSe, and Sn0.99Na0.01Se-STSe specimens showing the presence of Ag8SnSe6 as a second phase. Adapted with permission from ref. [245]. Copyright 2018, Wiley.
Figure 11. (A). The XRD results/patterns of (Sn1−xGex)Se: (a) powders, (b) planes perpendicular, (c) planes parallel to the direction of SPS pressing, and (d) SEM pictures of the SPS-sintered fractured surfaces of (Sn0.99Ge0.01)Se bulk specimen parallel to the direction of the press. Adapted with permission from ref. [216] Copyright 2016, The Royal Society of Chemistry. (B). (a) Powder XRD patterns of SnSe (1), Sn0.99Ag0.01Se (2), Sn0.99Na0.01Se (3), Sn0.98Ag0.01Na0.01Se (4), SnSeAg8SnSe6 (SnSe-STSe) (5), and Sn0.99Na0.01Se-Ag8SnSe6 (Sn0.99Na0.01Se-STSe) (6) specimens. (b) Expanded XRD results/patterns of SnSe, SnSe-STSe, and Sn0.99Na0.01Se-STSe specimens showing the presence of Ag8SnSe6 as a second phase. Adapted with permission from ref. [245]. Copyright 2018, Wiley.
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Figure 12. The changes of (A) S and (B) PF with respect to the σ of organic and composite/hybrid TE materials. Adapted with permission from ref. [257]. Copyright 2016, Nature.
Figure 12. The changes of (A) S and (B) PF with respect to the σ of organic and composite/hybrid TE materials. Adapted with permission from ref. [257]. Copyright 2016, Nature.
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Figure 13. (A). UV-Vis spectrum for the PEDOT:PSS (pristine), F-PEDOT:PSS, SFS−F−PEDOT:PSS, and BMIM−TFSI−SFS−F−PEDOT:PSS films. Adapted with permission from ref. [263] Copyright 2020, Frontiers. (B) UV−Vis spectrum of PEDOT:PSS films with various fabrication circumstances. Adapted with permission from ref. [264] Copyright 2013, The Royal Society of Chemistry. (C) UV−Vis absorbance spectra of PA and PAR films [83] Copyright 2023, Wiley. (D) UV-Vis-NIR spectrum for the untreated PEDOT:PSS, BSA-doped, DMSO-treated, and HZ-treated PEDOT:PSS films. Adapted with permission from ref. [82] Copyright 2018, Wiley. (E). UV−Vis spectrum of (d) pristine/untreated and after post-treated with (c) EG, (b) ChCl, and (a) DES. Adapted with permission from ref. [265] Copyright 2015, Wiley. (F) Vis−NIR spectrum of PP, DBAP, and TDBAP. Adapted with permission from ref. [77] Copyright 2025, Wiley.
Figure 13. (A). UV-Vis spectrum for the PEDOT:PSS (pristine), F-PEDOT:PSS, SFS−F−PEDOT:PSS, and BMIM−TFSI−SFS−F−PEDOT:PSS films. Adapted with permission from ref. [263] Copyright 2020, Frontiers. (B) UV−Vis spectrum of PEDOT:PSS films with various fabrication circumstances. Adapted with permission from ref. [264] Copyright 2013, The Royal Society of Chemistry. (C) UV−Vis absorbance spectra of PA and PAR films [83] Copyright 2023, Wiley. (D) UV-Vis-NIR spectrum for the untreated PEDOT:PSS, BSA-doped, DMSO-treated, and HZ-treated PEDOT:PSS films. Adapted with permission from ref. [82] Copyright 2018, Wiley. (E). UV−Vis spectrum of (d) pristine/untreated and after post-treated with (c) EG, (b) ChCl, and (a) DES. Adapted with permission from ref. [265] Copyright 2015, Wiley. (F) Vis−NIR spectrum of PP, DBAP, and TDBAP. Adapted with permission from ref. [77] Copyright 2025, Wiley.
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Figure 15. (A). ATR-FTIR spectra of PSSH and PSSNa: (h). Ring in-plane deformation vibrations (615 cm−1 region); (g) out-of-plane C=H deformation vibrations (833 cm−1, 776 cm−1, and 670 cm−1 regions); (f) symmetric stretch of SO3 (1035 and 1005 cm−1regions); (e) SO3 asymmetric stretching vibrations (1165 and 1125 cm−1 regions); (d) C=C stretching vibrations (aromatic): 1410, 1450, 1495, and 1600 cm−1 regions); (c) bending vibrations of O−H (1640 cm−1 region); (b) alkyl C-H stretch vibration (2925 cm−1 region); and (a) stretching vibration of O-H (3700–2965 cm−1 regions). Adapted with permission from ref. [277] Copyright 2012, The Royal Society of Chemistry. (B). ATR-FTIR spectra for pristine, LiOH-, NaOH-, and KOH-treated PEDOT:PSS films. Adapted with permission from ref. [278] Copyright 2019, The authors.
Figure 15. (A). ATR-FTIR spectra of PSSH and PSSNa: (h). Ring in-plane deformation vibrations (615 cm−1 region); (g) out-of-plane C=H deformation vibrations (833 cm−1, 776 cm−1, and 670 cm−1 regions); (f) symmetric stretch of SO3 (1035 and 1005 cm−1regions); (e) SO3 asymmetric stretching vibrations (1165 and 1125 cm−1 regions); (d) C=C stretching vibrations (aromatic): 1410, 1450, 1495, and 1600 cm−1 regions); (c) bending vibrations of O−H (1640 cm−1 region); (b) alkyl C-H stretch vibration (2925 cm−1 region); and (a) stretching vibration of O-H (3700–2965 cm−1 regions). Adapted with permission from ref. [277] Copyright 2012, The Royal Society of Chemistry. (B). ATR-FTIR spectra for pristine, LiOH-, NaOH-, and KOH-treated PEDOT:PSS films. Adapted with permission from ref. [278] Copyright 2019, The authors.
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Scheme 1. Different redox levels of PEDOT showing the transformation of PEDOT from bipolaron level to polaron, and then neutral level upon dedoping [264].
Scheme 1. Different redox levels of PEDOT showing the transformation of PEDOT from bipolaron level to polaron, and then neutral level upon dedoping [264].
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Figure 16. (A). Raman spectra of pristine and treated PEDOT:PSS films by various post-treatment techniques. Reproduced with permission from ref. [264] Copyright 2013, The Royal Society of Chemistry. (B). The Raman spectra of the untreated and formamide-treated PEDOT:PSS (Dip + Drop)2 films. Reproduced with permission from ref. [70] Copyright 2017, Wiley. (C) Normalized Raman spectra of the pristine, BSA-doped, and CSA-doped PEDOT:PSS films. Adapted with permission from ref. [82] Copyright 2018, Wiley. (D) Raman spectra of the untreated, MeOH-, DMSO-, and TFMSA−MeOH-treated PEDOT:PSS films. Adapted with permission from ref. [282] Copyright 2018, The Royal Society of Chemistry.
Figure 16. (A). Raman spectra of pristine and treated PEDOT:PSS films by various post-treatment techniques. Reproduced with permission from ref. [264] Copyright 2013, The Royal Society of Chemistry. (B). The Raman spectra of the untreated and formamide-treated PEDOT:PSS (Dip + Drop)2 films. Reproduced with permission from ref. [70] Copyright 2017, Wiley. (C) Normalized Raman spectra of the pristine, BSA-doped, and CSA-doped PEDOT:PSS films. Adapted with permission from ref. [82] Copyright 2018, Wiley. (D) Raman spectra of the untreated, MeOH-, DMSO-, and TFMSA−MeOH-treated PEDOT:PSS films. Adapted with permission from ref. [282] Copyright 2018, The Royal Society of Chemistry.
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Figure 17. (A). XPS spectrum of (a) untreated, (b) formic acid, (c) HZ (0.15 wt.%), and (d) normalized combination of untreated, formic acid-, HZ (various concentrations)-treated PEDOT:PSS films. Adapted with permission from ref. [69] Copyright 2020, The Royal Society of Chemistry. (B). S2p XPS spectra for (a) PEDOT:PSS films treated by different zwitterions; (b) PRA and PA; (c) PRAB and PAB; and (d) N1s XPS spectra of PRAB, PRA, PR, and R101. Adapted with permission from ref. [83] Copyright 2023, Wiley.
Figure 17. (A). XPS spectrum of (a) untreated, (b) formic acid, (c) HZ (0.15 wt.%), and (d) normalized combination of untreated, formic acid-, HZ (various concentrations)-treated PEDOT:PSS films. Adapted with permission from ref. [69] Copyright 2020, The Royal Society of Chemistry. (B). S2p XPS spectra for (a) PEDOT:PSS films treated by different zwitterions; (b) PRA and PA; (c) PRAB and PAB; and (d) N1s XPS spectra of PRAB, PRA, PR, and R101. Adapted with permission from ref. [83] Copyright 2023, Wiley.
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Figure 18. (A). AFM images for IL−SFS−F−PEDOT:PSS, SFS−F−PEDOT:PSS, and untreated films. Height images: (a) pristine, (b) 0, (c) 100 mM SFS in water for SFS−F−PEDOT:PSS, and (d) BMIM−TFSI (40 vol.%) for IL−SFS−F−PEDOT:PSS films. Phase images: (e) pristine, (f) 0, (g) 100 mM SFS in water for SFS−F−PEDOT:PSS, and (h) BMIM−TFSI (40 vol.%) for IL−SFS−F−PEDOT:PSS films. The scanned area was 1 μm × 1 μm for each picture. Adapted with permission from ref. [263] Copyright 2020, Frontiers. (B). The AFM pictures of PEDOT:PSS films captured by different treatments. Height images (upper row) and phase images (lower row). The scanning area was 1 × 1 mm2. Adapted with permission from ref. [264] Copyright 2013, The Royal Society of Chemistry. (C). Upper row: AFM pictures of the untreated, BSA-doped, and CSA-doped PEDOT:PSS films. The picture size was 2 μm × 2 μm. Lower row: pictures of the untreated, BSA-doped, and CSA-doped PEDOT:PSS films obtained by the camera of the stylus profiler. Adapted with permission from ref. [82] Copyright 2018, Wiley.
Figure 18. (A). AFM images for IL−SFS−F−PEDOT:PSS, SFS−F−PEDOT:PSS, and untreated films. Height images: (a) pristine, (b) 0, (c) 100 mM SFS in water for SFS−F−PEDOT:PSS, and (d) BMIM−TFSI (40 vol.%) for IL−SFS−F−PEDOT:PSS films. Phase images: (e) pristine, (f) 0, (g) 100 mM SFS in water for SFS−F−PEDOT:PSS, and (h) BMIM−TFSI (40 vol.%) for IL−SFS−F−PEDOT:PSS films. The scanned area was 1 μm × 1 μm for each picture. Adapted with permission from ref. [263] Copyright 2020, Frontiers. (B). The AFM pictures of PEDOT:PSS films captured by different treatments. Height images (upper row) and phase images (lower row). The scanning area was 1 × 1 mm2. Adapted with permission from ref. [264] Copyright 2013, The Royal Society of Chemistry. (C). Upper row: AFM pictures of the untreated, BSA-doped, and CSA-doped PEDOT:PSS films. The picture size was 2 μm × 2 μm. Lower row: pictures of the untreated, BSA-doped, and CSA-doped PEDOT:PSS films obtained by the camera of the stylus profiler. Adapted with permission from ref. [82] Copyright 2018, Wiley.
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Figure 19. (A). (a) Surface SEM of a PDOT:PSS/rhodamine 101 (R101) (PR) film. (b) Surface SEM of PRAB films. Adapted with permission from ref. [83] Copyright 2023, Wiley. (B). Cross-sectional SEM pictures of the following: (I) pristine, (II) CH3NO, (III) CH3NO−H2SO4, and (IV) CH3NO−H2SO4−NaBH4-treated PEDOT:PSS films. Adapted with permission from ref. [290] Copyright 2019, American Chemical Society. (C). SEM pictures of PEDOT:PSS thin films: (a) pristine, (b) 10 M formic acid, and (c) 26 M formic acid-treated films. The red arrow shows the segregated PSS. Adapted with permission from ref. [289] Copyright 2014, American Chemical Society. (D). SEM pictures of PEDOT:PSS films: (a) untreated; (b) treated by MeOH with the dipping technique; and (c,d) treated by MeOH with ther dropping technique. Adapted with permission from ref. [277] Copyright 2012, The Royal Society of Chemistry.
Figure 19. (A). (a) Surface SEM of a PDOT:PSS/rhodamine 101 (R101) (PR) film. (b) Surface SEM of PRAB films. Adapted with permission from ref. [83] Copyright 2023, Wiley. (B). Cross-sectional SEM pictures of the following: (I) pristine, (II) CH3NO, (III) CH3NO−H2SO4, and (IV) CH3NO−H2SO4−NaBH4-treated PEDOT:PSS films. Adapted with permission from ref. [290] Copyright 2019, American Chemical Society. (C). SEM pictures of PEDOT:PSS thin films: (a) pristine, (b) 10 M formic acid, and (c) 26 M formic acid-treated films. The red arrow shows the segregated PSS. Adapted with permission from ref. [289] Copyright 2014, American Chemical Society. (D). SEM pictures of PEDOT:PSS films: (a) untreated; (b) treated by MeOH with the dipping technique; and (c,d) treated by MeOH with ther dropping technique. Adapted with permission from ref. [277] Copyright 2012, The Royal Society of Chemistry.
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Figure 20. (A). (ad) GIWAXS pictures, (e) profiles for full integration lines, and (f) peak intensity at a q value of ca. 1.83 Å−1 (for PEDOT π−π stacking) with respect to the azimuthal angle (φ) for various alkali-treated DMSO/PEDOT:PSS films. The dip domains in the signal intensity profile in (e,f) correlated with the Pilatus detector gaps. qr is the parallel component of the scattering vector; qz is the scattering vector along the near-out-of-plane direction; and q is the scattering vector. Adapted with permission from ref. [278] Copyright 2021, Elsevier. (B). GIWAXS pictures of untreated PEDOT:PSS films and those doped by MeOH and TFSA, as well as those subjected to MeOH-TFSA dual treatments. Adapted with permission from ref. [304] Copyright 2025, Wiley. (C). 2D GIWAXS results/patterns for PEDOT:PSS specimens: (i,j) H2SO4, (g,h) HNO3, (e,f) HCOOH, (c,d) HCl, (b) EG treatment, and (a) untreated and their H2O-rinsed/washed counterparts. Adapted with permission from ref. [300] Copyright 2019, Wiley.
Figure 20. (A). (ad) GIWAXS pictures, (e) profiles for full integration lines, and (f) peak intensity at a q value of ca. 1.83 Å−1 (for PEDOT π−π stacking) with respect to the azimuthal angle (φ) for various alkali-treated DMSO/PEDOT:PSS films. The dip domains in the signal intensity profile in (e,f) correlated with the Pilatus detector gaps. qr is the parallel component of the scattering vector; qz is the scattering vector along the near-out-of-plane direction; and q is the scattering vector. Adapted with permission from ref. [278] Copyright 2021, Elsevier. (B). GIWAXS pictures of untreated PEDOT:PSS films and those doped by MeOH and TFSA, as well as those subjected to MeOH-TFSA dual treatments. Adapted with permission from ref. [304] Copyright 2025, Wiley. (C). 2D GIWAXS results/patterns for PEDOT:PSS specimens: (i,j) H2SO4, (g,h) HNO3, (e,f) HCOOH, (c,d) HCl, (b) EG treatment, and (a) untreated and their H2O-rinsed/washed counterparts. Adapted with permission from ref. [300] Copyright 2019, Wiley.
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Yemata, T.A.; Wubieneh, T.A.; Zheng, Y.; Chin, W.S.; Tadsual, M.K.; Beyene, T.G. Spectroscopic and Microscopic Characterization of Inorganic and Polymer Thermoelectric Materials: A Review. Spectrosc. J. 2025, 3, 24. https://doi.org/10.3390/spectroscj3040024

AMA Style

Yemata TA, Wubieneh TA, Zheng Y, Chin WS, Tadsual MK, Beyene TG. Spectroscopic and Microscopic Characterization of Inorganic and Polymer Thermoelectric Materials: A Review. Spectroscopy Journal. 2025; 3(4):24. https://doi.org/10.3390/spectroscj3040024

Chicago/Turabian Style

Yemata, Temesgen Atnafu, Tessera Alemneh Wubieneh, Yun Zheng, Wee Shong Chin, Messele Kassaw Tadsual, and Tadisso Gesessee Beyene. 2025. "Spectroscopic and Microscopic Characterization of Inorganic and Polymer Thermoelectric Materials: A Review" Spectroscopy Journal 3, no. 4: 24. https://doi.org/10.3390/spectroscj3040024

APA Style

Yemata, T. A., Wubieneh, T. A., Zheng, Y., Chin, W. S., Tadsual, M. K., & Beyene, T. G. (2025). Spectroscopic and Microscopic Characterization of Inorganic and Polymer Thermoelectric Materials: A Review. Spectroscopy Journal, 3(4), 24. https://doi.org/10.3390/spectroscj3040024

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