Emission Ellipsometry Study in Polymeric Interfaces Based on Poly(3-Hexylthiophene), [6,6]-Phenyl-C₆₁-Butyric Acid Methyl Ester, and Reduced Graphene Oxide

Abstract

We analyzed the interaction of three materials, reduced graphene oxide (RGO), [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), and poly(3-hexylthiphene) (P3HT), as well as the dependence of its photophysical properties within the temperature range of 90 to 300 K. The nanocomposite of the films was analyzed by optical absorption ultraviolet–visible (UV-Vis) and photoluminescence (PL) and emission ellipsometry (EE) as a function of sample temperature. The surface morphology was studied by atomic force microscopy (AFM). We noted that onset levels (E_onset) of the nanocomposite of P3HT and RGO are smaller than the others. The PL spectra showed the presence of anomalies in the emission intensities in the nanocomposite of P3HT and PCBM. It was also possible to determine the electron–phonon coupling by calculating the Huang–Rhys parameters and the temperature dependence of samples. Through EE, it was possible to analyze the degree of polarization and the anisotropy. We observed a high degree of polarized emission of the P3HT films, which varies subtly according to the temperature. For nanocomposites with RGO, the polarization degree in the emission decreases, and the roughness on the surface increases. As a result, the RGO improves the energy transfer between adjacent polymer chains at the cost of greater surface roughness. Then, the greater energy transfer may favor applications of this type of nanocomposite in organic photovoltaic cells (OPVCs) with enhancement in energy conversion efficiency.

1. Introduction

In the last decades, the study of the polymeric interface has proved to be increasingly crucial for developing organic optoelectronic devices [1,2,3,4,5,6,7], especially the donor/acceptor (D/A) interface. The D/A interface is necessary for the carriers to interpenetrate in the heterojunction and increase the probability of charge separation, i.e., facilitating the formation of excitons and improving the charge transfer between the electron transport layer (ETL) and hole transport layer (HTL) in the device [7], which led to a 15% PCE in an organic photovoltaic cell [8]. In this context, two semiconductors commonly used as D/A materials, respectively, poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), perform well as active layers in photovoltaic devices. P3HT is a regioregular thiophene-based semiconducting polymer with attached 6-carbon alkyl side chains (C₆H₁₃) in the 3-position of the thiophene rings. X-ray studies have shown that P3HTs share several common structural features: (i) a layered arrangement of polythiophene chains with the backbone determining the conjugation direction; (ii) π-stacking of polythiophene chains, which promotes interchain charge transport; and (iii) organization of the alkyl side chains, where they function as spacers between the polythiophene chains [9,10]. PCBM is one of the most widely used fullerene derivatives in organic photovoltaics (OPVs). The core of PCBM is a C60 fullerene, a spherical carbon molecule composed of 60 carbon atoms arranged in a truncated icosahedron structure, resembling a soccer ball [11]. PCBM is functionalized at the [6,6] position (where two hexagons share an edge) of the fullerene with a phenyl butyric acid methyl ester group. The functional group enhances the solubility of the fullerene in organic solvents and improves its miscibility with other organic materials, such as conjugated polymers [12,13]. However, they have problems, such as poor electronic correlation and morphological instabilities [1]. Many solutions are suggested to attempt to solve this apparent problem, such as chemical modifications in one of the species and the complete replacement of the donor or acceptor [1,2,7].

A significant advantage of semiconductor polymer thin films for organic electronics applications is their low cost, flexibility, designability, and slight toxicity, unlike Si-based solar cells [14,15,16]. On the other hand, low photogenerated current and fill factor are still challenging for organic optoelectronic devices. In this context, thin films with organic heterojunctions of the donor/acceptor type are used to increase the charge exchange performance of these devices. In this way, using P3HT/PCBM heterojunction increases carrier mobility [15,16] because their chemical structures have an adequate match both structurally and electronically. Through a heat treatment in preparing thin films, the film morphology of the P3HT/PCBM heterojunction composite is unique [14,15,16]. The heat treatment uniquely creates a highly organized and localized structure in P3HT and PCBM, providing a suitable junction for increasing carrier mobility. Light absorption causes excitons to be generated in P3HT and diffused to the interface between P3HT and PCBM. The match between the energy levels of P3HT and PCBM leads to increased carrier mobility.

The replacement of PCBM with 2D materials has been constantly suggested in the literature [1,2,17,18], mainly due to the difficulty of synthesizing side groups in fullerene, the precursor of PCBM [1,2,17,19]. Graphene, a carbon-based 2D material, is a promising material for photovoltaic devices with the potential to increase photoconversion efficiency [2]. Among all graphitic materials, graphene has the largest conjugated basal plane and is the most favorable binding to the various conjugated polymers found in photovoltaic cells. Graphene is flexible and exhibits high charge mobility (20,000 cm² V⁻¹ s⁻¹) and good conductivity (approximately 10⁸ S m⁻²) [20]. Its semi-metallic character makes graphene a potential candidate for being an excellent extractor or charge carrier; in other words, graphene can be used with ETL or HTL [2,20]. However, it is necessary to modify its work function and Eonset to match the energy levels of the D/A interface, which can be carried out through doping or chemical functionalization, allowing for HOMO-LUMO to coincide with the active layer. The most common procedure employed in laboratories to obtain graphene-like materials is the method to produce reduced graphene oxide (RGO), a graphene derivative. This method is cheap and quickly made at large scale [21]. It starts with synthesizing graphene oxide (GO) by the Hummers method [21] based on the oxidation of graphite in an acidic media. Subsequently, graphene oxide can be reduced into RGO, and the most common treatments used are hydrazine reduction and thermal annealing [22]. RGO’s unique properties make it an important material in various fields, such as energy storage, sensing, catalysis, and electronics [23,24]. During the reduction process, most oxygen-containing groups (such as hydroxyl, epoxy, carboxyl, and carbonyl groups) present in graphene oxide are removed, but some may remain, contributing to the incomplete recovery of the graphene structure [25]. These residual groups affect the electrical and chemical properties of RGO. O is composed of a carbon lattice similar to graphene, with a two-dimensional honeycomb structure formed by sp²-hybridized carbon atoms. However, unlike pristine graphene, RGO has various defects and residual oxygen-containing functional groups [26]. The level of reduction can be controlled, allowing for RGO’s electrical properties to be tuned for specific applications. Higher reduction levels lead to higher conductivity, though some defects may remain [27]. Polymer/GO nanocomposite structures have shown significant potential in applications such as photodiodes, supercapacitors, and solar cells, highlighting the importance of investigating GO-doped materials’ optoelectronic and electrical properties [28]. Recent studies on GO-doped ZnO and PCBM interlayers also revealed their potential to optimize Schottky diode performance by carefully adjusting GO content [28]. Liu et al. [29] were among the first to explore the use of solution-processable graphene modified with isocyanate as an acceptor in organic photovoltaics (OPVs). They prepared the active layer by spin-coating a mixture of P3OT and functionalized graphene in 1,2-dichlorobenzene, then annealing at 160 °C for 20 min. The resulting cell, configured as ITO/PEDOT/P3OT/LiF/Al, achieved an efficiency of 1.4% with a graphene concentration of 5 wt%. In a subsequent study [30], the same team used 10 wt% of the modified graphene with P3HT as the donor in a similar device, reaching a maximum efficiency of 1.1% after annealing at the same temperature for 10 min. The improvement in efficiency, compared to cells without graphene, was attributed to faster electron transport facilitated by the carbon nanomaterial. The use of PANI-g-rGO/P3HT nanohybrids showed enhancements in the morphological and photovoltaic stability of P3HT solar cells, exhibiting the least degradation after one month of air aging compared to other tested supramolecular systems [31]. Despite these advancements, further research is needed to fully understand and optimize the use of graphene and GO in photovoltaic devices, making it a fertile area for continued exploration.

In summary, the interface effects in the operating device conditions cause a significant loss of photo-excited carriers and can be investigated through conventional spectroscopy techniques [7,32], whereas the energy transfer processes depend on a series of photophysical processes, and it is generally necessary to correlate different experimental techniques for full clarification. Specifically, for studies involving energy transfer, emission ellipsometry (EE) [33] using Stokes parameter spectroscopy (SPS) [32] is a powerful technique that allows for fully describing the polarization state of the light emitted with the aid of Stokes’ theory [33]. Thus, it is possible to apply EE to study energy transfer from photo-excited carriers in polymeric and conjugated materials [32,34]. Furthermore, as EE describes all analyzed light polarization states, it is possible to obtain complete information on molecular ordering, anisotropy, and asymmetry factors [33].

Thus, this work aimed to study the photophysical properties of the interfaces in nanocomposite films of P3HT/PCBM, P3HT/PCBM/RGO, and P3HT/RGO. Our work highlights that the RGO has a role in improving energy transfer over the PCBM in the interface with the P3HT. In addition, it was observed that PCBM could function as a potential barrier between P3HT-adjacent chains, which makes energy transfer difficult, causing carriers to recombine in the same chains that were photo-excited, corroborating the low electronic correlation of the P3HT/PCBM interface. Conversely, this result can strengthen the role of PCBM as a good load separator.

2. Materials and Methods

Figure 1 presents the chemical structure of all materials used. The samples were prepared by spin-coating using a Laurell model WS-650MZ-23NPPB6 spin-coater. First, a layer of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS) as the HTL was deposited onto the indium tin oxide (ITO) substrate. ITO substrates were soaked in a piranha solution (five parts concentrated H₂SO₄ and two parts H₂O₂ (30%)) for 10 min and then rinsed, first in water, then in isopropanol and dried under N₂ flow. The PEDOT/PSS dispersed in water at 0.5–1 wt.% was bought from Sigma-Aldrich (CAS: 155090-83-8).

The P3HT regioregular conductive polymer and PCBM were purchased from Sigma-Aldrich with ≥90% and >99.5% purity, respectively. RGO was also used with the PCBM or replacing the PCBM. The RGO was obtained from the reduction of graphene oxide (GO), obtained by the method of Hummers and modified Offeman [21]. An aliquot of the aqueous GO dispersion was diluted in ethanol (2.8 mg mL⁻¹) containing 0.087 mol·L⁻¹ of tetrabutylammonium hydroxide to aid dispersion, and then 0.47 mol·L⁻¹ of hydrazine hydrate was added (N₂H₄·H₂O 80% v/v in H₂O, Merck S/A, Cotia, Brazil). The dispersion was stirred at room temperature (~25 °C) for 24 h to maximize the GO reduction following the procedure already described in the literature [35]. Figure S1 shows that the GO was reduced, and our RGO has a higher amount of sp2 carbons, which is desirable for reducing GO to RGO.

The P3HT (active layer) was deposited on top of HTL PEDOT/PSS, together with an electron- or acceptor-carrying material, thus forming a nanocomposite. We prepared the following composites: (i) P3HT/PCBM, (ii) P3HT/PCBM/RGO, and (iii) P3HT/RGO. Therefore, it was possible to obtain the following sets of samples, which consisted of ITO/PEDOT:PSS/ETL, as shown schematically in Figure S2; the thin films can be seen in Figure S3. The regioregular P3HT 1 molar and PCBM 0.1 molar were prepared in chlorobenzene and RGO of 0.1 mg/mL in water. Likewise, PEDOT/PSS and RGO were prepared at the same concentration of 1 molar in water for each composite. For the formation of ETL, 35 μL was deposited on ITO/HTL and rotated at 1000 rpm for 90 s. After forming the thin film, it was heated to 100 °C for 5 min in environmental conditions. HTL was deposited by spin-coating at 2000 rpm for 60 s using 200 μL of the precursor solution, and after deposition, the PEDOT/PSS thin film was heated in a muffle at 100 °C for 30 min in environmental conditions.

The measures of optical absorption in the ultraviolet–visible (UV-Vis) spectra were obtained at room temperature in an Agilent Cary 60 Absorption Spectrophotometer, operating in the 215–2500 nm range (wavelength accuracy ± 0.06 at 541.94 nm). Photoluminescence measurements (PL) were obtained for different sample temperatures, from 90 to 300 K, using a cryostat coupled to a closed helium system reaching 10⁻⁶ Torr pressures. A diode laser (LaserLine iZi 405) at 405 nm (3.06 eV) and 30 mW of power was used as a source of excitation. The luminescence of the polymer was captured by a CCD spectrophotometer USB2000 from Ocean Optics (standard error 0.125540214). For the emission ellipsometry (EE) spectroscopy technique, the optical path traveled is the same as that of PL, but with the addition of an analyzer polarizer and a quarter-wave plate, both achromatic. More details on the EE technique can be found in Reference [34]. The morphology of the films was studied with atomic force microscopy (AFM) in tapping mode using a Nanosurf FlexAFM 5 microscope (XY-Linearity Mean Error < 1.2%). A commercial Si cantilever with a spring constant between 20 and 100 Nm⁻¹ and a free oscillation interval between 250 and 300 Hz was used. Free Software Gwydion (version 2.66) (Department of nanometrology and technical length Czech Metrology Institute, Okružní, Czech Republic) calculated all AFM data analyses, such as the roughness, skewness, and kurtosis parameters [36]. Root mean square roughness is an essential tool that describes the standard deviation of surface roughness, which can lead to distinction between different surfaces. The skewness coefficient (R_sk) is the third term in the probability density function and is a parameter sensitive to high peaks or deep valleys. The kurtosis parameter is the fourth central profile of the probability density function (R_ku). It describes how flat the distribution curve is, which can also lead to separating surfaces and more high points or low points [37].

3. Results and Discussion

Figure 2 shows the optical absorption spectra UV-Vis for all analyzed samples; it is possible to observe that the maximum absorption of all the samples is centered at ~520 nm. Through the absorption spectra, it is possible to obtain onset levels (E_onset) similar to those in the Kubelka–Munk theory [38], which establishes a value of the cutting wavelength (λ_c) that corresponds to the wavelength at the end of the absorption spectrum [39]. Figure S4 shows how the λ_c was obtained for all samples, which coincides with the maximum emission in the PL spectra (see below in the PL experiment) [40].

Table 1. E_onset obtained by UV-Vis spectra.

Sample	Wavelength λc (nm)	Eonset (eV)
PEDOT:PSS/P3HT:PCBM	651.00 ± 0.06	1.90 ± 0.09
PEDOT:PSS/P3HT:PCBM:RGO	660.00 ± 0.06	1.88 ± 0.09
PEDOT:PSS/P3HT:RGO	665.00 ± 0.06	1.86 ± 0.07

shows their respective E_onset values. The value of E_onset can be explained to the P3HT in terms of (i) the conjugation length of the polymeric chains, the shorter the length of the conjugated polymeric segment, the greater the value of E_onset [33], and (ii) the feature regioregular of the polymeric main chain, that is, E_onset increases if the regioregularity decreases in the polymeric chains; thus, the sample that demonstrated the shortest conjugation length and the highest E_onset was PEDOT:PSS/P3HT:PCBM and the sample that presented the highest regioregularity was PEDOT:PSS/P3HT:RGO (Table 1) [41,42]. The data obtained for E_onset follows the literature [32] and concordance with Kinoshita et al., where the characteristic absorption range for the regioregular P3HT is 415 to 650 nm [42]. The spectrum presents three distinct peaks. The first ~511 to ~519 is related to the S₂ transition of P3HT (0–2, exciton + 2 phonons). The second peak at 552 with the S₁ transition of P3HT (0–1, exciton + 1 phonons), and the last peak at 605 with the S₀ transition of P3HT (0–0, exciton) [43]. For an explanation, the last affirmation is that long pristine P3HT chains typically have their absorption maximum centered at ~500 nm [14,16,44,45]. However, when PCBM is added to regioregular P3HT, there is a tendency for P3HT to undergo slight disorder in the structure [14,16], such that the disorder causes the absorption maximum to undergo a blueshift. When RGO is added to P3HT, this disorder is not as evident as when we have only PCBM; it does not electronically affect the absorption of P3HT in a significant way [46]. In other words, Figure 2 shows a broad line with a maximum of ~512 nm for PEDOT:PSS/P3HT:PCBM and a maximum of ~519 nm for PEDOT:PSS/P3HT:PCBM:RGO and PEDOT:PSS/P3HT:RGO composites. In the literature, these lines are denoted as 0–2, 0–1, and 0–0 transitions, respectively [14,47,48]. The shoulder at ~605 nm is associated with H-aggregated states of polymeric films [49,50]. The difference in the UV-Vis absorption broad line maximum in each film is related to how PCBM and RGO influence the structure of P3HT. When PCBM is added, the crystallinity of the P3HT film is altered in such a way that the effective conjugation length decreases, causing a blueshift. However, when RGO is added to PCBM, a redshift in the UV-Vis absorption spectrum is observed. The presence of RGO in the composite implies a more stable morphology, so P3HT has greater crystallinity, and the polymer chains have a greater effective conjugation length [14,47,48,49,50]. This factor is corroborated especially by E_onset, which also presents a redshift when RGO is added to the nanocomposite and when the composite is without PCBM.

The values in Table 1 show that the E_onset of P3HT decreases in the presence of reduced graphene oxide (RGO). This is due to the high electron transfer ratio of RGO, with non-hybridized states originating π orbitals outside the plane, providing greater electronic mobility. This characteristic is essential for applications in photovoltaic devices [20]. Here, the band that refers to the 0-0 transition is also highlighted for better visualization with E_onset. When PCBM is completely removed, a redshift from 651 nm to 665 nm is noted. The redshift is attributed to the fact that RGO implies a more stable morphology, so P3HT has greater crystallinity, and the polymer chains have a greater effective conjugation length [14,47]. Still, according to Figure 2, it is possible to observe that the three samples have wide spectra covering a good part of the visible light and had a maximum absorption in an approximate wavelength of 525 nm, consistent with the literature [5]. AFM images were obtained to investigate the morphology of the films. Figure 3 and

Table 2. Mean roughness, skewness, and kurtosis obtained from AFM measurements.

Sample	Mean Roughness	Skewness	Kurtosis
PEDOT:PSS/P3HT:PCBM	0.70 ± 0.01	5.32 ± 0.06	110 ± 1
PEDOT:PSS/P3HT:PCBM:RGO	1.26 ± 0.02	−0.19 ± 0.01	4.22 ± 0.05
PEDOT:PSS/P3HT:RGO	4.66 ± 0.06	−0.16 ± 0.01	3.40 ± 0.04

show the results of AFM analysis for the samples PEDOT:PSS/P3HT:PCBM, PEDOT:PSS/P3HT:PCBM:RGO, and PEDOT:PSS/P3HT:RGO. The mean square roughness values (σ_RMS) increase as the PCBM is replaced by the RGO, corroborating a solid change in the electronic properties of the nanocomposite films studied here.

PEDOT:PSS/P3HT:PCBM sample presents the lowest mean roughness of 0.70 nm (Figure 3a,e), which is higher than those of PEDOT:PSS/P3HT:PCBM:RGO (1.26 nm) (Figure 3b,e) and PEDOT:PSS/P3HT:RGO (4.66 nm) (Figure 3c,e). The height distribution along the sample is lower for PEDOT:PSS/P3HT:PCBM, presenting a narrow distribution centered at ~5 nm. However, it is possible to see points with much higher heights in the measured region of the sample. The height distribution for PEDOT:PSS:/P3HT:PCBM:RGO and PEDOT:PSS/P3HT:RGO are shifted to higher heights. PEDOT:PSS/P3HT:PCBM:RGO presents a narrow distribution centered at ~9 nm, whereas the height distribution of PEDOT:PSS/P3HT:RGO presents a very broad profile centered at ~25 nm. Skewness and kurtosis are two critical statistical parameters to describe a surface, and the former has a value of 0 (symmetrical height distribution) and the latter of 3 (moderate sharpness) for the surface heights under Gaussian distribution. A positive skewness means the surface has high peaks and filled valleys, while a negative one means deep scratches (grooves) and a lack of peaks. A kurtosis larger than 3 means the surface has high peaks and/or low valleys, while a smaller than 3 means low peaks and/or valleys [51]. So, adding RGO to the films disturbs the smooth morphology of the polymeric films, evidenced by the increase in the height distribution and roughness, along with the decrease in the skewness and kurtosis values. The morphology changes caused by adding RGO to the optical behavior of the films were further studied by the optical characterizations discussed below.

The results of the photoluminescence spectra in Figure 4 and Figure 5 were organized by intensity to analyze how the nanocomposite influenced the amount of light emitted and normalized to distinguish the regions of emission of each film. According to Janssen et al., PCBM causes a fluorescence suppression effect in P3HT films; the emission is extinguished using light, so the transfer of energy and charge is favored [52].

Figure 4a shows the PL spectra obtained for the sample PEDOT:PSS/P3HT:PCBM at different temperatures, ranging from 90 to 300 K. The spectra show that the maximum emission intensities vary with the sample temperature, exhibiting a blueshift as the temperature increases, with an 11 nm difference between 90 and 300 K. The PL spectra of PEDOT:PSS/P3HT:PCBM (Figure 4b) exhibit similar behavior, with intensity increasing and then decreasing as the temperature ranges from 90 to 300 K. This introduces an unexpected anomaly in the PL dependence on sample temperature [52]. As for the PL spectra of the sample consisting of PEDOT:PSS/P3HT:RGO (Figure 4c), unlike the other two samples, the emission intensity gradually decreases with increasing temperature. This is because the chains become stiffer as the temperature decreases, reducing energy transfer by non-radiative modes. Conversely, as the temperature increases, the thermal energy in each chain increases, enhancing energy transfer. In general, local disorder influences emission intensity reduction by increasing the availability of non-emissive modes for energy transfer, facilitating energy migration to structural defects and vibrational states, such as through resonance [53].

This anomaly in poly(3-alkylthiophene) (P3ATs) is explained in terms of the change in the conformation of the polymer main chain with temperature and its influence on the dynamics of the photo-excited species. With the excitation of P3AT by the Ar⁺ laser, electrons and holes are excited to the π* and π bands in a very short time. Luminescence in this polymer conductor can be considered by recombining these excited species. However, with increasing temperature, the excited species, electrons, and holes can move out of the excited region, increasing the probability of non-radiative recombinations and causing the luminescence intensity to be suppressed [40,54,55]. Furthermore, with increasing temperature, the effective conjugation length decreases due to the occurrence of a large torsion angle between the thiophene rings. Therefore, at high temperatures, due to the decrease in the effective conjugation length, the mobility of the excited carriers and their probability of escape from the excited region decrease, which can increase the probability of recombination and emission, implying an increase in the photoluminescence (PL) intensity. In the liquid phase, the torsion angle does not change significantly with temperature, meaning there is no notable change in the effective conjugation length, and the species’ confinement probability does not change significantly with temperature. Thus, the non-radiative recombination of excited species should increase with increasing temperature, which causes the luminescence intensity to be suppressed in the liquid phase [40,54,55]. The results indicate that having the RGO adjacent to the P3HT promotes the predictable behavior between luminescence intensity and sample temperature in organic luminescent semiconductors. This may be related to the changes in the morphology by the action of the RGO and/or the P3HT/RGO interface. The anomalous behavior presented by the samples with PCBM may be related to the P3HT/PCBM interface, which notoriously produces luminescence quenching and interferes with the free volume between the polymeric chains of P3HT, causing, with the increase in temperature, the anomaly of emission intensity [52].

Figure 5 displays the PL spectra normalized at the unit in each maximum for the three samples, shifted vertically to provide better visualization. For the three samples, it is possible to observe a blueshift in the emission maximum with the increase in temperature. This was expected and can be explained in terms of the effective conjugation length of the molecules, considering that there is an increase in the torsion angle between the vicinity of the thiophene ring of the P3HT polymer and the increase in temperature of the sample. Consequently, the effective conjugation length of the polymeric chain decreases, and this shape causes a blueshift in the emission. This effect arises due to this increase in thermal disorder, which favors the appearance of these distortions, such as folds and rotations, responsible for breaking the conjugated segments. All three samples have a maximum peak ~660 nm due to electronic transitions π-π*. The emission region occurs approximately between 500 and 875 nm. It is also observed in both samples that the band at 730 nm decreases dramatically with increasing temperature. The presence of this contribution represents a transition from the first phonon replica, whereas the peak of maximum emission represents the transition from zero phonon. These transitions are also known as transitions I (peak of maximum intensity in PL) and II (peak ~730 nm). Transition I occurs between LUMO and HOMO, whereas transition II occurs in the vibrational state, also known as HOMO + 1 [56]; the peak ~550 nm occurs due to conjugation-breaking and defects, causing shorts chains [57,58]. The loss of the definition of this contribution with the increase in temperature is associated with a loss of energy for the vibrational modes, which are activated as the temperature increases.

The spectra in Figure 5 are consistent with regioregular P3HT, as demonstrated by Clarck et al. [59]. As the temperature increases, the spectra are blueshifted and broadened, and the peak ~660 nm becomes relatively more intense. Temperature-dependent PL spectra of intramolecular excitations generally show a blueshift and broadening upon increasing temperature but no change in relative intensities of vibronic peak [57,59]. However, it is possible to observe that the PCBM influences directly on the PL broadening, and this effect increases with temperature. Comparing the spectra in Figure 5a,b with Figure 5c without PCBM, it is noted that the line shape is more resolved and peaks ~550 nm no show, making it evident that the PCBM influence in P3HT broadening.

On the other hand, RGO is not. In other words, as the temperature increases, the peaks are broadened, explained by the increase in the degrees of freedom of the P3HT polymer chains [14,47,48,57,58,59]. In cases where PCBM is present, there is also a quenching of the emission intensity in addition to the broadening. The proximity of PCBM to P3HT chains is explained by the quenching model [14,47,48,57,58,59]. The dynamics of the photo-excited carriers, shown in Figure 6, cause the electrons in the excited state, after the formation of the exciton, to be transferred to the PCBM, and recombination occurs in a non-radioactive manner. And the excitons formed directly in P3HT are relaxed to emitting states and recombine but at a much lower rate. In other words, the quantum efficiency of emission is small in nanocomposites with PCBM. For the case where RGO is present, as explained for the UV-Vis absorption analysis, The presence of RGO in the composite implies a more stable morphology, so P3HT has greater crystallinity. The polymer chains have a greater effective conjugation length [14,47,48,57,58,59], i.e., this interaction is different. It does not significantly alter the electronic structure of P3HT so the quenching model cannot be applied.

To quantify the change in the emission intensity of transition I concerning transition II, the Huang–Rhys parameter was estimated (S ≃ I₁/I₀), which denotes information about the electronic structure of the system through electron–phonon interaction [56].

Table 3. Huang–Rhys parameter (S≃I₁/I₀) for samples PEDOT:PSS/P3HT:PCBM, PEDOT:PSS/P3HT:PCBM:RGO, and PEDOT:PSS/P3HT:RGO at different temperatures. I₁ and I₀ correspond to the PL intensity (Figure 4) for transitions II and I, respectively.

S ≃ I1/I0
Temperature (K)	PEDOT:PSS/ P3HT:PCBM	PEDOT:PSS/ P3HT:PCBM:RGO	PEDOT:PSS/ P3HT:RGO
90 ± 1	0.74 ± 0.01	0.84 ± 0.02	0.73 ± 0.01
120 ± 1	0.86 ± 0.02	0.85 ± 0.02	0.87 ± 0.02
150 ± 1	0.91 ± 0.02	0.91 ± 0.02	1.12 ± 0.02
180 ± 1	0.93 ± 0.02	0.95 ± 0.02	0.96 ± 0.02
210 ± 1	0.95 ± 0.02	0.95 ± 0.02	0.96 ± 0.02
240 ± 1	0.95 ± 0.02	0.96 ± 0.02	0.92 ± 0.02
270 ± 1	0.96 ± 0.02	0.96 ± 0.02	0.94 ± 0.02
300 ± 1	0.95 ± 0.02	0.96 ± 0.02	0.93 ± 0.01

shows the S values for each temperature of each sample. In all samples, the lowest temperatures showed lower Huang–Rhys parameters. This may indicate that at low temperatures, the polymer at the interface does not have enough free volume to significantly change its conformation by vibrational modes, which causes the parameter to decrease S (<1).

Table 4. Difference in energy (eV) between the maximum of transition I and transition II of samples PEDOT:PSS/P3HT:PCBM, PEDOT:PSS/P3HT:PCBM:RGO, and PEDOT:PSS/P3HT:RGO in different temperatures.

Δν = ν0 − ν1 (eV)
Temperature (K)	PEDOT:PSS/ P3HT:PCBM	PEDOT:PSS/ P3HT:PCBM:RGO	PEDOT:PSS/ P3HT:RGO
90 ± 1	0.166 ± 0.003	0.165 ± 0.003	0.175 ± 0.004
120 ± 1	0.172 ± 0.003	0.171 ± 0.003	0.175 ± 0.004
150 ± 1	0.182 ± 0.004	0.174 ± 0.003	0.148 ± 0.003
180 ± 1	0.189 ± 0.004	0.183 ± 0.004	0.181 ± 0.004
210 ± 1	0.186 ± 0.004	0.189 ± 0.004	0.185 ± 0.004
240 ± 1	0.197 ± 0.004	0.166 ± 0.003	0.199 ± 0.004
270 ± 1	0.202 ± 0.004	0.188 ± 0.004	0.205 ± 0.004
300 ± 1	0.206 ± 0.004	0.209 ± 0.004	0.211 ± 0.004

shows the energy difference in eV between the value of the wavelength in transition I and in transition II [32,58,59]. The values of these energy differences, presented in Table 4, are between approximately 1300 and 1700 cm⁻¹ and correspond to the stretching vibration modes and deformations, among others, associated with the thiophene ring, mainly to the stretching of the C=C bond, which corresponds to the formation of polar and bipolar states, corroborating with the above analysis of the Huang–Rhys parameters showing electron–phonon coupling [32].

Figure 6 shows the simplified schematic energies diagrams for the PEDOT:PSS/P3HT:PCBM, PEDOT:PSS/P3HT:PCBM:RGO, and PEDOT:PSS/P3HT:RGO samples. Figure 6a shows the energy transfer between the samples and between interfaces based on photophysical analysis. Figure 6b shows the energy band for semiconductor interfaces with information already obtained. So, it is possible to observe in a simplified way the mechanism to photoexcited carriers into semiconductor interfaces with E_onset measured here for each sample.

EE measurements were performed to understand the mechanisms of energy transfer and the polarization states of light [33]. After analyzing the sample spectrum using the Fourier method, Stokes parameters S₀, S₁, S₂, and S₃ were determined. These parameters are related to the polarization states of light and thus are also related to the factors associated with the orientation of the sample’s polymer chains. The parameter S₀ is associated with the total amount of light emitted, S₁ provides the portion of light emitted that is linearly polarized vertically or horizontally, and S₂ represents the amount of light linearly polarized at +45° or −45° concerning the coupled polarizer before the spectrophotometer and S₃, which is a circularly polarized light patch on the right or left [33].

Figure 7 shows the EE spectra for a sample consisting of PEDOT:PSS/P3HT:PCBM:RGO at 90 K (a e c) and 300 K (b e d). The other samples’ analyses are similar (see Figures S5 and S6 in Supplementary Materials). It can be seen through spectra a and b that polarized light, P, is formed mainly by linearly polarized light. In the spectrum at a temperature of 90 K (Figure 7a), it is possible to observe that there were changes in the sample spectrum with the spectrum obtained at 300 K (Figure 7b). The main change is the change in the behavior of the curve (S₂/S₀), which is no longer null between 625 nm and 640 nm and in this range, it presented an amount of linearly polarized light rotated by −45°, indicating more significant rotation of the chains that have induced dipoles partially aligned with the excitation source. Although varying, the values of S₃/S₀ fluctuate within the experimental error, indicating that the film still does not emit circularly polarized light to the left or right. The S₁/S₀ curve, shown in Figure 7a, represents the linearly polarized light relative to the total light. It shows a negative value, indicating that the emitted light prefers the linear and vertical direction with the laboratory plane, which matches the direction of the excitation light polarization [33]. A polymeric chain has dipole moments that are preferentially excited when parallel to the excitation of linearly polarized light, facilitating exciton diffusion and energy transfer between polymeric chains [33]. Consequently, linear polarization predominates.

The degree of polarization, P (Equation (1)), indicates the amount of polarized light without differentiating between circularly or linearly polarized light [33]. The anisotropy factor, r (Equation (2)), shows whether the chains are ordered in a preferred direction. The asymmetry factor, g (Equation (3)), relates to circularly polarized light emission [32]. The greater the anisotropy factor, the greater the molecular ordering of the film and, consequently, the greater the degree of polarization in the emission, as this value is influenced by the molecular ordering of the film [60]. For the sample measured at 90 K, it is observed, from Figure 7c, that the degree of polarization has a maximum of ~90% at ~610 nm and subsequently decays to its minimum value of ~50% for the lowest energies. In the same way, for the sample measured at 300 K, it is observed, from Figure 7d, that the degree of polarization has a maximum of ~80% at ~610 nm and subsequently decays to its minimum value of ~70% for the lowest energies. As the wavelength increases, this decay of P and r may indicate a lesser alignment between the larger molecules. It is also possible to observe through the asymmetry factor curve, g, that there is no circularly polarized light emission in the sample. The spectra in Figure 7c, obtained for the sample analyzed at a temperature of 90 K, show that the degree of polarization increased for the wavelength range from 625 to 650 nm, concerning the spectrum obtained at 300 K.

$$P={\displaystyle \frac{I_{pol}}{I_{tot}}}={\displaystyle \frac{{\left(S_1^2+S_2^2+S_3^2\right)}^{\frac{1}{2}}}{S_0}}$$

(1)

$$r={\displaystyle \frac{-2\frac{S_1}{S_0}}{3+\frac{S_1}{S_0}}}$$

(2)

$$g=2{\displaystyle \frac{S_3}{S_0}}$$

(3)

Originally, polymeric films that present a polarized emission have molecular ordering in the same order as the polarization of the light emitted [33]. However, the degree of molecular ordering indicates that the samples do not have molecular ordering (see Figures S7–S9 in Supplementary Materials) [61]. We can observe in Figure 8 that the efficiency of energy (or charge) transfer, measured through the polarization of emitted light after excitation by linearly polarized light, depends on the sample’s temperature. The degree of polarization is expected to increase as the temperature decreases. Lower temperatures provide greater free volume and distance between chains, increasing the likelihood of radiative recombination within a single chain rather than energy transfer between chains. However, this behavior is not strongly visualized. The spectra in Figure 8b, obtained for the sample consisting of PEDOT:PSS/P3HT:PCBM:RGO, show that the measurements of the degree of polarization obtained for temperatures of 150 K, 180 K, 240 K, 270K, and 300 K, in the wavelength range from 625 nm to 650 nm, behave approximately the same (about 80% to 60%). After 650 nm, all these measurements remain approximately the same, except for the 300 K sample and the measurements taken at 120 K. The spectra of sample PEDOT:PSS/P3HT:PCBM (Figure 8a,d) behave similarly to the sample PEDOT:PSS/P3HT:PCBM:RGO. The degree of polarization has its maximum, approximately 75% and 98%, gradually decreasing for longer wavelengths until it stabilizes and becomes constant after 660 nm.

Comparing the spectra in Figure 8a–c, we can see an increase in the emission polarization as a function of the sample. The sample without PCBM presented a greater tendency for energy transfer and less polarization, as observed by nearly 10% of the degree of polarization and in the values of the anisotropy factor (Figure 8d). Generally, greater anisotropy indicates a higher molecular ordering of the sample, resulting in a higher degree of polarization in the emission. This value is influenced by the molecular ordering of the film [60]. However, deductions based on the anisotropy factor cannot be considered here, as the value of r calculated using the Stokes’ parameters is based solely on emission data obtained from a polarized light source. Corroborating, it is known that these samples do not have molecular ordering, as was shown by the measures of AFM and the factor of molecular order. Thus, the measurements obtained for these samples should show low anisotropy values. However, PCBM acts as a barrier between P3HT molecules, increasing the energy transfer time between molecules. This supports the use of PCBM as an effective charge separator in organic photovoltaic devices. So, due to this delay in transferring energy, the chance of a radioactive recombination becomes far greater than the chance of transferring energy to another chain. For this reason, the polarization is very high and thus interferes with the actual measurements of the EE anisotropy factor [33]. Compared to the other samples, the lower degree of polarization at all temperatures in the PEDOT:PSS/P3HT:RGO sample (Figure 8c) indicates that the lower the polarization, the greater the energy transfer. Thus, evidencing that the presence of RGO in the P3HT matrix with the absence of PCBM provides efficient dissociation of excitons in the P3HT:RGO interfaces; the greater the weight ratio of RGO to P3HT, the greater this effect will be, according to [47].

In other words, the apparent and unexpected contradiction between the values of r and β may be related to the accommodation of the PCBM between the side chains of the P3HT. PCBM is the ideal acceptor when combined with P3HT [62], improving charge transfer in organic photovoltaic devices even in polymeric films without induced ordering. This is explained by the longer lifespan of photo-excited carriers compared to P3HT alone [63]. Here, the EE results are similar. PCBM hampers energy transfer in chains that do not have their dipoles aligned with the excitation source, causing the recombination time of photo-excited carriers to be less than the energy transfer time between the main chains. Thus, recombination occurs in chains with dipoles aligned with the excitation source, resulting in high polarization in the emission.

Based on the above analysis, the PEDOT:PSS/P3HT:RGO sample shows better energy transfer from photo-excited carriers. Thus, it is possible to infer that the RGO can be a facilitator in the transfer of energy between one chain and another. Therefore, this analysis also shows that RGO is a potential candidate to replace PCBM as an n-type semiconductor in optoelectronic devices, such as organic photovoltaic cells (OPVCs) and organic light-emitting diodes (OLEDs). It should be noted that, in addition to good conductivity and an adequate E_onset, it facilitates the transfer of energy from photo-excited carriers between the polymeric chains of the P3HT.

4. Conclusions

The values of the E_onset calculated from the absorption spectra showed that the P3HT E_onset decreases in the presence of reduced graphene oxide due to the high electron transfer kinetics that this component presents. The photoluminescence spectra as a function of temperature showed anomalous behavior in the emission intensities of samples with the P3HT/PCBM interface. The behavior of the emission intensity of the sample with the P3HT/RGO interface increases with increasing temperature. The emission ellipsometry spectra showed that the temperature influences the degree of polarization of the sample, which influences changes along the excitation wavelengths. In addition, it showed that the sample emission has polarization and varies depending on the emission wavelength. Still, through the EE spectra, combined with the molecular ordering data (β), it is observed that the PCBM hinders the energy transfer of the photo-excited carriers, causing the carriers to recombine with high polarization, i.e., in the same direction of polarization as the excitation source. Thus, we can conclude that the PCBM, besides being a good acceptor, also acts as a barrier between the P3HT chains, favoring a longer lifetime for the photo-excited carriers, strongly implying its use in photovoltaic devices due to the favored separation of loads. Conversely, replacing PCBM with RGO, which has a slightly smaller E_onset, proves potentially favorable. The P3HT/RGO interface shows a greater energy transfer tendency than the P3HT/PCBM interface. RGO is a potential substitute for PCBM as an n-type semiconductor (acceptor) in photovoltaic devices. Besides having good conductivity and an adequate E_onset, RGO facilitates energy transfer from photo-excited carriers between the P3HT polymer chains.