A Comprehensive Review of Self-Assembled Monolayers as Hole-Transport Layers in Inverted Perovskite Solar Cells

Abstract

The hole-transport layer (HTL) plays a pivotal role in engineering high-performance inverted perovskite solar cells (PSCs), as it governs both hole extraction/transport dynamics and critically impacts the crystallization quality of the perovskite absorber layer in device architectures. Recent advancements have highlighted self-assembled monolayers (SAMs) as promising candidates for next-generation HTL materials in inverted PSCs due to their intrinsic advantages over conventional counterparts. These molecularly engineered interfaces demonstrate superior characteristics including simplified purification processes, tunable molecular structures, and enhanced interfacial compatibility with device substrates. This review systematically examines the progress, existing challenges, and future prospects of SAM-based HTLs in inverted photovoltaic systems, aiming to establish a systematic framework for understanding their structure–property relationships. The review is organized into three sections: (1) fundamental architecture of inverted PSCs, (2) molecular design principles of SAMs with emphasis on head-group functionality, and (3) recent breakthroughs in SAM-engineered HTLs and their modification strategies for HTL optimization. Through critical analysis of performance benchmarks and interfacial engineering approaches, we elucidate both the technological merits and inherent limitations of SAM implementation in photovoltaic devices. Furthermore, we propose strategic directions for advancing SAM-based HTL development, focusing on molecular customization and interfacial engineering to achieve device efficiency and stability targets. This comprehensive work aims to establish a knowledge platform for accelerating the rational design of SAM-modified interfaces in next-generation optoelectronic devices.

1. Introduction

With the gradual depletion of fossil fuels and the transformation of the global energy structure, solar cells as a sustainable, clean, and environmentally friendly energy source are receiving increasing attention. The history of solar cell development is epic, intertwined with material innovation and technological breakthroughs. Since Bell Labs developed the first practical crystalline silicon (c-Si) solar cell in 1954, photovoltaic technology has undergone three generations of material system iterations, continuously pushing the limits of physics and cost barriers. First-generation crystalline silicon technology laid the foundation for the photovoltaic industry. Monocrystalline silicon cells have achieved 26.8% efficiency through passivation technologies like PERC and TOPCon. Silicon, being non-toxic and abundant, dominates ~95% of the global photovoltaic market due to its mature commercialization [1]. However, the theoretical efficiency limit of single-junction silicon cells (~29%) means c-Si is approaching its efficiency ceiling. Second-generation thin-film batteries diversified development paths: Gallium arsenide (GaAs) single-junction cells, prized for exceptional radiation resistance, are widely used in spacecraft applications. Copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) offer low-cost advantages, yet face controversies over slow efficiency gains, toxicity, and scarcity of raw materials. Third-generation novel photovoltaics include dye-sensitized solar cells (DSSCs), organic solar cells (OPVs), quantum dot solar cells (QDSCs), and perovskite solar cells. Low-dimensional systems, such as 1D nanostructures, have attracted significant attention due to their unique optical and electronic properties [2,3]. Meanwhile, 2D quantum wells have demonstrated tunable exciton properties through compositional and structural modifications, profoundly impacting their applications in this field [4]. QDSCs, a novel photovoltaic technology utilizing quantum dots (QDs) as the light-absorbing layer. Their quantum confinement effect quantizes electron and hole energy levels, enabling tunable bandgaps and exceptional optoelectronic properties. Despite these advantages, QDSCs still face challenges such as low laboratory efficiency and unresolved stability issues under operational conditions [5].

Among various solar cell technologies, PSCs have become a research hotspot due to their high power conversion efficiency (PCE) and low-cost advantages. Since Miyasaka [6] and his colleagues first reported on PSCs in 2009, the PCE of PSCs has been continuously improving. Currently, the highest certified PCE in single junction PSCs reaches 26.81% [7] through molecular design and interface optimization. Homojunction PSCs have also been extensively studied in recent years. These devices directly construct p-n junctions by forming p-type and n-type regions through doping within a single perovskite material. The charge separation is driven by the built-in electric field of the p-n junction in the perovskite material, effectively reducing interface recombination due to better lattice matching at the interface as it consists of the same material. The reduced heterointerfaces may potentially enhance long-term stability [8]. Homojunction PSCs, which achieve more efficient charge collection through intrinsic p-n junctions, represent a crucial direction for breaking through efficiency limitations in the future. In recent years, finite element method (COMSOL) simulations coupling optical and electrical models have systematically optimized key parameters including doping concentration, layer thickness, carrier mobility, heterojunction defects, and contact barriers. Various homojunction device architectures have been designed through these simulations, providing theoretical guidance for the development of homojunction PSCs [9,10]. Simultaneously, the bandgap of perovskite materials can be precisely tuned by adjusting their chemical composition, such as through mixed halides or A-site cation engineering. By stacking a wide-bandgap (WBG) perovskite top cell with a narrow-bandgap (NBG) bottom solar cell—including NBG-PSCs, silicon solar cells, or organic solar cells—this tandem configuration overcomes the single-junction Shockley–Queisser efficiency limit, significantly enhancing the overall power conversion efficiency of solar cells [11,12,13,14].

Generally, a common PSC device consists of two electrodes, the perovskite light absorber, the electron-transport layer (ETL) and HTL. Moreover, PSCs can be categorized into two primary configurations—the conventional n–i–p structure and the inverted p–i–n architecture—depending on whether the ETL is positioned before or after the perovskite absorber layer (Figure 1). In the initial phase of n–i–p PSC development, researchers employed a dual-layer titanium oxide (TiO₂) configuration for electron transport. This bilayer TiO₂ approach represented a critical transition from dye-sensitized solar cell technology to perovskite photovoltaic devices, establishing foundations for charge extraction in n–i–p configurations. Meanwhile, the mesoporous structure resolved perovskite deposition and charge extraction challenges [15,16,17]. However, the necessity for elevated temperature processing and pronounced current–voltage hysteresis effects constrained their practical applicability in commercial photofvoltaic systems. The subsequent emergence of solution-processed tin oxide (SnO₂) as an alternative ETL material addressed these issues, but conventional n–i–p structured devices continue to encounter persistent challenges [18,19]. Compared with traditional n–i–p configurations such as [6]-phenyl-C61-butyric acid methyl ester (PCBM) or C60, inverted p–i–n PSC has several significant advantages, including stable metal electrodes exhibiting good stability, low hysteresis, low-temperature processing, and low cost. However, a persistent performance gap remains between the two architectures. Historical data reveals that single-junction p–i–n devices consistently achieve lower PCEs compared to their n–i–p counterparts. This efficiency disparity has motivated extensive research efforts to optimize interfacial engineering and material selection in inverted structures while maintaining their inherent stability and processability advantages [20,21,22,23,24].

This performance gap is mainly caused by a larger open-circuit voltage (VOC) loss, which is caused by non-radiative recombination within the bulk of the perovskite absorber layer and at its interfaces with charge-transport layers (CTLs) [15,25,26,27,28,29,30,31]. However, with the advancement of technology in recent years, the efficiency of inverted devices has surpassed that of formal devices through continuous innovation in materials and interface engineering [32]. Zhou et al. (2024) at Tsinghua University designed and synthesized a new HTM-T2, and the formal device achieved a photoelectric conversion efficiency of 26.41% and maintained 84% of the initial PCE after heating at 60 °C for 1500 h [33]. Chun-Du et al. [7] engineered the molecular 4PABCz and attained a certified PCE of 26.81% under reverse scanning conditions in inverted PSCs. Moreover, the hybrid SAMs composed of Me-4PACz and NA enhanced the wettability of the perovskite solution. As a result, the champion device achieved a PCE of 26.69% and maintained an initial PCE of 96.1% after more than 2400 h of operation at 1sun.

HTLs are a crucial element in inverted PSCs. They have a determinant function in hole extraction and transportation, surface passivation, and perovskite crystallization. At present, the pursuit of high-efficiency, stable, highly transparent, and cost-effective HTLs is of great significance for the commercialization of p–i–n PSCs. Currently, considering hole-transport capabilities and optical transparency, the most frequently utilized HTL materials in inverted PSCs include inorganic metal oxides like NiO_x and CuOx, and organic semiconductors like poly (3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT:PSS) and poly [bis (4-phenyl) (2,4,6trimethylphenyl) amine] (PTAA) [34,35].

While inorganic HTLs endow devices with greater stability compared to their organic-based counterparts, they come with certain drawbacks. Inorganic HTLs often require high-temperature processing, and their surfaces lack sufficient smoothness. This roughness impedes the formation of optimal interfacial contact between the HTL and the perovskite layer [36,37]. Previously, polymer-type hole transport materials (HTMs) such as PTAA and PEDOT:PSS were widely used due to their excellent conductivity and suitable highest occupied molecular orbital (HOMO) energy level for hole transport from perovskite to the electrode. However, they have significant drawbacks. To ensure the uniformity of the film, they need to be deposited to a specific thickness, which may increase manufacturing complexity and cost. Additionally, precise control over their HOMO levels and hole mobilities is crucial for effective charge transfer, yet this is difficult to achieve [13]. Therefore, researchers pay more attention to novel HTMs for inverted PSC devices.

Organic molecular assemblies termed self-assembled monolayers (SAMs) have emerged as a significant research focus due to their exceptional interfacial modulation capabilities. These assemblies, consisting of one or several layers of ordered organic molecules, can spontaneously adhere to solid substrates through anchoring groups [38]. SAM synthesis offers simplicity, enables direct molecular engineering, requires minimal material, and is suitable for the fabrication of large-area devices. The molecular structure of SAMs comprises three key components: anchoring, spacer, and terminal groups, with the terminal group playing a pivotal role in determining the material’s functionality. These distinctive attributes have facilitated the widespread application of SAMs [39].

When introduced as hole-transporting materials (HTMs) in perovskite solar cells (PSCs), SAMs can enhance interactions with upper materials through various functional groups and atoms. This facilitates perovskite formation and morphology control, modifies the surface work function, and precisely adjusts energy levels. As a result, charge extraction is enhanced, and interfacial carrier recombination is mitigated [38]. Additionally, SAMs can improve the wettability of the substrate surface, which aids in the formation of large-sized perovskite crystals, reduces trap density, and consequently elevates the performance of PSCs [38]. In terms of lower-layer materials, the ultrathin nature of SAMs allows them to be applied in extremely thin coatings on transparent conductive oxide (TCO) electrodes when functioning as HTMs [39]. Compared to alternative materials, SAMs exhibit minimal absorption of visible light [40] and establish a permanent dipole moment at the interface, thereby modulating the work function (WF) of the electrode [39]. Furthermore, the robust binding affinity between their anchoring groups and TCO effectively diminishes defect sites on the TCO surface, reduces interfacial charge recombination, and enhances charge transport capacity [39]. Overall, SAM-based HTMs are regarded as exceptional alternatives to polymers.

This review summarized the progress, challenges and outlook of SAMs used in inverted PSCs, being expected to present a comprehensive knowledge regarding the SAMs-based HTL. Herein, the review is composed of (1) the structure of inverted PSCs, (2) the composition of SAMs and the function of their head groups, and (3) the advancement of SAMs adopted as functional HTL and their modification effect on HTL. Furthermore, the review discusses both the advantages and challenges related to the application of SAMs in inverted PSCs. It also offers insights into the development trends of SAM-based HTLs, suggesting directions for enhancing the performance of inverted PSCs. By covering these elements, this review intends to offer a holistic understanding of SAMs in the context of inverted PSCs, facilitating further research and innovation in this field. Furthermore, it highlights emerging trends in SAM applications, particularly in tandem solar cells, where their low parasitic absorption and conformal substrate coverage offer significant efficiency gains. By consolidating mechanistic insights, performance benchmarks, and future research directions, this review serves as a valuable reference for advancing SAM-based PSCs toward commercialization. Its unique emphasis on buried interface engineering and tandem device integration distinguishes it from existing literature, providing a roadmap for further innovation in high-efficiency, stable perovskite photovoltaics.

2. The Structure of Inverted Perovskite Solar Cells and SAM Molecules

2.1. The Structure of Inverted Perovskite Solar Cells

The structure of the p–i–n PSC is as shown in Figure 1. The functions of each structure are as follows:

• Anode and cathode: PSC employs a dual-electrode system for efficient charge extraction and current generation. The front electrode typically consists of TCOs, such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) which are the most widely used. These TCO substrates maintain high optical transparency while providing effective electrical conductivity for charge collection. Opposing the TCO electrode, metallic contacts (commonly gold or silver) serve as the back electrode in the device architecture. This electrode configuration establishes a complete pathway for charge carriers
• HTL: In the p–i–n PSC, the HTL is initially fabricated onto the TCO substrate, serving as a fundamental component that significantly impacts overall device efficiency. This functional layer performs multiple essential roles, including facilitating hole collection, preventing electron recombination, and modulating the crystallization process and surface morphology of the perovskite active layer. To achieve optimal device performance, an effective HTL must meet several key criteria: the highest occupied molecular orbital (HOMO) level must align properly with the valence band maximum of the perovskite material to ensure selective hole extraction and effective electron blocking; excellent charge carrier mobility is required to ensure efficient hole transport and collection; appropriate surface properties, including controlled wettability, are necessary to facilitate uniform deposition of perovskite precursors and promote high-quality crystal growth and the material should exhibit straightforward processability and maintain stable interfacial characteristics during device operation and aging.
• Perovskite absorber layer: The perovskite layer serves as the fundamental component for photovoltaic conversion, performing three essential functions: photon capture, exciton generation, and charge separation. To achieve optimal device performance, the absorber layer must satisfy multiple critical criteria: dense, pinhole-free microstructure, homogeneous surface coverage. In p–i–n structured PSCs, the absorber layer’s morphological quality assumes particular significance. The surface topography directly influences the subsequent deposition of ultrathin ETLs, where conformal coverage and interfacial contact quality are paramount for efficient charge extraction. Superior film uniformity minimizes shunt pathways while maximizing interfacial contact area between functional layers.
• ETL: The ETL serves dual critical functions in perovskite photovoltaic devices: facilitating selective electron extraction and providing inherent encapsulation that mitigates environmental degradation of the perovskite absorber. In p–i–n devices, commonly employed ETL materials include: fullerene derivatives, n-type organic semiconductors, metal oxide compounds. To achieve optimal device performance, an efficient ETL must satisfy several essential criteria: proper band offset for electron extraction, sufficient energy barrier for hole blocking, high electron mobility, high transparency (particularly critical for tandem and bifacial configurations). These multifunctional requirements highlight the importance of careful ETL material selection and engineering in high-performance perovskite optoelectronic devices.

2.2. The Structure of SAM Molecules

Usually, SAM molecules are composed of three parts (Figure 2): (1) the anchoring group which has a strong affinity for the substrate and can lead to chemical adsorption of the surface. The anchoring group is typically a Brønsted–Lowry acid such as carboxylic acid (COOH) boric acid (B (OH)₂), sulfonic acid (SO₃H) and phosphonic acid (PO (OH)₂). Furthermore, the anchoring process significantly influences the molecular orientation with respect to the normal surface, the substrate’s contact resistance and W_F interfacial dipole, and energy offset between the Fermi level and energy of the frontier molecular orbital. (2) The spacer group serves as a structural bridge between the anchoring group and terminal group, playing a pivotal role in determining molecular organization through various non-covalent forces. Two common types of linkers are alkyl chains (nonconjugated) and aromatic groups (conjugated), with the latter usually showing higher charge transport rates than the former. Meanwhile, the composition of linker groups can exert influence over intermolecular interactions, including the self-assembly process and ultimate packing structure. Additionally, the variation of linker groups helps to regulate the molecular structure as well as the function of the entire SAM molecule. (3) The terminal group constitutes the primary contact interface with the perovskite photoactive layer and critically determines multiple interfacial properties. This group regulates surface wettability characteristics, modulates W_F properties, and governs charge extraction/transport dynamics across the interface. Common structural designs incorporate nitrogen-rich aromatic systems including carbazole, phenothiazine, and triphenylamine derivatives, which significantly enhance the charge-selective behavior of SAM-based interfaces. Through sophisticated organic synthesis strategies, researchers can systematically introduce diverse functional groups and heteroatoms (O, N, S, I, Br, Cl) into these terminal moieties. Such chemical modifications profoundly influence both the self-assembly process and the resulting monolayer morphology. Furthermore, these engineered terminal groups interact synergistically with the perovskite layer to precisely tune the surface W_F and optimize energy level alignment at the interface. SAM-based HTLs offer significant advantages for PSC applications. The exceptional hole transport capability of these molecular interlayers effectively mitigates charge carrier accumulation at critical perovskite/HTL junctions. The molecular engineering flexibility of SAMs permits precise W_F adjustment, enabling optimal energy level alignment with the perovskite absorber layer and ensuring compatibility with various electrode materials through uniform surface coverage. Beyond functioning as standalone HTLs, SAMs serve as effective interfacial modifiers when incorporated with conventional transport materials. This application prevents undesirable direct contact between the perovskite and underlying HTL, passivates interfacial defects, and reduces surface hydroxyl group concentration—all of which contribute to enhanced device stability and improved perovskite film quality. The tunable molecular structure of SAMs facilitates the creation of energetically favorable interfaces that minimize charge transfer losses and guide perovskite crystal growth toward superior morphological characteristics. This review systematically examines two primary implementation strategies for SAMs in perovskite photovoltaics: (1) as primary functional hole-transport layers and (2) as interfacial modification layers for conventional HTL materials, highlighting their respective roles in advancing device performance and reliability.

3. SAMs Adopted as Functional HTL

In 2018, Magomedov et al. [41] pioneered the application of SAMs as HTL in inverted PSCs. The research team designed an innovative molecular structure, (2-{3,6-bis[bis(4-methoxyphenyl)amino]-9H-carbazol-9-yl}[3ethyl)phosphonic acid (V1306), which incorporates a phosphonic acid anchoring moiety along with a carbazole-derived charge-transporting unit. Through a solution-based immersion method, these V1306 molecules formed an ordered monolayer on ITO electrodes, achieving a PCE of 17.8% in the fabricated photovoltaic devices. In the same year, the 4′-[bis (2′,4′-dimethoxybiphenyl-4-yl) amino]biphenyl-4-carboxylic acid (MC-43) SAM and 4,4″-bis (diphenyl amino) -1,1′:3′,1″-terphenyl-5′-carboxylic acid (TPA) SAM was also reported as the HTL for inverted PSCs, delivering a PCE of 17.3% and 15.9% [42]. After that, more and more SAM materials were designed based on these molecules. Although carbazole-based devices have achieved good efficiency, there is still a gap between the theoretical efficiency. How to optimize the carbazole-based HTL is a key issue to improve the performance and stability of the device.

3.1. The Design of SAMs for Advanced Devices

As illustrated in Figure 2, SAMs typically comprise three key components: (1) a terminal functional group responsible for charge transport and defect passivation, (2) a spacer group that connects the functional and anchoring moieties, and (3) an anchoring group that binds to metal oxide substrates. Through rational molecular design, researchers can systematically modify these structural components to precisely tune the physicochemical characteristics of SAMs, including energy level alignment, electrostatic potential distribution, and dipole moment orientation. One critical research direction involves establishing fundamental structure–property relationships between molecular architecture and photovoltaic performance to guide the development of optimized SAM materials.

3.1.1. Terminal Groups

The functional terminal group in SAMs interact directly with the adjacent perovskite layer, critically influencing interfacial characteristics. These interactions significantly determine both the crystallinity of the perovskite film during deposition and subsequent charge carrier dynamics at the heterojunction. Consequently, strategic modification of SAMs’ headgroup structures offers a promising approach to enhance both photovoltaic performance and operational durability in inverted architecture solar cells. Carbazole-based molecules, due to their exceptional hole transport capabilities, rigid main chain structures, and ease of functionalization, have emerged as an ideal choice for SAMs hole-transport layer materials [26]. These properties enable carbazole-based molecules to effectively modulate energy levels, optimize interface properties, and thereby enhance charge transport dynamics. Bin et al. [43] successfully developed a series of carbazole-based SAM materials (2-(9H-carbazol-9-yl) ethyl) phosphonic acid (2PACz) [2-(3,6-Dimethoxy-9H-carbazol-9-yl) ethyl]phosphonic Acid (MeO-2PACz) as depicted in Figure 3a, achieving high PCEs exceeding 21.1% in PSCs. Amran Al-Ashouri et al. [44] developed a novel SAM named Me-4PACz ([4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid), where methyl substituents were strategically incorporated into the molecular structure. This modified SAM demonstrated enhanced charge collection efficiency, as evidenced by accelerated hole transfer kinetics and reduced ideality factor in photovoltaic measurements. Systematic investigations were conducted by adjusting the carbon chain length connecting the carbazole donor moiety to the phosphonic acid anchoring group, revealing significant structure–property relationships. Haijun Bin et al. [43] found that 3PACz outperforms 2PACz, 4PACz, and PEDOT:PSS when used as a single-layer HTL in organic solar cells. Notably, all PACz single-layer HTLs exhibit higher optical transmittance and lower resistance compared to traditional materials like PEDOT:PSS, favoring the improvement of short-circuit current density (JSC) and fill factor (FF). Furthermore, the introduction of substituent groups into the carbazole moiety allows for modulation of the dipole moment, thereby improving surface wettability and facilitating the deposition of dense perovskite films [35].

The phenothiazine-based group is also a commonly used head group with high hole mobility and chemical stability. It is also well soluble in ordinary organic solvents and has good film-forming performance. Roberto Grisorio et al. [45] confirmed for the first time in 2017 that the phenothiazine-based structure serves as a hole transport material in solution-processed lead trihalide perovskite-based solar cells, revealing the potential multifunctionality of phenothiazine building blocks. At the same time, phenothiazine is a much cheaper building block than carbazole and triphenylamine. Cost-effective SAM materials demonstrate superior suitability for commercial-scale PSC applications. Recent studies indicate that sulfur incorporation effectively suppresses interfacial defects, minimizing charge carrier losses, leading to enhanced device stability and photovoltaic performance [46,47]. To systematically investigate the influence of chalcogen elements on perovskite interfaces, in 2022, Asmat Ullah et al. [48] developed a series of tricyclic aromatic compounds incorporating oxygen, sulfur, or selenium heteroatoms (Figure 3b). Photovoltaic characterization revealed peak PCEs of 22.73%, 21.63%, and 21.02% for devices incorporating Br-2EPSe, Br-2EPT, and Br-2EPO SAMs, respectively. Density functional theory simulations performed on the PbI₂-terminated (001) surface of MAPbI₃ demonstrated preferential binding between the Br-2EPX molecular series and undercoordinated lead atoms (Figure 3c). Notably, the enhanced interfacial binding energy correlated with decreased trap state concentrations and extended charge carrier diffusion lengths. Among the investigated chalcogen variants, selenium-functionalized SAMs exhibited the strongest interaction with the perovskite absorber, yielding the most significant reduction in interfacial defect density and consequent improvement in charge carrier lifetime.

The triphenylamine (TPA) moiety, characterized by its central nitrogen atom connected to three phenyl groups, adopted a distinctive propeller-shaped configuration. This three-dimensional molecular architecture effectively prevents close packing between adjacent molecules, facilitating the formation of well-ordered and homogeneous SAM-HTLs. Due to its exceptional electron-donating capability, TPA-based molecular units are frequently incorporated as donor components in the construction of donor-acceptor-type hole transport materials. Liao et al. [49] incorporated the D–A type backbone for the first time to build a series of novel SAMs as single-layer HSCs for PSCs and synthesized three D–A SAM molecules (MPA–BT–CA, MPABT–BA, MPA–BT–RA) with different anchor groups (benzoic acid (BA) 2-cyanoacrylic acid (CA) and rhodanine-3-propionic acid (RA)) (Figure 3d). Experimental results demonstrated that the anchoring groups significantly influenced both the photoelectric characteristics and molecular packing behavior of the SAMs. Specifically, the CA- and RA-terminated derivatives exhibited substantially enhanced molecular dipole moments relative to their BA-functionalized counterpart. Structural characterization revealed distinct assembly orientations—while MPA–BT–CA and MPA–BT–BA formed vertically aligned monolayers, MPA–BT–RA displayed a pronounced tilt angle (Figure 3e). This tilted configuration originated from the conformational flexibility of the methylene-linked sp³-hybridized carbons in the RA anchoring unit. Notably, ITO electrodes modified with MPA–BT–CA SAMs showed a marked increase in W_F, which contributed to achieving an outstanding PCE of 21.81% while maintaining excellent operational stability of the photovoltaic devices.

Rui Guo et al. [50] designed two d–a type self-assembled molecules with phosphonic acid-anchored groups as HTMs for p–i–n PSC in 2023 (Figure 3f). These designed molecules, PPA and PPAOMe, establish interfacial coordination with ITO substrates through their sulfur and oxygen heteroatoms. This specific interaction promotes a favorable face-on molecular arrangement, which effectively reduces the energy barrier for hole injection at the anode/hole-transport layer interface. In addition, PPA can enhance the stability of the interface by passivating the perovskite interface through its interaction with Pb²⁺. The final device achieves a stable efficiency of >23%. Furthermore, under thermal conditions (85 °C), PPA-based devices maintained 87% of the initial PCE after 1000 h of aging. In addition, the PPA-based PSC exhibited excellent operational stability, maintaining an efficiency of 91% after 1000 h of maximum power point (MPP) tracking under 1 solar illumination.

In addition to commonly studied carbazole, phenothiazine, and triphenylamine derivatives, researchers have explored various polycyclic aromatic systems for SAM applications. These extended π-conjugated frameworks demonstrate stronger intermolecular π–π stacking interactions, as exemplified by Py3 (Figure 3g). The rigid, planar structure of this peri-fused aromatic system forms stable molecular contacts, enabling devices to achieve remarkable PCEs up to 26.1% while exhibiting improved long-term stability [51]. Tan et al. [52] developed a dimethylacridine-phosphonic acid derivative (DMAcPA) that creates an energetically favorable p-type interface between perovskite and ITO electrodes (Figure 3h). This strategically designed SAM not only promotes efficient hole collection but also effectively suppresses electron recombination at the buried interface.

Figure 3. (a) Molecular structures of carbazole-based SAM-HTLs [43], (b) diverse hole selective molecules incorporating heteroatoms as SAMs for efficient p–i–n PSCs [48], (c) optimized structures of Br-2EX series molecules attached to the PbI₂- terminated perovskite surface [48], (d) structures of MPA–BT–CA, MPA–BT–BA, and MPA–BT–RA [49], (e) schematic illustration of potential molecular organization behavior in SAMs [49], (f) molecular structures of PPA and PPAOMe [50], (g) c-AFM images of Py3 [51], (h). energy-level diagrams for ITO/perovskite, ITO/DMAcPA/perovskite and ITO/perovskite (DMAcPA) WF, EF, Fermi level; CB, conduction band; VB, valence band [53].

Figure 3. (a) Molecular structures of carbazole-based SAM-HTLs [43], (b) diverse hole selective molecules incorporating heteroatoms as SAMs for efficient p–i–n PSCs [48], (c) optimized structures of Br-2EX series molecules attached to the PbI₂- terminated perovskite surface [48], (d) structures of MPA–BT–CA, MPA–BT–BA, and MPA–BT–RA [49], (e) schematic illustration of potential molecular organization behavior in SAMs [49], (f) molecular structures of PPA and PPAOMe [50], (g) c-AFM images of Py3 [51], (h). energy-level diagrams for ITO/perovskite, ITO/DMAcPA/perovskite and ITO/perovskite (DMAcPA) WF, EF, Fermi level; CB, conduction band; VB, valence band [53].

In inverted PSCs, the interfacial passivation between the perovskite layer and the hole-transport layer is crucial as it directly impacts the efficiency and stability of the device. The primary objective of interfacial passivation is reducing defects and non-radiative recombination, enhancing carrier transport efficiency and consequently boosting the efficiency and stability of the solar cell. Various strategies have been employed by researchers in interfacial passivation, including the introduction of passivating head groups in carbazole-based derivatives, the utilization of Lewis acid/base passivation strategies, and the design of asymmetric SAMs.

Jie Zeng et al. [54] discovered that enhancing wettability through introducing the methoxy (-OMe) group can optimize the nucleation and growth of perovskite films, leading to high-quality buried interfaces. In 2022, Ece Aktas et al. [55] synthesized three triphenylamine molecules (RC24, RC25, RC34) (Figure 4a) with different positions of methoxy substituents. They found the arrangement of methoxy substituents mainly affects the W_F, photophysical and surface properties of the molecule. The arrangement of methoxy substituents in TPA-based SAM affects the hydrophobicity and wettability of SAM, which affects the stability of SAM-based perovskite films (Figure 4b) and when the RC24 molecule is adsorbed to the ITO surface, it is more suitable to form a more compact, ordered, and stable monolayer relative to the RC34 molecule.

Recent research has demonstrated that halogenated SAMs can significantly enhance the performance of PSCs by forming halogen bonds with perovskite materials, leading to improved interfacial characteristics and enhanced device stability [56,57]. Asmat Ullah et al. [58] in 2021 designed a bromine-substituted SAM material, Br-2EPT (Figure 4c), incorporating an electron-withdrawing bromine substituent that exhibits optimal energy level alignment and effective defect passivation. The phenothiazine-derived structure, functionalized with bromide groups, ensures favorable band alignment with the perovskite absorber while efficiently suppressing electron recombination [26]. This molecular design facilitates rapid hole extraction and minimizes interfacial losses, contributing to an average PCE exceeding 22%. In addition, in 2024, Mingliang Li et al. [59] selected chlorinated phenothiazine as the head group and synthesized a versatile SAM molecule named TDPA-Cl (Figure 4c). Compared to Br-2EPT, TDPA-Cl SAM has the same PCE of 22.4% but higher JSC and FF.

Amran Al-Ashouri et al. [44] reported that methyl substituents possess advantages in both passivation and hole extraction. The single-junction device achieved FF of 84% and PCE of 20.8%. This is attributed to the ability of Me-4PACz to form a nearly undamaged interface between ITO and perovskite, effectively passivating and reducing non-radiative recombination, thereby enhancing the FF and overall performance of the solar cell device. To solve MeO-based SAM materials exhibiting a mismatch in HOMO levels with perovskite layer due to the strong electron-donating capability of methoxy group, Zhao-Chen et al. [60] introduced a methylthio (MeS) substituent that is superior to methoxy. Thus, this new substituent facilitates stronger interactions with the perovskite buried interface, enhances surface wettability, and provides better alignment of the HOMO energy level (Figure 4f). Furthermore, the configuration of MeS-CbzPh/perovskite achieves superior charge transport at the interface. Photovoltaic devices based on MeS-CbzPh HTL reached a champion efficiency of 26.01% under 1 sun, along with outstanding stability of 93.3% during 1000 h of MPPT.

The Lewis acid-base strategy has proven particularly effective in interfacial passivation of PSCs, and its integration with carbazole-based molecules is expected to further enhance the efficiency of inverted PSCs. In a 2024 study, Wenlin Jiang and colleagues [61] developed two novel asymmetric SAM molecules, CbzBF and CbzBT, as depicted in Figure 4d. These molecules effectively passivate buried interfaces through asymmetric conjugation extension and the introduction of Lewis basic oxygen and sulfur atoms in carbazole-based derivatives. The asymmetric structure of these molecules enhances intermolecular interactions, allowing for ordered and dense assembly of the SAMs at the interface, which reduces current leakage and interfacial recombination. The introduction of Lewis basic oxygen and sulfur atoms rich in lone pairs of electrons passivates uncoordinated Pb²⁺, reducing trap density and suppressing non-radiative recombination. These improvements led to PCE of 24.0% and FF of 84.41% for the PSC based on CbzBT, demonstrating the tremendous potential of precise molecular design in optimizing interfacial properties.

Certainly, the mere application of the Lewis acid-base strategy does not guarantee optimal passivation and device efficiency. For instance, in the same year of 2024, researchers including Wenlin Jiang [62] designed two SAM molecules, Cbz2S and Cbz2SMe, as depicted in Figure 4g. By modifying the carbazole structure, they achieved optimized Lewis basicity on the SAM-modified surface. Although both molecules exhibited similar energetic alignments, Cbz2S exhibited excessive reactivity due to overexposure of the sulfur atom, resulting in excessive accumulation of PbI₂ crystals at the buried perovskite interface. In contrast, Cbz2SMe achieved an appropriate exposure of the sulfur atom through the shielding effect of the methyl group, delicately balancing perovskite crystallization and Pb²⁺ passivation. This balanced approach effectively reduced trap-state density, maintaining good reactivity while avoiding excessive reactions, thereby harmoniously integrating interface passivation and hole-selective functionality. This subtle balance enabled Cbz2SMe to exhibit exceptional performance in PSCs, achieving a photoconversion efficiency of up to 24.42% and enhanced device stability. This work once again validates the feasibility of incorporating Lewis base passivating groups into SAM molecules and provides a new direction for future performance optimization of PSCs.

To enhance adsorption capabilities and optimize energy alignment, Wanhai Wang et al. [63] developed a dynamic self-assembly (DSA) technique, which culminated in the design of a carbazole-based derivative SAM-BCB-C4PA, as depicted in Figure 4h, the hole-transport layer. The increased molecular dipole moment resulting from the conjugated structure of BCB-C4PA enables exceptional modification of the W_F of the ITO substrate, achieving superior energy alignment with the perovskite valence band maximum (VBM).

Figure 4. (a) Structural characteristics of RC24, RC25, and RC34 [55], (b) measuring of contact angle on the RC24, RC25, and RC34 surfaces [55], (c) Molecular structures of Br-2PACz and TDPA-Cl [58], (d) molecular structures, calculation of dipole moments and HOMO energy levels of CbzPh, CbzBF, and CbzBT [61], (e) chemical structure and ESP of CbzPh, MeO CbzPh, and MeS CbzPh [62], (f) diagram of the boundary location of SAM HTLs under investigation, which is referred to as a vacuum level. EF and EVAC stand for the Fermi and the vacuum. EVBM stands for the Valency Peak Energy [60], (g) synthetic route of SAMs [62], (h) chemistry and molecule modeling for BCB-C4PA [63].

Figure 4. (a) Structural characteristics of RC24, RC25, and RC34 [55], (b) measuring of contact angle on the RC24, RC25, and RC34 surfaces [55], (c) Molecular structures of Br-2PACz and TDPA-Cl [58], (d) molecular structures, calculation of dipole moments and HOMO energy levels of CbzPh, CbzBF, and CbzBT [61], (e) chemical structure and ESP of CbzPh, MeO CbzPh, and MeS CbzPh [62], (f) diagram of the boundary location of SAM HTLs under investigation, which is referred to as a vacuum level. EF and EVAC stand for the Fermi and the vacuum. EVBM stands for the Valency Peak Energy [60], (g) synthetic route of SAMs [62], (h) chemistry and molecule modeling for BCB-C4PA [63].

WF modification of substrates is governed by two key factors: the inherent dipole moment orientation of SAM molecules and their packing density. Carbazole derivatives present a particular challenge in this regard; their planar, symmetric molecular architecture results in limited intrinsic dipole moments. While electron-donating substituents can enhance molecular dipoles, they frequently cause undesirable steric effects and alter the HOMO energy positions. These modifications often impair both the formation of well-ordered, densely packed SAMs and the efficiency of hole extraction. Interestingly, even minor structural alterations to carbazole cores can significantly influence their molecular packing behavior, stacking configuration, and electronic properties, ultimately impacting device performance. This highlights the need for innovative synthetic approaches that can effectively increase dipole moments in carbazole-based SAMs while maintaining optimal molecular packing density in the monolayer. To solve this problem, Jiang et al. [64] developed two innovative carbazole-based SAMs, CbzPh and CbzNaph, (Figure 5a). The researchers implemented asymmetric π extension for CbzPh and helical aromatic expansion for CbzNaph, both methodologies effectively enhancing molecular dipole moments while strengthening intermolecular π–π interactions. Comparative analysis revealed that CbzNaph exhibited superior characteristics, including: the most pronounced molecular dipole moment among the series, optimal HOMO energy alignment with the perovskite light-absorbing layer and tight intermolecular packing through efficient π-orbital overlap. Thus, the CbzNaph-based device demonstrated exceptional performance, reaching a record PCE of 24.1% while simultaneously showing enhanced operational stability compared to reference systems. By further utilizing the benefits of conjugated linkers in SAM design, Qu et al. [65] developed a functionalized SAM denoted 4-(7H-dibenzo[c,g]carbazol-7-yl) phenyl) phosphonic acid (Bz-PhpPACz) derived from (4-(9H-carbazol-9-yl) phenyl) phosphonic acid (PhpPACz) (Figure 5b) Theoretical and experimental investigations suggest that during inverted PSC fabrication, this molecular design enables the formation of an organized hydrophilic bilayer configuration under optimized processing conditions. This unique structure consists of a covalently bound SAM and a well-ordered secondary layer maintained through non-covalent interactions. Notably, the extended π-conjugation in PhpPACz strengthens intermolecular π–π stacking forces, promoting the assembly of this bilayer architecture with enhanced surface hydrophilicity (Figure 5c). This structural arrangement not only enables deposition of high-quality perovskite films over large areas but also improves charge collection efficiency at the interface. Device characterization demonstrated remarkable performance metrics, with certified PCEs reaching 26.39% (0.0715 cm²) and 25.21% (99.12 mm²) for small- and large-area devices, respectively. Furthermore, these devices exhibited superior operational stability, highlighting the practical viability of this molecular design approach.

In the same year, Jiang et al. [66] reported a highly crystalline self-assembled multilayer (SAMUL), (4-(7Hdibenzo[c,g]carbazol-7-yl) phenyl) phosphonic acid (CbzNaphPPA) (Figure 5d) used for hole-extraction in inverted PSCs. The intermolecular interaction of CbzNaphPPA was significantly improved to form H-aggregates as identified from its single crystals, significantly enhancing its hole mobility (Figure 5e) and the champion PSC employing CbzNaphPPA-SAMUL showed a high PCE of 26.07% with an extraordinary FF of 86.45% and excellent operational stability (retaining 94% of its initial PCE after 1200 h of continuous 1 sun illumination under MPP operation at 65 °C) demonstrating the advantage of employing CbzNaphPPA-SAMUL over the conventional 4PACz and CbzPPA with less ordered packing and lower rigidity. Furthermore, the champion PSC device based on a thermally evaporated CbzNaphPPA-SAMUL showed a record-high PCE of 23.50%, demonstrating its great potential for diverse fabrication of devices.

After that, to address thermal stability limitations associated with conventional SAMs, particularly concerning molecular desorption and insufficient interfacial adhesion, Dong et al. [67] connected TPA units with a phosphonic acid SAM via a methylene bridge and get self-assembled bilayers (SABs) as a new type of HTM (Figure 5f). Through the incorporation of rigid structural components that reinforce the monolayer framework and formation of adhesive interfacial contacts between TPA units and perovskite surfaces, this effectively suppresses thermo-mechanical degradation. The researchers demonstrated exceptional stability in inverted PSCs, attaining a certified PCE exceeding 26%. Remarkably, the encapsulated high-performance devices exhibited minimal degradation, retaining over 96% of their initial efficiency after 2000 h under harsh damp heat conditions (85 °C and 85% relative humidity) and maintaining 97% of their PCE following 1200 thermal cycles between −40 °C and 85 °C (Figure 5g). This outstanding stability can be attributed to the implementation of multilayer molecular contact interfaces, where covalent bonding between adjacent layers significantly improves the device’s resilience against thermal stress.

Du et al. [7] engineered the molecular 4PABCz by strategically extending the π-conjugation system of the 4PACz carbazole core. This structural modification promotes an optimal “face-on to face-on” molecular packing arrangement, where adjacent conjugated planes align in parallel orientation. Such well-ordered assembly enhances charge collection efficiency at the buried interface while simultaneously improving perovskite crystallization kinetics and mitigating interfacial strain. The optimized photovoltaic devices incorporating 4PABCz as a hole-selective layer demonstrated outstanding performance, attaining a certified PCE of 26.81% under reverse scanning conditions in inverted PSCs.

The head group led by the carbazole group has achieved very encouraging results, while other electron-rich polycyclic aromatic compounds such as DMAcPA and Py3 have shown a new trend due to their unique chemical properties. The modification of the head group from the initial method of halogen atom substitution to the current Π conjugated substitution shows that when we design the structure of molecules, we are no longer limited to parts, but pay more attention to the influence on the whole molecule and even the device.

Figure 5. (a) Molecular structures, calculated HOMO orbital distributions, dipole moments and HOMO energy levels of 4PACZ, CbzPh, and CbzNaph [64], (b) molecular design strategy and structure of Bz-PhpPACz [65], (c) proposed bilayer structure of BzPhpPACz inspired by the phospholipid bilayer and its enhanced hydrophilicity, evidenced by water drop contact angles of 92° for the single layer and 44° for the bilayer [65], (d) chemical structures of 4PACz, CbzPPA, and CbzNaphPPA and the proposed working mechanism of SAMUL in PSC [66], (e) molecular packing patterns in the crystal lattice of CbzNaphPPA, with dihedral angles measured through specific planes [66], (f) proposed mechanism for the coupling reaction between TATPA and 2PACz [67], (g) evolution of PCE of PSCs based on SAM and SAB interfaces under one-sun MPPT [67].

Figure 5. (a) Molecular structures, calculated HOMO orbital distributions, dipole moments and HOMO energy levels of 4PACZ, CbzPh, and CbzNaph [64], (b) molecular design strategy and structure of Bz-PhpPACz [65], (c) proposed bilayer structure of BzPhpPACz inspired by the phospholipid bilayer and its enhanced hydrophilicity, evidenced by water drop contact angles of 92° for the single layer and 44° for the bilayer [65], (d) chemical structures of 4PACz, CbzPPA, and CbzNaphPPA and the proposed working mechanism of SAMUL in PSC [66], (e) molecular packing patterns in the crystal lattice of CbzNaphPPA, with dihedral angles measured through specific planes [66], (f) proposed mechanism for the coupling reaction between TATPA and 2PACz [67], (g) evolution of PCE of PSCs based on SAM and SAB interfaces under one-sun MPPT [67].

3.1.2. Spacer Groups

The spacer group in SAM molecules serves as a crucial structural bridge between the terminal functional group and surface-anchoring unit, playing a pivotal role in defining both the electronic configuration and molecular packing behavior. Igal Levine and his team [68] found a notable decrease in hole transfer rates with increasing aliphatic chain length. This trend is attributed to the corresponding increase in the width of the tunneling barrier. Although effective in hole extraction and interface passivation, its hole transport capacity is limited by insulating alkyl linkers, and the unconjugated molecular structure is not conducive to the stability and charge transport ability of the electron-rich arylamine moiety due to the localization of electrons and charges, and its stability is poor. Shuo Zhang et al. [69] proposed in 2022 that the addition of conjugated linkers and intramolecular D–A groups to stabilize electron-rich arylamines through efficient electron/charge delocalization and energy level modulation was proposed, and they chose cyanoacetic acid (CA) as the anchor group to fabricate a series of conjugated SAMs materials, as shown in Figure 5a. The extended π-conjugation in the molecular architecture serves dual functions: facilitating efficient charge carrier mobility while stabilizing electron-donating arylamine moieties through effective charge delocalization. This structural feature additionally permits precise tuning of frontier molecular orbital energies, enabling optimal interfacial band alignment with adjacent layers. Through this structure–property optimization, the researchers identified MPA-Ph-CA as the most effective SAM, delivering an outstanding PCE of 22.53%.

In 2024, Qu et al. [70] developed an enhanced molecular design by modifying commercially available Me-4PACz through substitution of its aliphatic linker with a π-conjugated phenylene unit, yielding the novel Me-PhpPACz derivative for application in inverted PSCs. The photovoltaic devices incorporating this optimized SAM achieved unprecedented performance metrics, including a champion PCE of 26.17% (Figure 5b), an impressive FF reaching 86.79%, and remarkable operational stability. Compared with the conventional Me-4PACz, the aromatic phenylene linker in Me-PhpPACz confers superior photochemical stability and enhanced structural rigidity, and minimizes molecular deformation during monolayer formation, thus, improving molecular geometry promotes denser and more ordered SAM packing. Haoliang Cheng et al. [71] engineered two donor–π–acceptor (D–π–A) structured SAMs (FNE29 and DT-1) for ITO-based hole-transport layers in inverted architectures (Figure 5c). Systematic characterization demonstrated that DT-1, featuring an aromatic bridging unit, outperformed its non-fused counterpart (FNE29) in both charge carrier mobility and hole extraction efficiency. These findings strongly correlate the hole transport capabilities and photovoltaic performance of organic SAMs with the degree of π-conjugation in their molecular linkers. Consequently, fundamental studies investigating the structure–property relationships of these connecting moieties hold substantial importance for advancing organic optoelectronic materials.

Li et al. [72] found that the energy level alignment is determined by the orientation of 2PACz molecules on a different substrate. When the substrate has a stronger interaction with the carbazole group, the molecule tends to lie down, and even upside down, depending on the interaction strength. When the substrate has a stronger interaction with the phosphonic group, such as solution-cleaned ITO or oxygen plasma cleaned ITO, the molecule is assembled in an upright orientation with favorable hole extraction interfacial dipole. The oxygen plasma cleaned ITO showed additional upward band bending in the 2PACz layer, which could explain the usefulness of oxygen plasma treatment for better device performance. On the MAPbI₃ surface, 2PACz molecules show interaction with free MAþ ions, implying a protection effect in suppression of MAPbI₃ decomposition in vacuum (Figure 5d). Their study provided valuable knowledge on molecular orientation, band alignment, and interactions of the SAM molecule in the perovskite device.

In summary, spacer groups with aliphatic structures exhibit high molecular flexibility, often leading to structural deformation and torsional disorder in SAMs, thereby hindering the formation of well-organized molecular architectures. In contrast, π-conjugated aromatic spacers promote more ordered molecular packing and enable the fabrication of densely packed monolayer films. Furthermore, aromatic-containing SAMs demonstrate superior potential for practical applications due to their enhanced stability under various operational stresses, a characteristic attributed to the effective delocalization of π-electrons throughout the conjugated system.

3.1.3. Anchoring Groups

Anchoring groups serve as the critical interface between SAMs and metal oxide substrates, establishing robust chemical linkages through covalent or coordination bonds. Strategic modification of these anchoring units enables precise control over several key parameters: (1) the molecular dipole orientation of SAMs, (2) the interfacial adhesion strength between the substrate and HTL, and (3) the formation of continuous, pinhole-free monolayers. Researchers have explored numerous acidic functional groups, including phosphonic, carboxylic, and sulfonic acids, as effective anchoring units in SAM design for optoelectronic applications. Carboxyl-functionalized moieties (–COOH) have emerged as versatile components in the design of organic charge-transporting materials. Aktas et al. [73] designed EADR03 and EADR04 (Figure 6e), where the former demonstrated superior photovoltaic performance compared to conventional PTAA-based devices. This enhancement originated from multiple factors: precise energy level alignment for selective hole extraction, effective electron-blocking capability, improved perovskite crystallinity, and efficient defect passivation at interfaces. Hung et al. [74] synthesized a series of porphyrin derivatives functionalized with carboxylic acid groups, including mono-anchored AC-1 and di-anchored AC-3/AC-5 variants. The bidentate anchoring configuration of AC-5 promoted the formation of densely packed monolayers on ITO substrates and enhanced charge carrier mobility. These synergistic benefits resulted in photovoltaic devices exhibiting a remarkable PCE of 23.19% and an exceptional FF reaching 84.05%.

Figure 6. (a) Structures of MPA–BT–CA, MPA–BT–BA, and MPA–BT–RA [69], (b) molecular structures of Me-PhpPACz [70], (c) chemical structures of FNE29 and DT-1 based SAMs [71], (d) molecular orientation and interface dipoles at the various substrates [72], (e) chemical structures of EADR03 and EADR04 [73], (f) molecular structures of TPT-S6, TPT-C6, and TPT-P6 [75], (g) molecular structures and dipole moment of IDCz-3 [76], (h) illustration of molecular design concept for 3PATAT-C3 with multipodal strategy [75], (i) molecular structure of the amphiphilic MPA–CPA molecule [77], (j) schematic depiction of the bilayer stack of MPA–CPA molecules on an ITO-glass substrate [77], (k) the chemical structure of boric acid-based SAMs [78].

Figure 6. (a) Structures of MPA–BT–CA, MPA–BT–BA, and MPA–BT–RA [69], (b) molecular structures of Me-PhpPACz [70], (c) chemical structures of FNE29 and DT-1 based SAMs [71], (d) molecular orientation and interface dipoles at the various substrates [72], (e) chemical structures of EADR03 and EADR04 [73], (f) molecular structures of TPT-S6, TPT-C6, and TPT-P6 [75], (g) molecular structures and dipole moment of IDCz-3 [76], (h) illustration of molecular design concept for 3PATAT-C3 with multipodal strategy [75], (i) molecular structure of the amphiphilic MPA–CPA molecule [77], (j) schematic depiction of the bilayer stack of MPA–CPA molecules on an ITO-glass substrate [77], (k) the chemical structure of boric acid-based SAMs [78].

Phosphonic acid groups (−PO₃H₂) demonstrate enhanced suitability as anchoring moieties owing to their exceptional chemical stability. This advantage stems from their ability to exist in dianionic form, enabling the formation of strong bidentate coordination bonds with metal oxide surfaces in ITO substrates. Erpeng Li et al. [75] designed and synthesized a series of phenothiazine based molecular HTMs featuring different anchoring groups, that is, -SO₃H, -COOH, and -PO₃H₂ for TPT-S6, TPT-C6, and TPT-P6, respectively, as shown in Figure 6h. Computational modeling results revealed distinct adsorption behaviors among different anchoring groups on ITO substrates. The–PO₃H₂ functionalized molecules preferentially formed tridentate coordination bonds with the metal oxide surface, whereas both –COOH and –SO₃H terminated molecules exhibited bidentate adsorption configurations. These simulation results demonstrate that phosphate-based anchoring groups establish the most robust interfacial chemical bonding with ITO surfaces compared to other acidic moieties. (Figure 6f). The anchor group with stronger bond strength not only improves the assembly rate and adsorption density, but also imparts organic HTLs with high perovskite deposition tolerance, which greatly enhances the compactness of the ASA monolayer in the complete device. TPT-P6 based on PO₃H₂ shows the fastest and robust assembly, resulting in an orderly, dense and stable perforated monolayer on the ITO. With TPT-6P SAM, they achieved a PCE of 21.43% (0.09 cm²). When the device area is increased by a factor of 10 (1.0 cm²), a PCE of 20.09% is obtained, retaining 90% of the initial PCE after three months.

Multipodal SAMs demonstrate superior interfacial stability and molecular alignment compared to conventional single-anchor phosphonic acid systems. The bisphosphonate-functionalized IDCz-3 molecule exhibits an exceptionally large dipole moment (Figure 6g), enabling strong interactions with perovskite materials and facilitating the growth of highly crystalline absorber layers [76]. Spectroscopic characterization of FTO substrates modified with IDCz-3 revealed favorable energy level alignment at the interface, and this optimized electronic structure promotes efficient hole collection from the active layer (Figure 6h). Moreover, Truong et al. [79] developed the tripodal anchoring molecule 3PATAT-C3, which maintains a face-on orientation of its π-conjugated framework, and significantly reduces interfacial charge recombination and improves hole extraction efficiency through multiple phosphonic acid attachment points that precisely control molecular orientation.

In 2023, Guo et al. [78] developed novel SAMs incorporating boric acid anchoring groups functionalized with various arylamine derivatives. Their study revealed that highly acidic phosphonic acid moieties (−PO₃H₂) induce detrimental etching of ITO substrates, leading to compromised device reliability. Conversely, the moderately acidic MTPA-BA SAM demonstrated multiple advantages, including effective prevention of ITO surface degradation, maintenance of efficient charge transport properties and promotion of high-quality perovskite film growth (Figure 6k). The best device achieved a PCE of 22.62% and FF of 85.2% with enhanced storage (ISOS-D-1) and operational (ISOS-L-1) stabilities.

Shuo Zhang et al. [77], considered the amphiphilic nature of the perovskite precursor solution and proposed an amphiphilic hole transport material, MPA-CPA, with a new cyanophosphonic acid anchored group (Figure 6i). The introduction of the strong electron-withdrawing cyano group increases the deprotonation ability of phosphonic acid and the hydrophilicity of the anchor group, making the small molecule material have unique amphiphilic characteristics and be easily soluble in a variety of solvents of different polarities. Using the amphiphilic small molecule hole transport material, a “bilayer” membrane structure was constructed on ITO through dynamic anchored self-assembly (Figure 6j). The chemically anchored self-assembled ordered monolayer can firmly adhere to the ITO surface to ensure efficient hole selection and transport, while the unanchored chaotic and disordered overlay has superwetting characteristics on the surface due to the amphiphilia and easy dissolution characteristics of the molecule, which is not only conducive to the uniform preparation of large-area films in the upper layer, but also can effectively reduce the concentration of interlayer interface defects. The trans-structure PSCs prepared based on the new organic hole transport material have been certified by a third-party organization with an efficiency of 25.39%. In addition, the good wettability of the new organic hole transport material is very conducive to the fabrication of large-area devices, and the efficiency of 1 cm² device and 10 cm² module is 23.4% and 22.0%, respectively.

Another representative example is PTZ-CPA, which combines a phenothiazine donor unit with a cyanovinylphosphonic acid anchoring group [80]. This molecular design creates an ultra-smooth, highly wettable interfacial layer that simultaneously promotes the growth of highly crystalline FAPbI₃ films. As a result, PSCs utilizing FAPbI₃ as the light-absorbing layer achieved an impressive PCE of 25.35%, along with significantly improved long-term operational stability under continuous illumination.

In summary, achieving optimal device performance requires careful balancing between two critical factors: maximizing interfacial adhesion strength while minimizing substrate degradation. Recent advances demonstrate that both multipodal anchoring configurations and amphiphilic molecular designs offer promising solutions, exhibiting exceptional surface wettability characteristics coupled with enhanced long-term stability under operational conditions.

3.2. Other Strategies for Advanced Devices

3.2.1. Additive Strategy

Improving the intrinsic film quality of metal halide perovskites is very essential for boosting both the efficiency and stability of perovskite devices. Li et al. [81] demonstrated that incorporating a multifunctional compound, 4-guanidinobenzoic acid hydrochloride (GBAC), into the precursor solution facilitates the creation of an intermediate phase stabilized by hydrogen bonding. This additive serves a dual purpose: it controls crystallization dynamics during film formation and functions as a stable passivating agent in the thermally processed layer (Figure 7a). Their findings indicate that the crystallization process is influenced by the energy barriers associated with hydrogen bond dissociation and ion substitution between the intermediate complex and the perovskite matrix. This modified growth mechanism results in thin films with superior crystalline quality and expanded grain dimensions (Figure 7b). Furthermore, the aromatic moieties within the additive promote dense packing at grain boundaries through π–π stacking interactions, effectively mitigating defect formation. The optimized device architecture yielded remarkable performance metrics, including a record PCE of 24.8% and an impressive FF reaching 84.78%. Wang et al. [82] found that the formation of uniform amorphous phases in SAMs is governed primarily by steric effects and intermolecular forces, as demonstrated by comparative studies of crystalline and amorphous SAM variants. Specifically, Me-4PACz (a crystalline SAM, c-SAM) and (4-(3,6-diphenyl-9H-carbazol-9-yl)butyl)phosphonic acid (Ph-4PACz, an amorphous SAM, a-SAM) exhibit distinct structural properties due to differences in molecular packing. The amorphous nature of Ph-4PACz arises from its bulky molecular structure, which introduces significant steric constraints and weakens intermolecular attraction, preventing crystallization and promoting a highly disordered yet uniform morphology when deposited on TCO substrates (Figure 1b,c). This amorphous configuration enhances the homogeneity of the hole-transport layer, reducing defect density and improving charge extraction. Consequently, p–i–n PSCs incorporating a-SAMs achieve a record PCE of 25.20% for a 1-cm² device. Moreover, devices based on amorphous SAMs exhibit exceptional operational stability compared to their crystalline counterparts. Under continuous illumination (ISOS-L-1 protocol), a-SAM-based PSCs retain nearly 100% of their initial efficiency after 600 h, while under thermal stress at 85 °C (ISOS-T-2 protocol), they maintain 90% of their original performance (Figure 7c,d).

In addition to the molecular design strategies mentioned above, the addition of additives is also a common modification strategy and can also be used to adjust energy levels and passivate perovskite layer defects, so that the device can achieve better performance and stability.

Tang et al. [83] employed a homopiperazine-1,4-bis (2-ethanesulfonic acid) (denoted as HEA) molecule as a buried interface modifying layer to improve the PCE and operational stability of PSCs (Figure 7e). HEA with two sulfonic acid groups can anchor the vacancy unanchored by SAMs and provide a more uniform growth base for perovskite, and it can also passivate the uncoordinated Pb²⁺ defects. Consequently, the HEA-modified SAM layer shows a more uniform distribution, and the subsequent perovskite film exhibited a higher crystal phase purity without the excessive PbI₂ secondary phase and δ phase across the entire perovskite film (Figure 7f). Consequently, the resultant device attained an improved performance, achieving a champion PCE of 25.71% along with 1000 h of long-term operating stability.

Wang et al. [84] introduced a small molecule, 9-Fluorenylmethoxycarbonyl chloride (9-YT) to act on the buried interface of the p–i–n perovskite device. The 9-YT molecule interacts with uncoordinated Pb²⁺ through internal carbonyl groups and Cl atoms, passivating the perovskite defects. Furthermore, the π-conjugated groups in the 9-YT molecule can interact with the MeO-2PACz molecules in the HTL, forming a molecule bridge between the HTL and the perovskite layer. This can effectively improve the direct contact characteristics between perovskite and HTL. This significantly improves the hole extraction at the perovskite/hole-transport layer interface, optimizes the alignment of interface energy levels, and reduces the recombination of interface charges. Ultimately, a PCE of 24.82% was achieved, which is higher than the control device’s PCE of 23.37%, and also improves the stability of PSCs.

The amphiphilic nature of SAMs often promotes micelle formation with spherical morphology when processed in conventional alcohol-based solvents. This aggregation behavior during solution deposition frequently results in non-uniform film morphology. Furthermore, when these SAMs are coated onto transparent conductive substrates, their hydrophobic moieties become outwardly oriented. This surface arrangement adversely impacts both the crystalline perfection of subsequently deposited perovskite layers and the interfacial contact characteristics. To achieve an excellent interface, it is crucial to ensure the formation of a tightly ordered SAM layer as the substrate. Although carbazole-based molecules have demonstrated tremendous potential in photovoltaic devices, they may form micelles during processing and introduce additional energy barriers. To overcome this challenge and achieve optimal SAM growth conditions, Ming Liu et al. [85] developed a cosolvent strategy to disassemble micelles in the processing solution. The team introduced N,N-dimethylformamide (DMF) as a cosolvent into the isopropanol (IPA) solution of the SAM precursor. Leveraging the strong solvent–solute interaction between DMF and the carbazole group, the SAM micelles were effectively disassembled, thereby reducing the additional energy required to disrupt the micelles during SAM growth. As a result, the SAM-HTL demonstrated enhanced molecular packing density and superior substrate coverage when deposited on ITO electrodes. This optimized morphology promoted more favorable energy level matching at the interface, thereby facilitating enhanced hole collection efficiency while simultaneously suppressing charge recombination losses. Devices fabricated using this co-solvent approach achieved outstanding photovoltaic performance, with the highest-performing cell reaching a PCE of 24.98% and an impressive FF of 85.06%. After that, Luo et al. [86] found that the simultaneous modification of both the SAM and perovskite layer was accomplished through a simple post-treatment process using formamidinium chloride (FACl) dissolved in DMF. The DMF solvent effectively disrupts micellar aggregates through solvation effects, enabling reorganization of SAM molecules into a more ordered and compact arrangement, and this structural optimization yields a hole-transport layer with enhanced surface smoothness and improved charge carrier mobility. Residual FACl at the buried interface serves as a crystallization template, promoting the formation of perovskite grains with strongly preferred (001) orientation normal to the substrate surface. This well-aligned crystalline structure significantly enhances charge transport properties. Furthermore, the FACl treatment passivates interfacial defects while simultaneously improving both the bulk perovskite film quality and the electrical contact between the SAM and perovskite layers, thereby substantially reducing non-radiative recombination losses.

In the fabrication of perovskite solar modules (PSMs) for large-scale applications, the monolithic series-interconnection approach necessitates three laser patterning steps (P1-P2-P3). A key challenge arises from the substantial exposed glass surface at the P1 scribe lines after laser ablation. When these regions are coated with hydrophobic SAMs (e.g., Me-4PACz), the resulting surface properties lead to non-uniform perovskite film formation during solution deposition. This coating irregularity propagates structural defects from the scribe edges to the adjacent transparent conductive oxide (ITO) regions, creating localized current leakage paths between interconnected sub-cells that degrade module efficiency (Figure 8a). To overcome this limitation, Anil Reddy Pininti et al. [87] developed an innovative approach by modifying the Me-4PACz solution with the Lewis base additive 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU). The DMPU modification serves dual functions: (1) it suppresses molecular aggregation of the SAM molecules, and (2) enhances interfacial compatibility with perovskite precursor inks. This strategy effectively resolves the perovskite deposition challenges at critical P1 scribe zones while maintaining excellent charge transport properties (Figure 8b).

The additive strategy has become a powerful method to improve the intrinsic film quality of metal halide perovskite, significantly enhancing the efficiency and stability of PSCs. The addition of multifunctional additives such as GBAC, HEA, and 9-YT has been proven to regulate crystallization kinetics, passivate defects, and improve interfacial charge transfer. However, challenges still exist. Although additives can improve film quality, their long-term stability under operating conditions such as humidity and thermal cycling needs further validation. The use of the co-solvent strategy (DMF/IPA) and post-treatment method (FACl) effectively addresses the formation of micelles in SAM, improves molecular stacking, but introduces additional processing complexity. In addition, the manufacturing of large-area modules still faces obstacles, especially in laser ablation, where hydrophobic SAM may lead to uneven perovskite deposition, although DMPU modification provides a promising solution. In summary, although these additive based strategies have pushed PSC performance to new heights, future research should focus on scalability, long-term stability, and simplified manufacturing processes to promote commercialization. Balancing high efficiency and industrial feasibility remains a key challenge in perovskite photovoltaic power generation.

Figure 7. (a) Schematic of the GBAC-assisted film growth process [81], (b) surface and cross-section scanning electron microscope (SEM) images of the control and target perovskite films, with the scale bars of 1 μm [81], (c) continuous MPPT of PCE for encapsulated PSCs with c-SAM and a-SAM under the protocol of ISOS-L-1I. RH, relative humidity [82], (d) thermal stability measurement of PCE for PSCs with c-SAM and a-SAM under the protocol of ISOS-T-2I [82], (e) schematic representation of the chemical structure of HEA, device architecture and the interaction mechanism between HEA, SAM, and perovskite layers [83], (f) SEM images of the perovskite films based on ITO/SAM and ITO/SAM/HEA layers [83], (g) illustration of micelles formed from the amphiphilic SAM molecules and disassembled in co-solvent [85], (h) schematic diagram for evolution of SAMs configuration adjustment deposited from IPA or co-solvent [85].

Figure 7. (a) Schematic of the GBAC-assisted film growth process [81], (b) surface and cross-section scanning electron microscope (SEM) images of the control and target perovskite films, with the scale bars of 1 μm [81], (c) continuous MPPT of PCE for encapsulated PSCs with c-SAM and a-SAM under the protocol of ISOS-L-1I. RH, relative humidity [82], (d) thermal stability measurement of PCE for PSCs with c-SAM and a-SAM under the protocol of ISOS-T-2I [82], (e) schematic representation of the chemical structure of HEA, device architecture and the interaction mechanism between HEA, SAM, and perovskite layers [83], (f) SEM images of the perovskite films based on ITO/SAM and ITO/SAM/HEA layers [83], (g) illustration of micelles formed from the amphiphilic SAM molecules and disassembled in co-solvent [85], (h) schematic diagram for evolution of SAMs configuration adjustment deposited from IPA or co-solvent [85].

3.2.2. Co-Self-Assembly Strategy

By employing a co-assembly methodology, the surface energy characteristics can be precisely tuned to facilitate uniform deposition of hole-selective layers. This results in pinhole-free HTL films with optimized energy level matching at interfaces while effectively passivating trap states that would otherwise promote charge recombination. In 2019, Erpeng Li et al. [40] proposed an unprecedented anchor-based co-self-assembly strategy by selecting a TPA-PT-C6 triphenylamine derivative and a hydrophilic ammonium salt CA-Br for synergistic co-adsorption (Figure 8d). The CA-Br co-adsorbent has two functional ends, an anchored carboxylic acid group and a hydrophilic quaternary ammonium group. They found that the morphology and quality of perovskite films are improved, and some of the electron traps of perovskites may be passivated by the ammonium salt group in the molecular structure of CA-Br and improve the wettability of perovskite precursors on self-assembled HELs.

In addition, for the purpose of achieving completely covered perovskite layers, Hossain et al. [88] used PFNBr-Me-4PACz mixtures for uniform coating (Figure 8c). Moreover, the hybrid SAMs composed of Me-4PACz and NA enhanced the wettability of the perovskite solution [89]. As a result, the champion device achieved a PCE of 26.69%, with a certified value of 26.54%.

Recent advances in mixed SAM systems have significantly enhanced device performance through interfacial engineering. In 2022, Deng et al. [90] developed IAHA with methoxy substituents as a secondary component, effectively downshifting the HOMO level while enhancing molecular dipole moments. Zhang et al. [91] combined MeO-2PACz with FC-3283 (Figure 8e), achieving multifunctional improvements in film coverage and device performance (25.70% PCE). Su et al. [92] designed tBu-4PACz with tert-butyl groups (Figure 8f), suppressing molecular aggregation and yielding low-defect perovskite films, achieving PCE up to 26.25% and outstanding FF over 86%. In addition, Zhang et al. [93] established a SAM-4NPBA/FAI system (Figs. 8g, h) featuring boronic acid anchoring and dual passivation, achieving record FF (88.35%) and 25.3% PCE. These systematic studies address critical interfacial challenges including monolayer coverage, energy alignment, and defect passivation, providing new pathways for high-efficiency photovoltaic devices.

In addition to the above features, co-self-assembly is also capable of changing the way the SAMs combined with the substrate. Chang et al. [94] carefully designed the functionalized GDY derivative (PAG) based on carbazole structure with PA and triphenylamine structure to regulate the molecular orientation of 2- (9H-carbazol-9-yl) ethyl) phosphonic acid (2PACz). On the one hand, the π–π interaction between PAG and the carbazole ring of 2PACz increases the molecular order and reduce the agglomeration (Figure 8i). On the other hand, there is a hydrogen-bond interaction between the multi-site phosphoric acid groups of PAG and 2PACz, which induces 2PACz to form covalent bonds with the substrate in tridentate modes during the molecular self-assembly process, and then a homogeneous and highly ordered molecular packing mode is obtained. The subsequently deposited perovskite films exhibit enhanced crystallinity and a smooth buried interface. The inverted devices based on the co-assembly strategy deliver an optimal PCE of 26.10%.

There are also other ways to adjust the process method to improve the device performance. Donghoon Song et al. [95] demonstrate superior film formation in tin-based perovskites using sequential deposition rather than conventional single-step processing. Their findings highlight the critical influence of substrate wettability, with MeO-2PACz showing particular advantages as a SAM material because of its strong hydrophilic character and specific chemical affinity with SnI₂ precursors. The research team also observed that controlled thermal treatment of ITO substrates prior to deposition could further improve surface energy characteristics, leading to more homogeneous perovskite layer formation. Qiu et al. [96] developed an economical ambient-air technique to optimize SAM deposition on transparent conductive oxides. Their innovative method involves controlled substrate cooling under atmospheric conditions, which promotes uniform water adsorption from the surrounding environment. This process serves dual purposes: (1) generating additional surface hydroxyl groups and (2) decreasing oxygen defect concentrations. The resulting surface modification enhances SAM anchoring strength and significantly reduces current leakage pathways in completed devices.

The co-assembly strategy has proven to be a highly effective approach for optimizing HTLs and improving perovskite film quality in photovoltaic devices. By employing mixed self-assembled monolayer (SAM) systems, such as TPA-PT-C6/CA-Br, PFNBr-Me-4PACz, and PAG/2PACz, researchers have achieved uniform film coverage, enhanced energy level alignment, and defect passivation, leading to high efficiencies (26.69% PCE) and fill factors (88.35% FF). Additionally, sequential deposition and substrate pre-treatment methods have further enhanced perovskite film formation, particularly in tin-based perovskites and ambient-air processing. Despite these advancements, challenges remain. Mixed SAM systems require precise control over molecular ratios and deposition conditions, which may complicate large-scale manufacturing. The long-term stability of co-assembled layers under operational stress (e.g., thermal cycling, moisture exposure) needs further validation. Additionally, while substrate cooling and thermal treatments improve SAM anchoring, they introduce additional processing steps that may hinder industrial scalability. In summary, co-assembly and interfacial engineering represent a promising direction for high-performance perovskite solar cells, offering solutions to key challenges in film uniformity, charge extraction, and defect passivation. However, future research should focus on simplifying fabrication processes and ensuring long-term device stability to facilitate commercialization. By addressing these limitations, mixed SAM strategies could play a pivotal role in the next generation of perovskite photovoltaics.

Figure 8. (a) Diagram emphasizing block design and painting problems for Me-4PACz hole-selection contacts in P1 scrim regions, showing pinholes [87], (b) perovskite-type ink blade coating over the control (Me-4PACz) and target (Me-4PACz + DMPU) large area substrates, emphasizing the problem of dewetting of the perovskite-type ink at the glass/ITO interface on the control board, relative to the homogeneous perovskite-type wet film on the target substrate, corresponding photo images of large area blade-coated perovskite films on the control and target specimens. The visible pins on the P1 scribe lines are highlighted by a broken red ring under the control state [87], (c) the Me-4PACz, Me-4PACz: PFN-Br, and the perovskite-type layers [88], (d) the HTM TPA-PT-C6 self-assembling HTM TPA-PT-C6 and the co-sorbent CA-Br, schematically illustrating [40], (e) a diagram of the control and a mixture of SAMs [91], (f) MeO-2PACz, tBu-4PACz [92], (g) structural diagram of co-self assembly of DMAcPA and 4NPBA in perovskite-embedded base interface. XPS spectroscopy for SAM and SAM-4NPBA on ITO substrate [93], (h) schematic diagram based on the dual buried modification strategy [93], (i) schematic illustration of 2PACz prior to and following PAG co-assembly [94].

Figure 8. (a) Diagram emphasizing block design and painting problems for Me-4PACz hole-selection contacts in P1 scrim regions, showing pinholes [87], (b) perovskite-type ink blade coating over the control (Me-4PACz) and target (Me-4PACz + DMPU) large area substrates, emphasizing the problem of dewetting of the perovskite-type ink at the glass/ITO interface on the control board, relative to the homogeneous perovskite-type wet film on the target substrate, corresponding photo images of large area blade-coated perovskite films on the control and target specimens. The visible pins on the P1 scribe lines are highlighted by a broken red ring under the control state [87], (c) the Me-4PACz, Me-4PACz: PFN-Br, and the perovskite-type layers [88], (d) the HTM TPA-PT-C6 self-assembling HTM TPA-PT-C6 and the co-sorbent CA-Br, schematically illustrating [40], (e) a diagram of the control and a mixture of SAMs [91], (f) MeO-2PACz, tBu-4PACz [92], (g) structural diagram of co-self assembly of DMAcPA and 4NPBA in perovskite-embedded base interface. XPS spectroscopy for SAM and SAM-4NPBA on ITO substrate [93], (h) schematic diagram based on the dual buried modification strategy [93], (i) schematic illustration of 2PACz prior to and following PAG co-assembly [94].

4. SAMs Adopted to Modify HTL

Beyond serving as standalone HTLs in perovskite photovoltaics, SAMs have demonstrated significant potential as interfacial modifiers for conventional HTL materials. These molecular layers primarily function by optimizing the surface properties of existing HTLs, facilitating improved interfacial contact with both the perovskite absorber and transparent conductive substrates, which collectively contribute to enhanced device performance. Among all kinds of SAMs, the most popular one is MeO-2PACz, which can influence the energy level alignment at these interfaces and modify the W_F, resulting in an improved charge transport property [97]. Peng Xu et al. [98] first demonstrated the effectiveness of MeO-2PACz as an ultrathin interlayer between perovskite and poly(3-hexylthiophene) (P3HT). Their findings revealed that the SAM primarily enhances P3HT film formation on perovskite while simultaneously minimizing interfacial defect states, rather than passivating bulk perovskite defects. This interfacial optimization resulted in significantly suppressed charge recombination, enabling champion devices to achieve a PCE of 21.37% with the V_OC of 1.12 V. Current research indicates that SAM modification is most frequently applied to polymeric HTLs and NiO_x-based transport layers, where the molecular interlayers effectively address interfacial challenges while maintaining excellent charge transport properties.

4.1. SAM Modification for Enhanced Performance of Polymer HTLs

PEDOT:PSS is a widely adopted polymeric hole-transporting material in inverted PSCs, owing to its excellent visible-light transmittance and compatibility with low-temperature fabrication processes. However, the performance and long-term stability of PEDOT:PSS-based devices remain suboptimal due to inherent limitations, including mismatched energy levels, insufficient charge carrier mobility, unfavorable perovskite crystallization dynamics, and undesirable interfacial hydrophilicity. These material-level constraints collectively hinder the achievable PCE in such photovoltaic architectures [99].

The utilization of SAMs at the PEDOT: PSS HTL/perovskite (PVK) interface presents an effective approach to address the existing issues. By incorporating SAMs, the morphology of perovskite films can be enhanced, and the coverage of perovskite layers can be augmented. These improvements ultimately contribute to an elevation in the photoelectric conversion efficiency. Zhuowei Gu et al. [100] employed 3-aminopropionic acid as the SAM on the PEDOT: PSS HTL. This modification led to an improvement in the crystallinity and coverage of perovskite films (as depicted in Figure 9a,b). Consequently, the surface morphology was optimized, and the PCE increased from 9.7% to 11.6%.

Figure 9. (a) A C3-SAM modification PEDOT: PSS HTL. Emphasis is placed on the orientation of the additional permanent dipoles [100], (b) the sectional SEM of the manufactured apparatus [100], (c) the normalization PCE measured with MPPT at 20 °C in an N2 atmosphere at 20 °C and (d) the MPPT normalized PCE measured at 20 °C in an N2 atmosphere in an N2 environment, and (e) the configuration of the/PEDOT-PSS/2PACz/perovskite layer [101].

Figure 9. (a) A C3-SAM modification PEDOT: PSS HTL. Emphasis is placed on the orientation of the additional permanent dipoles [100], (b) the sectional SEM of the manufactured apparatus [100], (c) the normalization PCE measured with MPPT at 20 °C in an N2 atmosphere at 20 °C and (d) the MPPT normalized PCE measured at 20 °C in an N2 atmosphere in an N2 environment, and (e) the configuration of the/PEDOT-PSS/2PACz/perovskite layer [101].

Mengmeng Chen et al. [101] implemented a 2PACz SAM as an interfacial modifier on PEDOT:PSS hole-transport layers for wide-bandgap (1.62 eV) tin-based perovskite photovoltaic devices. This hybrid HTL system exhibited remarkable performance enhancement through several interconnected mechanisms: (1) substantial improvement in perovskite film compactness and crystallinity, (2) optimized energy level matching at the HTL/perovskite junction, and (3) significant suppression of non-radiative carrier losses. When configured as a PEDOT:PSS/2PACz bilayer architecture, comprehensive MPPT analysis revealed dramatically improved photostability compared to conventional single-layer HTLs (Figure 9c,d). The enhanced operational durability, coupled with superior charge extraction characteristics, stems from the synergistic combination of PEDOT:PSS’s bulk transport properties and 2PACz’s interfacial modification capabilities. This phenomenon might be accounted for by the fact that the 2PACz monolayer improved the stability of solar cells in several ways. It enhanced the compactness of the perovskite layer surface morphology, minimized non-radiative recombination, and inhibited direct contact between PEDOT-PSS and perovskite films.

Further research has shown that MeO-2PACz exhibits significant advantages in improving PSC performance and stability. In a study by SungWon Cho et al. [102], SAMs of MeO-2PACz and 2PACz were applied at the interface between PEDOT:PSS HTLs and Sn-based PVK films. Comparative analysis revealed that MeO-2PACz substantially outperformed its counterpart. X-ray photoelectron spectroscopy (XPS) data indicated that the phosphonate anchoring groups (P-O−) in the SAMs formed strong bonds with sulfur cations (S⁺) in PEDOT:PSS, leading to optimized energy level alignment (Figure 10b–d). Nuclear magnetic resonance (NMR) studies further elucidated that the methoxy terminal groups in MeO-2PACz coordinated with tin-oxygen octahedra in the perovskite lattice, effectively passivating interfacial defects. Time-resolved spectroscopic techniques, including femtosecond transient absorption (fs-TAS) and photoluminescence analyses, confirmed accelerated charge carrier extraction and reduced recombination losses in devices incorporating MeO-2PACz-modified interfaces (Figure 10f–h). As a result, the optimized architecture achieved a remarkable PCE of 12.16% (with an independently certified value of 11.60%), significantly exceeding the performance of control devices with 2PACz (8.78%) or unmodified interfaces (7.44%). Furthermore, accelerated aging tests under an inert atmosphere revealed negligible efficiency degradation after 2000 h, while devices maintained 92.6% of initial performance after 300 h under ambient conditions (30–50% RH), underscoring the critical role of MeO-2PACz in interfacial optimization for high-performance PSCs.

Another typical polymer HTL is PTAA, which is among the most frequently employed polymer HTLs. This is due to its outstanding electrical characteristics and chemical neutrality. Long Zhou et al. [104] developed a novel strategy for optimizing perovskite module performance through the implementation of a PTAA/Me-4PACz bilayer hole transport system. This architectural innovation significantly improved interfacial contact at the buried perovskite layer, simultaneously enhancing crystalline grain development and overall film morphology. The incorporation of Me-4PACz as SAM on PTAA optimized energy level alignment while effectively minimizing non-radiative losses at the critical interface. The composite transport system demonstrated superior electrical conductivity and reduced W_F characteristics, enabling more efficient charge collection. Photovoltaic devices incorporating this bilayer configuration achieved remarkable PCE of 23.52% for laboratory-scale cells and 20.18% for 65 cm² aperture area modules, representing substantial improvements over conventional PTAA-based architectures. The modified interfacial properties also yielded enhanced film uniformity and device-to-device consistency, particularly beneficial for large-area module fabrication.

SAMs have emerged as an effective interfacial modification strategy at polymer HTL/perovskite interfaces, demonstrating remarkable improvements in the following aspects: enhanced perovskite film morphology, optimized charge extraction efficiency, and improved device stability. Through precise molecular engineering, SAM modification enables better energy level alignment, significant reduction of non-radiative recombination losses, and enhanced device-to-device uniformity. However, despite these advancements, critical challenges remain in processing reproducibility, long-term thermal stability, and cost-effective manufacturing. Future research directions should prioritize the development of universal SAM designs with broader perovskite compatibility, and the implementation of scalable fabrication methodologies to bridge the gap between laboratory achievements and industrial production.

4.2. SAM Modification for Enhanced Performance of NiO_x HTLs

NiO_x has become a prevalent HTM in PSCs due to its exceptional optical transparency, superior hole conductivity, and robust chemical durability. Nevertheless, the presence of surface imperfections and hydroxyl groups in NiO_x films often compromises device efficiency and operational longevity [105]. Recent studies have employed SAMs to address these interfacial challenges. Qin Wang et al. [103] systematically investigated benzoic acid-derived SAMs for NiO_x surface passivation. Among various derivatives, 4-bromobenzoic acid demonstrated remarkable defect mitigation capabilities, simultaneously reducing trap states, optimizing energy level alignment, and modulating surface hydrophobicity to facilitate perovskite crystal growth. Dilpreet Singh Mann et al. [105] developed an alternative approach using (3-triethoxysilyl)propylamine (TSPA) as a molecular bridge. The amine-terminated SAM chemically anchored to NiO_x through hydroxyl interactions, effectively passivating interfacial traps while introducing a beneficial dipole moment. This interfacial engineering strategy significantly enhanced charge collection efficiency through improved energy level matching at the NiO_x/perovskite heterojunction. The photoelectric characteristics and device architectures of PSCs with diverse SAMs are presented in Table 1.

Table 1. Photoelectric characteristics and device architectures of PSCs with diverse SAMs.

SAM	Device Structure	VOC/(V)	JSC/(mA·cm−2)	FF/(%)	PCE/(%)	Refs.
4-bromobenzoic acid	ITO/NiOx with SAM/MAPbI3/PCBM/bis-C60/Ag	1.11	21.7	76.3	18.4	[103]
TSPA	ITO/NiO/TSPA/perovskite/PCBM/ZnO/Ag	1.11	22.8	80.3	20.21	[105]
3PAIDCz	PET/ITO/NiOx/SAMs/Perovskite/PCBM/BCP/Ag	1.145	24.68	81.64	23.07	[106]
Br-BPA	ITO/NiOx/SAM/perovskite/PC60BM/Ag	1.059	19.69	61.28	12.60	[107]
TBT-BA	ITO/NiOx/SAMs/perovskite/Spiro-OMeTAD/Ag	1.14	24.2	81.9	22.7	[108]
2PACz	ITO/NiOx-amine//2PACz/perovskite/PEAI/PCBM/BCP/Ag	1.12	21.39	74.0	18.04	[109]
2PACz	ITO/NiOx/SAMs/perovskite/LiF/C60/BCP/Cu	1.14	22.6	77.9	20.1	[110]
2PACz	ITO/NiOx wo/w modifications/CsFAMA perovskite/PEAI/PCBM/BCP/Ag	1.17	23.10	77.3	20.81	[109]
2PACz	FTO/NiO/interlayer/perovskite/LiF/C60/BCP/Cu	1.05	22.5	84.0	22.2	[111]
2PACz	FTO/HTL/perovskite/PCBM/BCP/Ag	1.12	24.96	82.89	23.25	[112]
MeO-2PACz	ITO/sp-NiOx/MeO-2PACz/perovskite/PC61BM/AZO/Ag	1.11	20.1	80.0	17.2	[113]
MeO-2PACz	ITO/NiOx/Me-4PACz/perovskite/spiro-OMeTAD/Ag	1.04	22.61	73.10	17.3	[114]
MeO-2PACz	ITO/NiOx/CH3NH3PbI3/PCBM/AZO/Ag	0.88	19.13	72.0	17.35	[115]
MeO-2PACz	ITO/NiOx/perovskite/LiF/C60/BCP/Cu	1.13	22.7	79.9	20.5	[110]
MeO-2PACz	ITO/NiOx/SAMs/perovskite/PCBM/BCP/Ag	1.10	24.9	83.0	20.8	[116]
MeO-2PACz	FTO/NiO/NR/SAM/Perovskite/C60/SnOx/Ag	1.16	25.57	86.35	25.66	[117]
Me-4PACz	ITO/NiOx/SAM/FAPbI3/PCBM/BCP/Ag	1.04	22.61	73.10	17.3	[114]
Me-4PACz	ITO/NiOx/SAMs/perovskite/LiF/C60/BCP/Cu	1.15	23.0	80.0	21.2	[110]
Me-4PACz	ITO/NiO/SAMs/FA0.95Cs0.05PbI3/PI/PC61BM/BCP/Ag	1.135	25.80	79.53	23.29	[118]
Me-4PACz and PC	ITO/NiOx/SAM/perovskite/PEABr/PCBM+ C60/BCP/Ag	1.175	25.88	82.54	25.09	[119]
Me-4PACz	FTO/NiOx/NR/MeO-4PACz/Cs0.05MA0.1FA0.85PbI3/C60/SnOx/Ag	1.162	25.67	86.35	25.66	[117]

Table 1. Photoelectric characteristics and device architectures of PSCs with diverse SAMs.

SAM	Device Structure	VOC/(V)	JSC/(mA·cm−2)	FF/(%)	PCE/(%)	Refs.
4-bromobenzoic acid	ITO/NiOx with SAM/MAPbI3/PCBM/bis-C60/Ag	1.11	21.7	76.3	18.4	[103]
TSPA	ITO/NiO/TSPA/perovskite/PCBM/ZnO/Ag	1.11	22.8	80.3	20.21	[105]
3PAIDCz	PET/ITO/NiOx/SAMs/Perovskite/PCBM/BCP/Ag	1.145	24.68	81.64	23.07	[106]
Br-BPA	ITO/NiOx/SAM/perovskite/PC60BM/Ag	1.059	19.69	61.28	12.60	[107]
TBT-BA	ITO/NiOx/SAMs/perovskite/Spiro-OMeTAD/Ag	1.14	24.2	81.9	22.7	[108]
2PACz	ITO/NiOx-amine//2PACz/perovskite/PEAI/PCBM/BCP/Ag	1.12	21.39	74.0	18.04	[109]
2PACz	ITO/NiOx/SAMs/perovskite/LiF/C60/BCP/Cu	1.14	22.6	77.9	20.1	[110]
2PACz	ITO/NiOx wo/w modifications/CsFAMA perovskite/PEAI/PCBM/BCP/Ag	1.17	23.10	77.3	20.81	[109]
2PACz	FTO/NiO/interlayer/perovskite/LiF/C60/BCP/Cu	1.05	22.5	84.0	22.2	[111]
2PACz	FTO/HTL/perovskite/PCBM/BCP/Ag	1.12	24.96	82.89	23.25	[112]
MeO-2PACz	ITO/sp-NiOx/MeO-2PACz/perovskite/PC61BM/AZO/Ag	1.11	20.1	80.0	17.2	[113]
MeO-2PACz	ITO/NiOx/Me-4PACz/perovskite/spiro-OMeTAD/Ag	1.04	22.61	73.10	17.3	[114]
MeO-2PACz	ITO/NiOx/CH3NH3PbI3/PCBM/AZO/Ag	0.88	19.13	72.0	17.35	[115]
MeO-2PACz	ITO/NiOx/perovskite/LiF/C60/BCP/Cu	1.13	22.7	79.9	20.5	[110]
MeO-2PACz	ITO/NiOx/SAMs/perovskite/PCBM/BCP/Ag	1.10	24.9	83.0	20.8	[116]
MeO-2PACz	FTO/NiO/NR/SAM/Perovskite/C60/SnOx/Ag	1.16	25.57	86.35	25.66	[117]
Me-4PACz	ITO/NiOx/SAM/FAPbI3/PCBM/BCP/Ag	1.04	22.61	73.10	17.3	[114]
Me-4PACz	ITO/NiOx/SAMs/perovskite/LiF/C60/BCP/Cu	1.15	23.0	80.0	21.2	[110]
Me-4PACz	ITO/NiO/SAMs/FA0.95Cs0.05PbI3/PI/PC61BM/BCP/Ag	1.135	25.80	79.53	23.29	[118]
Me-4PACz and PC	ITO/NiOx/SAM/perovskite/PEABr/PCBM+ C60/BCP/Ag	1.175	25.88	82.54	25.09	[119]
Me-4PACz	FTO/NiOx/NR/MeO-4PACz/Cs0.05MA0.1FA0.85PbI3/C60/SnOx/Ag	1.162	25.67	86.35	25.66	[117]

Table 1 comprehensively shows that different SAM systems result in significant variations in the device performance of PSCs. V_OC ranges from 0.88 to 1.19 V, with carbazole-based SAMs like 2PACz and MeO-2PACz generally achieving higher values by optimizing energy levels, enhancing band bending, and suppressing recombination. J_SC varies from 19.13–25.88 mA/cm², affected by charge extraction, perovskite crystallinity, and optical losses. FF is in the range of 61.28–86.35%, with MeO-2PACz based devices performing well due to reduced resistance and recombination. PCE values span from 12.60% to 26.54%, with composite SAM systems (Me-4PACz + PC and NA-Me) demonstrating optimal performance through synergistic interfacial modification. These hybrid systems combine the advantages of carbazole’s excellent hole transport, phosphonic acid’s robust anchoring, and co-adsorbents’ defect passivation capabilities.

Among all kinds of SAMs, the carbazole-based SAMs, especially MeO-2PACz, are the most popular choice. Juntao Zhao et al. [116] demonstrated that 2PACz and MeO-2PACz not only boost the crystallinity but also enhance hole extraction efficiency at the NiO_x/perovskite interface. Juanjuan Sun et al. [120] demonstrated through Fourier transform infrared spectroscopy (FTIR) and XPS measurements that the three tooth bonding between MeO-2PACz and NiO_x can achieve close contact, reducing the formation of pinholes and defects, as is evident from Figure 11a–c. AmiraR.M. Alghamdi et al. [113] reported that MeO-2PACz enhances the quality of the perovskite film in multiple ways. It enlarges the domain size of the perovskite film, which is beneficial for more efficient charge transport. Additionally, it reduces charge recombination occurring at the NiO_x/perovskite interface, which is crucial as it minimizes energy losses within the solar cell. Moreover, MeO-2PACz passivates the defects present on the NiO_x surface, helping to create a more stable and efficient interface between the NiO_x layer and the perovskite layer, as depicted in Figure 11f–i. Masatoshi Yanagida et al. [115] investigated the effect of MeO-2PACz modified NiO_x surface on the PCE of PSCs, and reported that the performance improvement was caused by two mechanisms. First, the SAM shields the exposed ITO surface within the thin NiO_x film, thus, preventing direct contact between ITO and perovskite. Second, it passivates interface defects that could otherwise function as non-radiative recombination hotspots.

Besides MeO-2PACz, other carbazole-based SAMs are also widely used. Xueliang Zhu et al. [111] demonstrated that 2PACz-functionalized NiO_x substrates substantially enhance charge collection efficiency while effectively suppressing interfacial trap states and halide ion diffusion, primarily through the reduction of non-radiative recombination pathways. Yamaguchi et al. [121] observed that 2PACz induces a pronounced vacuum-level shift at the NiO_x interface, attributed to molecular dipole formation and space-charge region development. This phenomenon increases the effective WF, subsequently strengthening band bending at the perovskite surface and significantly decreasing carrier recombination losses. Li Cao et al. [114] reported that the application of Me-4PACz not only passivates interfacial defects but also optimizes the energy alignment between NiO_x and FAPbI3, which in turn facilitates charge extraction, as illustrated in Figure 12a–c. Merve Tutundzic et al. [110] employed a blade-coating approach to deposit 2PACz and Me-4PACz SAMs onto sputtered NiO_x substrates. XPS analysis revealed that both SAM treatments induced a downward shift in the VBM position of NiO_x. As illustrated in Figure 12d, this energy level modulation effectively reduced the interfacial offset between the hole-transport layer and the perovskite absorber, thereby promoting more efficient hole extraction.

In addition to a single SAM layer, combined SAM materials have also been successfully applied in PSCs. Jingyang Lin et al. [109] reported that although surface modifications with only 2PACz yield comparable average efficiencies, the use of amine-2PACz for interface modification leads to a remarkable improvement in device stability. This enhancement can be ascribed to the enhanced quality of the perovskite layer. Ethanolamine contributes to additional defect passivation, and the combined surface modification also adjusts the energy level alignment. Qi Cao et al. [119] modify the buried interface of NiO_x-based PSCs. They doped Me-4PACz with phosphorylcholine chloride (PC) to form a combined SAM (Co-SAM) aimed at enhancing monolayer coverage. As illustrated in Figure 13, the synergistic action of phosphate groups and chloride ions (Cl⁻) in the PC effectively passivates surface defects in NiO_x. Concurrently, quaternary ammonium ions and Cl⁻ work cooperatively to heal organic cation and halide vacancies at the perovskite/NiO_x interface. This co-assembled molecular (Co-SAM) system serves dual functions. On the one hand, it facilitates the formation of high-quality perovskite crystals with reduced residual strain; on the other hand, it mitigates interfacial recombination losses while enhancing charge carrier mobility. The optimized devices incorporating this Co-SAM modification demonstrated exceptional photovoltaic performance, reaching a peak PCE of 25.09%. Furthermore, these devices exhibited remarkable operational durability, maintaining 93% of their initial performance after 1000 h of continuous illumination at AM 1.5G conditions.

Ziyue Feng et al. [117] proposed a multifaceted post-treatment method for NiO_x. Prior to the deposition of SAMs, they dissolved rubidium salt in ammonia water and applied it to NiO_x. This approach aimed to rectify the non-uniform coverage of the NiO_x/SAM HTL on the textured FTO substrate and to decrease the abundant defects in NiO_x nanoparticles. The device subjected to this post-treatment achieved a remarkable champion PCE of 25.66%. It also boasted an impressive FF surpassing 86.3% in a small device with an active area of 0.05 cm². The photovoltaic performance remained exceptional even at larger scales, with devices exhibiting a PCE of 23.97% for 1 cm² active areas. Remarkably, these devices showed excellent operational durability, retaining 95% of their initial performance after 1000 h when subjected to ISOS-L-2I aging protocols. Sanwan Liu et al. [89] developed an innovative approach by combining Me-4PACz with 4,4′,4″-nitrilotribenzoic acid (NA) to create composite SAMs (NA-Me) on NiO_x substrates. This molecular design strategy yielded multiple benefits: (1) the carboxylic acid functional groups in NA improved surface wetting characteristics, enabling better perovskite solution spreading and minimizing interfacial voids; (2) the triphenylamine-NA interactions prevented Me-4PACz aggregation, ensuring uniform molecular distribution; and (3) the optimized interface substantially enhanced charge collection while suppressing non-radiative losses. This interfacial modification strategy achieved record-breaking results, with laboratory-scale devices (0.1 cm²) demonstrating certified stabilized efficiencies reaching 26.54% (V_OC = 1.201 V). The technology showed excellent scalability, as evidenced by mini-modules (11.1 cm² aperture area) maintaining high efficiencies above 22.7% under certification conditions.

In recent years, researchers have utilized SAMs to address interface issues, with different SAMs exhibiting varying degrees of modification effects. Among them, carbazole-based SAMs have shown a particularly outstanding performance in optimizing energy levels and improving key performance parameters. In addition to a single SAM layer, the combination of SAM materials and post-treatment methods for NiO_x has a significant effect on improving device performance and stability. However, there are still many issues in the research of SAM-modified NiO_x. In terms of material research, studies on the modification effects of different SAMs are mostly based on experimental phenomena, with insufficient understanding of the microscopic mechanisms, which limits further optimization of material properties and the design of new systems. From the perspective of device applications, most research results are still in the laboratory stage, with a significant gap from actual production applications. In terms of device stability research, although there is a focus on performance improvement, there is relatively insufficient research on long-term stability. Future research on SAM-modified NiO_x in PSCs should focus on several key directions, such as advanced characterization process innovation, and holistic stability studies. By addressing these gaps, SAM-modified NiO_x can transition from lab-scale breakthroughs to industrially viable PSC technologies.

5. SAMs Used in Tandem Systems

SAMs possess distinct benefits for PSCs. They feature negligible parasitic absorption, low material usage, and a simplified fabrication process. The molecular structure of SAMs contains robust chemical anchoring groups. This allows SAMs to conformally coat the surfaces of diverse rough substrates. These properties make SAMs highly suitable for use in TSCs. In the architecture of TSCs, SAM-based HTLs play a crucial role. Their ability to conformally coat substrates enhances charge transfer efficiency, and low parasitic absorption maximizes the utilization of incident light. As a result, device performance is significantly improved, making SAMs a promising choice for researchers seeking to boost the efficiency and stability of perovskite-based TSCs [122].

5.1. Functional SAMs in Tandem Solar Cells

Among all kinds of TSCs such as all-perovskite TSCs, perovskite/CIGS TSCs, the perovskite/Si TSCs is the most popular choice for SAM HTL. The perovskite/Si TSCs consists of a bottom multi-crystalline Si cell and a top perovskite cell connected by a TCO, as shown in Figure 14a. Based on the advantages of low cost, simple fabrication process and tunable band gap of the top perovskite cells, perovskite/Si TSCs are regarded as a new generation of potential photovoltaic devices.

Mishima et al. [11] investigated the synergistic effects of mixed SAMs (MeO-2PACz and 2PACz) in perovskite-Si TSCs. The study rationale stemmed from the complementary properties of these carbazole-based molecules: MeO-2PACz’s tendency to leave uncoated regions on ITO surfaces contrasted with 2PACz’s excellent passivation capability and structural complementarity. Through comprehensive characterization, including XPS, CV, and impedance analysis, the researchers identified that MeO-2PACz’s incomplete surface coverage (Figure 14d–f) originated from steric hindrance effects. The strategic combination of both SAMs enabled 2PACz to occupy the exposed areas, thereby enhancing interfacial passivation. This molecular engineering approach yielded exceptional performance in inverted tandem architectures, with the optimized devices reaching 28.8% efficiency in laboratory tests and achieving a certified 28.3% PCE under MPPT conditions.

Currently, interfacial modulation is an important issue in developing perovskite/Si TSCs. Fortunately, SAMs can firmly anchor to the surface of TCO and form good interfacial contact, which is conducive to charge extraction and transport. Igal Levine et al. [68] reported that Me-4PACz-based SAMs not only enabled the fastest hole transfer rate constant to ITO, but also achieved the highest level of ITO passivation, with an extremely low density of interfacial electron traps. Guoliang Wang et al. [12] developed Ph-2PACz, a novel carbazole-based SAM-type HSL. The energy level alignment characteristics depicted in Figure 15c–h demonstrate that the developed hole-transport layer exhibits a reduced energy offset between its highest occupied molecular orbital and the valence band maximum of the perovskite absorber. This optimized interface, combined with improved surface wetting properties, facilitates the deposition of high-quality perovskite films with enhanced crystallinity and superior interfacial contact. The favorable energetics and morphology collectively enable more efficient charge carrier extraction from the photovoltaic active layer. Implementation of Ph-2PACz in single-junction devices with a 1.67 eV bandgap yielded remarkable performance parameters, including a certified PCE of 21.3%, an open-circuit voltage reaching 1.26 V, and an FF of 82.6%, representing the highest reported values for PSCs in this bandgap range. Furthermore, when integrated as the hole-selective contact in perovskite-silicon tandem architectures, the modified interface enabled devices with 1 cm² active area to achieve 28.9% efficiency and 1.91 V open-circuit voltage while demonstrating significantly improved operational stability relative to conventional PTAA- or 2PACz-based counterparts.

The pursuit of enhanced PSCl performance has driven the development of advanced interfacial engineering approaches, particularly through the implementation of multifunctional molecular modifiers. Shi-Chun Liu et al. [13] introduced an amphiphilic conjugated polyelectrolyte, poly(9,9-bis(3′-(N,N-dimethyl)-N-ethylammonium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) dibromide (PFN-Br) at the interface between SAMs and the perovskite absorber. This polymeric additive served dual functions by simultaneously reducing unpassivated regions on the ITO substrate and preventing void formation at the buried perovskite interface. Spectroscopic characterization (Figure 16a–f) demonstrated that PFN-Br incorporation elevated the Fermi level by 200 meV, generating favorable band alignment at the charge collection interface. These interfacial modifications produced significant gains in device performance, boosting the PCE from 12.0 ± 0.85% to 14.2 ± 0.67% while improving manufacturing reproducibility.

Chi Li et al. [14] devised a mixed SAM system that enhanced molecular adhesion to transparent electrodes while forming robust, hydrophobic charge transport layers with improved hole conduction properties. The optimized molecular assembly enabled PSCs with a 1.68 eV bandgap to achieve unprecedented efficiencies of 22.63% and FFs reaching 86.67%, as verified in Figure 16i–k.

The development of perovskite/silicon TSCs with SAM-based HTLs has significantly advanced photovoltaic performance. Mixed SAMs like MeO-2PACz and 2PACz enhance charge extraction and achieve high efficiencies (up to 28.8% in lab, 28.3% PCE under MPPT). Novel SAMs such as Ph-2PACz and Me-4PACz optimize energy levels and improve device performance (up to 21.3% PCE in single-junction, 28.9% in tandem). Amphiphilic polymers like PFN-Br at the interface also boost performance. However, challenges remain for commercialization: SAM deposition over large areas is difficult, long-term stability under real-world conditions is unvalidated, and complex synthesis and processing limit cost-effectiveness. Future research should focus on machine learning-guided SAM design, in-situ characterization for degradation mechanisms, and scalable fabrication methods to overcome these barriers and enable widespread deployment.

5.2. SAMs with NiO_x in Tandem Solar Cells

Similar to its applications in single junction PSCs, SAM can function as both a functional layer and a modification layer in TSCs. In TSCs, SAMs are predominantly utilized to modify NiO_x HTLs. Likai Zheng et al. [123] utilized 2PACz to modify the NiO_x HTL in an inverted PSC for V-shaped tandem applications. The SAM-treated NiO_x enhanced the PSC’s PCE by two percentage points. This improvement was achieved through passivating interface defects, aligning energy levels, and simultaneously obtaining a more uniform perovskite film. Radha K. Kothandaraman et al. [124] NiO_x enabled a strong connection with the SAM molecule. This strong binding enhanced the surface coverage of SAM. Additionally, the hydrophilic nature of the surface formed by this interaction improved the wettability of the perovskite ink. As a result, a perovskite absorber free of pin-holes was achieved, which is crucial for the efficient operation of perovskite-based devices.

Yamaguchi et al. [121] probed into the influence of 2PACz modification on NiO_x HSCs within the context of TSCs. The introduction of a 2PACz interlayer was found to significantly enhance the open-circuit voltage characteristics of the photovoltaic devices. As demonstrated through photoelectron yield spectroscopy analysis (Figure 17e,f), this performance improvement originated from a pronounced vacuum level shift caused by the molecular monolayer, which effectively increased the W_F of the charge transport layer. The elevated W_F induced stronger band bending at the perovskite interface, thereby suppressing surface recombination losses. Complementary theoretical calculations employing density functional theory revealed that the W_F modification stemmed partially from dipole formation at the 2PACz-modified interface. Electron spin resonance characterization further identified charge transfer phenomena between NiO_x and 2PACz, creating a space-charge region that amplified the vacuum level modification. These interfacial engineering effects collectively translated to superior device performance, with notable improvements in both V_OC and J_SC, as evidenced by the photovoltaic characterization data.

From certain viewpoints, NiO_x is introduced into devices to modify SAMs layers. Afei Zhang et al. [125] systematically examined how NiO interfacial layers optimize SAM formation on ITO substrates for wide-bandgap perovskite photovoltaics. Their mechanistic study identified three fundamental factors governing this enhancement: crystalline plane exposure, topographic smoothing, and hydroxyl group stabilization. The NiO interlayer preferentially exposes specific crystallographic orientations while increasing surface metal atom density, thereby providing abundant anchoring sites for molecular self-assembly. Nanoscale characterization revealed that NiO deposition significantly reduces substrate roughness, promoting more uniform SAM distribution and stronger interfacial adhesion. Thermal desorption experiments further demonstrated superior hydroxyl group retention on NiO-modified surfaces compared to pristine ITO, highlighting enhanced chemical stability. These combined effects contribute to improved molecular packing density and interfacial robustness in the resulting photovoltaic devices.

Researchers have also explored HTLs with a NiO_x:Cu + SAM structure. Radha K. Kothandaraman, et al. [124] examined the viability of NiO_x-based HTLs in TSCs. As depicted in Figure 18a, the NiO_x:Cu system functionalized with SAMs emerged as the optimal HTL for TSCs. The molecular modification effectively passivates interfacial defects in copper-doped NiO_x, simultaneously enhancing charge extraction kinetics while dramatically reducing trap state density at the critical interface. Compared to NiO_x:Cu alone, NiO_x:Cu + SAM also elevates the W_F of perovskite. This is likely a consequence of the dipole effect induced by SAM at the interface, leading to more favorable overall band alignment within the tandem device. When integrated with a CIGS bottom mini-module, the perovskite-CIGS 4-terminal tandem mini-module achieved record efficiencies of 20.5% for an aperture area of 2.03 cm² and 16.9% for 10.23 cm², as shown in Figure 18b. Ivona Kafedjiska et al. [126] reported that when a NiO_x:Cu + SAM HTL bilayer configuration is employed, the rough surface does not impede the tandem’s performance, as illustrated in Figure 18c,d. This accomplishment can be ascribed to the combination of the advantageous features of the two HTLs. First, it conformally encapsulates the textured surface of the underlying sub-cell, effectively eliminating potential shunting pathways between the aluminum-doped zinc oxide electrode and perovskite absorber. Concurrently, the SAM treatment passivates interfacial defects in the NiO_x:Cu layer, facilitating optimal charge transport characteristics required for high-performance photovoltaic operation.

SAM plays a key role in improving the performance of cascaded PSCs, especially when used together with NiO_x as HTL. Functioning as both a functional layer and a modification layer, SAM contributes significantly to passivating interface defects, aligning energy levels, and improving film uniformity. These characteristics highlight the key role of SAM and NiO_x as HTL to enhance the performance of PSCs.

In TSCs, SAMs have demonstrated dual functionality as both active charge transport layers and interfacial modification layers, with particular efficacy in enhancing the performance of NiO_x-based HTLs. Systematic investigations reveal that molecular engineering of NiO_x interfaces using phosphonic acid-based SAMs improves photovoltaic performance through the following mechanisms: effective passivation of interfacial trap states, optimization of energy level alignment at the HTL/perovskite interface, and promotion of highly uniform perovskite crystallization. Notably, the inherent properties of NiO_x, including its well-defined crystallographic orientation, nanoscale surface smoothness, and surface hydroxyl group density, synergistically facilitate the formation of high-quality SAM coatings on transparent conductive oxides. Recent advances in Cu-doped NiO_x (NiO_x:Cu) systems functionalized with SAMs have yielded record power conversion efficiencies in perovskite/CIGS tandem architecture, attributed to the complementary effects of enhanced charge extraction and improved interfacial band alignment. Future research should prioritize establishing structure–property relationships through atomic-scale interface characterization, developing accelerated testing protocols for interfacial stability assessment, and implementing machine learning approaches to optimize SAM molecular design. Addressing these challenges will be crucial for realizing the full potential of molecular interface engineering in next-generation photovoltaic technologies.

6. Conclusions and Outlook

This review provides a comprehensive overview of the advancements in SAMs utilized in inverted PSCs. SAMs exhibit superior characteristics relative to conventional HTLs in inverted PSCs, offering multiple benefits. These include tunable molecular architectures achieved by altering the terminal group, spacer group, and anchoring group, thereby optimizing their physicochemical properties. Furthermore, SAMs demonstrate versatile processing compatibility with multiple deposition techniques and solvent systems. Functionally, SAMs play a pivotal role in interfacial energy level modulation, enhancing charge carrier extraction kinetics, controlling perovskite film formation, and mitigating defect states at the bottom perovskite interface. These synergistic effects collectively improve the performance and operational stability of both single-junction inverted PSCs and perovskite-based tandem solar cells. The structural adaptability of SAMs, achieved through molecular engineering of their constituent groups, enables precise tuning of their electronic and morphological characteristics. In summary, SAM-based interfacial modifications represent a promising strategy for developing high-performance and durable perovskite photovoltaics.

Figure 19 showcases the evolution of PCE in inverted PSCs with SAM HTLs, highlighting key milestones. Based on this figure and the content of the article, several SAMs stand out as promising candidates for achieving high-performance inverted PSCs. 4PABCz is the best choice for single-junction devices, which achieves a record 26.81% PCE in single-junction devices by extending π-conjugation, enabling optimal molecular packing for enhanced charge collection and reduced interfacial strain. MeO-2PACz is the best for NiO_x-based device stability and scalability, which improves NiO_x-based devices by tuning energy levels, enlarging perovskite domains, and reducing recombination at interfaces. Ph-2PACz excels in perovskite/Si tandems, minimizing energy offsets and improving film quality, achieving 21.3% (single-junction) and 28.9% (tandem) PCE. These SAMs optimize interfacial properties, driving efficiency gains in both single and tandem architectures. As research progresses, the continuous evolution of SAM design, as depicted in Figure 19, indicates that further improvements are likely. Future SAMs may combine high efficiency with enhanced long-term stability, which is crucial for the commercialization of perovskite solar cells.

Compared with other competitive systems for solar cells, PSCs have unique advantages, according to Table 1. Silicon-based solar cells have long been dominant in the photovoltaic market, with commercial products having a PCE of 18–22% and lab-scale devices up to around 26.7%. The highest-performing SAM-based PSCs, like the NA-Me system with a 26.54% PCE, are approaching this record level, while their simpler solution-processing techniques offer cost and scalability benefits. Organic solar cells are lightweight, flexible and have a PCE of around 19–20%, and SAM-based PSCs with a PCE range of 12.60% to 26.54% outperform them on average due to better charge transport and light harvesting. Dye-sensitized solar cells usually have a PCE of 11–14% (up to 15–16% in advanced research), and SAM-based PSCs significantly surpass them in PCE, as perovskite layers can absorb more light and have better charge mechanisms.

Despite all the advantages of perovskite solar cells, there are still problems that need to be solved before they can be commercialized. One of the problems is the amplification of perovskite devices. Perovskite materials are usually composed of multiple components, and in the process of large-area preparation, due to the difference in crystallization kinetics and uneven solution coating, local impurities and component segregation are easy to occur. It is difficult to synchronize the crystallization process of large-area perovskite films, especially when the crystallization rate of narrow bandgap perovskites is too fast, resulting in an increase in defects at the bottom of the film and an increase in surface roughness. In addition, the toxicity and low evaporation rate of traditional solvents limit the promotion of green preparation processes. As the area increases, the interface defects between the perovskite and the charge transport layer increase significantly, resulting in the intensification of carrier recombination. The presence of impurities and defects will accelerate the decomposition of perovskite materials under humid heat or light conditions, resulting in the degradation of module efficiency. The spin coating method commonly used in the laboratory is difficult to scale-up, while large-area processes such as scraping and inkjet printing have higher requirements for material viscosity and crystallization kinetics. At present, Tan’s team uses a green solvent system of DMSO and acetonitrile (ACN) to delay the crystallization rate by adding glycamide hydrochloride to extend the film preparation time window by 10 times, thereby achieving an efficiency of 24.5% in a 20.25 cm² laminated module. The strategy also reduces bottom defects and improves uniformity through a self-healing mechanism. This solution can not only reduce the stress cracks inside the film and improve the mechanical stability, but also replace DMF with green solvent (DMSO/ACN), reduce toxicity and improve the processability of the solution, providing an environmentally friendly path for industrialization [127]. Zhao’s team has developed an interface engineering strategy based on low-dimensional perovskites (CHEA₂PbI₄) to convert impurities in three-dimensional perovskites (such as lead iodide) into two-dimensional perovskite layers with excellent conductivity. This layer not only passivates surface defects, but also improves charge transport efficiency while significantly inhibiting the penetration of moisture and oxygen. The module of Zhao Yixin’s team showed better stability than the traditional structure in the accelerated aging test, so that the efficiency of the 30 cm × 30 cm module reached 22.46% (certified value), which solved the restriction of impurity accumulation on large-area performance. In addition, Zhao’s team collaborated with CATL to apply CHEAI interface engineering to roll-to-roll production, verifying its feasibility in large-scale manufacturing [128].

Stability issues are a major obstacle to the successful commercialization of PSCs, and the instability issues that may be associated with PSCs are structural transitions, chemical and thermal instabilities, sealing issues, and moisture effects. The external factors that contribute to the degradation of PSCs and, thus, affect stability are mainly temperature, UV rays, and environmental (moisture, oxygen) exposure. Device degradation can be caused by degradation of the active layer, charge transport layer, or electrode. In addition, the quality of the film at each functional layer also affects the stability of the device, which is related to the intrinsic properties of the material. Most charge transport materials are chemically active and are a major source of instability due to interfacial degradation. An important way to improve device stability is to shield the fragile perovskite absorber with a protective layer. These protective layers can be composed of metal oxides (e.g., TiO₂, SnO₂, and ZnO₂), polymers (e.g., polyvinylpyrrolidone), and organic/inorganic hybrids (including SiO₂–TiO₂). In addition, high-efficiency PSCs often use precious metal materials (Au, Ag) as CE, which leads to high cost and metal ion diffusion that destabilizes the device. The development of high-efficiency non-precious metals can not only significantly reduce the cost of perovskite solar cells, but also alleviate the stability problem, but the current non-precious metal electrodes still face the problems of energy level mismatch at the interface with perovskite and low carrier transport efficiency, resulting in low PCE. In addition, ensuring proper device packaging and protection is critical to enhancing the stability of the PSC. Finally, for stability studies, future studies should focus more on improving long-term operational stability, especially under outdoor test conditions. For indoor laboratory testing, the stressor is usually fixed and isolated. In contrast, outdoor operation increases the unpredictability of equipment operation. For modules manufactured with standard striping techniques such as P1, P2, and P3, stability is much more complex. The main degradation mechanisms at the device level may differ at the module level. As a result, strategies to overcome stability challenges may shift from units to modules.

The advancement and optimization of SAMs are poised to profoundly influence future research trajectories in perovskite photovoltaics. Building upon current progress, we anticipate that the maturation of SAM-based technologies will unlock their full potential as high-performance HTLs in inverted PSCs. Particularly noteworthy is their dual capacity to simultaneously enhance charge extraction efficiency and interface stability—a critical requirement for next-generation photovoltaic devices. Furthermore, the versatile molecular engineering capabilities of SAMs present unprecedented opportunities for developing both single-junction and tandem architecture through tailored energy-level alignment. We envision this comprehensive review will serve as a valuable reference for researchers working on interfacial engineering strategies and accelerate the rational design of functional SAMs for high-efficiency photovoltaics.