Molecular Pinning Effect in Composite Self‐Assembled Monolayers Enabling Efficient Perovskite/TOPCon Tandem Solar Cells

ABSTRACT

Perovskite‐based single junction and tandem solar cells (TSCs) based on self‐assembled monolayers (SAMs) still suffer from interfacial losses related to poor wettability during film formation, defect states of perovskites, and suboptimal energy‐level alignment of devices. Herein, we propose a composite SAM (Co‐SAM) strategy based on tetrahedral‐symmetry‐driven molecular pinning effect of 4,4′,4″,4‴‐methanetetrayltetrabenzoic acid (4MA) with Me‐4PACz for efficient and durable wide bandgap perovskite solar cells (WBG PSCs) and perovskite/TOPCon TSCs. This design enhances monolayer uniformity on NiO_x, improves wettability of substrates, and regulates crystallization through Pb‐O coordination and hydrogen bonding, thereby passivating buried‐interface defects and optimizing energy‐level alignment. As a result, inverted 1.68 eV WBG PSCs achieve a champion efficiency of 23.52%, along with impressive stability, retaining over 80% after 1000 h of thermal aging at 85°C in an N₂ atmosphere. Furthermore, the Co‐SAM strategy demonstrates excellent generality, boosting the efficiency of 1 cm² TSCs to 32.26% (certified 32.10%). This work offers a versatile interfacial engineering approach toward highly efficient, stable, and scalable perovskite photovoltaics.

The 4MA‐integrated composite self‐assembled monolayer enhances monolayer uniformity, interfacial defect passivation, and energy‐level alignment via molecular pinning effect, achieving 23.52% efficiency in wide‐bandgap perovskite solar cells (32.26% in perovskite/TOPCon tandems) while retaining > 80% efficiency after 1000 h at 85°C under N₂ atmosphere, offering a versatile molecular strategy for efficient and durable perovskite photovoltaics.

1. Introduction

Wide‐bandgap perovskite solar cells (WBG PSCs) employing self‐assembled monolayers (SAMs) as hole‐transport layers (HTLs) still suffer from fundamental challenges in achieving high efficiency and stability [1, 2, 3], primarily due to suboptimal interfacial energy alignment, insufficient defect passivation, and poor film‐forming uniformity [4, 5, 6]. These issues are particularly evident in perovskite/silicon tandem solar cells (TSCs), a key technology for next‐generation photovoltaics [7, 8, 9, 10]. Despite the impressive certified efficiencies achieved with perovskite/silicon heterojunction (SHJ) TSCs [11, 12, 13, 14, 15], large‐scale production is hindered by the high manufacturing costs associated with SHJ production lines. In comparison, TOPCon‐based silicon solar cells dominate with over 60% market share owing to their cost‐effectiveness and scalability. However, perovskite/TOPCon TSCs still lag in performance compared to SHJ‐based counterparts [16, 17, 18, 19, 20, 21].

Addressing the efficiency gap requires innovative interface engineering through tailored SAMs [22, 23, 24, 25, 26, 27], which can form dense, ordered molecular layers on metal oxide substrates with functional groups like phosphonic or carboxylic acids. These SAMs precisely modulate work function and energy band alignment at the buried interface [28]. Moreover, functional terminal groups in SAMs also passivate interfacial defects, suppress non‐radiative recombination, and improve perovskite crystallinity [29, 30, 31, 32, 33]. Advanced strategies optimizing SAM molecular structure and composition (Co‐SAMs) have been developed to enhance charge extraction [34], minimize voltage losses [35], and improve stability of WBG PSCs, paving the way for cost‐effective, efficient, and scalable perovskite/TOPCon TSCs [36, 37, 38, 39, 40]. Notably, Co‐SAMs offer promising improvements in efficiency and stability over single‐component systems. However, their underlying interfacial mechanisms remain insufficiently elucidated. Critical questions persist regarding molecular‐scale interactions, dynamic energy‐level alignment, and interfacial defect passivation processes. This lack of in‐depth mechanistic understanding hampers the development of rational design strategies necessary for further performance optimization.

In this study, we employ a Co‐SAM strategy using 4,4′,4″,4‴‐methanetetrayltetrabenzoic acid (4MA), with tetrahedral symmetry and multiple carboxylic acid groups, combined with Me‐4PACz, to systematically investigate the structure‐function relationships and interfacial dynamics for constructing efficient and durable WBG PSCs and perovskite/TOPCon TSCs. The design rationale includes the effective vacancy occupation by the strong binding interaction between 4MA and Me‐4PACz, preventing direct contact between the perovskite layer and NiO_x substrate. The carboxylic acid groups of 4MA enhance the surface hydrophilicity and coordinate with the perovskite components to regulate the crystallization kinetics, leading to a denser perovskite film with a reduced defect density at the bottom interface. This synergistic interfacial modulation ensures efficient charge extraction and suppresses non‐radiative recombination at both upper and lower interfaces. Consequently, a 1.68 eV WBG PSC employing this Co‐SAM achieved a power conversion efficiency (PCE) of 23.52%, retaining over 90% of its initial efficiency after 1500 h of ambient storage (25°C, 30% RH) and over 80% after 1000 h of thermal aging at 85°C in a N₂ atmosphere. Additionally, a perovskite/TOPCon TSC with this Co‐SAM protocol delivered a stabilized efficiency of 32.26% (certified 32.10%) with an impressive stability.

2. Results and Discussion

We systematically investigated the interfacial modulation mechanism of the Co‐SAM system by integrating theoretical calculations with experimental characterizations. Figure 1a illustrates the optimized molecular geometries of Me‐4PACz and 4MA, along with their corresponding electrostatic potential (ESP) maps calculated by Gaussian theoretical methods. The computational results indicate that Me‐4PACz exhibits positive ESP primarily over the hydroxyl groups of the phosphonic acid moiety and carbazole unit, while the negative ESP centers on the oxygen atom of the P═O bond within the phosphonic acid group. In contrast, 4MA has its positive ESP largely distributed on the hydrogen atoms of the hydroxyl groups in the benzoic acid units, with negative ESP on the oxygen atoms in the carboxyl groups. This complementary ESP distribution highlights the potential for intermolecular interactions between Me‐4PACz and 4MA, forming a theoretical basis for enhanced composite SAM formation.

FIGURE 1.

(a) Chemical structures and ESP distributions of 4MA and Me‐4PACz. (b) Intermolecular interactions in the Me‐4PACz homodimer (Me‐Me), 4MA homodimer (4MA‐4MA), and 4MA/Me‐4PACz heterodimer (4MA‐Me). (c) Schematic depiction of Me‐4PACz and 4MA/Me‐4PACz composite SAM (Co‐SAM) assemblies on NiO_x substrates. (d) Schematic illustration of the perovskite film interfacial evolution enabled by the Co‑SAM strategy. (e, f) AFM‑IR images of the P═O bond distribution for (e) NiO_x/Me‐4PACz and (f) NiO_x/Co‐SAM films. (g, h) KPFM images of (g) Me‐4PACz and (h) Co‐SAM films. (i, j) XPS spectra of (i) NiO_x/Me‐4PACz and (j) NiO_x/Co‐SAM films.

To elucidate the molecular pinning effect of 4MA in the Co‐SAM, we calculated the molecular binding energies as illustrated in Figures 1b and S1 (see Supporting Information) [41]. Notably, the binding energy of the 4MA/Me‐4PACz heterodimer (4MA‐Me, −2.19 eV) surpassed that of both Me‐4PACz (Me‐Me, −0.88 eV) and 4MA (4MA‐4MA, −1.30 eV) homodimers. This indicates significantly stronger intermolecular interactions between the two components of Co‐SAM, which effectively mitigates the self‐aggregation tendency of Me‐4PACz. Consequently, these advantages promote the formation of a more uniform and densely packed composite monolayer, as depicted in Figure 1c. The adsorption energy between 4MA and NiO_x was calculated with a value of −1.69 eV, indicating a strong contact stability of 4MA in substrates (Figure S2).

Figure 1d illustrates the growth evolution of the perovskite film on the Co‐SAM substrate. The introduction of 4MA aims to mitigate Me‐4PACz aggregation, which arises from strong intermolecular interactions. The carboxyl groups of 4MA facilitate interfacial regulation through a dual mechanism: on one hand, they engage in hydrogen bonding with organic cations (FA⁺, MA⁺) and halide ions in the perovskite precursor, thereby promoting ordered nucleation; on the other hand, they enhance the coverage and interfacial stability of the SAM by coordinating with the underlying NiO_x substrate. This coordination is driven by the tetrahedral‐symmetry‐driven molecular pinning effect of 4MA, effectively improving the assembly of the Me‐4PACz SAM. This regulatory role is reflected in the resulting film morphology: the perovskite layer deposited on the Co‐SAM exhibited a smooth, mirror‐like surface, whereas the control film displayed incomplete coverage and poor spreading (Figure S3).

The uniformity of the SAM significantly affects its interfacial electrical properties. To examine this relationship, we conducted Kelvin probe force microscopy (KPFM) measurements on the films deposited on ITO substrates (Figure 1e,f). The Co‐SAM film exhibited the highest average contact potential difference (CPD) and narrowest CPD distribution among the samples (Figure S4). This homogeneous CPD profile indicates enhanced molecular packing density and improved morphological uniformity, while the increased average CPD corresponds to a higher work function of the modified surface [42]. Collectively, these results demonstrate that the incorporation of 4MA optimizes the assembly behavior of Me‐4PACz, leading to the formation of an ultrathin, defect‐minimized, and uniformly covered interfacial layer that serves as an ideal substrate for high‐quality perovskite film growth. Atomic force microscopy (AFM) further corroborated that the Co‐SAM strategy yielded a smoother surface topography, as reflected by a reduction in the average root mean square roughness (R _a) from 4.76 nm for Me‐4PACz to 4.60 nm for Co‐SAM (Figure S5). This reduction in roughness underscores the formation of a more uniform and densely packed monolayer, which is critical for suppressing interfacial recombination losses in photovoltaic devices. The homogeneous CPD distribution and reduced roughness thus serve as experimental evidence for the suppressed aggregation, corroborating the DFT‐based molecular pinning effect.

To further evaluate the molecular pinning mechanism at device‐relevant interfaces, AFM‐IR measurements were performed on the SAM layers (Figure 1g,h). The results reveal that, compared to the neat Me‐4PACz film, the P = O signal of Me‐4PACz in the Co‐SAM film is not only stronger but also more homogeneously distributed. This observation indicates that the incorporation of 4MA effectively suppresses the self‐aggregation of Me‐4PACz, promoting more uniform molecular packing across the monolayer. These findings are fully consistent with the DFT‐predicted heterodimer interaction and the homogeneous CPD distribution observed by KPFM, collectively supporting the proposed molecular pinning effect of 4MA within the Co‐SAM.

X‐ray photoelectron spectroscopy (XPS) was conducted to analyze the chemical states of the NiO_x substrates with Me‐4PACz and Co‐SAM modification (Figure 1i,j), providing insights into the molecular interactions at the interface. After deposition of the Co‐SAM, the Ni³⁺/Ni²⁺ ratio on the NiO_x surface increased to 2.05, compared to 1.91 for the NiO_x/Me‐4PACz reference. This change in the valence state ratio suggests an alteration in interfacial charge rearrangement and electronic structure, which may correlate with the charge transport properties at the interface [43]. To evaluate the charge transport behavior, current‐voltage (I‐V) measurements were performed on a NiO_x/SAM/Ag sandwich structure (Figure S6). The results show that the Co‐SAM modification leads to enhanced electrical conductivity compared to the Me‐4PACz reference. The trend observed in the I‐V measurements is consistent with the XPS‐derived chemical state changes, together implying a beneficial influence on interfacial hole extraction.

Density functional theory (DFT) calculations were performed to investigate the molecular‐scale interfacial interactions. The results revealed that the small molecule 4MA engages in pronounced chemical interactions with the perovskite precursor PbI₂ through its carboxyl (‐COOH) groups. As illustrated in Figure 2a, the carbonyl oxygen (C═O) of 4MA acts as a strong Lewis base, coordinating with the vacant orbitals of lead (Pb²⁺) in PbI₂ to form stable Pb‐O coordination bonds (Figure S7). The adsorption energy for this interaction (E _interactions) was calculated to be −2.96 eV, further confirming the formation of a stable chemical bond between 4MA and PbI₂.

FIGURE 2.

(a) Schematic illustration of 4MA adsorbed on a PbI₂‐terminated perovskite surface. (b) XPS spectra of Pb 4f core levels for Me‐4PACz/perovskite and Co‐SAM/perovskite films. (c) FTIR spectra of 4MA and the 4MA/PbI₂ mixture. (d, e) In situ UV‐vis absorption spectra recorded during thermal annealing of perovskite films deposited on (d) Me‐4PACz and (e) Co‑SAM substrates. (f, g) Top‐view SEM and (h, i) AFM images of Me‐4PACz/perovskite and Co‐SAM/perovskite films.

To further explore the chemical interactions between 4MA molecules and the perovskite interface, XPS measurements were conducted on the perovskite films deposited on different substrates. As shown in Figure 2b, the Pb 4f peaks of the pristine perovskite film (Me‐4PACz/perovskite) appear at binding energies of 136.46 eV (Pb 4f_7/2) and 141.34 eV (Pb 4f_5/2), which correspond to the typical Pb²⁺ state within perovskite. In contrast, the perovskite film grown on the Co‐SAM exhibited a systematic shift of the Pb 4f peaks toward lower binding energies. A similar shift was observed in the I 3d spectrum (Figure S8). These shifts indicate an increased electron density around the Pb atoms, which enhances the shielding effect on their core electrons and reduces the energy required for photoelectron emission from the Pb 4f and I 3d orbitals. Collectively, these findings provide direct experimental evidence of effective electronic interactions between the carboxyl groups of 4MA and Pb²⁺ within the perovskite.

Additionally, the interactions between 4MA and the perovskite precursors (PbI₂ and FAI) were systematically investigated using Fourier‐transform infrared (FTIR) spectroscopy. In the carboxyl‐stretching region (Figure 2c), 4MA exhibits a characteristic C═O vibration at 1695 cm⁻¹. Upon mixing with PbI₂, this peak underwent a distinct redshift to 1692 cm⁻¹, indicating an increased electron density in the C = O bond due to coordination with Pb²⁺. This observation confirms the formation of stable Pb‐O coordination bonds between the carbonyl oxygen of 4MA and the lead species in the precursor. In the amine‐stretching region (Figure S9), the N‐H vibration of FAI appears at 3340 cm⁻¹. After mixing with 4MA, this peak shifted to higher wavenumbers (3349 cm⁻¹), reflecting a decrease in electron density and strengthening of the N─H bond. This shift originates from the intermolecular hydrogen bonding between the ‐COOH of 4MA and the ‐NH₂ of FAI, which modifies the vibrational environment of the N‐H group.

The formation of hydrogen bonds between the ‐NH₂ groups of FAI and the C = O groups of 4MA was corroborated by ¹H NMR spectroscopy (Figure S10). Upon mixing FAI with 4MA, the characteristic N‐H proton signal of pure FAI, initially a single peak at 8.8 ppm, shifted and evolved into two distinct broad resonance peaks. This change suggests that the N‐H protons exist in two different chemical environments in the mixture: one representing the weakly interacting state, and the more downfield peak representing the N‐H groups strongly engaged in hydrogen bonding with the carbonyl oxygen of 4MA.

The wettability of the WBG perovskite precursor solutions on Me‐4PACz and Co‐SAM films was further assessed through contact angle measurements (Figure S11). The results revealed a notable reduction in the average contact angle from 40.4 ± 1.7° (standard deviation) for Me‐4PACz to 33.4 ± 1.9° for Co‐SAM, indicating improved surface wettability achieved via the composite strategy. This enhancement primarily originates from the hydrophilic carboxyl groups in the incorporated 4MA molecule, which effectively modulates the interfacial energy and mitigates the inherent wettability limitations of the single‐component Me‐4PACz.

In situ UV‐vis absorption spectroscopy was performed to monitor the real‐time perovskite phase evolution during annealing (Figure 2d,e). The results show that the Co‐SAM significantly retards the crystallization process: the complete conversion time increases from 18.9 s for the control (Me‐4PACz) to 23.1 s for the Co‐SAM. This kinetic delay is attributed to the C = O groups in the Co‐SAM, which interact with perovskite precursors and hinder rapid nucleation and growth. The retarded crystallization kinetics induced by the Co‐SAM, as evidenced by the extended conversion time, promote more ordered and defect‐tolerant perovskite film growth, leading to suppressed non‐radiative recombination and enhanced device performance.

The influence of the Co‐SAM on the morphology and crystallization quality of perovskite films was examined using scanning electron microscopy (SEM). Bottom‐view SEM images show that the perovskite film deposited on the Me‐4PACz substrate contains residual PbI₂ particles (Figure S12), indicating incomplete conversion during crystallization. In contrast, the film grown on the Co‐SAM displayed a fully converted, dense interface with no detectable PbI₂ residues. Surface‐view SEM images further illustrate that the perovskite grains formed on the Co‐SAM were notably larger, more uniform, and exhibited a compact microstructure (Figures 2f,g and S13). These morphological improvements stem from the synergistic effect of the Co‐SAM: first, the carboxyl groups of 4MA regulate crystallization kinetics through coordination and hydrogen‐bonding interactions with perovskite precursors; second, the introduction of 4MA suppresses molecular aggregation of Me‐4PACz, leading to a more uniform and densely packed SAM template. Together, these factors promote homogeneous nucleation and vertical growth of the perovskite layer.

To further validate the suppression of residual PbI₂ by 4MA, x‐ray diffraction (XRD) analysis was performed on perovskite films deposited on different SAM substrates (Figure S14). A distinct PbI₂ diffraction peak at 12.7° persisted in the film grown on the Me‐4PACz SAM, whereas no such peak was observed in the film prepared on the Co‐SAM. These findings confirm that the incorporation of 4MA promotes the complete conversion of perovskite and effectively eliminates unreacted PbI₂. The XRD findings are consistent with the absence of residual PbI₂ particles in the Co‐SAM sample as compared to Me‐4PACz (Figure 2f,g), which was also observed in the SEM images (highlighted within the white dashed ellipse in Figure S12). These results together confirm the critical role of Co‐SAM in regulating perovskite crystallization.

The surface morphologies of the perovskite films deposited on different SAM substrates were further evaluated using AFM. The R _a of the film grown on the Co‐SAM decreased significantly to 11.6 nm, compared to 15.7 nm for the film on the Me‐4PACz substrate. This reduction in roughness indicates that the Co‐SAM effectively promotes lateral grain coalescence and vertically uniform growth, leading to a smoother and more compact perovskite surface. These AFM results align well with the morphological improvements observed by SEM, confirming that Co‐SAM plays a key role in regulating crystallization kinetics and suppressing localized, inhomogeneous nucleation (Figure 2h,i).

UV‐vis absorption spectra of the ITO/NiO_x/SAM/perovskite structure show that both Me‐4PACz‐ and Co‐SAM‐based perovskite films exhibit similar absorption edges near 740 nm (Figure S15a). Tauc plot analysis derived from the absorption data yielded an identical optical bandgap of 1.68 eV for both systems (Figure S15b), indicating that the incorporation of 4MA did not alter the intrinsic band structure of the perovskite. Moreover, transmittance measurements of the bare SAM layers confirm that both Me‐4PACz and Co‐SAM possess highly consistent light transmission across the visible spectrum (Figure S16), demonstrating that the Co‐SAM strategy enables effective interfacial functionalization without compromising the optical properties of the perovskite film.

Steady‐state and time‐resolved photoluminescence (PL/TRPL) measurements were employed to examine carrier recombination dynamics. The Co‐SAM‐based perovskite film exhibited a substantially higher PL intensity and an extended average carrier lifetime of 997.60 ns, compared to 395.40 ns for that of Me‐4PACz (Figures S17 and 3a, Table S1), indicating effective suppression of non‐radiative recombination. Furthermore, the defect density was quantified using space‐charge‐limited current (SCLC) measurements on hole‐only devices (Figure 3b,c). The incorporation of 4MA resulted in a decrease in the trap‐filled‐limit voltage (V _TFL) from 0.913 to 0.801 V, with the corresponding defect density decreasing from 6.15 × 10¹⁵ cm⁻³ for the Me‐4PACz device to 5.08 × 10¹⁵ cm⁻³ for the Co‐SAM device (Figure S18). This reduced bulk trap density is consistent with the prolonged TRPL lifetime, which is sensitive to both bulk and interfacial defect states. These results demonstrate that the Co‐SAM strategy effectively passivates bulk defects (as indicated by SCLC) and interfacial defects (as indicated by TRPL and KPFM), thereby suppressing non‐radiative recombination and providing a mechanistic foundation for the improvements in open‐circuit voltage (V _OC) and fill factor (FF).

FIGURE 3.

(a) TRPL decay spectra of Me‐4PACz/perovskite and Co‐SAM/perovskite films. (b, c) SCLC measurements of hole‐only devices based on (b) Me‐4PACz and (c) Co‐SAM. (d, e) QFLS mapping images of (d) Me‐4PACz‐ and Co‐SAM‐based perovskite films. (f) Bar chart comparing the PLQY and QFLS values of Me‐4PACz‐ and Co‐SAM‐based perovskite films. (g, h) UPS spectra of the (g) ITO/NiO_x/Me‐4PACz and (h) ITO/NiO_x/Co‐SAM substrates. (i) Schematic energy level alignment diagram of HTLs and the perovskite film.

To further corroborate the reduced trap density, quasi‐Fermi level splitting (QFLS) and PL mapping measurements were performed. The QFLS results reveal that the Co‐SAM film exhibits a larger splitting (Figure 3d,e), directly quantifying the suppression of non‐radiative recombination losses. PL mapping further shows a more uniform and intense emission from the Co‐SAM film (Figure S19), spatially ruling out the presence of localized high‐defect regions. Specifically, the Co‐SAM film achieves a PLQY of 15.6%, significantly higher than the 7.1% observed for the Me‐4PACz reference (Figures 3f and S20). Correspondingly, the QFLS value increases from 1.251 eV (Me‐4PACz) to 1.272 eV (Co‐SAM). These results are consistent with the trends observed in TRPL and SCLC, collectively supporting the conclusion that the Co‐SAM strategy effectively reduces trap density. Collectively, these findings confirm that the Co‐SAM approach effectively suppresses non‐radiative recombination losses, providing a direct rationale for the enhanced V _OC observed in the corresponding devices.

Ultraviolet photoelectron spectroscopy (UPS) measurements were employed to examine energy level alignment (Figure 3g, h). Following 4MA modification, the Fermi level of Co‐SAM shifts from −4.41 to −4.73 eV, while its highest occupied molecular orbital (HOMO) level shifts downward from −5.32 to −5.36 eV (Table S2). This shift indicates the formation of an interfacial dipole oriented towards the interior at the NiOx/perovskite interface (Figure S21). Such induced dipoles not only optimize the energy level alignment of the HTL, reducing the hole‐extraction barrier, but also enhance electron blocking properties. These synergistic effects contribute to improved charge separation and collection [44], providing an energetic basis for the overall performance enhancement of the device (Figure 3i).

WBG PSCs were fabricated to systematically evaluate the effect of 4MA interface modification on perovskite solar cell performance (Figure 4a). An optimal Co‐SAM composition ratio of 4MA to Me‐4PACz of 2:5 (v/v, denoted as 0.4) was identified (Figure S22 and Table S3). Under this optimized condition, the champion device achieved a PCE of 23.52%, compared to 21.85% for the reference device (Figure 4b,c). All key photovoltaic parameters exhibited enhancement: the V _OC increased from 1.242 to 1.272 V, the short‐circuit current density (J _SC) rose from 21.35 to 21.67 mA cm⁻², and the FF improved from 82.40% to 85.41%. Furthermore, the devices based on Co‐SAM exhibited higher average values and superior batch‐to‐batch reproducibility in PCE (Figure 4c), V _OC, J _SC, and FF (Figure S23), confirming the reliability and effectiveness of this modification strategy.

FIGURE 4.

(a) Schematic device configuration of the 1.68 eV WBG PSCs. (b) Statistical distribution of PCEs and (c) J‐V curves of the best‐performing devices based on Me‐4PACz and Co‐SAM. (d) EQE and the corresponding integrated J _SC curves of the WBG PSCs. (e) Nyquist plots of the PSCs measured under dark conditions. (f) Mot‐Schottky analysis of the Me‐4PACz‐ and Co‐SAM‐based devices. (g) Long‐term storage stability and (h) thermal stability assessments of the unencapsulated devices based on Me‐4PACz and Co‐SAM.

External quantum efficiency (EQE) measurements further elucidated the performance enhancements. The Co‐SAM‐based device exhibited substantially improved photocurrent response in the 500–750 nm wavelength region (Figure 4d). The integrated J _SC derived from the EQE spectrum of the Co‐SAM was 21.54 mA cm⁻², surpassing the 21.34 mA cm⁻² measured for the Me‐4PACz‐based device. This trend aligns with the J _SC values obtained from J‐V measurements, reinforcing the conclusion that 4MA modification promotes increased generation and collection of photogenerated carriers, consistent with the observed improvements in film quality.

Electrochemical impedance spectroscopy (EIS) was conducted to further investigate the interfacial carrier dynamics (Figure 4e). The results show that the NiO_x/Co‐SAM‐based device exhibits lower charge‐transfer resistance (R _ct) and higher recombination resistance (R _rec), indicating that 4MA modification facilitates hole extraction and suppresses interfacial recombination. This conclusion is further validated by dark J‐V measurements (Figure S24), where the significantly reduced dark‐current density in Co‐SAM‐based devices reflects the effective suppression of leakage and non‐radiative recombination. Mott‐Schottky analysis reveals that the built‐in potential (V _bi) of the Co‐SAM‐based device reaches 1.06 eV (Figure 4f), higher than the 1.02 eV of the Me‐4PACz device. This increase denotes a stronger built‐in electric field at the NiO_x/perovskite interface, facilitating directional extraction of photogenerated holes while blocking electron backflow. These results provide an energetic rationale for the enhanced V _OC and FF, consistent with the electrical characterization results presented earlier.

The dependence of V _OC on light intensity was investigated, as represented in Figure S25a. The Co‐SAM‐based device exhibited a slope of 1.46 kT/q, approaching the ideal value of 1 kT/q, compared to 1.67 kT/q for the Me‐4PACz device. This reduced slope indicates effective suppression of trap‐assisted non‐radiative (Shockley‐Read‐Hall, SRH) recombination, further confirming that 4MA effectively passivates both bulk and interfacial defects. Additionally, a detailed loss analysis of the FF relative to the theoretical limit was also performed (Figure S25b). The difference between the maximum achievable FF_max and the Shockley‐Queisser limit (FF_SQ‐limit) was 5.69% for the Me‐4PACz device, decreasing to 4.14% for the Co‐SAM device. The smaller loss in the optimized device reflects the minimized nonradiative recombination and charge transport losses, which directly contribute to the improved FF observed in the WBG PSCs.

The stability of the unencapsulated devices was evaluated under environmental and thermal aging conditions (Figure 4g,h). After 1500 h of storage in ambient air (25°C, 30% RH), the Co‐SAM‐based WBG PSCs retained > 90% of their initial PCE, compared to only 71% for the Me‐4PACz devices. Under thermal aging at 85°C in a N₂ atmosphere, the Co‐SAM‐based devices retained > 80% of their initial PCE after 1000 h, significantly outperforming the control devices (retaining only 62%). These results collectively demonstrate that 4MA modification substantially enhances both environmental and thermal stabilities. The operational stability was further assessed by steady‐state photocurrent output (SPO) measurements over 180 s (Figure S26). The Co‐SAM device exhibited negligible current decay, confirming its robust operational performance under continuous one‐sun illumination.

The Co‐SAM strategy was successfully extended to perovskite/TOPCon TSCs, demonstrating its broad applicability in high‑performance photovoltaics. Figure 5a illustrates the device architecture, in which either Me‑4PACz or the Co‑SAM serves as a hole‑selective interlayer. A cross‑sectional SEM image reveals the well‑defined morphology and layered structure of the tandem device (Figure 5b). The J‐V characteristics of perovskite/TOPCon TSC based on Co‐SAM yielded a PCE of 32.26% (certified 32.10%, measured) (Figure 5c, Table S4, and Figure S27), with a V _OC of 1.973 V, J _SC of 20.25 mA cm⁻², and FF of 80.77% under reverse scan, and a PCE of 32.22%, V _OC of 1.973 V, J _SC of 20.26 mA cm⁻², and FF of 80.62% under forward scan, superior to those of the Me‐4PACz counterparts. To validate the generality of this approach, ten devices were fabricated on each type of SAM substrate. Statistical box‐plot analysis confirmed a tighter distribution and a marked enhancement in efficiency for the Co‐SAM‐based TSCs (Figure 5d), demonstrating the strategy's capacity to reliably optimize device performance.

FIGURE 5.

(a) Schematic device configuration of perovskite/TOPCon TSCs. (b) Cross‐sectional SEM image of the tandem device. (c) J‐V curves of the best‐performing Me‐4PACz‐ and Co‐SAM‐based TSCs. (d) Distribution statistics of PCE for Me‐4PACz‐ and Co‐SAM‐based devices. (e) EQE and the corresponding integrated J _SC curves of the devices. (f) SPO of the champion TSC based on Co‐SAM. (g) Long‐term storage stability and (h) MPP tracking stability assessments of the unencapsulated Me‐4PACz‐ and Co‐SAM‐based perovskite/TOPCon TSCs.

Figure 5e,f presents the EQE spectra and the SPO over 300 s for the Co‐SAM‐based TSCs. The SPO curve exhibited negligible photocurrent decay, indicating enhanced operational stability under continuous illumination. This stability under prolonged use marks a significant improvement over conventional device architectures, further underscoring the advantages afforded by the Co‐SAM strategy. The stability evaluation results of the unencapsulated devices are shown in Figure 5g. After 1700 h of storage in a N₂ atmosphere, the Co‐SAM‐based perovskite/TOPCon TSCs retained over 90% of their initial PCE, while the Me‐4PACz devices retained only 81%. Additionally, the operational stability under continuous one‐sun illumination at the maximum power point (MPP) was also assessed as shown in Figure 5h. After 650 h of continuous MPP tracking, the Co‐SAM‐based device retained 91% (76% for Me‐4PACz) of its initial PCE, demonstrating significantly superior performance compared to the control group. MPP tracking was further performed on unencapsulated perovskite/TOPCon TSCs under continuous illumination coupled with humidity stress (30% relative humidity in air, Figure S28). The results show that after 300 h of operation, the Co‐SAM‐based device retains 95% of its initial efficiency, markedly higher than the 70% retention observed for the Me‐4PACz‐based device. These findings demonstrate that the Co‐SAM strategy significantly enhances the operational stability of tandem devices under combined light and humidity stress, underscoring its practical relevance for real‐world photovoltaic applications.

3. Conclusion

In summary, we have developed a Co‐SAM strategy based on the molecular pinning effect of tetrahedral 4MA synergized with Me‐4PACz. This approach elucidates critical structure‐property relationships and interfacial dynamics for the construction of efficient and durable WBG PSCs and perovskite/TOPCon TSCs. The strong binding energy between 4MA and Me‐4PACz effectively suppresses molecular aggregation and facilitates the assembly of a dense, ordered monolayer. Through Pb‐O coordination and hydrogen‑bonding interactions with perovskite precursors, the Co‐SAM enables precise control of crystallization kinetics while optimizing interfacial energy‑level alignment and charge transport. This approach eliminates residual PbI₂, enhances film uniformity, and improves morphological compactness. Consequently, the WBG PSCs and perovskite/TOPCon TSCs employing the Co‐SAM achieved PCEs of 23.52% and 32.26% (certified 32.10%), respectively, underscoring the broad applicability and effectiveness of this molecular‐level approach across device architectures. This study offers novel insights into perovskite interface engineering and provides a robust foundation for the development of highly efficient and stable perovskite‑based optoelectronic devices.

Author Contributions

Zijia Li: resources. Tao Zhang: conceptualization, methodology, investigation, formal analysis, resources, writing – review and editing, data curation. Xinquan Wang: validation. Ruoyu Li: investigation, validation. Gang Xu: methodology, investigation. Yaxuan Yang: visualization. Zehang Liu: investigation, validation, data curation, resources. Yu Bao: validation. Kai Wang: resources. Yuzhou Wu: conceptualization, methodology, software, data curation, investigation, validation, formal analysis, writing – original draft, visualization, writing – review and editing. Qingquan He: supervision, resources, project administration, visualization, funding acquisition, methodology, conceptualization, investigation, writing – review and editing, validation, formal analysis. Jing Li: resources, supervision, writing – review and editing, funding acquisition. Chenxu He: resources. Jun Pan: funding acquisition, visualization, project administration, resources, writing – review and editing, conceptualization, methodology, supervision.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: anie72748‐sup‐0001‐SuppMat.docx.

Supporting File 2: anie72748‐sup‐0001‐data.zip

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.