Tailoring self-assembled monolayers with post-assembly nicotinic acids for efficient and ultraviolet stable inverted perovskite solar cells

Abstract

While inverted perovskite solar cells (PSCs) based on self-assembly monolayers (SAMs) have achieved record power conversion efficiencies (PCEs), two major challenges remain: incomplete SAM coverage on the substrate and inadequate defect passivation at the buried perovskite interface. This work demonstrates that a post-assembly nicotinic acid (NCA) interlayer effectively addresses these issues. The NCA treatment enhances the homogeneity and electrical conductivity of the underlying hole transport layer. Furthermore, the carboxylic acid groups in NCA form hydrogen bonds with the formamidinium cations and coordinate with Pb²⁺ ions, mitigating perovskite lattice strain and passivating defects at the buried interface. Consequently, devices incorporating a post-assembly NCA layer achieve a champion PCE of 25.98% and demonstrate significantly enhanced operational stability under UV exposure compared to control devices. These findings establish post-assembly modification as a potent strategy to overcome inherent limitations in state-of-the-art SAM-based PSCs.

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Peng Huang

Peng Huang is a Research Professor at Southwest Jiaotong University, where he leads the Smart Optoelectronics Device group. He earned his PhD from Soochow University in 2018 under the supervision of Prof. Yongfang Li. He was subsequently awarded a Marie Skłodowska-Curie Individual Fellowship, carrying out research at BCMaterials, Spain and the École Polytechnique Fédérale de Lausanne (EPFL). In 2023, he was selected for both the Sichuan Tianfu-Emei and the Chengdu Rongpiao Youth Talent Program. His research focuses on perovskite solar cells and emerging 2D optoelectronic materials. He has published over 40 papers in leading peer-reviewed journals.

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Introduction

Inverted perovskite solar cells (PSCs) have emerged as a promising platform for commercialization, with power conversion efficiencies (PCEs) approaching 27%, enhanced operational stability, and compatibility with flexible substrates and tandem architectures.^1–4 A major impetus behind the advancement of inverted PSCs has been the emergence of self-assembly monolayers (SAMs) as efficient hole transport layers (HTLs). Since their initial integration into PSCs in 2018,^5,6 SAMs have emerged as attractive candidates due to their tunable molecular structures, low material cost, and unique electrical properties.^7,8

Despite their considerable potential, persistent challenges hamper their performance and stability. The intrinsic amphiphilic nature of SAMs can lead to agglomeration in the precursor solution, which hinders the formation of high-density, tightly packed films atop transparent conductive oxide or nickel oxide (NiO_x) substrates.^9–11 Furthermore, large aromatic terminal groups (e.g., carbazole) in SAMs, which directly come into contact with the perovskite's buried interface, often exhibit limited ability to regulate this critical interface.^12,13 These limitations can result in interfacial non-radiative recombination and, ultimately, compromised device efficiency and stability. Crucially, these ultrathin, disordered SAMs and the associated uncontrolled buried interface defects make devices vulnerable to degradation under ultraviolet (UV) irradiation and thermal stress, imposing limitations on long-term operational stability.^14,15

To address these challenges associated with SAMs, strategies such as new SAM designs,^16–18 co-assembly techniques^19–21 and post-assembly methods^22,23 have been explored. Among these, post-assembly treatments using functional organic molecules offer an effective approach to mitigate SAM vacancies and passivate buried interface defects. For instance, Zhou et al. demonstrated that p-fluorobenzoic acid, used as a post-assembly molecule, effectively fills voids in the SAM and passivates buried defects in the perovskite.²⁴ Kyaw et al. employed ionic liquids as protective layers over SAMs, stabilizing film uniformity while passivating the perovskite interface and aligning energy levels.²⁵ Similarly, Li et al. reported that p-xylylenediphosphonic acid (p-XPA) enhances energy level alignment and facilitates interfacial hole transport.²⁶ Despite the promise of post-assembly strategies, the development of low-cost, facile methods capable of simultaneously optimizing SAM coverage, perovskite film growth and defect passivation remains essential.

Herein, we propose a dual-function post-assembly process to engineer the SAM HTL and facilitate the formation of high-quality perovskite films for efficient and stable inverted PSCs. We utilize (2-(3,6-dimethyl-9H-carbazol-9-yl)ethyl)phosphonic acid (Me-2PACz) as the foundational SAM and introduce nicotinic acid (NCA) as a post-assembly modifier. NCA molecules infiltrate the interstitial voids between Me-2PACz molecules, where their carboxylic acid groups chemically anchor to the NiO_x surface, thereby enhancing the uniformity and packing density of the interfacial layer. Simultaneously, NCA forms hydrogen bonds with FA⁺ cations and coordinates with undercoordinated Pb²⁺ ions at the buried interface. This dual interaction effectively passivates interfacial defects from both the HTL and perovskite sides. Consequently, this post-assembly method effectively reduces interfacial vacancies within the SAM HTL, releases the perovskite lattice strain, and passivates buried defects. The NCA-modified PSCs achieve a champion power conversion efficiency (PCE) of 25.98%, significantly surpassing the control device efficiency (24.88%). Moreover, unencapsulated target devices exhibit outstanding operational stability, particularly under UV illumination. This work offers a practical and broadly applicable strategy for SAM interface engineering, advancing both the performance and stability of PSCs.

Results and discussion

The post-assembly NCA layer was fabricated by spin-coating an NCA solution onto the NiO_x/Me-2PACz HTL, as illustrated in Fig. 1a. For clarity, the samples treated with NCA are referred to as target, while untreated samples are termed control. To investigate the interfacial interaction between NCA and the underlying layer, X-ray photoelectron spectroscopy (XPS) was performed. As shown in Fig. 1b, the C 1s spectrum of the target SAM film exhibits a prominent O–C=O peak at 288.5 eV, which is absent in the control sample (Fig. S1). This indicates the presence of carboxyl functional groups introduced by NCA. Additionally, the Ni 2p_3/2 peak in the target sample exhibits a negative shift of 0.29 eV (Fig. 1c), suggesting strong anchoring of NCA onto the surface. This interaction is further corroborated by a shift in the P 2p peak position (Fig. S2). Quantitative analysis shows an increase in the Ni³⁺/Ni²⁺ ratio from 3.42 (control) to 3.54 (target), suggesting enhanced p-type characteristics through improved hole carrier concentration.^27,28

Fig. 1 (a) Working mechanism of the NCA post-assembly strategy. XPS spectra of (b) C 1s core-level for the target SAM, (c) Ni 2p_3/2 core-level of the control and target HTLs. (d) I–V characteristics of the control and target HTL films. (e) Time-dependent water contact-angle evolution on the control and target HTL films. (f) AFM and (g) KPFM images of the control and target HTL films. (h) Potential distribution plots of control and target HTLs.

The electrical properties of the SAM HTL were further assessed through conductivity (σ) measurements, based on the equation σ = d/(A·R), where d is the layer thickness, A is the active device area, and R is the resistance derived from current–voltage (I–V) tests.^29–31 As shown in Fig. 1d, the target SAM film exhibited an enhanced conductivity of 4.45 × 10⁻³ mS cm⁻¹, compared to 2.84 × 10⁻³ mS cm⁻¹ for the control film. This improvement arises from NCA molecules occupying interstitial sites between Me-2PACz molecules, where their carboxyl groups anchor to the NiO_x surface. The enhanced interfacial anchoring reinforces the upper layer's interaction with NiO_x, thereby enhancing electrical conductivity.³² The NCA-modified interface exhibits a lower trap-filled limit voltage (V_TFL) of 0.22 V, compared to 0.30 V for the control device (Fig. S3). This reduction in V_TFL corresponds to a decrease in trap density (N_t) from 1.11 × 10¹⁸ cm⁻³ to 0.81 × 10¹⁸ cm⁻³, indicating effectively suppressed interfacial trap states.

Water contact angle measurements were conducted to assess the packing density of organic molecules. The static contact angle increased from 100.5° to 104° after introducing the post-assembly NCA layer (Fig. S4), indicating a more hydrophobic and densely packed surface. We further examined the change in water contact angle on Me-2PACz surfaces before and after DMF rinsing (Fig. S5). The results show that the NCA-modified SAM exhibits a significantly smaller change in contact angle (from 105.1° to 105.4°) compared to the unmodified SAM (100.1° to 104.6°). This minimal change demonstrates that the NCA layer remains intact after solvent exposure, confirming its effectiveness in enhancing interfacial stability. The time-dependent decay kinetics revealed an increase in the exponential decay constant from 1.30 to 1.56 min⁻¹ (Fig. 1e), supporting the formation of denser, more ordered organic molecular layers.

Atomic force microscopy (AFM) images revealed morphological changes induced by the post-assembly NCA (Fig. 1f). The root-mean-square (RMS) roughness decreased from 5.82 nm to 4.71 nm, demonstrating elimination of large agglomeration domains and resulting in more uniform surface. This smoother morphology improved interfacial contact with the overlying perovskite film. Kelvin probe force microscopy (KPFM) (Fig. 1g) provided further insights into surface potential uniformity and energy level alignment. The full-width-at-half-maximum (FWHM) of the contact potential difference (CPD) distribution narrowed from 8.2 mV to 7.3 mV after post-assembly NCA modification (Fig. 1h), reflecting improved surface potential homogeneity. Furthermore, the average CPD shifted from −258 mV to −291 mV, corresponding to a downward shift in the Fermi level. This shift enhanced the p-type characteristics, optimizing energy level alignment at the HTL/perovskite interface and potentially reducing non-radiative recombination losses.^33,34

To understand how the NCA molecule influences the perovskite films, we investigated its interactions with the key perovskite components. First, we examined interactions between NCA and the FAI using liquid-state proton nuclear magnetic resonance (¹H NMR) spectroscopy in dimethyl sulfoxide solution (DMSO-d₆). In the pure FAI solution, the nitrogen-bound protons appear at 8.81 ppm. After addition of NCA, this peak splits into two distinct signals (Fig. 2a, b and S6), indicating the formation of hydrogen bonds between the FA⁺ and NCA.³⁵ Additionally, the signal corresponding to the carboxylic acid proton (–COOH) in NCA shifted downfield by 0.018 ppm upon mixing with FAI, confirming hydrogen bonding interactions between the carboxylic acid group in NCA and FA⁺.

Fig. 2 ¹H NMR spectra of FAI, NCA treated with FAI and NCA in the chemical shift region of (a) 9.2–8.5 ppm and (b) 13.6–13.2 ppm. (c) ¹H NMR spectra of NCA and NCA treated with PbI₂ in the chemical shift region of 13.7–13.2 ppm. (d) FTIR spectra of NCA and NCA treated with PbI₂ over the wavenumber range of 1605–1800 cm⁻¹. (e) Pb 4f and (f) I 3d core-level spectra of the control and target perovskite films.

Next, we probed NCA interaction with PbI₂. In the ¹H NMR spectra (Fig. S7 and 2c), the carboxylic acid proton (–COOH) signal shifted downfield by 0.015 ppm upon mixing NCA with PbI₂, suggesting coordination bonding occurs between the –COOH group and Pb²⁺ ions.³⁶ Fourier transform infrared (FTIR) spectroscopy (Fig. 2d and S8) provided additional support: the characteristic vibrational peak of the –COOH group shifted from 1708 cm⁻¹ to 1717 cm⁻¹ upon binding with PbI₂. Electrostatic potential mapping (Fig. S9) revealed that the carboxylic acid group exhibits the highest electron density and strongest electron-donating capability in the NCA molecule. This suggests that it serves as the primary coordination site for Pb²⁺ ions, which may help passivate defects at the buried perovskite interface.

We analyzed the buried perovskite film using X-ray photoelectron spectroscopy (XPS). Inserting a post-assembly NCA layer caused distinct shifts in the binding energies of core levels associated with Pb and I elements (Fig. 2e and f). The Pb 4f peak of 4f_5/2 (4f_7/2) decreased by 0.17 eV, shifting from 143.21 eV (138.31 eV) to 143.04 eV (138.14 eV), respectively. Similarly, the I 3d peaks shifted to lower binding energies by 0.22 eV. Collectively, these results demonstrate that the NCA interacts with the perovskite film through two primary interactions: hydrogen bonding with the FA⁺ cation and coordination bonding with Pb²⁺ ions.

We further investigated the effects of above-mentioned chemical interactions of the NCA layer on the crystallization, morphology, and optoelectronic properties of perovskite films. X-ray diffraction (XRD) analysis revealed enhanced crystallinity in target perovskite films, evidenced by increased peak intensities compared to the control (Fig. 3a). Given the critical influence of lattice strain on device efficiency and stability, we quantified the residual micro-strain using the Williamson–Hall method³⁷ (Fig. 3b). The calculated micro-strain values (ε) decreased from 1.99 × 10⁻³ (control) to 1.38 × 10⁻³ (target), indicating effective alleviation of internal lattice strain by the NCA layer.

Fig. 3 (a) XRD patterns of the control and target perovskite films. (b) Williamson–Hall plots derived from the XRD data for the control and target perovskite films. GIXRD patterns at different ψ angles for (c) control and (d) target perovskite films. (e) Linear fit of 2θ-sin²(ψ) for control and target perovskites. (f) Schematic illustration of lattice strain mitigation in perovskite via post-assembly NCA treatment. Cross-sectional SEM images of (g) control and (h) target PSCs.

Grazing-incidence X-ray diffraction (GIXRD) analysis (Fig. 3c and d) reveals a systematic shift of the (012) perovskite diffraction peak toward higher 2θ angles with increasing tilt angle (ψ), quantitatively confirming the presence of compressive strain within the crystal lattice for both samples,^38–41 This shift arises from reduced interplanar spacing perpendicular to the substrate interface. Notably, NCA modification significantly mitigates this compressive strain, as evidenced by a smaller peak shift and the linear fitting of 2θ versus sin²(ψ) plots (Fig. 3e). The slope decreases from 0.055 in the control sample to 0.038 in the target perovskite films, quantitatively demonstrating the alleviation of residual compressive stress. This effect is attributed to optimized interfacial chemical interactions that suppress lattice distortion, thereby enhancing both the performance and stability of PSCs.

Grain sizes analysis of top-surface SEM images (Fig. S10) revealed average sizes of 265 for the control perovskite film and 284 nm for the target film, respectively. Cross-sectional SEM images (Fig. 3g and 2h) revealed significant morphological differences between the films. The control film exhibited pinholes and grooves at the buried interface, which are likely to promote interfacial charge recombination. In contrast, the NCA-treated perovskite film formed a uniform and continuous interface. This improvement can be attributed to the strong interaction between NCA and the perovskite, which enhances interfacial contact, reduces defects, and suppresses recombination losses.

To assess how these post-assembly NCA layers impact charge carrier behavior, we carried out steady-state photoluminescence (PL), time-resolved PL (TRPL), and photoluminescence quantum yield (PLQY) measurements. The target perovskite film showed significantly higher PL intensity than the control film (Fig. 4a), indicating reduced non-radiative recombination. UV-Vis absorption spectra and Tauc plot analyses (Fig. S11) confirmed that both films exhibited similar absorption characteristics and identical optical bandgaps of ∼1.55 eV. This ensures that the observed PL differences stem from recombination behavior rather than variations in light absorption.

Fig. 4 (a) Steady-state PL, (b) TRPL, and (c) PLQY spectra of the control and target perovskite films. (d) Schematic diagram of the inverted PSC device structure. (e) J–V characteristics and corresponding photovoltaic parameters (inset) for the control and target PSCs. (f) Stabilized power output and photocurrent density at the maximum power point (MPP) as a function of time for the target PSC. (g) EQE spectra and corresponding integrated J for the target PSC. Statistical distributions of the (h) PCE, (i) V_oc and (j) FF.

TRPL measurements provided further insight into carrier recombination dynamics. The decay curves were fitted using a bi-exponential model, yielding fast (τ₁) and slow (τ₂) decay components, typically associated with trap-assisted/interface recombination and bimolecular/bulk recombination, respectively (Fig. 4b and Table S1). The NCA-treated film demonstrated a longer average carrier lifetime (τ_avg. = 7.40 μs) than the control film (τ_avg. = 6.80 μs), reflecting suppressed non-radiative decay pathways. PLQY measurements quantified radiative recombination efficiency. In line with PL and TRPL results, the target perovskite film exhibited a substantially higher PLQY (0.35%) compared to the control (0.18%) (Fig. 4c). This enhancement confirms that the NCA layer effectively reduces non-radiative recombination losses and improves the overall optoelectronic quality of the perovskite film.

Motivated by the dual functional role of NCA in regulating SAM and perovskite films, we fabricated inverted PSCs to evaluate its influence on device performance. The device structure shown in Fig. 4d consists of glass/indium tin oxide (ITO)/NiO_x/Me-2PACz/NCA/perovskite/fullerene (C₆₀)/bathocuproine (BCP)/Ag. The perovskite absorber layer, with a composition of FA_1−xMA_xPbI_3−yCl_y, was deposited via a two-step sequential deposition method.^42,43 Based on optimization experiments (Fig. S12 and Table S2), the optimal NCA solution concentration was determined to be 0.5 mg mL⁻¹.

As shown in Fig. 4e, the current density–voltage (J–V) characteristics show consistent performance enhancements in the target devices, the most notable improvements are in the open-circuit voltage (V_oc) and fill factor (FF). The control PSC achieved a moderate PCE of 24.88%, V_oc of 1.178 V, and FF of 82.74%. In comparison, the target PSC obtained a champion PCE of 25.98% with a V_oc of 1.191 V and an FF of 85.31% (Table 1). Likewise, functionalizing the Me-4PACz surface with post-assembly NCA also boosts the device PCE (Fig. S13). To assess operational stability at the maximum power point (MPP), stabilized power output was recorded under continuous 1-sun illumination for 300 seconds (Fig. 4f). The target device exhibits a stabilized PCE of 25.67%, with an operating voltage of 1.01 V and a steady-state current density of 25.47 mA cm⁻², which are consistent with J–V curves measured under forward and reverse scans shown in Fig. S14 and Table S3, indicating negligible hysteresis and robust performance. The external quantum efficiency (EQE) curves are displayed in Fig. 4g. The integrated current of the target device was 24.52 mA cm⁻², in close agreement with the J_sc values obtained from the J–V curves, validating the accuracy and consistency of the results.

Table 1 Champion and average photovoltaic parameters of control and target PSCs

Condition		Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)
Control	Champion	1.178	25.53	82.74	24.88
	Average	1.187 ± 0.005	25.18 ± 0.14	83.09 ± 0.62	24.82 ± 0.052
Target	Champion	1.191	25.58	85.31	25.98
	Average	1.190 ± 0.004	25.57 ± 0.10	84.66 ± 0.39	25.76 ± 0.11

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To validate device reproducibility, we analyzed the statistical distribution of photovoltaic parameters across 15 independently fabricated devices (Fig. 4h–j and S15), where the average PCE improved from 24.82% for the control devices to 25.76% for the target devices. The narrower standard deviation in V_oc and FF highlights the role of the post-assembly NCA layer in promoting uniform interface quality and reducing batch-to-batch variation.

To systematically elucidate the impact of the post-assembly NCA on the performance of PSCs, a comprehensive set of electrical and spectroscopic characterization studies was performed. Electrochemical impedance spectroscopy (EIS) was employed to study interfacial charge transfer and recombination dynamics. The Nyquist plots (Fig. 5a), fitted with an equivalent circuit (results shown in Fig. S16), revealed that the NCA-modified PSC exhibited a lower series resistance (193 Ω vs. 645 Ω for the control; Table S4) and a higher recombination resistance (3501 Ω vs. 1921 Ω for the control).

Fig. 5 (a) EIS Nyquist plots, (b) Mott–Schottky plots, and (c) dependence of V_oc on light intensity for the control and target PSCs. (d) Calculated SQ-limit FF, analytical FF, and experimental FF values for the control and target PSCs. (e) Space charge limited current curves for hole-only devices for the control and target perovskites films. (f) tDOS spectra for control and target PSCs.

Capacitance–voltage (C–V) measurements were further carried out to evaluate the built-in potential (V_bi) of the PSCs. Based on Mott–Schottky plots (Fig. 5b), the V_bi of the control device was determined to be 0.93 V, whereas the target device exhibited a V_bi of 0.96 V. This enhancement suggests that the NCA modification promotes more effective separation and transport of photogenerated charge carriers due to the strengthened internal electric field, thereby contributing to the improved V_oc of the PSC.

Fig. 5c illustrates the relationship between V_oc and light intensity, from which the ideality factor (n) was extracted based on the slope of the fitted line.⁴⁴ This parameter serves as an important indicator of the dominant recombination mechanism in PSCs. The target device exhibited an ideality factor of 1.25, which is significantly lower than the value of 1.33 observed in the control device. This reduction indicates that defect-assisted non-radiative recombination is effectively suppressed in the target device.

To further elucidate the enhancement in FF, we analyzed the non-radiative recombination loss and charge transport loss that cause the FF to deviate from the Shockley–Queisser (S–Q) limit. The theoretical maximum FF (FF_max) without charge transport losses was estimated using the equation: where V_oc = qV_oc/nk_BT, k_B is the Boltzmann constant (1.38 × 10⁻²³ J K⁻¹), T is the temperature (300 K), q is the elementary charge (1.602 × 10⁻¹⁹ C), and n is the ideality factor. The S–Q limit FF (FF_S–Q) of the PSC with a bandgap of 1.55 eV is 91%. The calculated FF_max values are 87.08% and 87.78% for the control and NCA-modified devices, respectively (Fig. 5d). The deviations of the experimental FF from the FF_S–Q are 4.43% for the control and 2.52% for the target device, indicating that the target device suffers less from non-radiative recombination and charge transport losses.

To probe the defect states in the devices, space charge limited current (SCLC) measurements were conducted using hole-only device structures (ITO/NiO_x/Me-2PACz/NCA/perovskite/spiro-OMeTAD/Ag). The trap-state density (N_t) was quantified using the equation: N_t = (2εε₀V_TFL)/(eL²), where ε₀, ε, V_TFL, e and L are the vacuum permittivity, relative dielectric constant, trap-filled limit voltage, elementary charge, and perovskite layer thickness, respectively. As shown in Fig. 5e, the V_TFL values for the control and target devices were 0.41 V and 0.27 V, respectively, corresponding to a significant reduction in trap density from 3.61 × 10¹⁵ to 2.57 × 10¹⁵ cm⁻³. This result suggests that the NCA layer effectively passivates trap states in the perovskite film.

Additionally, the trap density of states (tDOS) was evaluated for both control and target PSCs (Fig. 5f). The equation is as follows: , where q is the elementary charge (1.602 × 10⁻¹⁹ C), k_B is the Boltzmann constant (1.38 × 10⁻²³ J K⁻¹), T is the temperature (300 K), ω is the applied angular frequency, V_bi is the built-in potential from the Mott–Schottky plot, C is the capacitance, and W is the depletion width. The demarcation energy (E_ω) is calculated using attempt-to-escape frequency (ω₀) and ω with the equation: E_ω = k_BT ln(ω₀/ω).⁴⁵ Consistent with the SCLC results, the target PSC exhibited reduced tDOS in the energy region above 0.3 eV, corresponding to mid-gap and deep-level defects. This observation confirms the effectiveness of the NCA interfacial layer in defect control, suggesting enhanced device stability.⁴⁶

To evaluate the impact of the post-assembly NCA layer on the operational stability of PSC, a series of stability tests were performed. As shown in Fig. 6a, the unencapsulated PSCs were stored in an inert nitrogen atmosphere at room temperature, following the ISOS-D-1I standard.⁴⁷ After 2000 hours of dark storage, the target devices retained 98% of their initial average PCE, with minimal standard deviation. In contrast, the control PSCs retained a lower percentage of their initial efficiency and exhibited a broader distribution of values.

Fig. 6 Storage stability assessed according to the (a) ISOS-D-1I and (b) ISOS-D-2I protocols at 65 °C. (c) Long-term operational stability under continuous ultra-violet irradiation (365 nm, 50 mW cm⁻²). Normalized in situ PL spectra of the (d) control and (e) target perovskite films under continuous laser irradiation.

To evaluate thermal stability, we performed accelerated aging tests following the ISOS-D-2I protocol at 65 °C under a nitrogen atmosphere. As shown in Fig. 6b, the target devices demonstrated superior stability, retaining 83% of their initial power conversion efficiency after 1000 hours of continuous thermal stress. In contrast, control devices showed significantly faster degradation, maintaining only 69% of their original efficiency under identical conditions. This improved thermal endurance highlights the robust stabilizing effect of the NCA interlayer under elevated temperature conditions.

Considering the long-term exposure of PSCs to solar irradiation in real-world applications, we evaluated UV-induced stability.⁴⁸ As shown in Fig. 6c, devices were continuously illuminated with a UV lamp (365 nm, 50 mW cm⁻²), at a significantly higher intensity than the UV portion of the AM1.5G solar spectrum (∼4.6 mW cm⁻²).⁴⁹ Under this harsh UV exposure for 200 hours, the target PSCs retained 82% of their initial PCE, whereas the control devices showed significant performance degradation, retaining only 66% of their original efficiency. These findings demonstrate that the NCA layer substantially enhances the UV stability of the devices, which is crucial for future commercialization.

To gain deeper insight into the photostability of the perovskite films, we performed in situ PL spectroscopy under inert conditions using a high intensity UV laser (355 nm, ∼850 mW cm⁻²). As shown in Fig. 6d and e, the results reveal that the PL intensity of the control perovskite films continuously declined to 47%, indicating progressive photo-induced degradation. In contrast, the target films maintained 70% of their initial intensity during exposure, suggesting enhanced resistance to UV-induced perovskite film damage. Overall, these results confirm that the post-assembly NCA interlayer significantly enhances the thermal and UV stability of PSCs.

Conclusion

In summary, this study demonstrates the significant enhancement of inverted perovskite solar cells through the strategic, post-assembly of NCA atop the SAM films. The NCA interlayer markedly improves surface homogeneity and electrical contact properties. Crucially, the carboxylic acid functional groups of NCA effectively passivate interfacial defects and alleviate lattice strain in the perovskite layer. This synergistic optimization yields a substantial performance boost, elevating the highest PCE from 24.88% for the control device to 25.98% for the post-assembly NCA-modified device. Moreover, the NCA-modified devices exhibit superior operational stability, particularly under accelerated UV aging conditions. Our work highlights the post-assembly modification as a powerful method to significantly enhance the interfacial and electronic properties of SAM contacts.

Author contributions

Zhijie Gao: conceptualization, methodology, investigation, formal analysis, writing – original draft, writing – review & editing, visualization. Jia Kou: investigation, visualization, writing – original draft. Wenlei Lv, Yansheng Chen, Lingying Ren: investigation. Peng Huang: conceptualization, investigation, supervision, project administration, funding acquisition, writing – original draft, writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

The following are available in the supplementary information: detailed experimental procedures, XPS, SCLC, NMR, and FTIR measurements. See DOI: https://doi.org/10.1039/d5ta04393j.

Acknowledgements

This work was supported by the Sichuan Science and Technology Program of China (2024NSFSC0998). We would like to thank the Analytical and Testing Center of Southwest Jiaotong University for SEM analysis.

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Footnote

† These authors contributed equally to this work.

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