Reversible crosslinking strategy for dynamic strain regulation in inverted perovskite solar cells

Domain: physics
Published: 2026-03-17
Editorial outcome: major_revision

Abstract
Perovskite solar cells tend to degrade much faster under day-night cycling conditions. One key reason lies in the periodic strain in perovskite film under cycling mode due to its soft crystal lattice nature. Here, we propose a dynamic reversible crosslinking strategy by introducing disulfide-mediated cross-linkable additive of 2-methacryloyloxyethyl-thioctate. Combining with carbon-carbon double bond, 2-methacryloyloxyethyl-thioctate crosslinks at grain boundaries at high temperature through disulfide bond ring-opening, inhibiting perovskite expansion under day conditions. While at room temperature, 2-methacryloyloxyethyl-thioctate de-crosslinks with inverse ring-closing reaction, promoting perovskite recovery under night conditions. As a result, periodic film strain is alleviated in PSCs during day-night cycling aging. Resulting devices show high efficiency of 26.5% with good cycling stability, retaining 95.7% of initial efficiency after MPP tracking for 1,800 h under day-night cycling mode.

Introduction
Perovskite solar cells (PSCs) have developed rapidly over the past decade, and the power conversion efficiency (PCE) has exceeded 27%1. However, the long-term operational stability is still a big issue, especially under day-night cycling mode, since it simulates the real operating conditions of PSCs 2-5. Previous work has demonstrated that PSCs tend to degrade much faster under natural day-night cycles than under continuous illumination3. Due to the soft lattice and high thermal expansion coefficient (3.3~8.4*10-5 K-1)6, perovskite film will expanse under day (illumination) conditions, while it will shrink under night (dark) conditions. In comparison with continuous illumination, periodically repeated strain will appear in perovskite film under day-night cycling mode. These periodic strains will induce deep defects in perovskite and accelerate ions migration7-9, thus leading to severe device degradation. Importantly, such film stain stems from internal perovskite itself and cannot be avoided by external encapsulation.

Various methods have been reported to control film strain in PSCs, such as molecular doping10-13, grains linking14-18 and interfacial bonding19-23. Besides, directly strengthening perovskite lattice with organic molecules anchoring or buffer layer introduction are reported to regulate perovskite strain and increase device cyclic stability2,3. However, these methods mainly focus on how to inhibit residual strain generated during perovskite film preparation or at fixed conditions. While under day-night cycling model, different types of strain are generated dynamically, and the methods to inhibit film expansion may be useless to inhibit film shrinking. Therefore, dynamic strain regulation needs to be developed to realize stable PSCs under day-night cycling aging conditions.

Herein, we propose a dynamic reversible crosslinking strategy to regulate strain in perovskite under day-night cycles by introducing cross-linkable additive of 2-methacryloyloxyethyl-thioctate (MTA, Fig. 1a). Different to previous only-polymerizable disulfide-bonded molecules,24-26 MTA can dynamically switch its state between cross-linking and polymerization, inducing dynamic regulation of perovskite strains. During day-night cycles, MTA cross-links in perovskite films at day model through thermal-initiated ring-opening reaction and form a robust 3D polymer network to link neighboring grains, thus inhibiting perovskite expansion. While at night model, the cross-linked MTA de-crosslinks through ring-closing reaction due to the decreased temperature, forming a flexible linear polymer to promote perovskite recovery and alleviate related strain (Fig. 1b). As a result, periodic film strain is alleviated in PSCs during day-night cycling aging. Resulting PSCs achieve a high efficiency of 26.5% with good operational stability, retaining ~95.7% initial efficiency after maximum power point (MPP) tracking for 1,800 h under day-night cycling mode (85℃-light aging for 12 h followed by 25℃-dark aging for 12 h).

Results
Reversible Crosslinking Strategy

Fig. 1 shows the chemical structure of MTA. MTA contains three functional groups: C=O, C=C and five-membered ring with disulfide bond. 1) C=O groups help MTA anchoring at grain boundaries (transmission electron microscope in Supplementary Fig. 1) due to its coordination with Pb2+, which can be confirmed by C=O vibration shifts in Fourier transform infrared (FTIR, Fig. 1c) and Pb signal shift in X-Ray photoelectron spectroscopy (XPS, Supplementary Fig. 2). 2) C=C bond promote MTA to polymerize during perovskite annealing crystallization, which can be confirmed by the disappearance of C=C characteristic peak in FTIR at ~1,600 cm-1 (Fig. 1c) and Raman at 1,635 cm-1 (Supplementary Fig. 3) and 1H signal from C=C in nuclear magnetic resonance hydrogen spectroscopy (1H NMR, Supplementary Fig. 4). 3) Five-membered ring with disulfide bond can induce the further cross-linking of poly-MTA through ring-opening reaction as shown in Fig. 1a. The Raman single peak at ~500 cm-1 is from disulfide ring in perovskite with MTA at 25 ℃. While it will split into double peaks at ~496 and ~504 cm-1 at 85 ℃, indicating the formation of the S-S single bonds induced by ring opening reaction. Such disulfide bond ring-opening will promote MTA cross-linking in perovskite, forming robust 3D polymer network. Interestingly, the splitting double Raman peaks from S-S bonds will recover into single peak if putting cross-linked perovskite with MTA at 25 ℃ again (Fig. 1d, one hour for example), indicating the de-crosslinking of MTA through ring closing reaction. Such phenomenon is reversible when temperature changes between 25 and 85 ℃, demonstrating the dynamic crosslinking of MTA through reversible ring opening / closing reaction. Besides, UV-Vis absorption spectroscopy also shows reversible variation in MTA absorption between 338 nm (disulfide ring) and 263 nm (S-S single bond), when temperature repeatedly changes between 25 and 85 ℃ (Supplementary Fig. 5). Such reversible ring-opening or ring-closing of disulfide bonds has been widely studied in dynamic polymerization27-29. Additionally, the liquid state MTA will become solid at 120 ℃ and then slowly changes to gel-like material when cooling to room temperature, further confirming the crosslinking and de-crosslinking process of MTA (Supplementary Fig. 6).

Dynamic Regulation of Perovskite Strain

Periodical light-heat variation during day-night cycling aging will induce periodical film strain in perovskite due to its soft nature (Fig. 2a)3. Grazing-incidence X-ray diffraction (GIXRD) is used to study the strain variation in perovskite film on ITO substrate during day-night cycles. Here we use illumination at 85 ℃ to simulate the day condition and dark state at 25 ℃ to simulate the night condition. Film expansion or strain will induce the angle shift in GIXRD, which can be evaluated with the slop (k) of 2𝜃-sin2(Ψ) linear fitting. As shown in Fig. 2b, when switching from night to day condition, the k value in control perovskite will obviously decrease from -0.195 to -0.371, which can only be partially recovered to -0.295 if further switching to night condition again. That’s to say, after one night-day cycle, the k value in control perovskite at night condition is ~50% decreased (-0.195 vs -0.295), which should be induced by the accumulated strain since film expansion at night condition can be ignored. With cycle times increasing, the periodic strain will gradually accumulate, and the k value will further decrease from initial -0.195 to -0.402 after 3 night-day cycles as shown in Fig. 2b (Supplementary Fig. 7). In perovskite with MTA, the dynamic cross-linking and de-crosslinking process of MTA will effectively inhibit perovskite expansion or shrinking during night-day cycles. AFM-based modulus mapping indicates that perovskite with MTA shows larger modulus (6.56 GPa) at day condition than that at nigh condition (4.05 Gpa, Supplementary Fig. 8), which can inhibit perovskite expansion at day condition. As a result, much smaller k value variation is observed (-0.123 vs -0.182) during night to day switching. When further switching to night again, the k value will recover to -0.132 (much close to initial value of -0.123), indicating that film strain in perovskite with MTA will be well released at night (Fig. 2c). Just due to the inhibited expansion and strain, perovskite with MTA shows much smaller k value fluctuation and better recovery than control perovskite during night-day cycles (Fig. 2d, Supplementary Fig. 9). This result indicates that the dynamic reversible cross-linking of MTA can indeed alleviate film expansion and strain in perovskite during day-night cycles, which is beneficial to perovskite stability. To further confirm the effect of reversible cross-linking on periodic film strain, an irreversible cross-linking additive of TMTA15 containing 3 C=C bonds is introduced as reference. Due to the robust 3D network structure, cross-linked TMTA in perovskite will inhibit film expansion with moderate k value variation (-0.169 vs -0.259) during night to day switching (Supplementary Fig. 10). However, cross-linked TMTA may block the perovskite recovery from already expanded state to initial state due to its robust nature. As a result, the expanded perovskite will not be well recovered when further switching to initial night condition, and ~40% of the expanded strain still retains in perovskite with cross-linked TMTA since the k value is only partially recovered from -0.259 to -0. 227. TEM is used to further study the strain in perovskite films. As shown in Fig. 2e, fresh perovskite shows uniform lattice fringe with lattice spacing of ~2.85 Å. After day-night aging (5 cycles), control perovskite shows enlarged but uneven lattice spacing ranging from 3.03 Å to 3.12 Å. Besides, some distortion or bending lattice fringes also appears in aged control perovskite (Fig. 2f), indicating the unignorable existence of lattice strain. While in aged perovskite with MTA, lattice fringes remain at ~2.85 Å uniformly (Fig. 2g), which may be induced by the release of strain with reversible cross-linking of MTA.

Perovskite Evolution under Day-Night Cycling Aging

Periodic film strains induce deep traps in perovskite during day-night cycling aging7,9. Transient photovoltage (TPV) and transient photocurrent (TPC) are used to study the traps generation in PSCs under cycling aging. As shown in Fig. 3a-c, control PSCs show a significantly reduced TPV response lifetime before and after aging for 7 or 15 cycles (from initial 1.12 μs to 0.95 μs or 0.82 μs), indicating the gradually increased trap-assisted recombination in PSCs. While in target PSCs, the periodic strain induced traps will be effectively inhibited due to the dynamic cross-linking of MTA. As a result, almost no change is observed in TPV lifetime before and after aging for 7 or 15 cycles (from initial 1.28 μs to 1.25 μs or 1.22 μs). Furthermore, the TPC response lifetime also shows little variation in target PSCs before and after aging for 7 or 15 cycles (from initial 0.85 μs to 0.87 or 0.89 μs, Fig. 3d-f), further demonstrating the inhibited strain-related traps. However, in control PSCs, TPC lifetime increases dramatically from 1.02 μs to 1.17 μs and 1.39 μs, indicating the gradually degraded carrier extraction. Additionally, trap state density (tDOS) is calculated to study the traps distribution in perovskite before and after day-night cycling aging though capacitance-voltage (C-V) and capacitance-frequency (C-F) fitting. As shown in Fig. 3g-i and Supplementary Fig. 11-12, PSCs usually show two trap regions: one centered at about 0.32 eV (shallow trap region: trap band I) and another centred at about 0.61 eV (deep trap region: trap band II). In PSCs before and after day-night cycling, the most obvious variation appears at trap band II region. In control PSCs, the trap density in perovskite is increased by ~40% from initial 4.45×1015 cm-3 eV-1 to 6.32×1015 cm-3 eV-1 after 7 cycles aging, and increased by ~90% after 15 cycles aging (8.61×1015 cm-3 eV-1), indicating the re-generation of deep traps in perovskite during day-night cycling aging. However, in target PSCs, almost no change is observed, and the trap density remains among 4.11~5.01×1015 cm-3 eV-1, This result further demonstrates that MTA can indeed inhibit the generation of periodic strain-induced deep traps through dynamic reversible crosslinking, agreeing with the results from TPC and TPV.

Activation energy (Ea) of ions migration is obtained from conductivity dependence on temperature in perovskite films30. As shown in Fig. 4a, The Ea of control perovskite decreases from 0.21 eV to 0.17 eV before and after for 15 cycles day-night aging, indicating that periodic strains will accelerate ion migration in PSCs. While in target perovskite, MTA can effectively alleviate the periodic strain and thus inhibit ion migration through reversible cross-linking reaction. As a result, almost no Ea change appears in target perovskite with MTA (0.24 eV vs 0.23 eV) before and after cycling aging (Fig. 4b). Due to the decreased Ea in perovskite after day-night cycling, iodine ions are more likely to migrate and react with the Cu electrode in control PSCs, which will further accelerate device degradation31-33. Cross-sectional scanning electron microscopy with energy-dispersive x-ray (SEM-EDX) is conducted to study the ion migration in aged PSCs under day-night cycles (perovskite morphology in Supplementary Fig. 13). As shown in Fig. 4c, iodine elements have surpassed the Pb regions in control PSCs, indicating inevitable iodine migration during cycling aging. In addition, element Cu elements have also diffused into the perovskite region in aged control PSCs, which should be induced by the iodine migration and its reaction with Cu electrode during device aging. However, in target PSCs, the ions migration is greatly inhibited and almost no iodine or Cu diffusion is observed (Fig. 4d).

Device Performance

Inverted PSCs are fabricated with configuration of ITO/Meo-2PACz/Perovskite/PDAI2/PEACl/PCBM/C60/TPBi/Cu (Fig. 5a). As shown in Fig. 5b, control PSCs show moderate PCE of 24.4% with short-circuit current density (JSC) of 26.1 mA cm−2, open-circuit voltage (VOC) of 1.14 V, and fill factor (FF) of 81.8 %. While in PSCs with MTA, device VOC and FF are greatly improved to 1.19 V and 85.1%, resulting in high PCE of 26.5% (J-V hysteresis in Supplementary Fig. 14). The improved performance can be ascribed to the defect passivation of C=O groups in MTA as confirmed previously34,35 and the inhibited lattice strain in perovskite as confirmed in Fig. 2. The stabilized power output reaches 26.0% (Fig. 5c). PSCs with MTA also show good reproducibility, with an average PCE of 26.1% for 10 independent devices (Supplementary Fig. 15). The integrated current from external quantum efficiency (EQE) is 25.3 mA cm-2 in target PSCs, which is close to the J-V measurement (Supplementary Fig. 16).

Long-term operational stability is a big challenge for PSCs under day-night cycling mode, since it simulates the real operating conditions and cannot be avoided by encapsulation. Generally, perovskites suffer periodic expansion and shrinking during day-night cycles, inducing inevitable deep traps as confirmed in Fig. 3 and accelerated ions migration as confirmed in Fig. 4. As a result, PSCs tend to degrade rapidly under day-night cycling MPP tracking, losing ~50% of initial efficiency merely within 430 h (Fig 5d and Supplementary Fig. 17, stability data is obtained from 3 separated devices). In PSCs with MTA, MTA cross-links into robust 3D network in day conditions through disulfide ring-opening reaction, thus inhibiting lattice expansion at high temperature. While in night condition, the cross-linked MTA de-crosslinks into flexible linear polymer, thus promoting lattice recovery at low temperature. Therefore, periodic lattice strain will be effectively alleviated in perovskite with MTA when aging under day-night cycles. Raman spectra, XPS and cyclic voltammetry (CV) tests confirm that the possible redox property of MTA shows ignorable effect on perovskite stability under day-night cycles (Supplementary Fig. 18). In contrary, target PSCs with MTA show good stability, retaining 95.7% of initial efficiency after MPP tracking for 1,800 h under day-night cycling mode (Supplementary Fig. 19, 85℃-light aging for 12 h followed by 25℃-dark aging for 12 h). Besides, we also fabricate PSCs with thioctic acid (TA), which contains similar disulfide ring with MTA but cannot cross-link due to the absence of C=C. PSCs with TA show similar rapid degradation behavior with control devices under day-night cycles (Supplementary Fig. 20), further confirming the effectiveness of dynamic crosslinking with MTA

Methods
Materials

Lead iodide (PbI2), Lead chloride (PbCl2), Caesium iodide (CsI), Methylammonium iodide (MAI), Methylammonium chloride (MACl), Formamidinium iodid (FAI), 1,3,5-tris (1-phenyl-1H-benzimidazol-2-yl) benzene (TPBi), 2-Phenylethylamine hydrochloride (PEACl), 6,6-Phenyl C61-butyric acid methyl ester (PCBM) and C60 are purchased from Xi’an Polymer Light Technology Corp. [2-(3,6-Dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (MeO-2PACz), 2-Methacryloyloxyethyl Thioctate (MTA) and 1,3-Diaminopropane Dihydroiodide (PDAI2) are bought from TCl. Isopropanol (IPA), Dimethylformamide (DMF) and Dimethyl sulfoxide (DMSO) are bought from Sigma-Aldrich. Poly[3-(4-carboxylatebutyl) thiophene] (P3CT) is purchased from the Rieke Metals. P3CT-N is synthesized through the reaction of 20 mg P3CT with 50 μL CH3NH2 in 1950 μL methanol solvent at 60 ℃ under stirring overnight.

Device fabrication

ITO substrates (20 mm × 20 mm) are cleaned with detergent, distilled water, acetone and isopropanol. The cleaned substrates are dried using N2 flow and treated with UV-Ozone for 20 min in air. MeO-2PACz solution is deposited on the ITO substrate by spin-coating at 3,000 rpm for 30 s and then annealed at 100 °C for 10 min. 1.3 M Cs0.05FA0.9MA0.05PbI3 perovskite precursor solution in DMF: DMSO (4:1) solvent is spin-coated on MeO-2PACz at 2,000 rpm for 10 s and 4,000 rpm for the 20 s. During spin-coating, 150 μL chlorobenzene (CB) is dropped on the substrate 12 s before the end of the second program. For the preparation of modified films, different concentrations of MTA are added into perovskite precursor solution. All perovskite films are annealed at 120°C for 20 min. PDAI2 and PEACl are deposited on the perovskite layer at 4,000 rpm for 30s and then anneal at 100 °C for 10 min. PCBM is deposited on the passivation layer at 2,000 rpm for 45s. The substrate is then transferred to a vacuum chamber. C60 (40 nm), TPBi (8 nm), and Cu (100 nm) are deposited sequentially by thermal evaporation. The device area is 0.09 cm2 and a mask of 0.0836 cm2 is used during I-V test.

Stability measurement.

For stability test, P3CT-N is used to replace MeO-2PACz as hole transport material, and MA-free Cs0.05FA0.95PbI3 is used as active layer. After perovskite crystallization, C60 (50 nm), ALD-SnOx (250 cycles) and Cu (100 nm) are sequentially deposited. The unencapsulated device is tested in N2 atmosphere at AM 1.5 G illumination in cyclic mode (85℃-light aging for 12 h followed by 25℃-dark aging for 12 h).

Characterization and measurements

The J-V curves are obtained under a solar simulator (Enlitech) using a Keithley 2,400 digital source meter under simulated AM 1.5 G illumination (100 mW cm−2). The J-V curves are all measured at a scan rate of 0.015 V s−1 under a forward scan from 1.25 to -0.2 V in a glove box at room temperature. The EQE measurement is carried out in ambient air using a QE system (SOFN Instruments Co., ltd) with monochromatic light focused on a device pixel. Fourier transform infrared spectroscopy (FTIR) is recorded in transmittance mode using IR spectrometer instrument (Thermo, Nicolet 6,700). X-ray photoelectron spectroscopy (XPS) measurement is carried out using Kratos AXIS ULTRA DALD XPS system. Raman spectroscopy (Raman) is measured by Bruclaman spectrometer, the light source is green. Scanning electron microscope (SEM) images and EDX are recorded using field emission scanning electron microscope (Zeiss GeminiSEM450) with an accelerating voltage of 10 kV. Capacitance-voltage spectra (C-V), Capacitance-frequency spectra (C-F), Transient photovoltage (TPV) and Transient photocurrents (TPC) are obtained on an all-in-one characterization platform paios developed by Fluxim AG, Switzerland in the laboratory of Zhipeng Kan at Guangxi University. The AM 1.5 G illumination is achieved with a Xenon lamp. Activation energy of ions migration (Ea) is recorded using semiconductor characterization system (Keithley 4,200-SCS). During the measurement, we first cool the devices to 173 K for 1 h and then heat to objective temperature.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have adequately addressed all reviewer comments, and the manuscript is now suitable for acceptance.", "reviewer_3": "Reviewer 3 - Comment 2\nThe author’s response does not adequately address the reviewer’s core concern.\nThe reviewer pointed out that lattice expansion and strain are distinct physical concepts and that temperature-dependent\nGIXRD alone cannot unambiguously distinguish between (1) reduced free thermal expansion and (2) reduced constraint-\ninduced strain. The authors’ response reiterates basic thermal-mismatch arguments—i.e., that perovskite films expand more\nthan ITO and thus experience strain—but this does not resolve the reviewer’s central issue.\nFirst, the authors do not explain how their measurements separate “less expansion” from “less strain.” They restate that ITO\nconstrains the film, but this is already known and does not demonstrate that the measured change is uniquely attributable to\nstrain relief.\nSecond, the authors’ justification that “GIXRD is widely used to study strain” is not sufficient. The reviewer’s point was not\nwhether GIXRD can detect strain, but whether temperature-dependent peak shifts can distinguish between intrinsic thermal\nexpansion and constraint-induced strain. This distinction remains unaddressed.\nThird, the revision to terminology (“lattice expansion” → “film expansion”) does not solve the mechanistic ambiguity. The\nreviewer’s concern was conceptual, not semantic. Changing wording does not clarify whether the interpretation of strain\nrelief is justified by the data.\nOverall, the response remains incomplete and does not convincingly resolve the mechanistic issue raised by the reviewer.\nThe response does not acknowledge or address the possibility raised by the reviewer—that MTA could simply make the film\nstiffer, resulting in reduced expansion rather than reduced strain. A more appropriate reply would explicitly acknowledge the\nlimitation of the current data and revise the claims accordingly or provide additional experimental evidence capable of\ndistinguishing between the two phenomena.\nReviewer 3 - Comment 3\nThe author’s response partially addresses the question but ultimately does not satisfactorily resolve the reviewer’s concern\nabout the redox activity of MTA.\nAlthough the authors conducted simple tests (UV–Vis of MTA + I₂ in chloroform; XRD of Pb film or I₂ powder + MTA in\nsealed containers), these experiments are not sufficient to rule out redox processes under realistic perovskite device\nconditions. Several issues remain as follows.\nFirst, the experimental environment does not reflect actual perovskite film conditions. The redox behavior of disulfide\ncompounds can differ drastically in the presence of halides, organic cations, moisture, light exposure, or under solid-film\nconfinement. Conducting tests in chloroform or with bulk Pb metal does not accurately replicate the perovskite environment.\nSecond, the analytical methods used are not sensitive enough. UV–Vis is not adequate to detect small changes in I₂/I⁻\nbalance. XRD is insensitive to surface-level oxidation or reduction of Pb⁰ (which may occur only at nanometer scale). More\nappropriate techniques (e.g., XPS, Raman, CV, or in-situ spectroscopies) would be needed to rule out redox activity.\nThird, the response does not address whether MTA may influence perovskite stability through redox chemistry inside the\ndevice. The reviewer’s concern was that the performance enhancement attributed to strain regulation might instead be due\nto a chemical stabilization mechanism. The author’s reply does not convincingly eliminate this possibility.\nIn summary, although the authors attempted to address the question, the provided experiments are not rigorous enough to\nexclude the possibility of redox-related effects. The statement “we will study it in future” is not sufficient for a mechanistic\nconcern in the current manuscript. The reviewer’s concern remains largely unresolved.\nThe responses to the other comments seem appropriate. It will be a better study if the responses to the above two comments\nare appropriately resolved."}, "author_response": "Reviewer #1\nThe authors have adequately addressed all reviewer comments, and the manuscript is now suitable for\nacceptance.\nResponse: Thanks for your support of our work.\nReviewer 3\nReviewer 3 - Comment 2:\nThe author’s response does not adequately address the reviewer’s core concern.\nThe reviewer pointed out that lattice expansion and strain are distinct physical concepts and that\ntemperature-dependent GIXRD alone cannot unambiguously distinguish between (1) reduced free\nthermal expansion and (2) reduced constraint-induced strain. The authors’ response reiterates basic\nthermal-mismatch arguments—i.e., that perovskite films expand more than ITO and thus experience\nstrain—but this does not resolve the reviewer’s central issue.\nFirst, the authors do not explain how their measurements separate “less expansion” from “less strain.”\nThey restate that ITO constrains the film, but this is already known and does not demonstrate that the\nmeasured change is uniquely attributable to strain relief.\nSecond, the authors’ justification that “GIXRD is widely used to study strain” is not sufficient. The\nreviewer’s point was not whether GIXRD can detect strain, but whether temperature-dependent peak\nshifts can distinguish between intrinsic thermal expansion and constraint-induced strain. This\ndistinction remains unaddressed.\nThird, the revision to terminology (“lattice expansion” → “film expansion”) does not solve the\nmechanistic ambiguity. The reviewer’s concern was conceptual, not semantic. Changing wording does\nnot clarify whether the interpretation of strain relief is justified by the data.\nOverall, the response remains incomplete and does not convincingly resolve the mechanistic issue\nraised by the reviewer. The response does not acknowledge or address the possibility raised by the\nreviewer—that MTA could simply make the film stiffer, resulting in reduced expansion rather than\nreduced strain. A more appropriate reply would explicitly acknowledge the limitation of the current\ndata and revise the claims accordingly or provide additional experimental evidence capable of\ndistinguishing between the two phenomena.\nResponse: Thanks for your comment. We feel sorry that we may misunderstand the comment from\nreviewer of first-round review. And now we may clearly understand the concern from reviewers after\nReviewer 3’s explanation. Thanks again for the kind comment.\nIndeed, thermal expansion and constraint-induced strain are two different concepts, and temperature-\ndependent GIXRD may not distinguish them clearly. In revised manuscript, we explicitly admit this\npoint in related description about GIXRD data. Free expansion and lattice strain may both contribute\nthe shift of diffraction and the slop (k) of 2𝜃-sin2(Ψ), especially under day condition (high\ntemperature). The AFM-based Young's modulus mapping (shown below) also confirms that perovskite\nwith cross-linked MTA under day condition show larger film modulus, which may inhibit perovskite\nfree expansion.\nBesides inhibited expansion, the lattice strain may also be inhibited with the introduction of MTA.\nAfter day-night cycles, control perovskite at night condition (without thermal-induced expansion) also\nshows larger absolute k value (diffraction shift) than perovskite with MTA as shown below, indicating\nthe inhibited film strain in target perovskite films. In addition, in comparison with fresh perovskite at\nnight, control perovskite at night still shows enlarged absolute k value after day-night cycles (Fig. 2d),\nwhich should be induced by the accumulated strain since film expansion at night condition can be\nignored. This result indicates that strain indeed exist in control perovskite during day-night cycles.\nTo further confirm the strain in perovskite films, Transmission electron microscope (TEM) is\nconducted in aged perovskite after day-night cycles. In TEM test, free expansion usually appears with\nuniform increase in lattice spacing, whereas film strain appears with the bending or distortion of lattice\nfringes due to the uneven changes in lattice spaceing. As shown below, fresh perovskite before aging\nshows uniform lattice fringe with lattice spacing of ~2.85 Å. After day-night aging, control perovskite\nshows enlarged but uneven lattice sapcing ranging from 3.03 Å to 3.12 Å. Besides, some distortion or\nbending lattice fringes also appear in aged control perovskite, indicating the unignorable existence of\nlattice strain. While in aged perovskite with MTA, lattice fringes remain at ~2.85 Å uniformly, which\nmay be induced by the release of strain with reversible cross-linking of MTA. The results from TEM\nindicate that lattice strain can indeed appears in perovskite films during day-night aging, and that\nMTA introduction can effectively reduce such lattice strain.\nReviewer 3 - Comment 3\nThe author’s response partially addresses the question but ultimately does not satisfactorily resolve the\nreviewer’s concern about the redox activity of MTA.\nAlthough the authors conducted simple tests (UV–Vis of MTA + I₂ in chloroform; XRD of Pb film or\nI₂ powder + MTA in sealed containers), these experiments are not sufficient to rule out redox\nprocesses under realistic perovskite device conditions. Several issues remain as follows.\nFirst, the experimental environment does not reflect actual perovskite film conditions. The redox\nbehavior of disulfide compounds can differ drastically in the presence of halides, organic cations,\nmoisture, light exposure, or under solid-film confinement. Conducting tests in chloroform or with bulk\nPb metal does not accurately replicate the perovskite environment.\nSecond, the analytical methods used are not sensitive enough. UV–Vis is not adequate to detect small\nchanges in I₂/I⁻ balance. XRD is insensitive to surface-level oxidation or reduction of Pb⁰ (which may\noccur only at nanometer scale). More appropriate techniques (e.g., XPS, Raman, CV, or in-situ\nspectroscopies) would be needed to rule out redox activity.\nThird, the response does not address whether MTA may influence perovskite stability through redox\nchemistry inside the device. The reviewer’s concern was that the performance enhancement attributed\nto strain regulation might instead be due to a chemical stabilization mechanism. The author’s reply\ndoes not convincingly eliminate this possibility.\nIn summary, although the authors attempted to address the question, the provided experiments are not\nrigorous enough to exclude the possibility of redox-related effects. The statement “we will study it in future” is not sufficient for a mechanistic concern in the current manuscript. The reviewer’s concern\nremains largely unresolved.\nResponse: Thanks for your comment. According to the recommendation, MTA’s potential redox\nchemistry in perovskite films is in-depth with Raman, cyclic voltammetry (CV) and XPS test. In\nRaman spectra, perovskite with extra I is used as reference to confirm the Raman peaks of I . As\n2 2\nshown below, I characteristic Raman signal appears at ~168 and 202 cm-1, while Pb-I signal from\n2\nperovskite appears at ~97 cm-1. In fresh perovskite with MTA (target), no I2 Raman signal exists.\nHowever, after 85℃ aging for 100 h, Raman signal of I will appear in targe perovskite, indicating\n2\nthat MTA may not well reduce I generated during perovskite aging.\nMTA. During our CV test, metalic Pb film on glass, Pt and Ag/AgCl electrode are used as working,\ncounter and reference electrodes respectively (shown below). NaClO aqueous solution (0.1 M) is used\n4\nas electrolyte. As shown below, Pb film shows reduction potential at -0.33 V. When adding MTA into\nelectrolyte solution, the reduction potential of Pb may change if MTA can oxidize Pb films. However,\nafter adding MTA, almost no change happens in the reduction potential of Pb films (~0.33 V),\nindicating that MTA may not well oxidize metallic Pb films.\nXPS is also conducted to study the MTA’s reaction in perovskite films under day-night cycling aging.\nIf MTA can indeed oxidize metallic Pb0 in perovskite films, Pb0 signal may not appear in aged\nperovskite films, and the S signal from MTA will change or shift after perovskite aging. In contrary,\nXPS test shows that Pb0 signal still appears at 141.54 eV and 136.75 eV in perovskite with MTA after\naging for 5 cycles. In addition, almost no changes happen in the S signal, and its XPS peaks remain at\n163.14 eV and 164.31 eV after aging for 5 cycles. These results indicate that MTA’s redox reaction\nmay not dominate in perovskite films during aging.\nFinally, to further study the possible effect of MTA’s redox reaction on device stability, we fabricate\nextra PSCs by using thioctic acid (TA, shown below) to repalce MTA. In comparison with MTA, TA\ncontains the same disulfide ring, which is the mian group for the possilbe redox function of MTA.\nHowever, TA contains no C=C, and cannot cross-link under thermal contitions (day condition). That’s\nto say, TA owns very similar redox ability with MTA, but cannot dynamically crosslink for strain\nrelease. As shown below, after day-night cycling aging for 300 h, control PSCs degrades rapidly, and\nonly retians 67.6% of initial efficiency. PSCs with TA show similar rapid degradation behaviour,\nmerely retaining 76.1% of initial efficiency. While PSCs with MTA show greatly improved stabiltiy\nand retain 99.8% of initial efficiency. This resutlt further confirms that dynamic crosslinking, rather\nthan redox reaction, is the core factor for stabilty improvement in PSCs with MTA."}, {"round": 2, "reviewer_comments": {"reviewer_3": "All concerns raised during the review process have been properly resolved, and the manuscript, in its current form, merits\nacceptance."}, "author_response": ""}]

Decision letter