Violet/ultraviolet light-induced depassivation in halide perovskite solar cells

Domain: physics
Published: 2025-12-17
Editorial outcome: minor_revision

Abstract
Ammonium-terminated ligands, used directly or in the form of two-dimensional perovskites, are leading defect-passivating agents for halide perovskites and have significantly contributed to achieving the highest efficiencies across diverse perovskite solar cells. However, even these state-of-the-art perovskite solar cells suffer from rapid degradation during operation, raising concerns over the durability of the passivation. In this work, we unveil a mechanism of violet/ultraviolet light-induced depassivation that universally affects ammonium ligands. Exposure to violet/ultraviolet light triggers a charge-carrier transition from ammonium-terminated ligands to the halide perovskite framework, leading to the deprotonation of the ammonium group. This deprotonation disrupts the ammonium-perovskite interaction, resulting in depassivated perovskites susceptible to photodegradation, as validated in cells with 26.44% efficiency. This updated understanding surpasses existing ligand failure models limited to thermal intolerance and iodide oxidation, highlighting an essential perspective for enhancing the long-term efficacy of passivating agents.

Introduction
Halide perovskite (HP) solar cells have garnered significant research interest over the past decade, achieving a remarkable power conversion efficiency (PCE) of 27.0% in single-junction, small-area configurations1. One key factor in this improvement is the effective passivation of defects. Although defects in HPs are relatively benign compared to those in traditional semiconductors, they still considerably limit the photovoltaic performance2,3. Significantly, defects also drive degradation by facilitating ion migration4, environmental erosion5, and phase transition6. To combat these issues, passivating agents that interact with and neutralize defects are essential.

Ammonium-terminated ligands (R-NH3+, R represents alkyl or aryl groups) have been the leading defect-passivating agents. They can serve directly as passivating agents themselves, acting as Lewis acids to neutralize negatively charged defects such as common uncoordinated halogen anions and Pb-I anti-sites, through hydrogen bonding and Coulomb interactions7,8. Alternatively, they are widely used to generate two-dimensional (2D) HP derivatives, which offer passivation and affect carrier extraction at the 3D/2D HP interface9–11. Merits like flexible molecular structure12, strong chemical affinity for HPs13, broad environmental adaptability across HPs with diverse compositions, sizes, dimensionalities, and bandgaps14–17, as well as excellent reproducibility in laboratories worldwide, set ammonium ligands apart. They have contributed to realizing the highest PCEs in various solar cell types, including (lead HP) p-i-n cells18, (lead HP) n-i-p cells19, tin-lead HP cells20, pure-tin HP cells21, all-HP tandem cells22, HP/silicon tandem cells17, HP/organic tandem cells23, HP/CuInSe2 tandem cells24, large-area HP solar modules (715.1 cm²)25, indoor HP photovoltaics26, and air-fabricated HP cells27.

However, contradictory to the excellent defect passivation by ammonium ligands, rapid degradation during operation remains a critical challenge for these state-of-the-art HP cells, which raises concerns about the durability of ammonium ligands. Increasing attention is paid to studying the depassivation of ammonium ligands. Meng et al. showed that deprotonation of organic cations can occur in the precursor stage with the oxidation of iodide ions28. Wang et al. reported that ammonium ligands with a low acid dissociation constant can be thermal-intolerant and might fail passivation under high-temperature light soaking29. The poor thermal/light tolerance of typical ammonium ligands was also confirmed by Pei et al.30. However, the depassivation mechanisms of ammonium ligands are still insufficiently understood and warrant further exploration.

In this work, we reveal a violet/ultraviolet (UV) light-driven depassivation mechanism that universally affects ammonium-terminated ligands. Contrary to the common belief that these ligands play a negligible role in photogenerated carrier dynamics, we find that violet/UV photons induce an electron transition from the ammonium group to the HP framework, which subsequently causes the deprotonation of the ammonium group. This deprotonation disrupts the ammonium-HP interaction and leads to depassivated HP susceptible to photodegradation, as validated in cells with 26.44% PCE. These results highlight the vulnerability of the prevailing passivation strategy with ammonium ligands toward sunlight, providing crucial insights into the development of more robust solutions.

Results
Ligand-HP interfacial carrier dynamics
To effectively investigate the proposed mechanism, a sample with a high density of well-ordered ammonium-HP bonds is essential. The ammonium-passivated 3D HP system is not an ideal platform due to the sparse and disordered distribution of the ligands on the 3D HP surface31. Instead, we utilized Ruddlesden-Popper 2D HPs with the formula L2PbI4, where L represents ammonium-terminated ligands. As exemplified by BA2PbI4 (BA denotes butylammonium, Fig. 1A), the -NH3+ head of the BA ligands occupies positions typically held by cesium, methylammonium, or formamidinium in 3D HPs, connecting to the HP framework via the (-NH3+)-I- bonding in the same manner as in ammonium-passivated 3D HPs32. The BA ligands assemble orderly on the two sides of the HP framework, creating atomically dense and organized ammonium-HP connections to make L2PbI4 an ideal sample for mechanistic exploration. Moreover, the well-defined ammonium-HP interface enables a clear electronic structure that facilitates characterization.

Notably, in L2PbI4, both the conduction band minimum (CBM) and valence band maximum (VBM) arise from the inorganic HP sublattice33. In contrast, ammonium-terminated ligands like BA act as electronic barriers, confining charge carriers within the HP sublattice and forming quantum wells. Therefore, these ligands have been considered to have negligible participation in photogenerated carrier dynamics34. However, our experimental findings deviate from this interpretation. To characterize the carrier dynamics of a BA2PbI4 film, broadband ultrafast transient absorption (TA) spectroscopy was employed, using a pump energy of 3.82 eV (325 nm). The well-defined structure of BA2PbI4 is confirmed by the X-ray diffraction (XRD) pattern (Fig. S1), the steady-state absorption spectrum (Fig. S2), and the photoluminescence (PL) spectrum (Fig. S2). As shown in Fig. 1B, three distinct photobleaching (PB) signals can be identified. The PB signal in the range of 2.43–2.57 eV corresponds to the inherent excitonic absorption of BA2PbI4, resulting from transitions between the VBM and the exciton state (Ex)35. However, the additional two PB signals observed in the near-UV range of 2.90–3.10 eV and 3.20–3.40 eV are rarely reported, and their origin remains unclear, warranting further investigation.

We first rule out the possibility that the near-UV PB signals arise from raw reagents, i.e., PbI2 or BAI, as their absorption peak positions do not match the observed PB signals (Fig. S3)36. Furthermore, we conducted TA characterization on a series of 2D HP films made with various ammonium-terminated ligands, including hexylammonium (HA), octylammonium (OA), dodecylammonium (DA), phenylethylammonium (PEA), and phenylpropylammonium (PPA). These ligands feature aliphatic and aromatic side groups with varying chain lengths. The results reveal that the near-UV PB signals are not unique to BA2PbI4 but are a universal feature across these 2D HP systems (Figs. S4–S9). The XRD patterns (Fig. S4) suggest that some of these 2D HP films may contain minimal unidentified impurities, but these impurities are inconsistent across all films. Other 2D HPs such as PPA2PbI4 and OA2PbI4 are highly phase-pure with no discernible impurities. These observations indicate that the near-UV PB signals do not arise from specific impurities or 2D HP/impurity heterojunctions. Therefore, we infer that the near-UV PB signals originate directly from the 2D HPs.

The lower-energy near-UV PB signal’s change in absorbance (ΔA) is comparable to that of the excitonic PB signal, indicating significant transition strength, while the higher-energy PB signal’s ΔA is weaker. Notably, we notice a subtle but discernible correlation between the lower-energy near-UV PB signal and the excitonic PB signal. The initial decay of the near-UV PB signal coincides with a slight rise of the excitonic PB signal (Fig. 1C). This suggests a charge transfer process from certain higher energy states to the VBM or the Ex. Therefore, the near-UV PB signal is unlikely to stem from excited-state absorption or the Auger process, as both mechanisms promote transitions to high-energy levels by depleting excited-state carriers, which would result in a reversed rise-decay relationship. Furthermore, even upon quasi-resonance excitation for the excitonic absorption (using pumping light at 2.48 eV, 500 nm), the near-UV PB signals featuring higher energies are detected as anti-Stokes PB signals across all the 2D HPs (Fig. S10). Thus, the near-UV PB signals could arise from carrier transitions involving the VBM or Ex. The observation also excludes the scenario that the near-UV PB signals are from hot carriers. Otherwise, it should be unobservable upon the lower-energy excitation.

To better understand the origin of the near-UV PB signal, we quantitatively depict the electronic structure of BA2PbI4 with a highlight of key energy levels, including the CBM, Ex, VBM, lowest unoccupied molecular orbital (LUMO) of the ligand layer, and highest occupied molecular orbital (HOMO) of the ligand layer, in Fig. 1D. The VBM is measured with ultraviolet photoelectron spectroscopy (UPS) characterization (Fig. S11). The obtained VBM (−5.82 eV) and the accordingly inferred CBM based on a bandgap of 2.7 eV agree with the previous report by Silver et al37. Ex is determined per the excitonic absorption energy (Fig. S2, 2.41 eV). The HOMO and LUMO also refer to the report by Silver et al. obtained with density functional theory (DFT) calculations37. Based on the energy diagram, the energies of the two near-PB signals (2.9–3.1 eV for the low-energy one and 3.2–3.4 eV for the higher-energy one) closely match those of the HOMO-to-Ex (3.1 eV) and HOMO-to-CBM (3.4 eV) carrier transitions, respectively. The minor deviations are reasonable considering that the DFT calculation adopted for the HOMO in the previous report usually overvalues the result. A supportive piece of evidence might be the emergence of the onset of a new band in the Fermi region of the UPS spectrum at approximately 0.43 eV below the VBM (Fig. S11), which can tentatively be attributed to the HOMO. As such, the true HOMO might be within the range of −6.25 eV (−5.82 eV minus 0.43 eV) to −6.50 eV versus the vacuum level, so the HOMO-Ex gap and the HOMO-CBM gap could be in the range of 2.85–3.10 eV and 3.15–3.40 eV, justifiably the above deviations. Notably, the HOMO-to-Ex and HOMO-to-CBM transitions are also consistent with the above observation of anti-Stokes PB signals.

The HOMO-to-Ex/CBM transitions will generate a dipole at the ligand-HP interface due to the separation of electrons and holes. To investigate the formation of this interfacial dipole, we performed steady-state optoelectrical characterizations based on the following perspective: violet/UV photons have a limited penetration depth in BA2PbI4 films, approximately 113 nm for 400 nm photons38. Therefore, in a thick film where violet/UV photons cannot penetrate the entire thickness, photoexcitation occurs near the illuminated front surface but not at the unilluminated back surface. If photoexcitation induces the HOMO-to-Ex/CBM transition, dipoles will exist at the ligand-HP interface in the illuminated front, while the non-illuminated back will lack these dipoles. This creates a global potential difference (ΔV) across the thick film, as illustrated in Fig. 2A. Conversely, in a thin film where violet/UV photons can penetrate its entire thickness, photoexcitation occurs throughout the film, so this ΔV should be absent.

We then employed a thick film of ~337 nm and a thin film of ~29.5 nm (Fig. S12) for comparison. The BA2PbI4 films were tested in a vertical ITO/BA2PbI4/Cu device configuration. The indium tin oxide (ITO) and copper (Cu) electrodes have similar work functions (ITO in the range of 4.6–4.75 eV and Cu at 4.7 eV), rendering energetic symmetry across the device. The BA2PbI4 films exhibit an out-of-plane orientation (Fig. S1), along which direction the ligand layer and the HP sublattice alternate.

The current-voltage (I–V) measurements for the ITO/BA2PbI4/Cu devices demonstrated an ohmic contact feature (Fig. S13), regardless of the BA2PbI4 film thickness. The presence of ΔV can be confirmed by analyzing the symmetry of the overall I–V characteristic. Specifically, an ideally symmetrical energy landscape without ΔV should result in a symmetrical I-V characteristic. In this case, the same absolute bias (Vabs) would generate equal currents in the forward and reverse directions (denoted as Ifor and Irev, respectively), leading to a zero-sum of Ifor and Irev (Ifor + Irev = 0). Conversely, when ΔV is present, the (Ifor + Irev) versus Vabs curve would display a diode-like characteristic.

In the dark, the 337 nm-thick BA2PbI4 film exhibits a quasi-exponential (R2=0.956) decrease in the (Ifor + Irev) as the applied bias Vabs increases (Fig. 2B). The imperfect exponential dependence indicates the presence of weak built-in potentials at contacts, which can be attributed to the slight difference in built-in potential between the near-ITO side and the near-Cu side, as illustrated in Fig. S14. The built-in potential disparity is fitted to be 57.8 ± 7.2 meV, which aligns with the reasonable work function difference between the Cu and ITO electrodes. Intriguingly, this I–V dependence reverses upon simulated solar illumination. The (Ifor + Irev) instead increases exponentially with Vabs, and it follows a perfect exponential dependence (R2=0.993), suggesting the emergence of an inverse ΔV upon illumination, which is consistent with the effect of the interfacial dipole. To further confirm that the photogenerated ΔV is induced by the violet/UV photons, a long-pass filter was used to pre-screen the simulated solar illumination, allowing only photons with wavelengths greater than 420 nm (energy < 2.95 eV) to pass. Indeed, under this filtered illumination, the diode-like characteristic is significantly mitigated, indicating the absence of the photogenerated ΔV. These observations confirm the presence of a global potential difference ΔV induced by violet/UV photons in the thick BA2PbI4 film. Notably, under the filtered illumination, the (Ifor + Irev) versus Vabs characteristic does not revert to that in the dark. This is because the photoinjected carrier density is sufficient to eliminate the built-in potentials at both the near-Cu side and the near-ITO side of the film, as illustrated in Fig. S15. The above I-V measurements were double-tested to ensure data reliability (Figs. S16, S17). Furthermore, similar optoelectrical characterizations were conducted on a series of 2D HPs, which confirmed the universality of the observed results. These additional measurements on the 2D HP systems are presented in Figs. S18–S22.

Violet/UV light-induced deprotonation of the ammonium group
The HOMO level in these 2D HPs is contributed by the C–H σ-bonds, C–C σ-bonds, and N–H σ-bonds37,39. The direct attachment of the ammonium group to the HP sublattice facilitates efficient orbital overlap40, making the ammonium group the primary contributor to the HOMO-to-Ex/CBM electron transition process. In contrast, the aryl/alkyl group of the ligands extends outward and is not connected to the HP sublattice, unlikely to participate in the transition. Indeed, for 2D HPs by ammonium ligands with different aryl/alkyl groups, the energies of the near-UV PB signals are consistent (Figs. S5–S9), confirming the negligible impact of the aryl/alkyl groups. In the following, we term the HOMO-to-Ex/CBM electron transition the ammonium-to-HP electron transition to facilitate the discussion.

The ammonium group (−NH3+) is formed by protonating a neutral amine (R–NH2) with a proton (H+). The protonation occurs when the nitrogen atom in the amine group shares a lone pair of electrons with H+, which results in three chemically equivalent N–H bonds within the ammonium group, with the positive charge delocalized across the entire structure. Thus, the ammonium-to-HP electron transition under violet/UV light could deprotonate the ammonium group, disrupting its interaction with the HP sublattice. This mechanism is consistent with the well-documented easy degradation of 2D HPs upon UV exposure41,42.

To confirm this deprotonation of the ammonium ligands due to the ammonium-to-HP electron transition, we designed a photoreaction system where the deprotonation of ammonium results in measurable H2 production. Specifically, 2D HPs in the form of nanoplates are dispersed in hydroiodic acid (termed HI hereafter) to trigger the following reactions upon violet/UV illumination:
I- + h+ -> 1/2 I2 (1)
1/2 R-NH3+ + 1/2 H+ + e- -> 1/2 R-NH2 + 1/2 H2 (2)
The H+ and I- ions are from the HI solution, and the proton in the (−NH3+) group participates in the photoreactions. These photoreactions are framed by the DFT simulation results (Fig. S23, Supplementary Note 1). A crucial link between the photoreactions and the deprotonation dynamics of the ammonium group is that the nitrogen atom serves as the active site for both photoreactions (Fig. 3A), as the adsorption of H+ and I- is most energetically favorable at the nitrogen location.

The two photoreactions work together in a cyclic mechanism that allows for a continuous production of H2. Reaction (1) is endergonic (ΔG>0, ΔG refers to the Gibbs free energy change), while reaction (2) occurs spontaneously (ΔG<0). This means energy input from violet/UV photons is necessary to drive the cyclic process. The overall cycle of the violet/UV light-driven photoreaction can be summarized as follows (Fig. 3B):
1. Violet/UV illumination excites the ammonium-to-HP electron transition;
2. Then, the photogeneration of holes in the ammonium groups drives the endergonic oxidation of I- ions (as I3-) in the solution to I2;
3. Subsequently, H+ ions from the surrounding solution react with the ammonium groups to produce H2;
4. Consequently, the ammonium groups are deprotonated to -NH2 moieties;
5. Finally, the -NH2 moieties are re-protonated by H+ ions in the solution, resetting the cycle to the initial state.

Accordingly, the production of H2 is a direct result of the dynamic deprotonation of the ammonium groups. The vessel used for the photoreactions was evacuated to minimize interference from other potential oxidants and reductants. The temperature of the vessel was fixed at 10 degrees Celsius during the test to prevent interference from heating. The 2D HP is stable throughout the test as discussed in the Supporting Information file (Fig. S24, Supplementary Note 2). H2 production is only detectable under illumination, confirming that photoexcitation is essential for initiating hydrogen generation. Using a Xe lamp as the light source (250 mW cm−2), we measured H2 production rates under two different illumination conditions, as shown in Fig. 3C: (1) filtered illumination, which excludes violet/UV photons with wavelengths below 420 nm (approximately 5% of the total illumination power); and (2) full-spectrum, unfiltered illumination. The experimental results reveal a significant enhancement in the hydrogen evolution activity upon the full-spectrum, unfiltered illumination. Specifically, the approximately 5% of violet/UV power not filtered out lead to a dramatic 186% increase in the hydrogen evolution rate, rising from 1.69 mmol g−1 h−1 to 4.83 mmol g−1 h−1. This finding highlights the vital role of violet/UV photons in driving the photoreaction cycle, confirming the violet/UV light-induced deprotonation of the ammonium groups in 2D HPs. The minor degree of H2 production upon visible illumination (photon wavelength > 420 nm) could arise from the photocatalytic H2 generation by the inorganic HP moiety. Specifically, the direct transfer of photogenerated holes and electrons from the inorganic HP framework to the I-/H+ ions in the solution may contribute to this residual H2 evolution (Fig. S25, Supplementary Note 3)43.

In contrast to in the HI solution, re-protonation is unlikely to occur in solid-state solar cells due to the absence of free H+ ions. Consequently, deprotonation of the ammonium group would disrupt the ammonium-HP interaction, leading to the degradation of 2D HPs. A detailed analysis of the violet/UV light-induced degradation of the 2D HP BA2PbI4 is provided in Fig. S26, Supplementary Note 4. This degradation pathway underscores a potential failure mechanism for the commonly employed strategy of utilizing 2D HPs as passivating agents in HP solar cells.

Violet/UV light-induced depassivation in HP solar cells
Since 2D and 3D HPs exhibit the same chemical interactions between ammonium-terminated ligands and the HP framework, the violet/UV light-activated ammonium-to-HP electron transition and the resulting disrupted ligand-HP interaction should also apply to ammonium-passivated 3D HPs. To explore this concept in the 3D HP system, we first conducted TA spectrum measurements on a multiphasic quasi-2D HP film. This film follows the general formula BA2(CH3NH3)n-1PbnI3n+1 and contains both 2D and 3D HP phases with a gradient of dimensionality44,45. We used excitation light of 2.48 eV (500 nm) to photoexcite all the 2D and 3D HP phases. As shown in Fig. 4A, PB signals were observed for n=3, 4, 5, and >5 phases, consistent with the absorption spectrum (Fig. S27). Importantly, a broadened PB signal is detected in the violet and UV regions. The energy of this broadened PB signal is higher than the 2.48 eV excitation light, which aligns with the proposed ammonium-to-HP electron transition mechanism. The broadening of the PB signal can be attributed to a series of energy gaps between the organic HOMO and inorganic Ex/CBM in the multiphasic film. Consistently, the broadened PB signal is also observed in a series of quasi-2D HPs with various ammonium-terminated ligands, confirming the generality of this finding (Figs. S28–S31). These results demonstrate that the violet/UV light-activated ammonium-to-HP electron transition applies to quasi-2D HPs with increasing dimensionality and is reasonably extendable to 3D HPs.

To support this point, we compared the absorption spectra of a pristine 3D HP (MAPbI3, MA denotes methylammonium) film and a surface-treated 3D HP film with BA in Fig. 4B. To avoid interference from 2D HPs, the dose of ammonium-terminated ligands (BAI in this case) for surface treatment was carefully controlled. The absence of 2D HPs is demonstrated in Fig. S32. The two absorption spectra exhibit perfect overlap within the tested wavelength range, except between 300 and 400 nm (corresponding to photon energies between 3.10 eV and 4.13 eV), where the absorption of the ligand-treated film is enhanced compared to the pristine film. Since MAPbI3 is mono-cationic (containing only the CH3NH3+ cation, no Cs+ or CH(NH2)2+) and mono-anionic (containing only the I- anion), the observed enhanced absorption is unlikely to arise from phase separation. Moreover, the enhanced absorption does not originate from BAI (as evidenced in Fig. S3). Instead, the energy range of the enhanced absorption coincides with the violet/UV light-induced ammonium-to-HP electron transition mechanism. Furthermore, consistent with the universality of the mechanism, the enhanced absorption in the 300 to 400 nm range is also observed in 3D HP films treated with other types of ammonium-terminated ligands, as shown in Fig. S33. These results confirm the applicability of the proposed mechanism to 3D HPs. Notably, resolving the ammonium-to-HP electron transition signal in surface-treated 3D HP films using TA characterization is challenging due to its overlap with the intrinsic high-energy absorption signals of 3D HPs, as well as difficulties in eliminating interference from 2D HPs (Fig. S34).

Consequently, exposure to violet/UV photons causes the deprotonation of ammonium ligands, disrupting the ammonium-HP interaction. To verify this, we conducted Fourier-transform infrared (FTIR) spectroscopy on both UV-irradiated (365 nm @ 25 mW/cm² for 5 h) and non-irradiated ammonium-passivated MAPbI3 samples. During irradiation, the 3D HP powder was gently stirred and dispersed in chlorobenzene with ligands. It is well established that HPs adopt an X-type ligand binding configuration, where Lewis acid and Lewis base moieties attach to the surface of HP in pairs31,46. Herein, we employed BA and oleic acid (1:1 in molar ratio) as the ligand pair. The 3D HP powder was collected every hour, followed by isolation for the FTIR characterization. As shown in Fig. 4C, the 2854 cm−1 peak arises from the symmetric stretching vibration of -CH2- groups. It originates from both the butylammonium and oleic acid segments in the ligand pair but is not detected in MAPbI3. Notably, as the irradiation time increases, the -CH2- peak wanes gradually, indicating the detachment of ligands from the HP surface. After 5 h of irradiation, this characteristic peak becomes barely discernible. In contrast, for the non-irradiated sample, the 2854 cm−1 peak remains after the 5 h test. This contrast confirms that the 365 nm irradiation is the driving factor for ligand detachment, in agreement with our mechanism.

These results show that violet/UV photons in natural solar illumination will nullify the passivation effect of ammonium ligands in HP solar cells. The depassivated HP could be susceptible to defect formation that causes photodegradation47. For instance, depassivated iodide ions will act as reservoirs for binding sites to form I0 species, which can easily combine to generate I2, leading to defect formation to compromise device performance48. To verify the susceptibility to defect formation following the depassivation, we fabricated inverted solar cells with the configuration ITO/4PADCB/Al2O3/Cs0.05FA0.85MA0.1PbI3/LiF/C60/BCP/Ag, adopting the high-efficiency HP formula Cs0.05FA0.85MA0.1PbI3 widely used in the community49. As shown in Fig. 4D, the pristine solar cell without any ammonium-based passivation exhibited a champion PCE of 17.23% (open-circuit voltage Voc = 1.10 V, short-circuit current Jsc = 22.09 mA/cm2, fill factor FF = 70.71%). In contrast, proper passivation of the HP layer using PPA and octane-1,8-diammonium significantly improved the device performance, resulting in a champion PCE of 26.44% (Voc = 1.179 V, Jsc = 26.22 mA/cm2, FF = 85.49%) in our lab. The certified PCE by a third party is 26.15% (for reverse scan, Voc = 1.188 V, Jsc = 25.86 mA/cm2, FF = 85.12%, Fig. S35). This contrast confirms the beneficial passivation effect induced by the ammonium ligands. Part of the improvement also arises from the crystallization modulation effect of the ammonium ligands50.

The harm caused by violet/UV photons to HP solar cells can occur in several ways. To isolate the effect of the depassivation mechanism here, we subjected the passivated solar cells to a fatigue test with intermittent light-dark alternation. A detailed discussion can be found in Supplementary Note 5. During each replication, the solar cell was first illuminated for an I–V scan for 7 s, and then allowed to recover in the dark for 3 s for the self-healing of metastable states51. Illumination will evoke a defect-annihilation process (I−-Vi+ Frenkel pairs annihilation) and a defect generation process (two I0 combining into I2), and the defect generation process tends to occur in a defective region where I− is poorly stabilized48,52. The built-in potentials of the heterojunctions vary during the I-V scan in each replication, causing periodic changes in the built-in electric field that affect the ions in the depletion region (Fig. S36).

We used two illumination conditions for comparison: violet/UV light-filtered solar illumination (using a >420 nm long-pass filter) and unfiltered illumination. The solar simulator we used was measured to contain 12.78% of its total irradiance at wavelengths below 420 nm, slightly higher than that in the standard AM 1.5G irradiation (~5%). The filtered light intensity was calibrated to compensate for the filtering loss. Besides, to exclude interference from ambient factors, we conducted the tests in an N2-filled glovebox. As shown in Fig. 4E, for both illumination conditions, the PCEs increase after the initial several replications, indicating the dominance of the defect annihilation process initially48,52. The device subjected to the unfiltered illumination manifests a weaker PCE increase, which implies a less favored defect annihilation process. This result is consistent with the violet/UV light-induced depassivation mechanism, i.e., the ligand anchoring for I− sites is disrupted, so the defect generation process is exacerbated. Indeed, the absorption spectrum characterization throughout this test reveals a significantly more pronounced decline in light absorption for the unfiltered device (Fig. S37), which can be ascribed to more pronounced defect generation. Subsequently, the PCE starts to decline as the defect formation process gradually takes over. The decline happens first in the unfiltered device. After 47 replications, the device under the filtered illumination retains 98.0% of its initial PCE, while the value significantly drops to 91.4% for the unfiltered device. In another test protocol, after being subjected to continuous illumination for 24 h, the unfiltered device exhibits a more severe decline in Voc and FF compared to the filtered device (Fig. S38, Table S1), a pattern indicating defect formation as Voc and FF are sensitive to defects (Supplementary Note 6). These results show that the light-disrupted ammonium-HP interaction mechanism depassivates HP to make it susceptible to defect formation under illumination, causing photodegradation.

Methods
Materials
The materials used in this study include lead(II) iodide (PbI2, >99.999%); cesium iodide (CsI, 99.999%); methylammonium iodide (MAI, 99.5%); methylammonium hydrochloride (MACl, 99.5%); butylammonium iodide (BAI, 99.0%); octylammonium iodide (HAI, 99.5%); dodecylammonium iodide (DDAI, 99.5%); phenethylammonium iodide (PEAI, 99.5%); phenylpropylammonium iodide (PPAI, 99%); octane-1,8-diammonium iodide (ODADI, 99.5%); lead(II) bromide (PbBr2, 99.99%); methylammonium lead iodide (MAPbI3, 99%); C60 (99%); bathocuproine (BCP, 99%); and phenylpropylammonium bromide (PPABr, 99%), which were purchased from Xi’an Polymer Light Technology Corporation.
Additional materials including lithium fluoride (LiF, 99.995%); dimethylformamide (DMF, anhydrous, 99.8%); dimethyl sulfoxide (DMSO, anhydrous, ≥99.9%); octadecylammonium (ODA, ≥99% (GC)); chlorobenzene (CB, anhydrous, 99.8%); ethyl alcohol (anhydrous, ≥99.5%); acetone (≥99.5%); isopropanol (IPA, anhydrous, 99.5%), and Al2O3 dispersion (nanoparticles, <50 nm particle size (DLS), 20 wt.% in isopropanol) were sourced from Sigma-Aldrich.
Formamidinium iodide (FAI, >99.99%) was purchased from Greatcell Solar Material. 4PADCB was purchased from Suzhou Li Wei Tech Co., Ltd. Acetic acid (HAc, ≥99.8% (GC)) was sourced from Aladdin.
Unless otherwise stated, these reagents were used directly without further purification.

2D HP film fabrication
The ITO substrates were sequentially cleaned using an ultrasonic bath. Each cleaning step consisted of a detergent solution, deionized water, acetone, and IPA, with each step lasting 15 min. After cleaning, the substrates were placed in a drying oven to ensure complete evaporation of all solvents. Before spin coating, the substrates received a 15-min UV treatment.
For the preparation of the BA2PbI4 film, the precursor was created by mixing PbI2 and BAI in a molar ratio of 1:2, dissolving the mixture in a solvent blend of DMF and DMSO at a volume ratio of 7:3. The lead(II) concentration in the precursor was set to 1 M for thick films and 0.1 M for thin films. The precursor solution was then deposited onto the substrate using spin coating. Specifically, 50 μL of the precursor was spin-coated onto the substrate at a speed of 4000 rpm for 30 s, with a ramp-up rate of 2000 rpm per second. Following spin coating, the film was annealed at 100°C for 10 min. All fabrication procedures were conducted within a nitrogen-filled glovebox.
For the synthesis of HA2PbI4, OA2PbI4, DA2PbI4, PEA2PbI4, and PPA2PbI4, the preparation of the precursor and film fabrication procedures was identical, with the only variation being the specific ammonium iodide reagents used in the precursor solutions, tailored to the target 2D HP type.

ITO/2D HP/Cu device fabrication for the steady-state optoelectrical characterizations
The cleaning of substrates, the preparation of various 2D HP precursors, and the spin coating parameters for the 2D HP film fabrication are the same as above. After obtaining the 2D HP layer, a 100 nm-thick Cu electrode was thermally deposited onto the 2D HP layer to complete the device.

3D HP (MAPbI3) film fabrication
The precursor was prepared by dissolving stoichiometric amounts of PbI2 (461.01 mg, 1 mmol) and MAI (158.97 mg, 1 mmol) in 1 mL of DMF. The substrate cleaning procedure was consistent with the previous steps. The film fabrication employed a two-step spin coating process, first at a speed of 1000 rpm for 10 s and then at 5000 rpm for 30 s. The ramp-up rate for both steps was 2000 rmp/s. For each spin coating iteration, 40 μL of the precursor was used. Additionally, 800 μL of CB was deposited onto the wet HP film as an antisolvent at the 15th second during the second spin coating step. Following spin coating, the film was annealed at 100°C for 10 min. All these fabrication procedures were carried out within a nitrogen-filled glovebox.

Synthesis of ODA2PbI4 nanoflakes for the UV light-driven photoreaction
The precursor solution was prepared in two steps. First, CsI (519.62 mg, 2 mmol) and PbI2 (461.01 mg, 1 mmol) were dissolved in 15 mL of DMSO to create precursor A. Separately, 0.5 g of ODA was dissolved in 10 mL of HAc to create precursor B.
Under vigorous stirring, 1.5 mL of precursor A was injected into 10 mL of precursor B. This triggered an instantaneous color change of the transparent solution to yellow due to the formation of the ODA2PbI4 nanoflakes. The resulting solution was then subjected to 15 min of ultrasonic treatment.
After that, the ODA2PbI4 precipitation was extracted via centrifugation. The precipitate was then washed twice with HAc and once with toluene.
All these preparations and purification procedures were carried out in the ambient air.

Theoretical calculations
All first-principle calculations in this work were carried out by using the Vienna Ab initio simulation package (VASP) based on density DFT with the projector augmented wave (PAW) method59–61. The generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) was used for the exchange-correlation function62. A cutoff energy of 500 eV for the plane-wave basis set is adopted in all computations. Each unit cell with a 5×5×1 Γ centered k-point grid was employed for geometry optimization and energy calculations. More details are presented in Fig. S23, Supplementary Note 1.

UV light-driven photoreactions
UV light-driven photoreactions were conducted in a custom-designed double-layered Pyrex vessel. The HI aqueous solution was prepared by mixing 16 mL of HI, 4 mL of H3PO2, and 20 mL of deionized water. Next, 30 mg of ODA2PbI4 powder was added to the HI solution, which was then subjected to ultrasonic treatment for 30 min before transferring to the vessel.
To ensure accurate hydrogen production and eliminate interference from O2 in the air, the reaction system was vacuumed prior to irradiation. A Xe lamp (Ushio-CERMAX LX300) served as the light source, with the light intensity fixed at 250 mW cm−2. The irradiation spectrum was controlled using a 420 nm long-pass filter and an AM 1.5G filter. The amount of evolved H2 was measured using gas chromatography. During the test, the temperature of the vessel was fixed at 10 degrees Celsius to exclude the interference from heat.

HP solar cells fabrication
The solar cell device structure was configured as follows: ITO/4PADCB/Al2O3/Cs0.05FA0.85MA0.1PbI3/LiF/C60/BCP/Ag. Before thin-film deposition, the ITO substrates underwent a thorough cleaning process as described earlier.
For the hole-transport layer, 4PADBC was dissolved in ethyl alcohol to a concentration of 0.5 mg/ml. The solution was dropped onto the substrate and left to stand for 10 s. Then, the solution was spin-coated at 3000 rpm for 30 s, and the ramp-up rate was 2000 rpm/s. After that, the coated layer was annealed at 100°C for 10 min. Following the 4PADBC layer, Al2O3 nanoparticles were deposited to increase hydrophilicity. The Al2O3 dispersion was obtained by diluting the purchased one 50 times with IPA. The dispersion was vigorously shaken before use. It was deposited onto the 4PADBC layer by spin-coating at 5000 rpm for 30 s, and the ramp-up rate was 3000 rpm/s. After the deposition, the film was annealed at 100°C for 10 min.
The HP precursor was prepared by dissolving the following materials in a mixture of DMF (0.8 mL) and DMSO (0.2 mL): PbI2 (760.70 mg), FAI (219.30 mg), MAI (23.80 mg), CsI (19.5 mg), and MACl (12.70 mg). ODADI (0.48 mg) was also added as a bulk passivating agent. The HP precursor solution (50 μL) was then spin-coated onto the substrate at 4000 rpm for 40 s, with a ramp-up rate of 800 rpm/s. At the 35th second of the spin-coating process, 150 μL of CB was deposited onto the substrate as an antisolvent. Subsequently, the ITO/4PADCB/Al2O3/Cs0.05FA0.85MA0.1PbI3 substrate was annealed at 100°C for 30 min. In addition to the bulk passivation, surface passivation was also applied. Specifically, a PPABr/IPA solution (0.5 mg mL−1, 30 μL) was dynamically spin-coated onto the HP film at 4000 rpm for 30 s, with a ramp-up rate of 2000 rpm/s. Finally, the film was annealed at 100°C for 5 min.
Subsequently, a series of thin films was thermally evaporated under high vacuum conditions to complete the device structure. First, a 1 nm-thick layer of LiF was deposited, followed by a 24 nm-thick layer of C60, and then a 4.5 nm-thick layer of BCP. Finally, a 100 nm-thick Ag electrode was thermally deposited to finalize the device fabrication.

FTIR spectra measurement of UV light-irradiated and non-irradiated 3D MAPbI3 (with ammonium passivation)
100 mg MAPbI3 powder were dispersed in 5 mL CB, and 0.162 mmol BA/oleic acid pairs were used as ligands. The target sample was subjected to UV irradiation (365 nm @ ~25 mW/cm2) and gently stirred with a magnetic stirrer at the same time. The reference sample was treated the same way, except it was encapsulated with aluminum foil to prevent irradiation. The irradiation test lasted for 5 h. After every hour, part of MAPbI3 was extracted and isolated by centrifugation and vacuum treatment. Finally, the irradiated MAPbI3 sample, the non-irradiated MAPbI3 sample, and the pristine MAPbI3 were sent for the FTIR spectroscopy characterization.

Characterizations
Time-resolved photoluminescence (TRPL) characterizations for 2D HP films were conducted with a universal streak camera (Hamamatsu). Two excitations of 2.76 eV (1 kHz, 100 fs, for excitonic photoexcitation) and 3.82 eV (1 kHz, 100 fs, for the ammonium-to-HP carrier transition) were used for comparison, with a similar injected carrier density of ~2×10−12 cm−2;
Steady-state UV–vis absorption spectra were obtained from a UV–vis/NIR spectrophotometer (JASCO V-770EX);
The TA characterizations were conducted using a HELIOS TA spectrometer. An excitation light of 3.82 eV (325 nm, 1 kHz, 100 fs) was used for 2D HP films, and the power was 0.318-2.29 mW/cm2 (recorded in the figure caption for each sample);
The steady-state PL spectra were obtained by a Cary Eclipse Fluorescence Spectrophotometer.
The XRD measurements were carried out with a Bruker D8 Advance XRD system and an Ultima IV (Rigaku) system.
The scanning electron microscope (SEM) images were obtained by an FEI field emission electron microscope, Quanta 250F.
The UPS and X-ray Photoelectron Spectroscopy (XPS) measurements were implemented using an electron analyzer (ESCALAB 250Xi, Thermo Fisher Scientific), which is equipped with a UV source (hυ = 21.2 eV) for the UPS measurement and a monochromatic X-ray source (hυ = 1486.7 eV) for the XPS measurement. The UPS and XPS measurements were carried out in a vacuum (1.0×10−10 mbar). For the UPS characterization, a bias voltage of −5.0 V was applied.
J–V curves were extracted under AM 1.5G illumination, enabled by a solar simulator (Newport IEC/JIS/ASTM) equipped with a 450 W xenon lamp and a Keithley 2400 source meter. The intensity of the simulated solar light was standardized by a standard Si photodiode detector calibrated at the National Renewable Energy Laboratory. The Keithley 2400 source measure unit was used to record the device parameters.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper reveals for the first time the mechanism of ammonium-terminated ligand depassivation induced by violet/ultraviolet light, filling the previous research gap in light-induced ligand failure. The authors clarify the electron transfer from ammonium-terminated ligands to perovskite lattices under UV excitation and the subsequent deprotonation process, providing new insights into understanding light degradation in high-efficiency perovskite solar cells. However, several issues require clarification and revision. Please address the following questions before acceptance:\n1. The introduction mentions existing studies only focus on thermal instability and iodine oxidation, but fails to further compare the relative contributions of violet/ ultraviolet light-induced failure with other mechanisms (e.g., thermal stress). This weakens the necessity justification for this research.\n2. In Figure 2, the ΔV differences between thick/thin films demonstrated through I-V curves are used as evidence for light-induced interfacial dipoles. Could electrode contact differences or uneven distribution of photogenerated carriers interfere with ΔV? For instance, limited light penetration depth in thick films might cause carrier accumulation near electrode regions.\n3. While the conclusion briefly summarizes the UV-induced de-passivation mechanism of ammonium ligands and mentions its potential implications for other phenomena (e.g., ion migration, phase separation), these discussions lack in-depth exploration and data support in the main text, appearing vague.\n4. The photocatalytic hydrogen production experiment (Figure 3) indirectly speculates deprotonation of ammonium ligands through H2 monitoring. Why weren't direct characterizations (e.g., XPS or NMR) to confirm the chemical conversion from -NH3+ to -NH2?\n5. What scientific question does the absorption spectrum data in Figure 4B aim to address? Please elaborate on this section.\n6. In Figure 4C, the weakened ligand signals in FTIR alone are insufficient to convincingly prove the disruption of ammonium-HP interactions and attribute this process to ligand deprotonation. Please provide further justification.", "reviewer_2": "Research by Yu et al. discovered that violet/ultraviolet light can induce depassivation of ammonium -terminated ligands. These ligands become deprotonated, thereby leading to the depassivation phenomenon. This provides a novel insight into the violet/ultraviolet light-induced degradation of highly efficient perovskite solar cells prepared with meticulous passivation, aiding in understanding the dynamic behavior at the two-dimensional/three-dimensional efficient photovoltaic interface. However, the experimental evidence for some key conclusions still needs to be strengthened, especially in terms of the universality of the mechanism, the characterization of dynamic processes and the influence of environmental factors. While the perspective offered by this research work is novel, several points require clarification, as detailed below:\n1. The authors claim that violet/ultraviolet light induces the deprotonation of amines, and this deprotonation disrupts the ligand-perovskite interaction, triggering the depassivation of the perovskite making it susceptible to light degradation. However, the issue is that UV light itself can induce the degradation of perovskite, regardless of whether it is passivated or not. Furthermore, UV light also damages electron/hole transport layer materials. The reduction in perovskite solar cell performance is the result of the combined effects of UV light.\n2. When measuring the device IV curves under filtered/non-filtered light conditions, the performance decline in non-filtered devices is primarily due to reductions in VOC and FF, while JSC is less affected. Why is this? Correspondingly, the authors show in Figure 4D that the short-circuit current density of the unpassivated device is only 22.09 mA/cm², while that of the passivated device significantly increases to 26.22 mA/cm². How do the authors explain this?\n3. The authors propose the phenomenon of violet/ultraviolet light-induced depassivation in passivated perovskite solar cells but do not provide specific methods to address this problem in order to obtain UV-stable perovskite solar cells.\n4. It would be beneficial if the authors could provide the EQE of the perovskite solar cells before and after UV aging, along with corresponding analysis, to gain a deeper understanding of the perovskite's photo-response.\n5. From the data in Figure 4E, it can be seen that under illumination of 1.064 suns, the performance of the perovskite solar cell decreased by nearly 10% in less than 300 seconds of light exposure. This cannot simply be attributed to ultraviolet light in the solar spectrum; the perovskite cell itself has severe intrinsic light stability issues. Even under UV-filtered light, the performance of the perovskite cell also showed a significant decrease within the same timeframe, dropping from a peak of about 1.05 to around 0.98.\n6. Another intriguing phenomenon: during the light/dark cycling test, the efficiency of the perovskite solar cells increased significantly under both UV-filtered and UV-retained light, but the increase was greater under UV-filtered light. Why is this? Additionally, what was the initial efficiency of the devices used for the light/dark cycling test?\n7. It is mentioned in the text that the depassivation mechanism induced by violet/ultraviolet light is generally applicable to ammonium-based ligands, but currently, near-ultraviolet light bleaching (PB) signals have only been observed in a few 2D perovskites and quasi-2D systems such as BA2PbI4. It is suggested to supplement more 3D perovskite data of ammonium ligands with different structures (such as those containing aromatic rings, long-chain alkyl groups, etc.), especially the direct evidence of electron transfer at the ligand-perovskite interface in the pure 3D MAPbI3 system, to confirm the universality of the mechanism in different dimensions and ligand structures.\n8. The near-ultraviolet PB signal is attributed to the electron transfer from ammonium to perovskite, but the influence of impurities (such as unreacted PbI2 or ligand decomposition products) is not completely excluded in the manuscript. Although XRD shows that the purity of some samples is relatively high, it is necessary to further confirm through characterization methods such as high-resolution transmission electron microscopy (HRTEM) or time-of-flight secondary ion mass spectrometry (TOF-SIMS) that the signals only come from the ligand-perovskite interface, rather than residual precursors or degradation products.\n9. In the manuscript, the HOMO/LUMO energy levels of ligands are defined based on the DFT calculation results of Silver et al. However, different calculation methods (such as functional selection and basis set size) may lead to deviations in energy values. It is suggested to supplement the first-principles calculations for specific ligands (such as BA+), clarify the consistency degree between the energy difference of HOMO-exciton (Ex) and the energy of the near-ultraviolet PB signal observed experimentally, and discuss the possible sources of the deviation between the calculation and the experiment (such as solvent effect, exciton binding energy).\n10. Figure 4E shows that the device efficiency decreases under ultraviolet light, but the current data only infer the formation of defects through the I-V curve. It is suggested to supplement in-situ characterization methods, such as time-resolved photoluminescence (TRPL) under light or spatially resolved surface photovoltage spectroscopy (SPS), to directly monitor the density changes of non-radiative recombination centers caused by ligand depassivation, and establish the real-time correlation between photocurrent attenuation and ammonium dissociation.\n11. In the light reaction experiment, the increase in the generation rate of H2 indicates the deprotonation of the ammonium group, but there is a lack of kinetic curves under different light intensities (such as the variation of H2 generation amount with time). It is suggested to supplement the ultraviolet light intensity gradient experiment to quantify the relationship between photon flux and deprotonation rate, and analyze the activation energy of this process through the Arrhenius equation to distinguish light-induced electron transfer from thermally driven ligand decomposition.\n12. Fatigue tests were conducted in a nitrogen glove box, but in practical applications, water vapor and oxygen may accelerate the degradation of perovskite. It is suggested to supplement the comparative experiments in the air environment to clarify whether the depassivation induced by violet/ultraviolet light is independent of environmental factors or has a synergistic effect with water vapor. Furthermore, it is necessary to explain whether the intensity of the ultraviolet light used in the test matches the ultraviolet component in the actual sunlight (for example, the ultraviolet proportion in the AM 1.5G spectrum is approximately 5%).\n13. The FTIR results show that the ligand signal weakens after ultraviolet irradiation, but it is only a static characterization. It is recommended to use in situ FTIR to monitor the real-time changes of ammonium characteristic peaks (such as N-H scaling vibration) during the irradiation process, clarify the dynamic path and reversibility of deprotonation (-NH3+→-NH2), and rule out the possibility of physical desorption of ligands."}, "author_response": "Response to Reviewer #1: .............................................................................................. 2\nComment #1 ............................................................................................................... 2\nComment #2 ............................................................................................................... 3\nComment #3 ............................................................................................................... 9\nComment #4 ............................................................................................................. 11\nComment #5 ............................................................................................................. 14\nComment #6 ............................................................................................................. 16\nResponse to Reviewer #2: ............................................................................................ 20\nComment #1 ............................................................................................................. 20\nComment #2 ............................................................................................................. 23\nComment #3 ............................................................................................................. 27\nComment #4 ............................................................................................................. 28\nComment #5 ............................................................................................................. 31\nComment #6 ............................................................................................................. 33\nComment #7 ............................................................................................................. 35\nComment #8 ............................................................................................................. 40\nComment #9 ............................................................................................................. 43\nComment #10 ........................................................................................................... 46\nComment #11 ........................................................................................................... 49\nComment #12 ........................................................................................................... 52\nComment #13 ........................................................................................................... 55\nThis paper reveals for the first time the mechanism of ammonium-terminated\nligand depassivation induced by violet/ultraviolet light, filling the previous\nresearch gap in light-induced ligand failure. The authors clarify the electron\ntransfer from ammonium-terminated ligands to perovskite lattices under UV\nexcitation and the subsequent deprotonation process, providing new insights into\nunderstanding light degradation in high-efficiency perovskite solar cells. However,\nseveral issues require clarification and revision. Please address the following\nquestions before acceptance:\nComment #1. The introduction mentions existing studies only focus on thermal\ninstability and iodine oxidation, but fails to further compare the relative\ncontributions of violet/ultraviolet light-induced failure with other mechanisms\n(e.g., thermal stress). This weakens the necessity justification for this research.\nResponse: We sincerely thank you for dedicating your valuable time to reviewing our\nmanuscript and for providing insightful comments. We have carefully revised the\nmanuscript to address your feedback to the best of our ability. We hope the refined\nversion more effectively conveys our scientific findings and contributes meaningfully\nto the perovskite research community.\nWe agree that the justification in the introduction needs to be enhanced. While we agree\nthat there are many degradation paths in halide perovskites, including ultraviolet light-\ntriggered ones, we would like to clarify that we would like to focus on the passivating\nfailure aspect, which is a different perspective from general degradation.\nAs we disused in the manuscript, the current understanding of the passivation failure of\nammonium ligands, the most widely used passivating agents in halide perovskites, is\nlimited to iodide oxidation (in the precursor stage) and thermal/light intolerance (in\nformed perovskite films). To the best of our knowledge and with your kind agreement,\nour work represents the first report on ultraviolet/violet light-induced depassivation in\nhalide perovskites.\nTo strengthen the discussion and argument, we have made revisions in the revised\nmanuscript (on page 4) following your comment. The revisions are highlighted in\nyellow, as follows:\n“However, contradictory to the excellent defect passivation by ammonium ligands,\nrapid degradation during operation remains a critical challenge for these state-of-the-\nart HP cells, which raises concerns about the durability of ammonium ligands.\nIncreasing attention is paid to studying the depassivation of ammonium ligands. Meng\net al. showed that deprotonation of organic cations can occur in the precursor stage with\nthe oxidation of iodide ions.[28] Wang et al. reported that ammonium ligands with a low\nacid dissociation constant can be thermal-intolerant and might fail passivation under\nhigh-temperature light soaking.[29] The poor thermal/light tolerance of typical\nammonium ligands was also confirmed by Pei et al.[30] However, the depassivation\nmechanisms of ammonium ligands are still insufficiently understood and warrant\nfurther exploration.”\nReferences\n28. Meng, H. et al. Inhibition of halide oxidation and deprotonation of organic cations\nwith dimethylammonium formate for air-processed p–i–n perovskite solar cells.\nNat. Energy 9, 536-547 (2024).\n29. Wang, M. et al. Ammonium cations with high pKa in perovskite solar cells for\nimproved high-temperature photostability. Nat. Energy 8, 1229-1239 (2023).\n30. Pei, F. et al. Inhibiting defect passivation failure in perovskite for perovskite/Cu(In,\nGa)Se monolithic tandem solar cells with certified efficiency 27.35%. Nat. Energy\n2\n10, 824-835 (2025).\nWe sincerely hope the above reply addresses your concern.\nComment #2. In Figure 2, the ΔV differences between thick/thin films\ndemonstrated through I-V curves are used as evidence for light-induced interfacial\ndipoles. Could electrode contact differences or uneven distribution of\nphotogenerated carriers interfere with ΔV? For instance, limited light penetration\ndepth in thick films might cause carrier accumulation near electrode regions.\ndifferences or uneven distribution of photogenerated carriers do exist, but they do not\naffect the interpretation of the light-induced global potential difference (ΔV).\nFIRST, in terms of the potential impact of electrode contact differences, we used\nan ITO/2D perovskite/Cu MSM device configuration for the measurement. ITO and Cu\nare selected due to their similar work functions (ITO in the range of 4.6-4.75 eV and\nCu at 4.7 eV), minimizing the interference from the contact difference.\nFigure R1. I-V characteristic of the ITO/thick BA PbI /Cu device, which shows an\n2 4\nOhmic contact for both metal/semiconductor contacts on the two sides.\nAll devices based on different types of 2D perovskites show an Ohmic feature. Take\nBA PbI (BA denotes butylammonium) as an example, the I-V characteristic measured\n2 4\nin the dark reveals an Ohmic feature, as shown in Figure R1 (presented as Fig. S13 in\nthe prior manuscript). The I-V characteristics for other 2D perovskite-based devices\nthat also show an Ohmic feature can be found in Fig. S18 -S22 in the Supplementary\nInformation file. Notably, the Ohmic contact feature indicates that the Schottky\nbarrier heights (from the BA PbI side to the metal electrode side) for the two\n2 4\nmetal/semiconductor contacts are minor. Otherwise, a Schottky contact feature\n4\nshould be observed. Additionally, the I-V characteristic measured under standard AM\n1.5G illumination shows no photovoltaic effect (no photovoltage), which also\nconfirms a negligible impact of the electrode Fermi level difference, as a high disparity\nin the work functions of the two electrodes should induce a photovoltaic effect.\nTherefore, the impact of electrode contact difference should be trivial. Further\nclarification is embedded in the following discussion of the uneven distribution of\nphotogenerated carriers.\nFigure R2. I-V characteristics of the thick BA PbI film in the dark, under full-\n2 4\nspectrum one-sun illumination, and under one-sun illumination excluding violet/UV\nphotons (energy>2.95 eV), measured in the device configuration of ITO/BA PbI /Cu.\n2 4\nSECOND, in terms of the potential impact of uneven distribution of\nphotogenerated carriers, we believe the uneven distribution exists and will impact the\nenergy landscape across the device, but it will not interfere with the interpretation of\nthe result.\nFigure R2 shows the I-V symmetry characteristic of the ITO/thick BA PbI /Cu device.\n2 4\nWhen in the dark, the (I + I ) versus V characteristic shows a mixed\nfor rev abs\nexponential/linear feature, as the exponential fit (single exponential is the best fit)\nshows a non-ideal R2 value of 0.95616. The exponential feature corresponds to a\nSchottky contact, while the linear feature corresponds to an Ohmic contact. The\nimperfect exponential fit agrees with the dominant Ohmic contact feature present in\ncontact properties.\nWhen the device is under full-spectrum simulated AM 1.5G illumination, the (I + I )\nfor rev\nversus V dependence reverses, and the (I +I ) instead increases with V .\nabs for rev abs\nThis\ndependence can be well single-exponentially fitted with an R2 value of 0.99295, which\nsuggests the emergence of an inverse ΔV upon illumination. This result is consistent\nwith the effect of the interfacial dipole arising from the light-induced ammonium-to-\nperovskite carrier transition in our mechanism. To further validate that this phenomenon\nis due to violet/ultraviolet photons, we used a long-pass filter that filters out photons\nwith a wavelength shorter than 420 nm. Indeed, the I-V characteristic becomes almost\nflat with a non-ideal exponential fit (R2= 0.93034), suggesting significantly mitigation\nof ΔV. This change indicates that the emergence of ΔV is indeed due to violet/ultraviolet\nphotons, in line with our mechanism.\nFigure R3. UPS spectrum of the BA PbI film measured under weak illumination. The\n2 4\nFermi level is about -4.6 eV versus the vacuum level, which is close to the center of the\nbandgap. The CBM of BA PbI is -3.4 eV, and the VBM is -5.8 eV.\n2 4\nNotably, the I-V characteristic under the filtered illumination does not revert to\n6\nthat in the dark. We believe this is where the uneven charge carrier distribution\ntakes effect. That is, the photoinjected electron-hole pairs (excitons in 2D perovskites)\nwould concentrate near the illuminated side due to limited penetration depth, and they\nwould screen the built-in potential, despite being weak, in the ITO/BA PbI junction.\n2 4\nTo confirm this claim, we have carried out ultraviolet photoelectron spectrum (UPS)\ncharacterization of the BA PbI film in the dark and under illumination. The\n2 4\nillumination source in this UPS measurement is broad-spectrum visible white light,\ncovering approximately 400-700 nm, with a color temperature of around 3000-5000 K,\nand it is significantly less intense compared to the simulated AM 1.5G illumination\n(~0.1% intensity suggested by online data). Even with this low intensity illumination,\nthe Fermi level of BA PbI shifts to the middle of the bandgap upon illumination\n2 4\n(Figure R3), which means the photoinjected carrier density is already enough to fully\nscreen the built-in potential near the illumination side.\nImportantly, even with this screening effect due to uneven charge carrier\ndistribution, the interpretation of the I-V characteristic is still valid to support our\nmechanism. The energy landscape across the device when in the dark and under\ndifferent illumination conditions can be illustrated as below in Figure R4:\nFigure R4. Schematic illustration of the energy landscapes across the device when in\nthe dark and under different illumination conditions.\n2 4 2 4\npotentials, but the Ohmic property is dominant. The minor work function difference\nbetween ITO and Cu causes non-perfect symmetry across the energy landscape, as\nillustrated in Figure R4a. The downward band bending is based on the weak p-type\nproperty of BA PbI (Fermi level at 4.74 eV versus the vacuum level, closer to VBM,\n2 4\nshown in Figure S11 in the manuscript). When the device is under filtered simulated\nAM 1.5G illumination, in which photons with a wavelength shorter than 420 nm are\nfiltered out, uneven charge carrier concentration occurs due to limited penetration depth\nof the lights (and also limited diffusion length of excitons, referring to Magdaleno, A.\nJ. et al. Mater. Horiz. 8, 639-644 (2021)). Consequently, the built-in potential at the\nITO/BA PbI contact is screened, while that of the BA PbI /Cu contact on the back will\n2 4 2 4\nbe less impacted. The corresponding energy landscape is illustrated in Figure R4b.\nWhen the device is under full-spectrum illumination that maintains the <420 nm\nphotons, the energy barrier is imposed due to the organic-to-inorganic carrier transition\nmechanism, as illustrated in Figure R4c. These energy landscapes agree with the I-V\nsymmetry characteristic analysis in Figure R2.\nAccordingly, we have made revisions in the manuscript to strengthen the argument. The\nrevision is on page 11 in the revised manuscript, as below:\n“In the dark, the 337 nm-thick BA PbI film exhibits a quasi-exponential (R2= 0.956)\n2 4\ndecrease in the (I +I ) as the applied bias V increases (Fig. 2B). The imperfect\nfor rev abs\nexponential dependence indicates the presence of weak built-in potentials at contacts,\nwhich can be attributed to the slight difference in built-in potential between the near-\nITO side and the near-Cu side, as illustrated in fig. S14. The built-in potential disparity\nis fitted to be 57.8±7.2 meV, which aligns with the reasonable work function difference\nbetween the Cu and ITO electrodes. Intriguingly, this I-V dependence reverses upon\nsimulated solar illumination. The (I +I ) instead increases exponentially with V ,\nfor rev abs\nand it follows a perfect exponential dependence (R2= 0.993), suggesting the emergence\nof an inverse ΔV upon illumination, which is consistent with the effect of the interfacial\ndipole.”\nWe sincerely hope the above reply addresses your concern.\npassivation mechanism of ammonium ligands and mentions its potential\nimplications for other phenomena (e.g., ion migration, phase separation), these\ndiscussions lack in-depth exploration and data support in the main text, appearing\nvague.\nResponse: Thank you for your helpful comment. We do believe the light-induced\ndepassivation mechanism in this work can contribute to the understanding of many\nimportant phenomena in this field, including the dynamism of the 2D/3D perovskite\ninterface, phase segregation in mixed-halide perovskites, ion migration and hysteresis\nin devices, enhancements in photocatalytic activity, and the photoluminescence\nblinking/quenching of perovskite nanocrystals. While we acknowledge this claim is\nlargely based on our understanding of perovskites rather than data, we would also like\nto clarify that the theme of this work is to introduce this unknown mechanism. The\nstatement you commented on is for discussion, not an argument, which is why we place\nit in the Discussion section. Proving these phenomena requires huge efforts, and we\nbelieve it is unrealistic to include them in this self-contained work.\nTo make a clearer statement, we have expounded this discussion in the revised\nmanuscript (on pages 21-22). The text is shown below:\n“We believe the mechanism revealed here could also contribute to the understanding of\nmany other important phenomena in this field, such as the dynamism of the 2D/3D HP\ninterface,[13, 53, 54] phase segregation in mixed-halide HPs,[4] ion migration and\nhysteresis in devices,[55] enhancements in photocatalytic activity,[43] and the\nphotoluminescence blinking/quenching of HP nanocrystals.[56] Specifically, the 2D/3D\nHP configuration, wherein a 2D HP layer serves as a protective and passivating coating\nover a 3D HP absorber, exhibits a dynamic dimensionality change at the interface over\ntime.[54] This dynamism causes deviations from the initial design and compromises\ndevice efficiency and stability. This dynamic behavior leads to deviations from the\ninitial structural design, compromising both device efficiency and stability. Previous\nstudies have observed the migration of ammonium ligands under illumination,\nconsistent with the mechanism proposed herein;[57] Ion migration, a prevalent\nphenomenon in HPs, underpins critical device behaviors such as hysteresis.[55] While\n9\nanchoring ammonium ligands can mitigate ion migration, this effect may be\nexacerbated under illumination.[58] This photoresponse aligns with the proposed\nmechanism; For photocatalytic applications of 2D HPs, such as H evolution,[43]\n2\nefficient separation of electrons and holes is essential. Our mechanism suggests that\nthis separation is facilitated by an organic-to-inorganic carrier transition induced by\nviolet/UV photons, thereby enhancing photocatalytic activity; Additionally, ammonium\nligands are commonly employed as surface-passivating agents for HP nanocrystals.\nHowever, a light-induced depassivation mechanism may trigger ligand dynamics at the\nsurface, exposing active sites and introducing trap states. This process could contribute\nto charge trapping/detrapping phenomena, resulting in photoluminescence blinking or\nquenching.[56] Nevertheless, further experimental investigations are required to\nsubstantiate these hypotheses.”\nReferences\n4. Meggiolaro, D., Mosconi, E. & De Angelis, F. Formation of surface defects\ndominates ion migration in lead-halide perovskites. ACS Energy Lett. 4, 779-\n785 (2019).\n13. Park, S. M. et al. Engineering ligand reactivity enables high-temperature\noperation of stable perovskite solar cells. Science 381, 209-215 (2023).\n43. Wang, H. et al. Mechanistic understanding of efficient photocatalytic H\n2\nevolution on two-dimensional layered lead iodide hybrid perovskites. Angew.\nChem., Int. Ed. 60, 7376-7381 (2021).\n53. Teale, S., Degani, M., Chen, B., Sargent, E. H. & Grancini, G. Molecular cation\nand low-dimensional perovskite surface passivation in perovskite solar cells.\nNat. Energy 9, 779-792 (2024).\n54. Sutanto, A. A. et al. Dynamical evolution of the 2D/3D interface: a hidden\ndriver behind perovskite solar cell instability. J. Mater. Chem. A 8, 2343-2348\n(2020).\n55. Alharbi, E. A. et al. Atomic-level passivation mechanism of ammonium salts\nenabling highly efficient perovskite solar cells. Nat. Commun. 10, 3008 (2019).\n56. Tian, Y. et al. Giant photoluminescence blinking of perovskite nanocrystals\nreveals single-trap control of luminescence. Nano Lett. 15, 1603-8 (2015).\n57. Mathew, P., Cho, J. & Kamat, P. V. Ramifications of ion migration in 2D lead\nhalide perovskites. ACS Energy Lett. 9, 1103-1114 (2024).\n58. deQuilettes, D. W. et al. Photo-induced halide redistribution in organic-\ninorganic perovskite films. Nat. Commun. 7, 11683 (2016).\nabove phenomena and contribute to advancing their understanding.\nWe sincerely hope the above reply addresses your concern.\nComment #4. The photocatalytic hydrogen production experiment (Figure 3)\nindirectly speculates deprotonation of ammonium ligands through H monitoring.\nconversion from -NH + to -NH ?\n3 2\nResponse: Thank you for your insightful comment. We tried to organize our manuscript\nin a step-by-step logic. First, we revealed the HOMO-to-E (organic-to-inorganic)\nx\ncarrier transition (Figure 1-2 in the manuscript) under violet/ultraviolet excitation\nwith the transient absorption spectrum characterization and the steady-state\noptoelectronic characterization; Then, we designed the photoreaction system to prove\nthe ensuing deprotonation process (Figure 3 in the manuscript); Finally, we\nconfirmed the disruption of the ligand-perovskite interaction and its impact on\nsolar cells (Figure 4 in the manuscript) that causes depassivation. In this way, we\nhope to present this mechanism clearly. The Figure 3 you mentioned is used to confirm\nthe deprotonation process, while the change of the -NH + is related to the disruption of\n3\nthe ligand-perovskite interaction.\nTo directly validate the disrupted ammonium ligand-perovskite interaction, we have\ncarried out experiments on both 2D and 3D perovskite systems. The details of the 3D\nperovskite system by the Fourier Transform Infrared (FTIR) characterization are\nintroduced in the response to Comment #6 below. The details of the 2D perovskite\nsystem are as follows.\ndriven deprotonation. (a) Schematic diagram of electron transitions upon the ~2.76\neV (450 nm) and ~3.40 eV (365 nm) monochromatic excitations. Comparative\nabsorption spectra of the BA PbI films at 0 min and 60 min upon (b) the ~2.76 eV\n2 4\nirradiation and (c) the ~3.40 eV irradiation, respectively. The intensities of the two\nirradiations are both 20 mW cm-2. The test was implemented in the ambient air. (d)\nComparison of the Urbach energy evolutions of the 2D perovskite film in different\nillumination conditions (~3.40 eV, ~2.76 eV, and darkness). Overall, the evolving\nabsorption profile and the increase in Urbach energy agree with the (near) UV light-\ninduced deprotonation and detachment of the ammonium from the perovskite\nframework.\nWe carried out a comparative irradiation test on BA PbI perovskite films and used the\n2 4\nX-ray diffraction (XRD) measurement to characterize the products. Specifically, we\nirradiated BA PbI films with monochromatic lights of ~2.76 eV (450 nm) and ~3.40\n2 4\neV (365 nm), respectively. The ~2.76 eV irradiation is for the band-edge\nphotoexcitation, while the ~3.40 eV irradiation provokes the ammonium-to-perovskite\ncarrier transition (Figure R5a). The irradiation intensities of both sources were\n12\ncontrolled to be 20 mW cm-2. As shown in Figure R5b, the absorption profile of the\nfilm by the ~2.76 eV irradiation barely changed within the first 60 minutes, indicating\nhigh photostability. In stark contrast, the ~3.40 eV irradiation caused a conspicuous\ndecline in the resonance intensity (Figure R5c) after 60 min. Meanwhile, the absorption\nedge also became blunted after the irradiation, implying the appearance of mid-gap trap\nstates.\nTaking the Urbach energy as the quantitative metric, the ~2.76 eV and ~3.40 eV\nirradiations impose vastly different degradation kinetics on the BA PbI films.\nAs\n2 4\ncompared in Figure R5d, the degradation upon the ~3.40 eV irradiation started\nimmediately and featured a short τ of 12.2 min. However, a discernible degradation\nupon the ~2.76 eV irradiation only appeared after 60 min and featured a much longer τ\nof 55.6 min. Such a difference in the characteristic time indicates distinct\ndecomposition routes triggered by the two irradiations. The mild photolysis by the\nband-edge photoexcitation was unveiled previously to arise from a photooxidation\nprocess, while the acute photodegradation by the ~3.40 eV irradiation could be\nassigned to the (near) UV light-activated deprotonation and detachment of the\nammonium.\nFig. R6. The XRD pattern of the BA PbI film after the ~3.40 eV (365 nm) irradiation\n2 4\nfor 60 min.\n2 4\nfilm exposed to the (near) UV irradiation (Figure R6). BAI was detected, agreeing with\nthe disrupted BA-perovskite interaction arising from our mechanism.\nTo summarize, the photoreaction system, designed with DFT simulations, is used to\nprove the deprotonation caused by our mechanism, contributing to a step-by-step\nargument. The direct characterizations (XRD, FTRI) are also provided, but in the\nfollowing sections. Thank you very much for your understanding.\nWe sincerely hope the above reply addresses your concern.\nComment #5. What scientific question does the absorption spectrum data in\nFigure 4B aim to address? Please elaborate on this section.\nResponse: Thank you for your helpful comment. Figure 4B (shown below as Figure\nR7a) is the steady-state UV-vis absorption spectrum of a MAPbI perovskite film\n3\nsurface-treated with BAI. It shows that after the surface treatment, an enhanced\nabsorption can be found in the range of 300 to 400 nm (corresponding to photon\nenergies between 3.10 eV and 4.13 eV), which aligns with our proposed ligands-to-\nperovskite carrier transition mechanism. The higher the concentration of ammonium,\nthe stronger the characteristic absorption, as can be checked in Fig. S32a in the\nSupplementary Information file, and is present as Figure R7b below.\nFigure R7. (a) Comparative absorption spectra of 3D perovskite (MAPbI ) films before\n3\n14\nand after surface treatment with ammonium ligands (butylamine iodide). (b) Steady-\nstate absorption spectra of 3D perovskite (MAPbI ) films surface-treated by butylamine\n3\niodide solutions with a gradient of concentrations.\nThe UV-vis absorption characterization is to confirm the viability of the mechanism in\nthe ligand-passivated 3D perovskites. Prior characterizations in the manuscript are\ncarried out in (quasi-) 2D perovskites considering the abundant ligand-perovskite bonds\nand the easy characterizations, and it is important to extend this mechanism to the 3D\nperovskite system.\nWe have provided the corresponding statement in the prior manuscript version for\nclarification (on page 18 in the manuscript), as shown below:\n“To support this point, we compared the absorption spectra of a pristine 3D HP\n(MAPbI₃) film and a surface-treated 3D HP film with BA in Fig. 4B. To avoid\ninterference from 2D HPs, the dose of ammonium-terminated ligands (BAI in this case)\nfor surface treatment was carefully controlled. The absence of 2D HPs is demonstrated\nin fig. S32. The two absorption spectra exhibit perfect overlap within the tested\nwavelength range, except between 300 and 400 nm (corresponding to photon energies\nbetween 3.10 eV and 4.13 eV), where the absorption of the ligand-treated film is\nsignificantly enhanced compared to the pristine film. Since MAPbI is mono-cationic\n3\n(containing only the CH NH + cation, no Cs+ or CH(NH ) +) and mono-anionic\n3 3 2 2\n(containing only the I- anion), the observed enhanced absorption is unlikely to arise\nfrom phase separation. Moreover, the enhanced absorption does not originate from BAI\n(as evidenced in fig. S3). Instead, the energy range of the enhanced absorption coincides\nwith the violet/UV light-induced ammonium-to-HP electron transition mechanism.\nFurthermore, consistent with the universality of the mechanism, the enhanced\nabsorption in the 300 to 400 nm range is also observed in 3D HP films treated with\nother types of ammonium-terminated ligands, as shown in fig. S33. These results\nconfirm the applicability of the proposed mechanism to 3D HPs. Notably, resolving the\nammonium-to-HP electron transition signal in surface-treated 3D HP films using TA\ncharacterization is challenging due to its overlap with the intrinsic high-energy\nabsorption signals of 3D HPs, as well as difficulties in eliminating interference from\n2D HPs (fig. S34).”\nComment #6. In Figure 4C, the weakened ligand signals in FTIR alone are\ninsufficient to convincingly prove the disruption of ammonium-HP interactions\nand attribute this process to ligand deprotonation. Please provide further\njustification.\nResponse: Thank you for your insightful comment. To verify the disrupted ligand-\nperovskite lattice interaction, we have carried out characterizations in both 2D and 3D\nperovskite systems. For the 2D perovskites, we have carried out irradiation tests and\nXRD characterizations, as detailed in the response to Comment #4. The FTIR\ncharacterization you commented on is used to confirm the disrupted ligand-perovskite\ninteraction in the 3D perovskite system.\nWhile we totally agree that the prior FTIR characterization result is not convincing\nenough, we do believe it is direct evidence that shows the detachment of ligands from\nthe perovskite surface due to the photoinduced depassivation mechanism. To further\nstrengthen this argument, we have measured the irradiation time-dependent FTIR\nspectrum. The dynamic FTIR characterization should be more convincing than the prior\nstatic FTIR characterization. The details are introduced below.\nMAPbI was selected as the perovskite sample as it is single-anion, single-cation, and\n3\nphase-stable at room temperature. Otherwise, there could be a phase segregation or a\nphase transition that interferes with the result interpretation. The ligand pair of\nbutylammonium and oleic acid was used as the passivating agent. For the experiment,\nthe MAPbI polycrystalline powder was dispersed in a benign chlorobenzene solution\n3\ncontaining the ligands. The doses are as follows: 100 mg MAPbI polycrystalline\n3\npowder was dispersed in 5 mL chlorobenzene that contains 11.9 mg butylammonium\nand 45.8 mg oleic acid. The molar ratio of butylammonium to oleic acid is 1:1. The\nsolution was gently stirred by a magnetic stirrer during the test. Meanwhile, a\nmonochromatic illumination of 365 nm was applied to the solution with a power density\nof ~25 mW/cm2. The 365 nm irradiation is sufficient to trigger the depassivation\nmechanism. The dispersion was collected every hour. Then, the MAPbI powders were\n3\nisolated by centrifugation and vacuum drying. In comparison, in another set, the same\nMAPbI dispersion was subjected to the same test except that it was covered with\n3\n16\naluminum foil to avoid irradiation. The irradiated sample with different irradiation\ntimes, the non-irradiated sample, and the pristine MAPbI sample were sent for the\nFigure R7. The irradiation time-dependent FTIR result of the passivated perovskite\nsample.\nAs shown in Figure R7, the FTIR characterization confirms the gradual detachment of\nligands under the 365 nm irradiation. The 2854 cm⁻¹ peak arises from the symmetric\nstretching vibration of -CH₂- groups, which originate from both the butylammonium\nand oleic acid segments in the ligand pair, while it is not detected in MAPbI . Therefore,\n3\nthis characteristic FTIR peak can be used to monitor the residue of ligands on the\nsurface of MAPbI . It should be noted that the peak of the N-H scaling vibration may\n3\nnot be an ideal observation target, as the MAPbI perovskite intrinsically shows a strong\nImportantly, as the irradiation time increases, this characterization peak of -CH₂- wanes\ngradually, indicating the detachment of ligands from the perovskite surface. This is in\nline with the photoinduced depassivation mechanism introduced in our work. After 5 h\nof irradiation, this characteristic peak becomes barely discernible. In contrast, for the\n17\nnon-irradiated sample, the 2854 cm⁻¹ peak remains after the 5h test. This contrast rules\nout the impact of physical detachment and confirms that the 365 nm irradiation is the\ndriving factor of the ligand detachment, agreeing with the photoinduced detachment\nmechanism.\nWe would also like to clarify that it is challenging to characterize the ligand detachment\nin the 3D perovskite system. This is because the dose of ligands needs to be prudently\ncontrolled. Otherwise, 2D perovskite would form to interfere with the analysis. For\nexample, in the UV-vis characterization discussed in the response to Comment #5, the\ndoses of ammonium ligands for the surface treatment were controlled to prevent the\nformation of 2D perovskites, so the enhanced absorption in the UV range can be\nascribed to the ligand-to-perovskite carrier transition. The low content of ligands on the\nsurface of 3D perovskites makes them difficult to detect. To support this claim, we have\ncarried out X-ray Photoelectron Spectroscopy (XPS) characterizations on the surface\npassivated MAPbI films, as shown in Figure R8 below.\n3\nbutylammonium iodide and pristine MAPbI films for (a) the Pb element, (b) the I\n3\nelement, and (c) the N element.\nThe concentration of the butylammonium for the surface treatment is 2.5 mg/ml\n(dissolved in isopropanol), which is close to the upper limit for avoiding the formation\nof 2D perovskites based on our observation. However, the XPS spectra show negligible\ndifference between the surface-treated and pristine MAPbI films. This result shows it\n3\ncan be challenging to detect the ammonium ligands even for the XPS characterization.\nTherefore, the dynamic FTIR characterization in the 3D perovskite system, along with\nthe XRD characterization in the 2D perovskite system, serves to confirm the disrupted\ninteraction between ammonium ligands and the perovskite lattice due to the\n18\ndepassivation mechanism. Accordingly, we have made revisions in the revised\nmanuscript (on pages 18-19), as shown below:\n“Consequently, exposure to violet/UV photons causes the deprotonation of ammonium\nligands, disrupting the ammonium-HP interaction. To verify this, we conducted Fourier-\ntransform infrared (FTIR) spectroscopy on both UV-irradiated (365 nm @ 25 mW/cm²\nfor 5 h) and non-irradiated ammonium-passivated MAPbI samples. During irradiation,\n3\nthe 3D HP powder was gently stirred and dispersed in chlorobenzene with ligands. It is\nwell established that HPs adopt an X-type ligand binding configuration, where Lewis\nacid and Lewis base moieties attach to the surface of HP in pairs.[32, 46] Herein, we\nemployed BA and oleic acid (1:1 in molar ratio) as the ligand pair. The 3D HP powder\nwas collected every one hour, followed by isolation for the FTIR characterization. As\nshown in Fig. 4C, the 2854 cm⁻¹ peak arises from the symmetric stretching vibration of\n-CH - groups. It originates from both the butylammonium and oleic acid segments in\n2\nthe ligand pair but is not detected in MAPbI . Notably, as the irradiation time increases,\n3\nthe -CH - peak wanes gradually, indicating the detachment of ligands from the HP\n2\nsurface. After 5 h of irradiation, this characteristic peak becomes barely discernible. In\ncontrast, for the non-irradiated sample, the 2854 cm⁻¹ peak remains after the 5h test.\nThis contrast confirms that the 365 nm irradiation is the driving factor for ligand\ndetachment, in agreement with our mechanism.”\nIn the end, we sincerely thank you again for dedicating your valuable time to reviewing\nour work and for providing insightful feedback. We hope our responses adequately\naddress your concerns and align our manuscript with the high standards of Nature\nCommunications. We have made every effort to refine our work and welcome any\nfurther comments or suggestions in subsequent reviews. Thank you once again for your\nthoughtful contributions.\nResearch by Yu et al. discovered that violet/ultraviolet light can induce\ndepassivation of ammonium -terminated ligands. These ligands become\ndeprotonated, thereby leading to the depassivation phenomenon. This provides a\nnovel insight into the violet/ultraviolet light-induced degradation of highly\nefficient perovskite solar cells prepared with meticulous passivation, aiding in\nunderstanding the dynamic behavior at the two-dimensional/three-dimensional\nefficient photovoltaic interface. However, the experimental evidence for some key\nconclusions still needs to be strengthened, especially in terms of the universality of\nthe mechanism, the characterization of dynamic processes and the influence of\nenvironmental factors. While the perspective offered by this research work is\nnovel, several points require clarification, as detailed below:\nComment #1. The authors claim that violet/ultraviolet light induces the\ndeprotonation of amines, and this deprotonation disrupts the ligand-perovskite\ninteraction, triggering the depassivation of the perovskite making it susceptible to\nlight degradation. However, the issue is that UV light itself can induce the\ndegradation of perovskite, regardless of whether it is passivated or not. Furthermore,\nUV light also damages electron/hole transport layer materials. The reduction in\nperovskite solar cell performance is the result of the combined effects of UV light.\nResponse: We sincerely thank you for dedicating your valuable time to reviewing our\nmanuscript and for providing insightful comments. We have carefully revised the\nmanuscript to address your feedback to the best of our ability. We hope the refined\nversion more effectively conveys our scientific findings and contributes meaningfully\nto the perovskite research community.\nWe totally agree that the harm caused by UV lights to perovskite solar cells is not\nlimited to the depassivation reported here, but also to the perovskite itself (passivated\nor not) and charge-transport materials. Therefore, we organize our manuscript in a step-\nby-step logic. First, we revealed the carrier dynamics to reveal the ammonium-to-\nperovskite carrier transition (Figure 1 and Figure 2 in the manuscript). Second, we\nconfirmed the cause of deprotonation of the ammonium group (Figure 3). Finally, we\nshowed disrupted ammonium-perovskite interaction (depassivation) and its impact on\n20\nperovskite solar cells (Figure 4). By this step-by-step argument, we focus on the\nviolet/ultraviolet light-induced depassivation mechanism, minimizing the interference\nfrom other parallel mechanisms.\nFurthermore, when it comes to the solar cell device performance characterization,\nwhich is to help validate the depassivation mechanism, the impact of parallel\nmechanisms caused by UV irradiation should be minor in our work. The reasons are\ndetailed below:\nFirst, as you commented, UV lights also jeopardize charge-transport layers and the\nperovskite itself. In this work, since a p-i-n inverted device configuration is used, the\nimpact on the hole-transport layer (SAM, 4PACZ) should be considered. According to\nprevious reports, the jeopardization arises from 1) the decomposition of aromatic rings\nin SAM molecules and the debonding between the SAM and ITO (Sci. Adv. 11,\neadu3493 (2025)), and 2) the degradation of the SAM/perovskite interface (Science 384,\n1126-1134 (2024)). However, these harms are reported to only become discernible\nin hours or tens of hours. In our test protocol with intermittent light-dark alternation,\nthe solar cell in every cycle was first illuminated for an I-V scan that lasts 7 seconds\nand was then allowed to recover in the dark for 3 seconds for self-healing. The number\nof cycles implemented was only several hundred cycles, amounting to an irradiation\ntime of less than one hour. Therefore, during our test, the parallel impacts of UV lights\nshould be minor.\nSecond, we have carefully designed the solar cell performance testing protocol, i.e., the\nlight-dark alternation testing protocol, to minimize the impact from parallel\nmechanisms induced by UV irradiation. The details are as introduced in Supplementary\nText 5 in the Supplementary file, as shown below:\n“Supplementary Text 5:\nIt should be noted that UV lights affect HPs in several ways. Previous reports showed\nthat long-term, continuous UV irradiation would result in irreversible chemical\ndecomposition of HPs.[9, 10] Short-term illumination would generate metastable mid-\ngap states in HPs, but these metastable states could disappear after a self-healing\nprocess in darkness.[11, 12] To highlight the mechanism proposed here, the testing method\nneeds to minimize interference by other parallel impact mechanisms. Therefore, we\nadopted the short-term, alternating illumination/darkness test method proposed by\nMotti et al with modifications.[13]\nrecovery process in darkness (3 s). During illumination, a defect annihilation process\nand a defect generation process would be triggered simultaneously:\n1) The photo-driven annihilation of Frenkel defects (I--V+ Frenkel pairs) that enhances\ni I\nthe photovoltaic performance.[13, 14]\n2) The reaction between two filled traps I0 to form one I molecule that compromises\ni 2\nthe photovoltaic performance.[13]\nThe defect generation process tends to occur at a defective region where the I- is poorly\nstabilized, but can be suppressed upon passivation by ammonium-terminated ligands.\nThe two processes are ion-migration-related, thus are cumulative and barely reversible\nin darkness.[13] Therefore, the alternating testing can minimize the interference from\nparallel mechanisms.\nUV lights could also jeopardize SAMs, decomposing their aromatic rings and\nweakening the interaction between them and ITO.[15] However, the harm to SAMs by\nUV irradiations only becomes discernible in hours or tens of hours,[15, 16] while in our\ntest protocol with intermittent light-dark alternation, the solar cell in every cycle was\nfirst illuminated for an I-V scan that lasts 7 seconds, and was then allowed to recover\nin the dark for 3 seconds for self-healing. A significant distinction in performance\ndegradation was observed after 47 cycles (as shown in Figure 4E in the main text),\namounting to only 329 s of irradiation. During this time slot, the impact of UV lights\non the SAM layer could be minimal.\nBased on the above discussion, the texting protocol here allows effective isolation of\nthe light-induced depassivation effect on HP solar cells.”\nReferences:\n9. Farooq, A. et al. Spectral dependence of degradation under ultraviolet light in\nperovskite solar cells. ACS Appl. Mater. Interfaces 10, 21985-21990 (2018).\n10. Wang, Y. et al. Tautomeric molecule acts as a \"sunscreen\" for metal halide\nperovskite solar cells. Angew Chem. Int. Ed. Engl. 60, 8673-8677 (2021).\n11. Nie, W. et al. Light-activated photocurrent degradation and self-healing in\nperovskite solar cells. Nat. Commun. 7, 11574 (2016).\n22\n12. Khenkin, M. V. et al. Dynamics of photoinduced degradation of perovskite\nphotovoltaics: from reversible to irreversible processes. ACS Appl. Energy\nMater. 1, 799-806 (2018).\n13. Motti, S. G. et al. Controlling competing photochemical reactions stabilizes\nperovskite solar cells. Nat. Photonics 13, 532-539 (2019).\n14. Mosconi, E., Meggiolaro, D., Snaith, H. J., Stranks, S. D. & De Angelis, F.\nLight-induced annihilation of Frenkel defects in organo-lead halide perovskites.\nEnergy Environ. Sci. 9, 3180-3187 (2016).\n15. Liu, T. et al. Efficient perovskite solar modules enabled by a UV-stable and\nhigh-conductivity hole transport material. Sci. Adv. 11, eadu3493 (2025).\n16. Fei, C. et al. Strong-bonding hole-transport layers reduce ultraviolet degradation\nof perovskite solar cells. Science 384, 1126-1134 (2024).\nWe sincerely hope the above reply addresses your concern.\nComment #2. When measuring the device IV curves under filtered/non-filtered\nlight conditions, the performance decline in non-filtered devices is primarily due\nto reductions in V and FF, while J is less affected. Why is this?\nOC SC\nCorrespondingly, the authors show in Figure 4D that the short-circuit current\ndensity of the unpassivated device is only 22.09 mA/cm², while that of the\npassivated device significantly increases to 26.22 mA/cm². How do the authors\nexplain this?\nResponse: Thank you for your insightful comment.\nFIRST, your comment pointed out a careless statement we made in the main text, as\ndetailed below.\nthe HP solar cell after being continuously illuminated by filtered and unfiltered lights\nfor 24 h.\nThe J-V characteristics shown in Figure R9 (Fig. S38 in the manuscript) are those of\nthe devices aged under filtered and unfiltered illuminations. They show larger\ndistinctions in V and FF, while J values are less discrepant. However, compared to\noc sc\nfresh devices, all J , V , and FF have significantly declined after aging, as shown\nsc oc\nin Figure R10 below.\nfiltered illumination and (b) unfiltered illumination. For filtering the illumination, a 420\nnm long pass is used.\nTherefore, it is incorrect for us to claim that “J is less affected” in the main text. The\nsc\ncorrect statement should be “J is less discrepant compared to V and FF after the\nsc oc\naging”. We made a careless, misleading statement in the main text. But this was not\nintentional, as we stated it correctly in Supplementary Text 6:\n“After 24 hours, the solar cell under unfiltered illumination exhibits a significant\ndecline in V to 0.906 V and FF to 57.15%, while the device under filtered illumination\noc\nretains a much higher V of 1.103 V and an FF of 64.72%. The residual J values are\noc sc\nless distinct between the two sets of solar cells, measuring 19.23 mA/cm² for the\nunfiltered and 20.50 mA/cm² for the filtered illumination.”\nWe are so sorry for this mistake and have revised the statement in the new manuscript\nversion, as shown below (on page 21):\n“In another test protocol, after being subjected to continuous illumination for 24 h, the\nunfiltered device exhibits a more severe decline in V and FF compared to the filtered\noc\ndevice (fig. S38 and Table S1), a pattern indicating defect formation as V and FF are\noc\nsensitive to defects (Supplementary Text 6).”\nIn principle, J is less sensitive to defects compared to V and FF. This is because J\nsc oc sc\nis largely determined by light absorption and charge transport, where defects primarily\n25\ncause recombination with minimal impact on these processes. In contrast, V and FF\noc\nare highly sensitive to defects due to their strong dependence on recombination losses.\nThe reason the aging causes the J decline here can be ascribed to defect-induced\nsc\ndegradation of the interface between the perovskite layer and the charge transport layers\n(ACS Nano 10, 218-224 (2016)).\nSECOND, the reason the unpassivated device shows a much lower J than the\nsc\npassivated device is that the ammonium ligands, besides serving as passivating agents,\nalso play a role in the crystallization process and improve the crystallinity of the\nperovskite layer (Nature 616, 724-730 (2023)).\nIn our recipe, as we stated in the experimental section, we used octane-1,8-diammonium\nand phenylpropylammonium as the passivating agents. Octane-1,8-diammonium is\nincorporated in the perovskite precursor for bulk passivation, and\nphenylpropylammonium is for surface passivation. Besides bulk passivation, octane-\n1,8-diammonium also modulates the crystallization of the perovskite layer to improve\nits crystallinity (Adv. Mater. 36, e2400105 (2024)). Assuming smooth carrier extraction,\nJ of the solar cell is decisively determined by the light absorption and carrier transport\nsc\nof the perovskite layer. A higher crystallinity of the perovskite layer would contribute\nto both light absorption and carrier transport. Therefore, the passivated device shows a\nmuch higher J than the unpassivated device.\nsc\nTo clarify, we have added the following statement in the revised manuscript (on pages\n19-20), as shown below:\n“As shown in Fig. 4D, the pristine solar cell without any ammonium-based passivation\nexhibited a champion PCE of 17.23% (open-circuit voltage V = 1.10 V, short-circuit\noc\ncurrent J = 22.09 mA/cm2, fill factor FF = 70.71%). In contrast, proper passivation of\nsc\nthe HP layer using PPA and octane-1,8-diammonium significantly improved the device\nperformance, resulting in a champion PCE of 26.44% (V = 1.179 V, J = 26.22\noc sc\nmA/cm2, FF = 85.49%) in our lab. The certified PCE by a third party is 26.15% (for\nreverse scan, V = 1.188 V, J = 25.86 mA/cm2, FF = 85.12%, fig. S35). This contrast\noc sc\nconfirms the beneficial passivation effect induced by the ammonium ligands. Part of\nthe improvement also arises from the crystallization modulation effect of the\nammonium ligands.[50]”\n50. Jia, P. et al. Intermediate phase suppression with long chain diammonium\nalkane for high performance wide-bandgap and tandem perovskite solar cells.\nAdv. Mater. 36, e2400105 (2024).\nWe sincerely hope the above reply addresses your concern.\nComment #3. The authors propose the phenomenon of violet/ultraviolet light-\ninduced depassivation in passivated perovskite solar cells but do not provide\nspecific methods to address this problem in order to obtain UV-stable perovskite\nsolar cells.\nResponse: Thank you for your insightful comment.\nThis work focuses on unveiling the vulnerability of the widely used ammonium ligands\nagainst violet/ultraviolet irradiation and the underlying mechanism. Although we are\nunable to offer immediate solutions, this work could open the avenue to designing the\nnext-generation passivating agents to advance the operational durability of perovskite\nsolar cells.\nBy taking a close look at the depassivation mechanism reported here, it works because\nviolet and ultraviolet photons are sufficient to pump the carrier transition from the\nammonium HOMO to the perovskite E /CBM states. Therefore, the light-induced\nx\ndepassivation can be prevented if the HOMO-E /CBM gap increases beyond the violet-\nx\nultraviolet photo energy. Also, given that this mechanism renders a trend for\ndepassivation, multi-site anchoring could also alleviate the light-induced depassivation.\nAccordingly, we believe the following directions might merit consideration:\n1) Ammonium ligands exhibit adjustable chemical properties. Functional groups on\nthese ligands can be engineered to increase the ionization potential of the\nammonium moiety.\n27\n2) Given that this mechanism tends to disrupt the interaction between ammonium\nligands and the perovskite lattice, multi-site anchoring could help alleviate the light-\ninduced depassivation.\n3) Instead of using ammonium ligands directly as passivating agents, 2D perovskites\ncan be a sturdier alternative. 2D perovskites can be tailored not only in the\nammonium ligands but also in the perovskite sublattice. By decreasing the electron\naffinity of the perovskite lattice (e.g., by using non-lead 2D perovskites or alloying),\nthe HOMO-E /CBM gap can also be increased.\nx\n4) New types of ligands beyond ammonium-terminated ligands should be developed\nfor perovskite solar cells. For instance, phosphorus-, oxygen-, and sulfur-centered\nligands, such as phosphonium salts, carboxylic acids, and thiols, show great\npromise. They are not as widely applicable in perovskites as ammonium ligands,\nwarranting further exploration.\nWe have added the above discussion in the revised manuscript in the “Discussion”\nsection (on pages 23-24). We hope this modification addresses your concern and helps\ninspire possible solutions. Thank you for your understanding.\nWe sincerely hope the above reply addresses your concern.\nComment #4. It would be beneficial if the authors could provide the EQE of the\nperovskite solar cells before and after UV aging, along with corresponding analysis,\nto gain a deeper understanding of the perovskite's photo-response.\nResponse: Thank you for your insightful comment. We have reimplemented the ageing\ntest to supplement data in response to this comment. Below, we show the EQE data for\nthe UV aging in both the ambient air and the N -filled glovebox. Other complementary\n2\ndata can be found in responses to other comments.\nFor the ageing test carried out in the ambient air, the depassivation mechanism has\na synergetic effect with the air. This is because the depassivation mechanism pumps the\nelectrons from the HOMO level in the ammonium ligands to the CBM/E states in the\nx\nperovskite lattice, forming an excited state that is vulnerable to attack by redox\n28\nsubstances in the air, such as oxygen.\nFigure R11. The I-V characteristics of passivated solar cell devices subjected to (a) the\n365 nm irradiation and (b) the 450 nm irradiation in the ambient air, respectively. (c)-\n(d) The corresponding EQE spectra for the two devices before and after the ageing test.\nWe carried out the tests in a fume hood with two irradiation sources (365 nm and 450\nnm) for comparison. The 365 nm irradiation can trigger the depassivation mechanism,\nwhile the 450 nm irradiation cannot. The powers of both irradiations were set to 20\nmW/cm2. We irradiated both devices for 30 min and then restored them in a N -filled\n2\nglovebox in the dark for 5 h.\nAs shown in Figure R11a and Figure R11b, the J of both devices barely changes. The\nsc\nimpact of the irradiations is reflected in V and FF. Notably, the FF of the device\noc\nsubjected to the 365 nm irradiation shows a much more serious decline in FF (from\n85.16% to 69.98%) compared to the device subjected to the 450 nm irradiation (from\n85.19% to 76.60%). This observation suggests the generation of more defects by the\n365 nm irradiation, which is in line with the depassivation mechanism.\nsc\nnot exhibit a conspicuous change for both devices, the EQE spectrum of the 365 nm-\nirradiated device shows that the EQE values decrease in the short-wavelength region\nbut increase in the long-wavelength region. This could be ascribed to slight phase\nsegregation that changes the light-responsive pattern. As such, this phenomenon agrees\nwith the depassivation mechanism, as the passivation failure will destabilize ions to\nfacilitate phase segregation.\nFigure R12. EQE spectra of solar cell devices subjected to (a) a simulated AM 1.5G\nillumination with photons of <420 nm wavelength filtered and (b) a non-filtered,\nfull-\nspectrum AM 1.5G irradiation.\nFor tests carried out in an N -filled glovebox, the solar cell devices were subjected\n2\nto two illumination conditions for comparison: 1) simulated AM 1.5G illumination with\nphotons of <420 nm wavelength filtered; 2) non-filtered, full-spectrum AM 1.5G\nillumination. The EQE spectra of the devices before and after the ageing test are shown\nin Figure R12. Consistent with the results obtained in the ambient ageing test, the J\nsc\nbarely changes after the ageing test in the N -filled glovebox. Besides, due to the\n2\nabsence of air, the phase segregation phenomenon is also alleviated, and the EQE\nprofiles for both devices barely change before and after the test.\nThe above EQE data reveal that J is less sensitive to the depassivation mechanism,\nsc\nagreeing with the statement in our manuscript. This is because J is essentially less\nsc\nsensitive to defects compared to V and FF. The barely impacted J also confirms that\noc sc\nthe tests we carried out do not cause a comprehensive degradation of the perovskite\nsolar cell, making the argument of defect depassivation more convincing.\nComment #5. From the data in Figure 4E, it can be seen that under illumination\nof 1.064 suns, the performance of the perovskite solar cell decreased by nearly 10%\nin less than 300 seconds of light exposure. This cannot simply be attributed to\nultraviolet light in the solar spectrum; the perovskite cell itself has severe intrinsic\nlight stability issues. Even under UV-filtered light, the performance of the\nperovskite cell also showed a significant decrease within the same timeframe,\ndropping from a peak of about 1.05 to around 0.98.\nResponse: Thank you for your insightful comment, which helps us improve our\nargument.\nTo understand this phenomenon, the mechanisms underlying the testing protocol\nwarrant clarification, which is detailed in the response to Comment #1. To isolate the\neffect of the depassivation, we adopted an intermittent testing protocol with light-dark\nalternations. The solar cell in every cycle was first illuminated for 7 seconds, during\nwhich an I-V scan was applied, and was then allowed to recover in the dark for 3\nseconds for self-healing. As detailed in the response to comment #1, a defect\nannihilation process (annihilation of I--V+ Frenkel pairs) and a defect generation\ni I\nprocess (two I0 combining into I ) would be triggered simultaneously during the\ni 2\nillumination period, and both are ion migration-related (Nat. Photonics 13, 532-539\n(2019).\nThe defect generation process tends to occur at a defective region where iodide ions are\npoorly stabilized (Nat. Photonics 13, 532-539 (2019)). According to the depassivation\nmechanism reported here, violet/ultraviolet lights will disrupt the ligand-perovskite\ninteraction, so the depassivated sites (including I- sites) would lose anchoring and tend\nto migrate. The I-V scans are applied not only to record the performance evolution\nbut, more importantly, also to drive the ion migration, so that the defect evolution\ncan be reflected in performance.\nheterojunction during I-V scans.\nNotably, the cyclic I-V scan test is not like the maximum power point (MPP) tracing\ntest commonly adopted in perovskite solar cells. It is a dynamic testing condition. As\nillustrated in Figure R13, when at V , the built-in potentials are mitigated by the\noc\napplied bias voltage to the extent that the drift current equals the diffusion current; when\nat J , no bias voltage is applied, and the built-in electric fields are at their maxima.\nsc\nTherefore, during the I-V scans, ions within the depletion region would be subject to\ndynamic built-in electric fields. As such, ion migration is more easily triggered\ncompared to that in an MPP tracing test, in which the built-in electric field is constant.\nWe have made modifications in the manuscript on page 20 to clarify, as shown below\n(fig. S36):\n“The built-in potentials of the heterojunctions vary during the I-V scan in each\nreplication, causing periodic changes in the built-in electric field that affect the ions in\nthe depletion region (fig. S36).”\nAdditionally, we would also like to clarify that the common MPP test was not adopted\nbecause it may unintentionally alleviate the harm caused by UV lights for two reasons:\n1) Encapsulation is often applied to enhance protection. These encapsulation\nmaterials (e.g., UV-curable epoxy, polyisobutylene, glass, ethylene vinyl acetate,\n32\nionomer) often effectively isolate UV lights (either by absorption or reflection);\n2) The light sources for the long-time MPP testing are usually inorganic light sources\nthat contain much less UV light in the spectrum (Science 384, 1126-1134 (2024)).\nTherefore, the MPP testing exhibits a longer device lifetime. For your reference, the\nammonium ligand-passivated solar cells here can realize a T of 1400 h (unpublished\n82\ndata, used as a control device in another work in preparation), confirming its quality.\nTo summarize, the seemingly rapid performance drop is not only because of\nillumination but is a result of combined illumination and dynamic electric field stress.\nThe performance drop is attributed to a defect generation process exacerbated by the\ndepassivation mechanism.\nWe sincerely hope the above reply addresses your concern.\nComment #6. Another intriguing phenomenon: during the light/dark cycling test,\nthe efficiency of the perovskite solar cells increased significantly under both UV-\nfiltered and UV-retained light, but the increase was greater under UV-filtered light.\nWhy is this? Additionally, what was the initial efficiency of the devices used for the\nlight/dark cycling test?\nResponse: Thank you for your insightful comment.\nThe efficiency evolution during the cyclic test is a result of the competition between\nthe light-induced defect annihilation (annihilation of I--V + Frenkel pairs) and\ni I\nlight-induced defect generation (two I0 combining into I ) processes (Nat.\ni 2\nPhotonics 13, 532-539 (2019)). The defect generation process tends to occur in a\ndefective region where I- is poorly stabilized. In the filtered device, the depassivation\nof ammonium ligands is not triggered, leaving I- sites better stabilized, so the defect\nannihilation process is favored, improving the efficiency. In contrast, in the unfiltered\ndevice, the depassivation mechanism is activated by violet/ultraviolet photons to\ndisrupt the ligand-perovskite interaction. Therefore, more I- sites are exposed,\n33\nexacerbating the defect generation process. Consequently, the unfiltered device shows\na slower efficiency increase initially, as well as a faster efficiency drop later.\nThis clarification was embedded in Supplementary Text 5. In the revised version, we\nhave added the following text in the main text to clarify (on page 20):\n“…The harm caused by violet/UV photons to HP solar cells can occur in several ways.\nTo isolate the effect of the depassivation mechanism here, we subjected the passivated\nsolar cells to a fatigue test with intermittent light-dark alternation. A detailed discussion\ncan be found in Supplementary Text 5. During each replication, the solar cell was first\nilluminated for an I-V scan for 7 seconds, and then allowed to recover in the dark for 3\nseconds for the self-healing of metastable states.[51] Illumination will evoke a defect-\nannihilation process (I--V+ Frenkel pairs annihilation) and a defect generation process\ni I\n(two I0 combining into I ), and the defect generation process tends to occur in a\ni 2\ndefective region where I- is poorly stabilized.[48, 52] The built-in potentials of the\nheterojunctions vary during the I-V scan in each replication, causing periodic changes\nin the built-in electric field that affect the ions in the depletion region (fig. S36)…”\nAbout the discrepancy in the PCE rise (on pages 20-21):\n“…As shown in Fig. 4E, for both illumination conditions, the PCEs increase after the\ninitial several replications, indicating the dominance of the defect annihilation process\ninitially.[48, 52] The device subjected to the unfiltered illumination manifests a weaker\nPCE increase, which implies a less favored defect annihilation process. This result is\nconsistent with the violet/UV light-induced depassivation mechanism, i.e., the ligand\nanchoring for I- sites is disrupted, so the defect generation process is exacerbated.\nIndeed, the absorption spectrum characterization throughout this test reveals a\nsignificantly more pronounced decline in light absorption for the unfiltered device (fig.\nS37), which can be ascribed to more pronounced defect generation. Subsequently, the\nPCE starts to decline as the defect formation process gradually takes over. The decline\nhappens first in the unfiltered device. After 47 replications, the device under the filtered\nillumination retains 98.0% of its initial PCE, while the value significantly drops to 91.4%\nfor the unfiltered device…”\ndevices subjected to (a) the non-filtered, full-spectrum simulated AM 1.5G solar\nillumination, and (b) the simulated AM 1.5G solar illumination with photons of <420\nnm wavelength filtered.\nThe initial efficiencies of the solar cell devices subjected to the fatigue test are 23.12%\nfor the non-filtered illumination and 22.31% for the filtered illumination, as shown in\nFigure R14. They were fabricated from one batch to ensure a fair comparison, despite\na slight discrepancy in performance.\nWe sincerely hope the above reply addresses your concern.\nComment #7. It is mentioned in the text that the depassivation mechanism induced\nby violet/ultraviolet light is generally applicable to ammonium-based ligands, but\ncurrently, near-ultraviolet light bleaching (PB) signals have only been observed in\na few 2D perovskites and quasi-2D systems such as BA PbI . It is suggested to\n2 4\nsupplement more 3D perovskite data of ammonium ligands with different\nstructures (such as those containing aromatic rings, long-chain alkyl groups, etc.),\nespecially the direct evidence of electron transfer at the ligand-perovskite interface\nin the pure 3D MAPbI system, to confirm the universality of the mechanism in\n3\ndifferent dimensions and ligand structures.\nResponse: Thank you very much for your insightful comment.\nbleaching signals in a series of n=1 2D perovskites constructed with different organic\nligands, including butylammonium, hexylammonium (HA), octylammonium (OA),\ndodecylammonium (DA), phenylethylammonium (PEA), and phenylpropylammonium\n(PPA). These ammonium ligands include aliphatic ammoniums and aromatic\nammoniums, and they feature different lengths of the aliphatic and aromatic groups.\nBesides, we have also detected the PB signals in all their mixed-n quasi-2D perovskite\nsystems, except dodecylammonium whose hydrophobicity inhibits the formation of its\nquasi-2D perovskite film. The data have all been shown in the Supplementary\nInformation file, Figure S6~S10 for n=1 series, Figure S28-S31 for mixed-n\nsystems. Please kindly refer to the Supporting Information file for details.\nThe reason we did this research with 2D perovskite systems is that 3D-ligand systems\noffer surface ligand density, making the detection of this mechanism challenging. When\ntrying to increase the surface ligand density, 3D perovskites easily transition into (quasi-)\n2D perovskites to interfere with the interpretation of the results. The 3D-to-2D\ntransition can even be exacerbated in the sample for the transient absorption spectrum,\nbecause the MAPbI film sample for the transient absorption spectrum measurement\n3\nhas to be very thin, which facilitates the 3D-to-2D transition. Therefore, preventing the\nformation of 2D perovskites requires a prudent control over the ligand dose. We have\nprovided this statement in the previous manuscript version (on page 6):\n“…To effectively investigate the proposed mechanism, a sample with a high density of\nwell-ordered ammonium-HP bonds is essential. The ammonium-passivated 3D HP\nsystem is not an ideal platform due to the sparse and disordered distribution of the\nligands on the 3D HP surface.[32] Instead, we utilized Ruddlesden-Popper 2D HPs with\nthe formula L PbI , where L represents ammonium-terminated ligands...”\n2 4\nEven for 3D-ligand systems without any 2D perovskite side-products, detecting the\nphotobleaching signal can be challenging. This is because the energy of the ligand-to-\nperovskite transition overlaps with that of the high-energy absorption of pure 3D\nMAPbI , making the interpretation of the signal difficult. The corresponding statement\n3\nand data (Figure S34) in the prior manuscript version are shown below (on page 18):\n36\n“…Notably, resolving the ammonium-to-HP electron transition signal in surface-\ntreated 3D HP films using TA characterization is challenging due to its overlap with the\nintrinsic high-energy absorption signals of 3D HPs, as well as difficulties in eliminating\ninterference from 2D HPs (fig. S34)…”\nFig. S34. TA spectra of 3D MAPbI HP films treated with BAI. The surface treatment\n3\nwith ammonium ligands can easily cause the formation of 2D HPs, which complicates\nthe interpretation of the PB signals. The formation of 2D HPs is even exacerbated in\nthe 3D HP film for the TA measurement, as the film needs to be very thin, so the 3D-\nto-2D conversion is easier to occur. It was not until the concentration of BAI was\nreduced to 0.3 mg/ml that the formation of 2D HPs was prevented. However, MAPbI\n3\nitself exhibits high-energy absorption signals, one of which broadly distributes in the\nultraviolet region and overlaps with the ammonium-HP interaction signal. This overlap\nmakes it challenging to resolve the ammonium-HP interaction signal in ammonium-\npassivated 3D HPs using TA spectrum measurements.\nAlthough we are unable to provide the transient absorption spectrum characterization\nin 3D perovskite systems due to the abovementioned challenges, we have provided\nsteady-state UV-vis absorption spectra to verify the mechanism. Take the BA surface-\n37\ntreated MAPbI film as an example, we observe an absorption enhancement in the 300\n3\nnm - 400 nm range, peaked at around 350 nm (3.10 eV - 4.13 eV, peaked around 3.54\neV). This absorption enhances with increasing BA concentration. The energy does not\nmatch the intrinsic absorption of BA itself (Fig. S3 in the manuscript) but agrees with\nthe depassivation mechanism:\nFigure R15. (Figure S32a in the manuscript). (a) Steady-state absorption spectra of 3D\nHP (MAPbI ) films treated by BAI solutions with various concentrations on the surface.\nammonium ligands, including hexylammonium, octylammonium, dodecylammonium,\nphenylethylammonium, and phenylpropylammonium. These ligands include aliphatic\nand aromatic types with a series of alkyl/aromatic chain lengths, verifying universality.\n3D HP MAPbI before and after surface treatments with various ammonium ligands,\n3\nincluding (a) PPA (in the form of PPAI), (b) PEA (in the form of PEAI), (c) DA (in the\nform of DAI), (d) OA (in the form of OAI), (e) and HA (in the form of HAI). Insets:\nZoom-in regions of absorption spectra in the range of 300-400 nm.\nWe can assess the validity of the energy range of the enhanced absorption by\ndrawing a comparison with BA PbI . In BA PbI , the butylammonium (BA+) ligands\n2 4 2 4\nare an integral part of the crystal structure, orderly arranged and strongly interacting\nwith the inorganic [PbI ]4- perovskite layers through electrostatic forces and hydrogen\n6\nbonding. This strong, highly ordered environment leads to an effective HOMO of the\nBA+ within this 2D system reported at -6.5 eV relative to the vacuum level. In contrast,\nthe attachment of BA+ ligands to the surface of 3D MAPbI perovskites is typically\n3\nmore random and dynamic. This implies a less extensive and less structured interaction\nbetween the -NH + group of the ligand and the underlying 3D perovskite surface,\n3\ncompared to its role as a structural spacer in 2D perovskites. This reduced perturbation\nfrom the perovskite lattice allows the N-H bonds within the BA+ cation to retain more\nof their intrinsic strength. Consequently, the electrons in the HOMO of the ammonium\ngroup are held more tightly within the ligand itself. This leads to a deeper (more\nnegative) HOMO energy level for the ammonium group when adsorbed on the 3D\nMAPbI surface, compared to its more delocalized and stabilized state within the\n3\nhighly ordered 2D BA PbI lattice. Considering that the typical CBM of MAPbI films\n2 4 3\nis -3.9 eV versus the vacuum level, and the absorption peak revealed by the absorption\n39\nspectra is 3.5 eV, we can estimate the HOMO level of the ammonium group on the 3D\nperovskite surface to be -7.4 eV. This value is indeed more negative than that in\nBA PbI (-6.5 eV), which aligns with our reasoning. Therefore, ascribing the enhanced\n2 4\nabsorption behavior to the ammonium-to-perovskite carrier transition is tenable.\nTo conclude, directly observing the ammonium-to-perovskite carrier transition in the\nligand-3D perovskite system with the transient absorption spectrum characterization is\nchallenging. This is because the dose of the ammonium ligands needs to be very low to\nprevent the interference of 2D perovskites, which easily form during the surface\ntreatment. The low ligand coverage on the 3D perovskite surface weakens the\nphotobleaching signal. Besides, the photobleaching signal overlaps with the intrinsic\nhigh-energy absorption of the 3D perovskite film to obscure the data interpretation.\nHowever, confirmed in n=1 and mixed-n 2D perovskite systems, the mechanism is in\nprinciple extendible to 3D perovskites. Experimentally, the steady-state absorption\nspectrum characterization shows an enhanced absorption in the ultraviolet region after\nthe 3D perovskite film is surface-treated with ammonium ligands. The absorption\nenhancement strengthens as the ligand concentration increases. This is observed for\nvarious ammonium ligands, confirming universality. The enhanced absorption and the\nenergy range align with the mechanism here. In the manuscript, follow-up\ncharacterizations, including FTIR and device characterizations, all agree with the\nmechanism reported here.\nWe sincerely hope the above reply addresses your concern. Thank you very much for\nyour understanding.\nWe sincerely hope the above reply addresses your concern.\nComment #8. The near-ultraviolet PB signal is attributed to the electron transfer\nfrom ammonium to perovskite, but the influence of impurities (such as unreacted\nPbI or ligand decomposition products) is not completely excluded in the\n2\nmanuscript. Although XRD shows that the purity of some samples is relatively\nhigh, it is necessary to further confirm through characterization methods such as\nhigh-resolution transmission electron microscopy (HRTEM) or time-of-flight\n40\nsecondary ion mass spectrometry (TOF-SIMS) that the signals only come from the\nligand-perovskite interface, rather than residual precursors or degradation\nproducts.\nResponse: Thank you for your comment.\nWe agree with the necessity to strengthen the conclusion that the near-UV\nphotobleaching signal is from 2D perovskites rather than potential impurities. We agree\nthat HRSTEM and TOF-SIMS can help understand the major components of the\nsamples, but respectfully, these characterizations may be less convincing in excluding\nimpurities. For example, even if an HRSTEM image is present to show high phase\npurity, the conclusion only applies to this specific region of observation, and it is\nimpractical to exclude all potential impurities in the entire film. This is why we adopted\nthe XRD characterization, as it provides a comprehensive analysis of the entire film.\nFurthermore, in response to your comment, we have added the following analysis to\nfurther strengthen the argument.\nFigure R17. (a) Pseudocolor ultrafast TA spectrum of the BA PbI film under 3.82 eV\n2 4\n(325 nm, 1 kHz, 100 fs, ~510 µW/cm2) laser excitation, showing two near-UV PB\nsignals in addition to the excitonic PB signal. (b) Schematic illustration of the electronic\nstructure of BA PbI with a highlight of key energy levels relative to the vacuum level.\n2 4\nThe transient absorption spectrum characterization can provide additional details to\nconfirm that the near-UV photobleaching signals are truly from 2D perovskites\nthemselves. In the previous version of transient absorption spectra, we only showed the\nphotobleaching signals caused by the HOMO-to-E (E refers to the excitonic state)\nx x\ncarrier transition. In fact, two photobleaching signals can be identified, and the other\n41\none matches the HOMO-to-CBM transition perfectly.\nTake BA PbI for an example, the detailed TA spectrum is shown in Figure R17a. In\n2 4\naddition to the excitonic absorption photobleaching in the range of 2.43-2.57 eV, there\nare two photobleaching signals in the near-UV region. The low-energy one at 2.90-3.10\neV was introduced in the prior manuscript version. It corresponds to the HOMO-to-E\nx\ncarrier transition. In addition, there is another high-energy PB signal centered at ~3.3\neV. The energy level distribution of the three photobleaching states well matches the\nenergy band structure of BA PbI , as shown in Figure R17b.\n2 4\nThe two PB signals in the near-UV region are also observed in a series of other 2D\nperovskites constructed with various ammonium-terminated ligands (hexylammonium\n(HA), octylammonium (OA), dodecylammonium (DA), phenylethylammonium (PEA),\nand phenylpropylammonium (PPA)), as shown in Figure R18 below:\nFigure R18. Pseudocolor ultrafast TA spectrum of a series of 2D perovskites\nconstructed with various ammonium-terminated ligands. In all these samples, two PB\nsignals in the near-UV region can be identified. These 2D perovskites are (a) PPA PbI ,\n2 4\n(b) PEA PbI , (c) DA PbI , (d) OA PbI , (e) HA PbI . Notably, the signals in DA2PbI4\n2 4 2 4 2 4 2 4\nare relatively weak, which can be ascribed to a less crystalline nature of the film due to\nthe ultrahigh hydrophobicity of DA.\nThe observation of two PB signals with their energies closely matching the HOMO-to-\nE carrier transition and the HOMO-to-CBM carrier transition, along with the XRD\nx\n42\ncharacterization, provides strong support that the PB signals are from 2D perovskites\nthemselves. Accordingly, we have revised the corresponding statement in the\nmanuscript (on page 8), as shown below:\n“Based on the energy diagram, the energies of the two near-PB signals (2.9 - 3.1 eV for\nthe low-energy one and 3.2 - 3.4 eV for the higher-energy one) closely match those of\nthe HOMO-to-E (3.1 eV) and HOMO-to-CBM (3.4 eV) carrier transitions,\nx\nrespectively. The minor deviations are reasonable considering that the DFT calculation\nadopted for the HOMO in the previous report usually overvalues the result. A\nsupportive piece of evidence might be the emergence of the onset of a new band in the\nFermi region of the UPS spectrum at approximately 0.43 eV below the VBM (fig. S11),\nwhich can tentatively be attributed to the HOMO. As such, the true HOMO might be\nwithin the range of -6.25 eV (-5.82 eV minus 0.43 eV) to -6.50 eV versus the vacuum\nlevel, so the HOMO-E gap and the HOMO-CBM gap could be in the range of 2.85 -\nx\n3.10 eV and 3.15 - 3.40 eV, justifying the above deviations. Notably, the HOMO-to-E\nx\nand HOMO-to-CBM transitions are also consistent with the above observation of anti-\nStokes PB signals.”\nWe sincerely hope the above reply addresses your concern. Thank you for your kind\nunderstanding.\nWe sincerely hope the above reply addresses your concern.\nComment #9. In the manuscript, the HOMO/LUMO energy levels of ligands are\ndefined based on the DFT calculation results of Silver et al. However, different\ncalculation methods (such as functional selection and basis set size) may lead to\ndeviations in energy values. It is suggested to supplement the first-principles\ncalculations for specific ligands (such as BA+), clarify the consistency degree\nbetween the energy difference of HOMO-exciton (E ) and the energy of the near-\nx\nultraviolet PB signal observed experimentally, and discuss the possible sources of\nthe deviation between the calculation and the experiment (such as solvent effect,\n43\nexciton binding energy).\nResponse: Thank you very much for your comment.\nThere is very limited report on the detailed electronic structure of BA PbI 2D\n2 4\nperovskites, and the reference (Silver, S. et al. Adv. Energy Mater. 8, 1703468 (2018))\nwe provided in the manuscript is the only report we found that offers details on it,\nespecially the HOMO level position. This work is widely cited in the 2D perovskite\nresearch community (115 times cited), verifying wide acknowledgement. The limited\nsource we can access does not allow us to provide a more precise simulation than this\nwork.\nHowever, we can provide some additional experimental evidence and analysis to\nsupport the claim that the observed near-UV PB signal corresponds to the HOMO-to-\nEx carrier transition, as listed below:\n1) First, we detailed in the response to the above comment (comment #8). There are\nactually two near-UV PB signals. For the archetypal BA PbI sample exhibited in\n2 4\nthe main text, the lower-energy one is in the 2.90 - 3.10 eV range, and the higher-\nenergy one is in the 3.20 - 3.40 eV range. The energy gap between them matches\nthe reported exciton binding energy range for BA PbI (e.g., 260 meV reported in\n2 4\nSilver, S. et al. Adv. Energy Mater. 8, 1703468 (2018); 380 meV reported in Hong,\nH. L. et al. Nat. Commun. 15 (2024)).\n2) According to the electronic structure of BA PbI , the two near-UV PB signals\n2 4\nclosely match the HOMO-to-E and HOMO-to-CBM transitions, respectively. The\nx\nelectronic structure suggests a HOMO-to-E gap of 3.1 eV and a HOMO-to-CBM\nx\ngap of 3.4 eV. The two values reach the upper end of the energies of the two PB\nsignals observed. The deviations can be ascribed to an overvaluation of the HOMO\nposition common for DFT simulations (deeper than the actual value), and might be\njustified by our UPS characterization. As shown in Figure R19 below, the UPS\nspectrum reveals the emergence of the onset of a new band at approximately 0.43\neV below the VBM, close to but less deep than the simulated HOMO level. As such,\nthe HOMO-E gap and the HOMO-CBM gap could be in the range of 2.85 -\nx\n3.10 eV and 3.15 - 3.40 eV, justifying the above deviations.\nBA PbI film. (a) Fermi region and (b) secondary electron region of the UPS spectrum\n2 4\nof BA PbI . (c) Identification of the possible VBM-HOMO energy gap in the\n2 4\nlogarithmic scale of the Fermi region of the UPS spectrum.\nBased on the above analysis, we have strengthened the corresponding argument in the\nmain text (on page 8), as shown below:\n“…To better understand the origin of the near-UV PB signal, we quantitatively depict\nthe electronic structure of BA PbI with a highlight of key energy levels, including the\n2 4\nCBM, E , VBM, lowest unoccupied molecular orbital (LUMO) of the ligand layer, and\nx\nhighest occupied molecular orbital (HOMO) of the ligand layer, in Fig. 1D. The VBM\nis measured with Ultraviolet Photoelectron Spectroscopy (UPS) characterization (fig.\nS11). The obtained VBM (-5.82 eV) and the accordingly inferred CBM based on a\nbandgap of 2.7 eV agree with the previous report by Silver et al.[31] E is determined\nx\nper the excitonic absorption energy (fig. S2, 2.41 eV). The HOMO and LUMO also\nrefer to the report by Silver et al. obtained with density functional theory (DFT)\ncalculations.[31] Based on the energy diagram, the energies of the two near-PB signals\n(2.9 - 3.1 eV for the low-energy one and 3.2 - 3.4 eV for the higher-energy one) closely\nmatch those of the HOMO-to-E (3.1 eV) and HOMO-to-CBM (3.4 eV) carrier\nx\ntransitions, respectively. The minor deviations are reasonable considering that the DFT\ncalculation adopted for the HOMO in the previous report usually overvalues the result.\nA supportive piece of evidence might be the emergence of the onset of a new band in\nthe Fermi region of the UPS spectrum at approximately 0.43 eV below the VBM (fig.\nS11), which can tentatively be attributed to the HOMO. As such, the true HOMO might\nbe within the range of -6.25 eV (-5.82 eV minus 0.43 eV) to -6.50 eV versus the vacuum\nlevel, so the HOMO-E gap and the HOMO-CBM gap could be in the range of 2.85 -\nx\n3.10 eV and 3.15 - 3.40 eV, justifying the above deviations...”\n31. Silver, S. et al. Characterization of the valence and conduction band Levels of n =\n1 2D perovskites: a combined experimental and theoretical investigation. Adv.\nEnergy Mater. 8, 1703468 (2018).\nWe sincerely hope the above reply addresses your concern.\nComment #10. Figure 4E shows that the device efficiency decreases under\nultraviolet light, but the current data only infer the formation of defects through\nthe I-V curve. It is suggested to supplement in-situ characterization methods,\nsuch as time-resolved photoluminescence (TRPL) under light or spatially resolved\nsurface photovoltage spectroscopy (SPS), to directly monitor the density changes\nof non-radiative recombination centers caused by ligand depassivation, and\nestablish the real-time correlation between photocurrent attenuation and\nammonium dissociation.\nResponse: Thank you for your insightful comment. We agree that the suggested\nevidence will strengthen the argument in our manuscript.\nWe have carried out the TRPL test as suggested. However, the rapid degradation of the\ndevice during the TRPL test significantly interferes with the result interpretation. The\ndetails of the test are as shown below: In order not to activate the depassivation\nmechanism during the TRPL test, we used a 450 nm excitation wavelength. The\nexcitation power is ~70 μW. The device was in a configuration of\nITO/4PADCB/Al O /Cs FA MA PbI /LiF/C /BCP/Ag, the same device\n2 3 0.05 0.85 0.1 3 60\nconfiguration for the aging test. For the TRPL test, the device was tested unencapsulated,\nas encapsulation materials usually screen violet/ultraviolet photons, which is undesired\nin our control experiment. In our measurement, the device performance dramatically\ndeclines after only one round of the TRPL test. The TRPL result and the change in\nthe I-V characteristic before and after the TRPL test are shown in Figure R20 below:\nmeasured before and after the TRPL test.\nThe severe performance degradation is expected to arise from the synergetic effect\nimposed by the excitation light and the ambient air. Based on this observation, we are\nafraid this degradation would interfere with the analysis of the depassivation\nmechanism in this work.\nAlternatively, we measured the absorption spectra of the solar cell device during the\nlight-dark alteration test. Compared with the TRPL test, the absorption spectrum\nmeasurement is milder and more friendly for the devices, preventing damage as in the\nTRPL test.\nFor each cycle during the ageing test, the device was subjected to a reverse scan from\n1.2 V to -0.05 V under illumination, and was then followed by a recovery in the dark.\nThe illumination time is 7 s, and the dark recovery time is 3 s. Two illumination\nconditions were applied for comparison: one is non-filtered, full-spectrum simulated\nAM 1.5G solar illumination, and the other is filtered simulated AM 1.5G solar\nillumination that eliminates photons of wavelength below 420 nm.\ndevices subjected to (a) the non-filtered, full-spectrum illumination and (b) the filtered\nillumination, respectively, during the fatigue test.\nWe measured the evolution of the absorption spectrum of solar cell devices subjected\nto the two testing conditions. Due to the high reflectivity enabled by the Ag electrode,\nthe overall baseline of the absorption is high. But still, the absorption feature of the\nperovskite layer can be identified for analysis. The baseline-corrected absorption\nspectra are shown in Figure R21. For the device subjected to the non-filtered,\nfull-spectrum simulated AM 1.5G solar irradiation, which contains photons with\nwavelengths below 420 nm that can trigger the depassivation mechanism, a severe\ndegradation of the absorption feature is observed as the fatigue test proceeds. This can\nbe ascribed to the ion migration driven by the varying built-in electric field during the\nfatigue test, which exacerbates defect generation to compromise the light absorption.\nIn stark contrast, the absorption of the device subjected to the filtered illumination\nshows a significantly mitigated degradation feature. This result is in line with the\ndepassivation mechanism, as one of the key effects of ammonium ligands is to anchor\nhalide ions.\nAccordingly, we have added this discussion in the revised manuscript (on pages 20-21)\nto strengthen the argument, as follows:\n“We used two illumination conditions for comparison: violet/UV light-filtered solar\nillumination (using a >420 nm long-pass filter) and unfiltered illumination. The solar\n48\nsimulator we used was measured to contain 12.78% of its total irradiance at\nwavelengths below 420 nm, slightly higher than that in the standard AM 1.5G\nirradiation (~5%). The filtered light intensity was calibrated to compensate for the\nfiltering loss. Besides, to exclude interference from ambient factors, we conducted the\ntests in an N₂-filled glovebox. As shown in Fig. 4E, for both illumination conditions,\nthe PCEs increase after the initial several replications, indicating the dominance of the\ndefect annihilation process initially.[48, 52] The device subjected to the unfiltered\nillumination manifests a weaker PCE increase, which implies a less favored defect\nannihilation process. This result is consistent with the violet/UV light-induced\ndepassivation mechanism, i.e., the ligand anchoring for I- sites is disrupted, so the defect\ngeneration process is exacerbated. Indeed, the absorption spectrum characterization\nthroughout this test reveals a significantly more pronounced decline in light absorption\nfor the unfiltered device (fig. S37), which can be ascribed to more pronounced defect\ngeneration. Subsequently, the PCE starts to decline as the defect formation process\ngradually takes over. The decline happens first in the unfiltered device. After 47\nreplications, the device under the filtered illumination retains 98.0% of its initial PCE,\nwhile the value significantly drops to 91.4% for the unfiltered device.”\nReferences\nperovskite solar cells. Nat. Photonics 13, 532-539 (2019).\nhalide perovskites. Energy Environ. Sci. 9, 3180-3187 (2016).\nWe sincerely hope the above reply addresses your concern.\nComment #11. In the light reaction experiment, the increase in the generation rate\nof H indicates the deprotonation of the ammonium group, but there is a lack of\n2\nkinetic curves under different light intensities (such as the variation of H\n2\ngeneration amount with time). It is suggested to supplement the ultraviolet light\nintensity gradient experiment to quantify the relationship between photon flux\nand deprotonation rate, and analyze the activation energy of this process through\nthe Arrhenius equation to distinguish light-induced electron transfer from\n49\nthermally driven ligand decomposition.\nResponse: Thank you for your insightful comment. We totally agree that the thermally\ndriven ligand decomposition should be excluded to strengthen the argument. In fact, we\nwould like to clarify that the H generation experiment was implemented at a fixed\n2\ntemperature (10 degrees Celsius) throughout the measurement. We feel sorry for not\nincluding this detail in the experimental section. Taking measurements at a fixed\ntemperature is a routine practice in the photocatalysis-related experiments, and its aim\nis exactly to exclude the impact of heating.\nAccordingly, we have added this detail to the revised manuscript (on page 15), as shown\nbelow:\n“Accordingly, the production of H is a direct result of the dynamic deprotonation of\n2\nthe ammonium groups. The vessel used for the photoreactions was evacuated to\nminimize interference from other potential oxidants and reductants. The temperature of\nthe vessel was fixed at 10 degrees Celsius during the test to prevent interference from\nheating. The 2D HP is stable throughout the test as discussed in the Supporting\nInformation file (fig. S24 and Supplementary Text 2).”\nThe precise measurement of the photon flux and the deprotonation rate, while surely\nwould contribute to the discussion, can be challenging in practice. In this experiment,\n2D perovskites in the form of nanoplates disperse in an HI aqueous solution, which is\nin a glass container (vacuumed and temperature fixed). Illumination is from an external\nlight source, so incident photons have to pass the glass container (reflection), be\nabsorbed by the HI aqueous solution (absorbs photons with a wavelength below 400\nnm), and be scattered by the 2D perovskite nanoplates themselves before being\nabsorbed by the 2D perovskites. These processes are not linear with intensity.\nirradiation and the 500 nm irradiation, respectively.\nAlternatively, to strengthen the argument, we measured the external quantum efficiency\n(EQE) of the photocatalytic H generation upon monochromatic irradiations of 365 nm\n2\nwavelength and 500 nm wavelength, respectively. The 365 nm is energetically\nsufficient to trigger the deprotonation mechanism, while the 500 nm irradiation\nresonates with the absorption of the excitonic absorption of 2D perovskites. EQE is the\nratio of the number of useful reaction events (H generation in this case) by a\n2\nphotocatalyst to the total number of photons that strike it from an incident light source.\nGenerally, UV photons above the bandgap should deliver a lower EQE due to thermal\nenergy loss, and UV photons are more prone to loss by the abovementioned adverse\nprocesses (reflection, absorption, scattering). However, we found that the 365 nm\nirradiation still delivers a much higher EQE (43.4%) than the 500 nm irradiation\n(23.2%), which aligns with the photon-induced deprotonation effect in our work. The\nresidual H generation by the 500 nm excitation arises from the direct exciton transfer\n2\nfrom 2D perovskites to H+/I- ions, as detailed in the manuscript.\nWe sincerely hope the above reply addresses your concern.\npractical applications, water vapor and oxygen may accelerate the degradation of\nperovskite. It is suggested to supplement the comparative experiments in the air\nenvironment to clarify whether the depassivation induced by violet/ultraviolet\nlight is independent of environmental factors or has a synergistic effect with water\nvapor. Furthermore, it is necessary to explain whether the intensity of the\nultraviolet light used in the test matches the ultraviolet component in the actual\nsunlight (for example, the ultraviolet proportion in the AM 1.5G spectrum is\napproximately 5%).\nResponse: Thank you for your comment.\nThe light-induced depassivation mechanism should have a synergetic effect with the\nlight-assisted erosion of oxygen/moisture in the air. This is because the violet-\nultraviolet photons will pump the ammonium-to-perovskite electron transition,\npotentially exacerbating the redox erosion by oxygen/moisture in the air to boost the\ndepassivation process.\nExperimentally, we used solar cells with the configuration\nITO/4PADCB/Al O /Cs FA MA PbI /LiF/C /BCP/Ag. It should be noted that\n2 3 0.05 0.85 0.1 3 60\nthis is the same proof-of-concept device configuration in the manuscript, but in our\nprevious version, the device configuration is mistakenly written as ITO/NiO /Meo-\nx\n4PACZ/Cs FA MA PbI /LiF/C /BCP/Ag due to multi-person editing, and we are\n0.05 0.85 0.1 3 60\nso sorry for this careless mistake, but it does not impact the core of this work. The\nperovskite layer is passivated with octane-1,8-diammonium and\nphenylpropylammonium. One cell was subjected to 365 nm irradiation in the air for 30\nmin. The irradiation power was 6.4 mW/cm2. Another solar cell was subjected to 450\nnm irradiation in the air with the same irradiation period and irradiation intensity. The\n365 nm irradiation can activate the depassivation mechanism, while the 450 nm\nirradiation cannot. After the irradiation test, both samples were stored in the glovebox\nin the dark for 5 h’s recovery. The recovery process eliminates the impact of metastable\ntrap states.\nthe 365 nm irradiation test. (b) Initial and after-ageing I-V characteristics of the solar\ncell subjected to the 450 nm irradiation test. (c) The EQE spectra of the device in (a)\nbefore and after ageing. (d) The EQE spectra of the device in (b) before and after ageing.\nTable R1. Performance metrics of solar cells before and after ageing\nIrradiation Sample J (mA/cm2) V (V) FF (%) PCE (%)\nsc oc\nInitial 22.70 1.140 85.16 22.04\n365 nm\nAgeing 22.80 1.091 69.98 17.41\nInitial 23.31 1.143 85.19 22.70\n450 nm\nAging 23.41 1.097 76.60 19.67\nThe device performance before and after the ageing process is shown in Figure R23\nand Table R1. Both ageing processes cause an efficiency drop. Notably, the 365 nm\nirradiation causes a significant decline of FF from 85.16% to 69.98%. In contrast, the\ndevice subjected to the 450 nm irradiation only manifests an FF drop from 85.19% to\n76.60%. This result indicates the emergence of additional shunts in the 365 nm-\nirradiated device, which is in line with the depassivation mechanism proposed. The J\nsc\nvalue does not change conspicuously for both devices before and after ageing, and the\nV value drops are almost identical.\noc\nFor the 365 nm-irradiated device, the EQE spectra show that the EQE values in the\nshort-wavelength region decrease and those in the long-wavelength region increase\nafter the ageing. This could arise from phase segregation that generates wider-bandgap\nphases near the bottom and narrower-bandgap phases in the bulk. In contrast, for the\n450 nm-irradiated device, the EQE profile barely changes before and after ageing. This\ncontrast is consistent with the depassivation mechanism. That is, the ammonium-\nligands fail to anchor ions upon the 365 nm irradiation to facilitate the phase segregation.\nBased on the above analysis, we infer that the light-induced depassivation mechanism\nshould have a synergetic effect with the erosion by oxygen and moisture in the air.\nThe relative contribution of photons with wavelengths shorter than 420 nm to the total\nirradiance of the solar simulator in our lab, a Newport 94023A solar simulator, was\ndetermined using a Newport 91150V reference cell and a Thorlabs FGL420M long-\npass filter. Based on our measurement, the Newport 94023A solar simulator produces\napproximately 12.78% of its total irradiance at wavelengths below 420 nm,\nequivalent to 7.95% Suns under the photoresponse of the Newport 91150V reference\ncell. In standard AM 1.5 G solar irradiation, the irradiance should indeed be ~5%,\nas you commented. Our solar simulator for the solar cell device test contains a higher\nproportion of the irradiance at wavelengths below 420 nm. The effective inclusion of\nthis violet/ultraviolet irradiation makes the comparative solar cell characterization\nconvincing. Although this might cause deviation in measuring the device performance,\nthe contribution from the violet/ultraviolet part is relatively minor. Besides, we have\nalso provided certificated solar cell performance (Fig. S36 in the Supplementary\nInformation file) to validate the device quality for this study.\nTo clarify the irradiance proportion in the solar simulator we used, we have added the\nfollowing clarification in the revised manuscript (on page 20):\n“We used two illumination conditions for comparison: violet/UV light-filtered solar\nillumination (using a >420 nm long-pass filter) and unfiltered illumination. The solar\n54\nsimulator we used was measured to contain 12.78% of its total irradiance at\nwavelengths below 420 nm, slightly higher than that in the standard AM 1.5G\nirradiation (~5%).”\nWe sincerely hope the above reply addresses your concern.\nComment #13. The FTIR results show that the ligand signal weakens after\nultraviolet irradiation, but it is only a static characterization. It is recommended\nto use in situ FTIR to monitor the real-time changes of ammonium characteristic\npeaks (such as N-H scaling vibration) during the irradiation process, clarify the\ndynamic path and reversibility of deprotonation (-NH + → -NH ), and rule out the\n3 2\npossibility of physical desorption of ligands.\nResponse: Thank you for your insightful comment. Following your comment, we have\nmeasured the irradiation time-dependent FTIR spectra. The details are shown below:\nWe selected MAPbI as the perovskite sample, as it is single-anion, single-cation, and\n3\nphase-stable at room temperature. Otherwise, there could be a phase segregation or a\nphase transition that interferes with the interpretation. The ligand pair butylammonium\nand oleic acid was used. 100 mg MAPbI polycrystalline powder was dispersed in a 5\n3\nmL chlorobenzene solution containing 11.9 mg butylamine and 45.8 mg oleic acid (1:1\nin molar ratio). The solution was gently stirred with a magnetic stirrer during the test.\nIn the meantime, we applied a monochromatic illumination of 365 nm to the solution\nwith a power density of ~25 mW/cm2. The 365 nm irradiation can trigger the\ndepassivation mechanism. We extracted the dispersion every hour and isolated the\nMAPbI powders by centrifugation and vacuum drying. In comparison, the same\n3\naluminum foil to avoid irradiation. The irradiated sample with different irradiation\ntimes, the non-irradiated sample, and the pristine MAPbI sample were sent for FTIR\n3\nmeasurement.\nsample.\nThe FTIR characterization confirms the gradual detachment of ligands under the 365\nnm irradiation. As shown in Figure R24, the 2854 cm⁻¹ peak arises from the symmetric\nstretching vibration of -CH - groups and can originate from both the butylammonium\n2\nand oleic acid segments in the ligand pair, while it is not detected in non-passivated,\npristine MAPbI . Notably, the characteristic peak of the N-H scaling vibration may not\n3\nbe an ideal observation target, as non-passivated MAPbI perovskite intrinsically shows\n3\na strong N-H scaling vibration signal as well. Importantly, as the irradiation time\nincreases, this characterization peak wanes gradually, indicating the detachment of the\nligands from the perovskite surface. This is in line with the photoinduced depassivation\nmechanism introduced in our work. After 5 h of irradiation, this characteristic peak\nbecomes barely discernible. In contrast, for the non-irradiated sample, the 2854 cm⁻¹\npeak remains after the 5 h test. This contrast rules out the impact of physical detachment\nand confirms that the 365 nm irradiation is the driving factor of the ligand detachment,\nagreeing with the photoinduced detachment mechanism.\nAccordingly, we have made modifications in the revised manuscript (on pages 18-19),\n56\nas shown below:\n“Consequently, exposure to violet/UV photons causes the deprotonation of ammonium\nligands, disrupting the ammonium-HP interaction. To verify this, we conducted Fourier-\ntransform infrared (FTIR) spectroscopy on both UV-irradiated (365 nm @ 25 mW/cm²\nfor 5 h) and non-irradiated ammonium-passivated MAPbI samples. During irradiation,\n3\nthe 3D HP powder was gently stirred and dispersed in chlorobenzene with ligands. It is\nwell established that HPs adopt an X-type ligand binding configuration, where Lewis\nacid and Lewis base moieties attach to the surface of HP in pairs.[32, 46] Herein, we\nemployed BA and oleic acid (1:1 in molar ratio) as the ligand pair. The 3D HP powder\nwas collected every one hour, followed by isolation for the FTIR characterization. As\nshown in Fig. 4C, the 2854 cm⁻¹ peak arises from the symmetric stretching vibration of\n-CH - groups. It originates from both the butylammonium and oleic acid segments in\n2\nthe ligand pair but is not detected in MAPbI . Notably, as the irradiation time increases,\n3\nthe -CH - peak wanes gradually, indicating the detachment of ligands from the HP\n2\nsurface. After 5 h of irradiation, this characteristic peak becomes barely discernible. In\ncontrast, for the non-irradiated sample, the 2854 cm⁻¹ peak remains after the 5h test.\nThis contrast confirms that the 365 nm irradiation is the driving factor for ligand\ndetachment, in agreement with our mechanism.”\nReferences\n32. De Roo, J. et al. Highly dynamic ligand binding and light absorption coefficient\nof cesium lead bromide perovskite nanocrystals. ACS Nano 10, 2071-81 (2016).\n46. Nenon, D. P. et al. Design principles for trap-free CsPbX nanocrystals:\n3\nenumerating and eliminating surface halide vacancies with softer lewis bases. J.\nAm. Chem. Soc. 140, 17760-17772 (2018).\nIn terms of the dynamic deprotonation path, we have to admit that we are unable to\nelucidate it due to its complex nature. Even without considering the interference of air,\nthe ligand dynamics could involve not only the redox reaction of ligand pairs\nthemselves but also ions in the perovskite lattice (halide ions), as follows:\nBA+ + OLA- + MAPbI ⇄ BAI + MA(OLA) (2)\n3\nwhere BA denotes butylammonium, OLA refers to oleic acid. We believe that detailed\ndynamic paths require detailed exploration, which is unlikely to be incorporated into\nthis work. We sincerely hope for your kind understanding.\nIn the end, we sincerely thank you again for dedicating your valuable time to reviewing\nour work and for providing insightful feedback. We hope our responses adequately\naddress your concerns and align our manuscript with the high standards of Nature\nCommunications. We have made every effort to refine our work and welcome any\nfurther comments or suggestions in subsequent reviews. Thank you once again for your\nthoughtful contributions.\n58"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The paper can be published now since all the questions have been well addressed though the the degradation interference from other mechanisms were not separated.", "reviewer_2": "The authors have responsed the propoesed questions. This manuscript can be accepted now."}, "author_response": ""}]

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