Sequential Organic Ligand Modifications to Dedicatedly Restructure Grain Boundary and Surface of Perovskites

Domain: physics
Published: 2025-12-13
Editorial outcome: minor_revision

Abstract
The durability of perovskite solar cells (PSCs) is substantially limited by defects and imperfect structures at grain boundaries (GBs) and grain surfaces (GSs) of perovskites, where common single-step ligand passivation struggles due to ligand heterogeneous distributions and distinct chemical environments. Herein, we demonstrate a two-step sequential dedicated-ligand (TSS-DL) strategy that selectively targets GBs and GSs through spatially resolved interactions of the dedicated ligands. We identified two classes of dedicated ligands, including Ligand X, notably 2,2-difluoroacetamide bromide (DFABr) for GBs to restructure them into stable 2D perovskites and suppress defects, and Ligand Y, featuring 4-aminopiperidinium dibromide (4APPBr2) for GSs to create a rigid 2D perovskite overlayer that shields 3D perovskites from external stresses. TSS-DL-treated PSCs retain >93% of their power conversion efficiency (PCE) after 3,000 hours of operation and >90% after 4,000 hours under high humidity (>85% RH). By sequentially restructuring GBs and GSs using dedicated ligands, the work provides an effective way to overcome the instability of 3D PSCs while maintaining high performance.

Introduction
There has been significant interest from industry and academia in three-dimensional (3D) perovskite solar cells (PSCs), among the most promising photovoltaic technologies1,2. as the power conversion efficiency (PCE) has dramatically risen to over 27% today3–5. However, intrinsic instability and longevity of PSCs under real-world conditions (e.g., high humidity, continuous light soaking, and high temperature) remain significant challenges due to high defect densities at the grain boundaries (GBs) and grain surfaces (GSs) of polycrystalline perovskite film6,7. These defects may include undercoordinated halogen ions (X-) or Pb2+ ions, lead clusters, Pb1-anti-site defects, I1- or MA+, FA+, and Cs+ vacancies8. Various approaches have been reported to suppress these defects2,6,8,9. One of the recent attractive approaches is to use organic ammonium ligands that mostly form a 2D perovskite layer on 3D perovskite to not only passivate but also shield 3D perovskite surfaces from degradation induced by moisture and oxygen10, passivating defects, and suppressing ion migration8,9,11,12. However, the stability of 2D perovskite overlayers will vary or even degrade under thermal stress, highly dependent on the rigidity of the 2D perovskite which is seldom studied13,14. Besides, those 2D overlayers or surface passivation commonly formed by a single-step ligand process fail to uniformly cover both perovskite GBs and GSs comprehensively, causing incomplete treatment and degrading device efficiency and stability6,15. More importantly, in many studies, perovskites with different GBs and GSs are often treated by one kind of ligand, and improvements in device performance observed may be due to passivating defects of GSs rather than addressing the issues of GBs16.

Notably, GBs and GSs have distinct chemical properties and structural environments. GBs are a homogeneous junction with a narrow spatial gap. They exhibit higher defect densities due to the misalignment of crystal lattices, which can lead to localized states that trap charge carriers. In contrast, GSs are a heterogeneous junction17–19. They primarily interact with their adjacent layer materials, leading to chemisorption or physisorption processes that alter their surface states. Therefore, heterogeneous distributions of the ligand and distinct chemical environments in GBs and GSs necessitate spatially tailored passivation strategies by dedicated ligands/ligands. Particularly, penetrability is important for GB additives because it affects how well they penetrate and interact with the defects within the GBs16,20. However, existing works predominantly employ mixed ligands in a single step, which do not show the spatial resolution to address the different defect chemistries of GBs and GSs21,22. Consequently, it is highly desirable to develop a strategy that effectively suppresses defects at both GBs and GSs with distinct chemical and structural characteristics to enhance stability and performance.

In this work, we introduce a two-step sequential dedicated-ligand (TSS-DL) strategy to independently address the distinct defect chemistries of GBs and GSs through their spatially resolved interactions of the dedicated ligands. Two classes of dedicated ligands in our TSS-DL strategy, including Ligand X and Ligand Y, are designed to target these specific defects by forming stable 2D perovskites and restructuring the GBs and GSs defective sites that enhance the perovskite material’s overall performance. In the first step, Ligand X represented by 2,2-difluoroacetamide bromide (DFABr) with a small size and three distinct functional groups, can effectively penetrate GBs and suppress buried multiple defects, resulting in the formation of 2D perovskites via the three distinct functional groups, preventing water infiltration at the GBs. In the second step, class Y ligands, especially 4-aminopiperidinium dibromide (4APPBr2), predominantly suppress GS defects and form a rigid 2D perovskite overlay on top of 3D perovskites as a protective barrier, significantly improving the film’s resistance to external stressors such as heat, light, moisture, and oxygen, thereby enhancing its overall stability. Importantly, we find that the sequence of this process not only reconfigures the structure of GBs and GSs with fewer defects but also enhances overall optoelectronic properties, reduces non-radiative recombination, and improves energy level alignment while reversing the TSS-DL sequence limits the structure reconfiguration, leading to considerable PCE and stability drop. Differently, the TSS-DL-treated devices demonstrate outstanding stability, retaining >93% of their initial PCE after 3000hr. continuous operation at maximum power point (MPP) and over 4000hr. under high humidity (>85% RH). Even under aggressive damp-heat testing (85°C and 85% RH), they maintain 93% efficiency after 2600hr. Overall, by restructuring GBs and GSs using the TSS-DL strategy, this work illustrates spatially resolving interactions between dedicated ligands that stabilize GBs and GSs independently, and overcomes the limitations of single-step treatment, achieving high efficiency and stability in PSCs.

Results
To address open-ended issues of various defects both at GBs and GSs (Fig. 1a), we propose the use of the TSS-DL strategy to ensure comprehensive coverage simultaneously both at the GBs and GSs of the perovskite film (detailed schematic procedure of this method is given in Supplementary Fig. 1).

While there are narrow dimensions of GBs and different chemistries than GSs, we explored various organic ligands that are not only highly penetrable but also multifunctionally address multiple defects in the GBs in the first step of TSS-DL (Fig. 1b). Interestingly, we find that certain ligands, designated here as Ligand X (notably DFABr), are particularly dedicated to GB defects and exhibit superior durability and performance when used as in the first step (Fig. 1c). In the second step we find that some ligands, referred to here as Ligand Y (notably 4APPBr2) mainly passivate the GSs defects and achieve the best performance in the second step. Therefore, in the following discussion, we will delve deeper into the effects of these two ligands and the sequence of two steps of the TSS-DL strategy on 3D perovskite film and thus on optoelectronic properties, performance, and durability of PSCs.

We carefully applied each ligand in separate and sequential steps without prior mixing before deposition (Supplementary Fig. 2). This technique contrasts with traditional methods, where ligands are often mixed before deposition using single-step treatment, which can lead to uncontrolled interactions and aggregation. By applying each ligand separately and sequentially, we precisely control the distinct chemistries of GBs and GSs and achieve a more uniform and effective suppression of multiple defects simultaneously. While optimizing the TSS-DL strategy, the selection of appropriate ligands is critical to achieve the targeted stabilization of GBs and GSs. Both DFABr and 4APPBr2 were identified from the Ligand X and Ligand Y groups in Fig. 1b based on their distinct chemical properties, which align with the specific stabilization needs of GBs and GSs, respectively. DFABr was selected as Ligand X for its ability to effectively target GBs due to its strong bonding capabilities via triple functional groups and small molecular size configuration (Supplementary Fig. 3), which allows it to penetrate and adhere to the narrow GBs more effectively23–25. This configuration facilitates the precise alignment of DFABr molecules, enhancing their interaction with GBs and acting as an all-in-one ligand24. As a result, DFABr can significantly reduce multiple defect sites at GBs, improve intrinsic stability and overall efficiency and stability. 4APPBr2 was chosen as Ligand Y, due to its rigid backbone that can selectively interact with GSs, forming a rigid 2D perovskite passivating layer that enhances structural stability and minimizes surface degradation26. By restructuring GSs, 4APPBr2 ensures that GSs remain stable over time, maintaining their performance. This complementary action with DFABr targeting of GBs ensures comprehensive defect management and long-term efficiency improvement of the PSCs. The detailed explanations of the effects of different ligands and their sequence in the treatment will be described below.

We first conducted time-of-flight secondary ion mass spectrometry (TOF-SIMS) to understand the penetration of ligands into the perovskite film. In the Perovskite/X/Y sample, X ligands are predominantly detected throughout the thickness of the perovskite films, while Y Ligands are principally detected on the surface (Fig. 2a). As a result, X ligands can uniquely penetrate the GBs. In the perovskite/Y/X, both ligands are distributed on the surface (Fig. 2b)6. This notable observation can be attributed to the steric hindrance, which is significantly larger in the Y cation due to its bulky ring structure. The bulky ring structure of the Y cation limits its ability to penetrate deeper into the film, causing it to remain primarily on the surface. On the other hand, the X cation, characterized by its smaller size and electron-withdrawing CF2 group, is induced to penetrate profoundly within the GBs27. However, in the Perovskite/Y/X sample, the Y cation obstructs this deep penetration of the X cation.

Meanwhile, we employed X-ray photoelectron spectroscopy (XPS) to verify potential chemical interactions between Ligand X and Ligand Y with the perovskite structure, and to elucidate their sequential effects at GBs and GSs. When Ligand X, which contains O and F groups, introduced prior to Ligand Y, the Pb 4f peaks exhibited a more substantial shift toward lower binding energy (by 0.23 eV), suggesting increased electron density at Pb²⁺, likely due to direct coordination with the oxygen and fluorine atoms in Ligand X (Fig. 2c). Additionally, the F 1s and O 1s peaks shifted to higher binding energies (Fig. 2d and Supplementary Fig. 4), indicating that these atoms are involved in strong chemical bonding with Pb²⁺ or other positively charged defects28. This pronounced shift in the XPS spectra when Ligand X is applied first implies that Ligand X has a strong and direct chemical interaction with Pb²⁺ sites, which are abundant at GBs due to their higher defect density compared to GSs16,23. In contrast, (Perovskite/Y/X), less pronounced binding energy shifts for Pb 4f, O 1s, and F 1s were observed, suggesting that initial treatment of the perovskite film by Ligand Y at the GSs hinders subsequent interactions of Ligand X with the perovskite structure, particularly at the GBs.

To confirm the dedicated distributions of Ligand X at GBs and Ligand Y at GSs, local dark conductivity is assessed using conducting atomic force microscopy (c-AFM) to measure Ctrl-Perovskite and TSS-DL-treated perovskite films at designated locations, as the AFM topographic image in Fig. 2g. The GSs are denoted with square symbols, and the GBs are marked with triangular symbols. The tip bias is swept from 2.5 V to −2.5 V at a rate of 0.15 V/s. For the Ctrl-Perovskite, the observed dark currents at both GBs and GSs are the same as in previous reports29,30 (Supplementary Fig. 5a). However, for the Perovskite/X, the dark current at the GSs is the same as the Ctrl-Perovskite, but at the GBs, the dark current is significantly higher by almost three times than that of the Ctrl-Perovskite (Figs. 2e and 2f). This indicates a substantial enhancement in conductivity at the GBs by Ligand X treatment, suggesting the successful incorporation of Ligand X at the GBs31. For the Perovskite/Y and Perovskite/X/Y samples, the dark currents at the GSs are markedly higher than those of the Ctrl-Perovskite and Perovskite/X samples (Supplementary Figs. 5b-7), confirming the predominant localization of Ligand Y at the GSs. Statistical analysis of dark current measurements taken at 40 different points in both the GBs and GSs for all samples is summarized in Fig. 2h, demonstrating the robustness and reproducibility of these results.

Kelvin Probe force microscopy (KPFM) is also used to analyze the effects of Ligand X and Y surface potential changes in perovskite films. The Ctrl-Perovskite film exhibited numerous GBs black pits (represented by white circles in Fig. 2g). A 1D height profile extracted from the topography indicates an average depth of ~70 nm between the GS and these pits (Supplementary Fig. 8). In the Perovskite/X, the number of black pits decreased significantly, and the average depth reduced to ~40 nm. While for the Perovskite/X/Y, nearly all pits were eliminated, with the average depth further reduced to ~20 nm. These results suggest that the TSS-DL method effectively fills the pits, indicating successful reconstruction of GBs and GSs sequentially. Moreover, the KPFM measurements showed a reduction in contact potential difference (CPD) from ~400 mV (Ctrl-perovskite) to ~150 mV for Perovskite/X and ~10 mV for perovskite//X/Y). This sharp decline in CPD correlates with the filling of GB pits, suggesting improved surface homogeneity. As a result, the Perovskite/X/Y sample displayed a uniform potential profile than both the Ctrl-perovskite and Perovskite/X. Consequently, the comprehensive suite of TOF-SIMS, XPS, c-AFM, and KPFM characterizations collectively demonstrates that the choice and sequence of ligand treatments profoundly influence the spatial distribution, chemical interaction, and electronic properties of the perovskite films. Specifically, ligand X, owing to its smaller size and electron-withdrawing properties, effectively penetrates and forms 2D perovskite to reconstruct GBs, enhancing local conductivity and passivating defect sites. Meanwhile, while the stability of 2D perovskite overlayers highly depends on its rigidity, the bulkier ligand Y is preferentially localized at the GSs, where it further forms rigid 2D perovskite to passivate surface defects as well as contributes to improved film uniformity26. Sequential application of X followed by Y enables complementary passivation of both GBs and GSs, resulting in reduced surface potential variations, enhanced electronic homogeneity, and overall superior film quality compared to either ligand alone or reversed treatment order.

Dedicatedly restructure grain boundaries and surfaces of perovskites

The effects of two classes of ligands, ‘X’ and ‘Y’, and their sequence on perovskite morphology and crystal structure are further investigated to understand how they restructure GBs and GSs and increase perovskite durability. The scanning electron microscopy (SEM) top-view and cross-sectional images reveal that Ctrl-perovskite films exhibit clear GBs (Fig. 3a). However, these GBs become almost imperceptible following a single-step deposition process using Ligand X (Supplementary Fig. 9), resulting in the formation of a dense, pinhole-free, and more uniform polycrystalline perovskite film, where the Ligand X predominantly modifies the GBs. We find that Perovskite/X film treated again with X ligand in the second step, there is no noticeable change, but its aggregate remains in the same position, as shown in Supplementary Fig. 10). Interestingly, when we treat the Perovskite/X sample again with Ligand Y (Perovskite/X/Y; Fig. 3a), it results in an even more uniform film with no visible GBs, suggesting that Ligand Y primarily passivates the GSs defects (Supplementary Fig. 11). To validate our findings, we examined the morphology of the Perovskite/Y/X film. In the Perovskite/Y/X sample, it is evident that the X ligand predominantly remains on top of the Perovskite/Y film without penetrating the GBs. It confirms that Ligand Y mainly modifies the perovskite GSs and highlights the importance of the sequence in imprinting the morphology uniformity.

To evaluate the crystallinity and modified structural properties of GBs and GSs after treatment, we conducted grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements on TSS-DL-treated perovskite films and elucidated the sequential change of the two steps (Fig. 3a). The Ctrl-Perovskite film shows a distinct diffraction peak at qz (~1.2 Å−1), representing the (100) main peak of 3D perovskite with an intensive PbI2 peak at qz (~0.93 Å−1). This excess PbI2 on the 3D perovskite surface is beneficial for the 2D perovskite formation9. The diffraction map of the Perovskite/X film (Supplementary Fig. 12) shows a weak PbI2 peak and new strong diffraction peaks at lower qz (~0.74 Å−1) representing the (002) planes of the 2D perovskite at the GBs (n=1)9. Stronger and additional discrete Bragg spots are observed at a higher q (qz ~0.85 Å−1 and qxy ~1.8 Å−1) in the Perovskite/X/Y sample. This indicates the formation of an additional 2D perovskite phase with vertical orientation with respect to perovskite GSs corresponding to the (282), (121), and (082) planes, as highlighted with red dashed outlines. By contrast, we do not observe the difference between peaks at lower qz and discrete Bragg spots in Perovskite/Y/X samples. However, the PbI2 peak intensities in the Perovskite/Y/X sample are weaker than Perovskite/X/Y and show some passivation effect, but without additional Bragg spots covering some untreated defects at GBs. In addition, the GIWAXS results are consistent with conventional X-ray diffraction (XRD) analysis (Fig. 3b), where a small low-angle peak at 11.50° can also be observed for Perovskite/X (Supplementary Fig. 13), further confirming the formation of 2D perovskite structures. Furthermore, the high-resolution transmission electron microscopy (HR-TEM) of the Perovskite/X/Y sample also confirmed the formation of well-oriented 2D perovskite (Supplementary Fig. 14). This well-oriented structure is crucial for enhancing the material’s optoelectronic properties, such as improved charge transport and reduced energy loss.

Optoelectronic properties of TSS-DL-treated perovskite

We investigate the potential for defect suppression and enhancement of charge carrier dynamics at the GBs and GSs in TSS-DL-treated perovskite films by examining their absorption and photoluminescence (PL) emission properties. UV-visible spectroscopy measurements (Fig. 3c) revealed that X ligands did not significantly alter the UV-vis absorption spectra after a single-step treatment, while Y ligand treatment in the first step resulted in decreased absorption. This highlights the importance of using X ligands in the first step. When Y ligands are applied in the second step, the Perovskite/X/Y absorption improves, indicating high-quality perovskite films. Furthermore, the PL intensity of Perovskite/X improved noticeably compared to the Ctrl-Perovskite and Perovskite/Y, suggesting a significant reduction in defects, and suppressed non-radiative charge recombination primarily at the GBs (Fig. 3d). Notably, for perovskite/X/Y the PL intensity further increased, and the low-wavelength peak shifted to 620 nm, indicating the possible presence of a mixture of two types of 2D perovskites (Fig. 3d inset)32–36. Conversely, for perovskite/Y/X the PL intensity also improved but much weaker than Perovskites/X/Y. Additionally treating with the same ligand in the second step also did not improve PL intensities (Fig. 3e and 3f), further confirming the importance of sequential deposition of ligand X and Ligand Y. Furthermore, the low-wavelength peak at 555 nm for Perovskite/X and at 620 nm for Perovskite/X/Y can also be observed from the backside, although with relatively weaker intensity compared to the front side (Supplementary Fig. 15). This observation confirms that Ligand-X predominantly penetrates at the GBs. In contrast, for the Perovskite/Y and Perovskite/Y/X samples, no low-wavelength peak is observed from the backside, suggesting that the Ligand-Y primarily reacts at the GSs and forms a 2D perovskite layer at the top. These results also align with the above c-AFM, SEM, XPS, and GIWAXS findings. To further validate the steady-state PL results, we conducted photoluminescence quantum yield (PLQY) measurements, which provide absolute values of emission efficiency. As shown in Supplementary Fig. 16, the PLQY of Perovskite/X/Y increased from 8.1% in the Ctrl-Perovskite and 13.1% in Perovskite/X to 17.8% in Perovskite/X/Y. In contrast, Perovskite/Y/X exhibited a reduced PLQY of 11.5%. These results indicate that non-radiative recombination was significantly suppressed by employing the TSS-DL method37.

A time-resolved photoluminescence (TRPL) measurement is further conducted to examine the effects of the TSS-DL method on the charge carrier dynamics properties of perovskite films (Fig. 3g). The emission decreased sharply in the Ctrl-Perovskite films due to non-radiative carrier recombination at the GBs and GS. Upon single-step treatment (Perovskite/X), there is only a slight improvement in carrier lifetime, attributed to its ability to suppress defect-induced recombination at the GB. In contrast, the TSS films (Perovskite/X/Y) exhibited a significantly slower emission decay, indicating reduced non-radiative recombination at both GB and GS. These results suggest that the TSS-DL method effectively enhances the optoelectronic properties by improving carrier lifetime.

Finally, we explored the impact of the TSS-DL method on interface energy using ultraviolet photoelectron spectroscopy (UPS) (Fig. 3h and Supplementary Fig. 17). The Perovskite/X/Y sequence also resulted in a significant reduction of the work function. This adjustment caused an upshift in the Fermi level and a downshift in the conduction band minimum (CBM), leading to an excellent alignment of energy levels with C60. Consequently, this facilitated efficient electron-selective charge transfer38. Additionally, a deeper valence band maximum (VBM) can reduce electron-hole recombination at the interface by blocking hole transport. In contrast, Perovskite/Y/X sequences caused Fermi-level downshifts and CBM upshifts, resulting in a significant barrier to electron transport at the perovskites/C60 interface. These results further highlight the importance of introducing these ligands in the correct order.

Photovoltaic performances and stability

We further conducted an in-depth study on the effect of the TSS-DL strategy on device performance. Fig. 4a illustrates cross-sectional SEM images of p-i-n champion devices fabricated by this method. The J-V curves of champion devices are shown in Fig. 4b. As discussed above, initially, we evaluated all ligands first in the step-I and then in step-II. We find that Ligand X, especially DFABr at a concentration of 2 mg mL⁻¹, exhibits the best performance in the first step (Supplementary Fig. 18a and Supplementary Table 1). In the second step, Ligand Y, especially 4APPBr2 at 0.4 mg mL⁻¹, demonstrates superior efficacy (Supplementary Fig. 18b and Supplementary Table 2). Ctrl-Perovskite-based devices show a PCE of 22.52%, a Voc of 1.10 V, a short-circuit current density (Jsc) of 25.14 mA/cm2, and a fill factor (FF) of 80.53%. In contrast, the Perovskite/X-based devices exhibit an improved PCE of 24.23% with a Voc of 1.15 V, a Jsc of 25.47 mA/cm², and an FF of 83.24%. Compared to Ctrl-Perovskite and Perovskite/X-based PSCs, Perovskite/X/Y-based devices have shown further improvements, achieving a champion PCE of 26.06%. This was primarily due to enhancements in Voc at 1.19 V, Jsc at 26.16 mA/cm², and FF at 83.92%. Interestingly, when changing the sequence, the PCE of the Perovskite/Y/X-based PSCs drops, with a maximum PCE of 23.18%, a Jsc of 24.4 mA/cm², and an FF of 81.8%. The external quantum efficiency (EQE) spectra shown in Fig. 4c provide integrated Jsc values that are comparable to those obtained from the J-V curves. Fig. 4d illustrates the stabilized performance of four types of photovoltaic devices, while Fig. 4e presents a statistical analysis of the PCEs of 40 devices of each type, confirming their reproducibility. The champion devices based on perovskite/X/Y also exhibited negligible hysteresis when measured in both the reverse and forward scan directions (Supplementary Fig. 19). To evaluate the feasibility of large-scale production of PSCs, 1 cm² devices are frequently employed as an intermediate step toward scaling up the device area39–41. Supplementary Fig. 20 shows the J-V curve of the champion device based on Perovskite/X/Y with an active area of 1 cm². A champion PCE exceeding 24% was achieved using our TSS-DL method, further demonstrating its scalability.

To evaluate the effect of the TSS-DL method on device stability, we further conducted three different types of standardized stability tests on our unencapsulated devices following the ISOS (International Summit on Organic PV Stability) protocols42. We investigate the stability of unencapsulated devices under continuous light illumination at MPP aging at 65°C in ambient air with over 85% RH (ISOS-L2). The Perovskite/X/Y-based device retains over 93% of its initial PCEs after 3000 hr. of continuous operation, whereas the Ctrl-Perovskites, Perovskite/X, and Perovskite/Y/X devices show efficiency losses of over 20%, 30%, and 40% respectively (Fig. 4f). Additionally, we conducted a damp heat test at 85 °C and 85% RH (ISOS-D3), inside a humidity chamber (Supplementary Fig. 21). The Perovskite/X/Y-based device retained 94% of its maximum PCE after 2600 hours, compared to Perovskite/X, and Perovskite/Y/X devices, which retained only 20%, 40%, and 60% of their efficiencies, respectively. Moreover, the Perovskite/X/Y-based device demonstrated no efficiency degradation after even 4000 hr. of dark storage in ambient air at room temperature (ISOS-D1) (Fig. 4g). To further assess the moisture stability of the perovskite/X/Y films, contact angle measurements were conducted using water droplets (Supplementary Fig. 22). The results showed an increase in contact angle from 42.62° for the Ctrl-Perovskite, to 53.77° for Perovskite/X, and up to 67.13° for Perovskite/X/Y, indicating that the TSS-DL treatment effectively enhances the hydrophobicity of the surface1. We then evaluated long-term moisture stability by exposing Ctrl-Perovskite, Perovskite/X, and Perovskite/X/Y films to ambient air with a relative humidity of 80-90%. After three months, the Ctrl-Perovskite film had completely turned yellow, while Perovskite/X exhibited a slight brown coloration. In contrast, the Perovskite/X/Y films retained their original pure black phase under the same conditions (Supplementary Fig. 23). These findings further highlight the superior moisture resistance of Perovskite/X/Y films compared to the Ctrl-Perovskite and Perovskite/X films. This suggests that the TSS-DL treatment confers enhanced protection against moisture-induced degradation in perovskite films.

Methods
Materials
Lead(II) iodide (PbI2, 99.9%), 2,2-Difluoroacetamide (DFA, >98.0% GC), 2,2,2-Trifluoroacetamide (TFA, >98.0% GC), 4-Aminobenzotrifluoride (4ACF3, >98.0% GC&T), 4-(Trifluoromethyl)benzamide (4TB, >98.0% GC), 4,4-Difluorocyclohexanecarboxamide (95%, 4,4-DCB), 4-Aminopyridine (4AP, >99.0% GC&T), and 4-Aminopiperidine (4APP, >99.0% GC&T) were all obtained from TCI. Formamidine iodide (FAI) was obtained from GreatCell solar materials in Australia. Dimethyl sulfoxide (DMSO, extra dry, 99%), N,N-Dimethylformamide (DMF, extra dry, 99%), isopropyl alcohol (IPA), Chlorobenzene (CB, extra dry, 99.8%), Hydrobromic acid (HBr, 48 wt.% solution in H2O) and aluminum oxide (Al2O3) nanoparticles (with a particle size of <50 nm, 20 wt.% solution in isopropanol) were all sourced from Sigma-Aldrich. PC60BM was acquired from Solarmer Energy, while C60 was purchased from Organtec Ltd in China. Bathocuproine (BCP) was obtained from Acros Organics in the USA, and silver pellets were sourced from Kurt J. Lesker, also in the USA. Additionally, cesium iodide (CsI), [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACZ), and phenethyl ammonium bromide (PEABr. 99.9%) were purchased from Xian Polymer Light Technology Corp in China. All chemicals were used as received, without any additional purification steps.

Organic salt synthesis. All organic salts/spacers in this work are synthesized by dissolving equimolar amounts of organic spacer precursors in 2 mL of deionized water/ethanol, then add equimolar amounts of HBr aqueous solution dropwise. After stirring for 2 hours at 0°C, the precipitate is obtained by rotary evaporation, washed three times with ethanol and ether, and finally dried in an oven at 60°C for 12 hours to obtain.

Device fabrication
Patterned Glass/ITO substrates (15 Ω sq-1) were consecutively cleaned by sequential ultrasonication for 30 minutes with ethanol, acetone, and then stored in ethanol for further use. The cleaned substrates were dried with a high-pressure Nitrogen gas flow and then treated with UV-ozone for 30 minutes (ITO site facing up) before being transferred into an N2-filled glove box device for fabrication. Me-4PACz (SAM) (0.33 mg/ml) in IPA was spin-coated on the glass/ITO substrate at 4,000 rpm for 30 seconds and then annealed at 100°C for 10 minutes. Subsequently, Al2O3 (diluted by taking 60 μl from the stock in 2450 μl IPA) was spin-coated on ITO/SAM at 5000 rpm for 30 seconds and then annealed at 100°C for 5 minutes.

Fabrication of TSS-DL-based perovskite film. Cs0.05(FA0.79MA0.21)0.95PbI3 (Eg 1.55 eV)-based perovskite precursors were prepared in the mixed solvent of DMF and DMSO with a volume ratio of 4:1. The precursors were then filtered by a 0.2 µm PTFE syringe filter before use. The mixed perovskite precursor was spin-coated on the ITO/SAM/Al2O3 layer at 1000 rpm for 8s and 27 seconds for 6000 rpm, and 200 μL of CB was dropped onto the spinning samples at the 10th second of the second step to form a brown film. Then the film was annealed at 110°C for 25 min. The film was cooled down before the next step. Next in step 1, Ligand-X (2 mg/mL) in IPA:DMSO (200:1) was spin-coated dynamically on ITO/SAM/Al2O3/perovskite at 5000 rpm and then annealed at 100°C for 5 minutes. The same procedure was repeated in step 2 with class Y salt (0.4 mg/mL). Next, C60 (40 nm), BCP (8 nm) and 120 nm Ag were thermally evaporated on treated/untreated perovskite films under a vacuum of 2×10−6 Torr in the high-vacuum deposition system, with a device area of 0.08 cm² or 1 cm2.

Device measurements
All fresh and aged device performance measurements, excluding MPP stability tests, were conducted inside a N2-filled glove box to ensure controlled conditions. All current vs voltage (J-V) measurements were performed in both forward and reverse scan directions over the voltage range of −0.1 V to 1.2 V with a step size of 0.01 V and a scan rate of 100 mV/s. These measurements were carried out under standard 1 sun illumination (AM1.5G) using an Enlitech solar simulator (SS-F5-3A) linked with a Keithley 2400 source meter. The light intensity was calibrated using a standard silicon solar cell (SRC-2020, Enlitech). The EQE was measured in ambient air using a solar cell spectral response system (QE-R, Enlitech).

Thin film characterizations
Ultraviolet-visible (UV-vis) absorption spectra of Ctrl-Perovskite, Perovskite/X, Perovskite/Y, Perovskite/X/Y and Perovskites/Y/X-based films were measured in air at room temperature using a Shimadzu UV-Visible Spectrophotometer (UV-2600). All films were prepared on Glass/ITO substrates. X-ray Diffraction analysis (XRD) of all perovskite films was measured using a Rigaku SmartLab diffractometer with Cu Kα radiation (λ = 1.5406 Å) at a voltage of 40 kV and 200 mA currents. The scanning range was 5.0°−60° with a scan rate of 5°/min and 0.02°/step. The perovskite samples for XRD were prepared on cleaned Glass/ITO substrates and measured under ambient conditions without any additional protocol. X-ray Photoelectron spectroscopy (XPS) measurements were performed in an ultra-high vacuum analysis chamber with a base pressure of approximately 1×10⁻¹⁰ Torr. A monochromatic Al Kα X-ray source (photon energy = 1486.6 eV) was used for excitation. The photoelectron spectra were acquired using a hemispherical electron energy analyser (PHOIBOS 150, SPECS). The samples were prepared on cleaned glass/ITO substrates inside an N₂-filled glove box. Following fabrication of perovskite films, the samples were transferred to the XPS facility in vacuum-sealed plastic bags and measured on the same day to minimize exposure to air and potential contamination. Ultraviolet photoelectron spectroscopy (UPS) measurements were performed using an ultrahigh vacuum surface analysis system equipped with a ULVAC-PHI 5000 VersaProbe III spectrometer and a helium I (HeI, 21.2 eV) photon source. The samples were biased at −5 V during measurement. The pass energy was set to 5.0 eV to ensure high-resolution spectra. Calibration of the UPS system was conducted using a clean gold (Au) substrate before measurements. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyses were conducted on a NanoTOF-II instrument (ULVAC-PHI, Japan) employing a Bi₃⁺ primary ion beam (30 kV) for high-resolution spectral acquisition and an Ar⁺ sputter beam (3 keV, 100 nA) for depth profiling. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were conducted at the BL03HB beamlines of the Shanghai Synchrotron Radiation Facility (SSRF), utilizing an X-ray energy of 10 keV (wavelength 1.2398 Å) with a PLATUS detector, which was positioned vertically from the sample. The experiment was set with a grazing incidence angle of 0.4° and an exposure time of 10 seconds. The perovskite films for GIWAXS measurement were prepared on a Glass/ITO substrate. The contact angles of all samples were tested by the JC2000C1 contact angle meter of Powereach at room temperature on the same day. 1.0 μl of deionized water droplets was dropped carefully onto the perovskite films. Kelvin probe force microscopy (KPFM) and conductive-atomic force microscopy (c-AFM) data were collected concurrently using an atomic force microscope (NT-MDT NTEGRA) on the same day. A Pt-coated tip (Pt thickness: 30-35 nm; tip height: 14-16 µm) was employed for these measurements. To minimize noise and moisture interference, nitrogen gas continuously flowed into the chamber during measurement. The steady-state photoluminescence (PL), Time-resolved photoluminescence (TRPL) spectra, and PL quantum yield (PLQY were measured using a fluorescence spectrophotometer (PicoQaunt Fluo Time 300). A laser diode with a peak wavelength of 405 nm was used as the excitation source. The films were measured from the front side of the sample (perovskite side). High-resolution SEM (Hitachi S-4800) and TEM (Thermo Scientific Talos F200X STEM) were used to observe the morphology of the perovskite films.

Stability measurement
The ISOS-L2 and ISOS-D3 stability tests were conducted by storing the devices in a humidity chamber equipped with precise humidity and temperature control, a calibrated LED light source providing one-sun illumination, and an automated device stability tracking system. This system utilized custom MATLAB software integrated with a Keithley 2635 source meter for real-time performance monitoring. For the ISOS-L2 test, the temperature was maintained at 65°C, with the chamber door left open to ambient air at a relative humidity of 85 ± 5%. The maximum power point (MPP) was set based on the Vmax measured under 1-sun illumination for fresh devices: Ctrl-Perovskite (0.96 V), Perovskite/X (0.98 V), Perovskite/X/Y (1.03 V), and Perovskite/Y/X (1.01 V). All current vs voltage (J-V) measurements were performed over the voltage range of −0.1 V to 1.2 V with a step size of 0.01 V and a scan rate of 100 mV/s. For the ISOS-D3 (damp heat test), devices were exposed to a constant temperature of 85°C and relative humidity of 85%, with performance measurements taken at regular intervals to assess degradation under accelerated aging conditions. For dark storage (ISOS-D1), unencapsulated devices were stored in the dark under ambient conditions with a relative humidity of 85 ± 5%. Device performance was periodically monitored throughout the storage period to evaluate shelf-life stability. The J-V scanning parameters were kept consistent across all stability measurements. For each aging condition, three pieces were tested, each featuring five fingers, and the reported data represent the average performance of these devices.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors demonstrated a two-step sequential dedicated-ligand (TSS-DL) strategy to address the persistent stability issue\nby restricting both grain boundaries (GBs) and surfaces (GSs). The selection of Ligand X and Ligand Y showcases a deep\nunderstanding of their dedicated interactions with GBs and GSs, respectively, with direct experimental evidence. Overall,\nthis work offers a promising pathway toward more stable and commercially viable perovskite solar cells, and it sets a new\nbenchmark for future research in this area. I would recommend the publication of this work in Nature Communications after\naddressing the following issues:\n1. (a) What are the advantages of applying each ligand separately and sequentially in the TSS-DL strategy compared to\nmixing ligands before deposition? (b) Do the chemical properties of different ligands influence their specific targeting of grain\nboundaries (GBs) and grain surfaces (GSs), respectively?\n2. Can the authors explain more in detail (a) why the steric hindrance imposed by the bulky ring structure of Ligand Y limits\nits penetration depth, as evidenced by TOF-SIMS? (b) Can the authors explain why DFABr is more efficient than TFABr, and\n4APPBr than 4APBr?\n3. How does the formation of well-oriented 2D perovskite phases influence the efficiency and exciton dynam-ics?\nAdditionally, how the two types of 2D perovskites improve the overall stability of target perovskite films.\n4. The authors are suggested to provide more detail in Fig.1a for all the cases about different defects and label them\nproperly. Additionally, label different cations of perovskite.\n5. The authors shall provide more detail on how the order of ligand application (X then Y vs. Y then X) affects the\noptoelectronic properties, as special PL spectra show obvious differences for both cases.\n6. It is recommended to investigate a range of concentrations and assess their impact on the device perfor-mance to\nestablish optimal conditions and enhance overall efficiency.\n7. What is the underlying reason for the increase in dark current observed at the GBs and GSs following treat-ment with\nligands X and Y, respectively? Additionally, does the sequence of the two-step ligand application influence the dark current\nlevels?", "reviewer_2": "In this manuscript entitled “Sequential Organic Ligand Modifications to Dedicatedly Restructure Grain Boundary and Surface\nof Perovskites”, the authors present a two-step sequential dedicated-ligand (TSS-DL) strategy targeting grain boundaries\n(GBs) and grain surfaces (GSs) in perovskite solar cells (PSCs), resulting in impressive operational stability under harsh\nconditions. The work is timely and of potential interest to the perovskite research community. The double modification\nstrategy is utilized at perovskite/C60 interface, with in-depth analysis on each modification. I would like to recommend the\npublication of the paper upon the following issues are addressed properly to strengthen the scientific rigor and broaden the\nimpact of the study:\n1. While Ligand Y is claimed to enhance moisture resistance, the underlying mechanism is not well explained. Additional\ndiscussions or experimental evidence (e.g., contact angle measurements) are needed to clarify how the ligand contributes to\nmoisture protection.\n2. The influence of ligand concentration on the final device performance is not explored. Optimization studies would help\ndetermine process sensitivity and further validate the robustness of the method.\n3. The description of Figure S8 (left panel) does not match the actual content shown in the figure. The authors should revise\neither the caption or the figure itself to ensure consistency and avoid confusion.\n4. From the Pb⁺ signal intensity, it appears that the perovskite film thicknesses in Figures 2a and 2b differ significantly. The\nauthors should explain what causes this variation and whether it may influence the TOF-SIMS analysis results or\ninterpretations.\n5. Figure 2j lacks a scale bar. Please add an appropriate scale bar.\n6. In line 220, GIWAXS data is referenced as being in Figure 2b, while it is actually shown in Figure 2j.\n7. Figures 3c, 3d, and 3e lack clear legends indicating what each color represents. The authors should revise these figures\nto include labels that clarify the meaning of the different colored traces or data points.\n8. Figures 4c, 4d, 4e, 4f, and 4g also omit the definition of each color used in the plots.\n9. In Figure 2a and 2b, the signal of Pb2+ shows quite differences between each other. This phenomenon is quite rare in the\npast. Please make sure the test of these two samples are made in single batch, along with same test parameters.\n10. In Figure 3a inset, for a PL test on a perovskite film with 2D/3D mixed structure, the laser incidence direction may greatly\naffect the results. (Nat. Rev. Mater., 2023 8, 533) Therefore, please provide detailed testing methods.\n11. In Figure S11, please provide the information of the bandgap of the perovskite film.\n12. In Figure 4c, please provide the integrated JSC.\n13. The caption of Figure 4g is not correct.", "reviewer_3": "The manuscript submitted by Ahmad, Choy et Al. presents a double treatment aimed to passivate grain-boundary defects\nand create a protective 2D overlayer on 3D polycrystalline Perovskite Solar Cells.\nThe main novelty of the manuscript lies in the materials chosen, since the choice of ligands has not been found in other\npublications. After the treatment(s) the authors report very high (>26%) efficiencies and stabilities (>90% after 4,000 hours\nunder high humidity >85% RH).\nThough it is possible to navigate through the manuscript and understand the message that the authors seek to transmit, the\nmanuscript in its current form suffers from numerous minor errors of form and a few errors of substance that deprive it of the\nquality and rigour expected in this journal. In particular, there is an enormous lack of details regarding the experimental and\ncharacterization methods, without these details the reproducibility of this work, and hence the contribution to the wider\ncommunity of perovskite researchers is greatly diminished and turn the exercise of reading the paper in an exercise of faith.\nBecause of these reasons I cannot recommend to publish the paper in its current form. The following is a non-exhaustive list\nof the issues found in the manuscript:\n1) The main argument of the manuscript is that ligand X passivates grain boundaries while ligand Y passivates GS creating\na 2D layer. This is mainly supported by the ToF-SIMS data that shows a higher concentration of ligand X in the bulk of the\nperovskite when the order of the treatment is X/Y rather that Y/X, this is an indicative however not a solid proof.\n2) The XPS analyses are very poor in quality (images too small) and their contribution to the overall results is of dubious\nrelevance. The authors should consider that XPS gets information out of the outermost layers of the material, usually 2 nm\nand very often no more than 5, and given that the AFM suggest welss between 20 and 70 nm, it is not clear where the\ninformation we are seeing is coming from. Appart from that, knowing exactly the history of the sample and the conditions of\nthe analysis (before the analysis and inside the chamber, transport in vacuum?, cleaning?, ion bombardment?, samples\nwere analysed the same day? Etc) is of extreme importance to judge the relevance of the results shown.\n3) The dark-current statistical analysis of figure S5 seems one of the more conclusive results of the manuscript, how many\ndifferent samples of each kind have been analyzed?, the data was collected the same day with the same tip?\n4) The navigation of the manuscript between the main text and the supporting file turns complicated, since there is a lot of\ninformation and images in the supporting file that are actually very relevant to understand the main text and they should be\nthere. Supporting files should have information that are in-principle not required in a first read of the manuscript, but the\nauthors continuously compare results of the main manuscript and the supporting files, obliging the reader to have both\ndocuments open and continuously switching from one to the other. This is another reason why the current\n5) In general, all the images are too small, the authors are strongly encouraged to reconsider the current image sizes and\ndistribution they have chosen. At the moment, all the images lack the size and quality required to make an informed\njudgement.\n6) Sustainability: The authors propose to pass from a single spin-coating process to a double process. This doubles the\namount of solvent required, time and energy to produce a device. Today, when perovskite devices are in the brink of\nindustrial deployment these questions are extremely relevant. Do the authors have estimated how much solvents and\nenergy these additional steps require and how it translates to the cost, CO2 impact and lyfe-cycle-assesment of potentially\nmillions of devices measuring millions of m2?, it would be interesting to have a brief statement on this regard, since we are\nworking on creating a sustainable technology we should aim to use only the most sustainable processes and materials.\n7) Lines 117-122 include statements that would require citation of sources. For example is there any reference to the library\nof organic ligands mentioned? is it published? are there previous experimental works on this regard?. While the statements\nabout the adequacy of ligand X based on its size also seem rather arbitrary without further experimental evidence to prove\nthese claims.\n8) The AFM images of Figure 2 have a scale bar in mm, probably it should micrometres (um), on the other hand the SEM\nimages lack of scale bar at all, something recurrent in the manuscript (and supporting file) with most of the SEM images.\n9) Line 245: “PL intensity improved by over 50%”…an increase of the PL counts does not necessarily means an\nimprovement. In order to judge that, the structure of the device analysed should be known but this is not mentioned in any\npart of the manuscript or SI file, is there any transport layer in these devices?, this is particularly important to understand the\nTRPL results. On the other hand, since the PL images don’t have a proper scale-bar, the reader most believe the 50%\nstatement. The authors are encouraged to include a proper scale-bar rather than using the easy but unprofessional resource\nof the arbitrary units.\n10) The stability measurements lack a lot of information to judge their relevance, for example scanning frecuency and\nscanning resolution and speed are very important factors to rule out ionic effects. The authors are encouraged to follow\nthese directions: https://www.nature.com/articles/s41560-019-0529-5\n11) All the experimental analysis technique lack information (XRD, XPS, UPS, etc), just stating the brand and model of the\nequipment used is not enough, it is advisable to state the particular conditions of the measurement like the energy used,\nions, chamber conditions, calibration procedures, etc. Publishing is an act of sharing information with the rest of the\nscientific community, being transparent is the real contribution to the community, otherwise it can be seen as an exercise of\nshowcasing dubious results. Read for example https://doi.org/10.1016/j.apsadv.2023.100534\n12) The devices analysed are extremely small, nowadays this is less acceptable in perovskite research. The authors are\nencouraged to either show results in bigger devices (>1 cm2) or at least state why they have not done so or why they\nconsider these are not relevant.\n13) Line 63: TSS-DS should read TSS-DL"}, "author_response": "Response to reviewer comments: We thank the reviewer for their insightful and encouraging comments\non our work. We are honored to receive the positive evaluation that our experimental approach paves\nthe way for the stability and commercial viability of perovskite solar cells. We also appreciate your\nconstructive suggestions. We have addressed all the raised issues accordingly in the revised manuscript.\nBelow is the point-by-point response to the reviewer's concerns and comments.\nQ1. (a) What are the advantages of applying each ligand separately and sequentially in the TSS-DL\nstrategy compared to mixing ligands before deposition? (b) Do the chemical properties of different\nligands influence their specific targeting of grain boundaries (GBs) and grain surfaces (GSs),\nrespectively?\nA1. Thanks to the reviewer for your valuable comments. (a) Applying each ligand separately and\nsequentially in the TSS-DL (two-step sequential dual-ligand) strategy offers better control over the\nformation and spatial distribution of 2D perovskite phases. This approach allows each ligand to\nselectively passivate specific defect sites; Ligand X primarily at GBs and Ligand Y at GSs, leading to\nmore effective defect passivation and improved film quality. In contrast, mixing ligands before\ndeposition can cause competitive binding and uneven coverage, reducing passivation efficiency and\npotentially causing phase mixing or aggregation.\n(b) Yes, the chemical properties of the ligands play a key role in their selective targeting, especially for\neffective suppression of multiple defects at the GBs. Ligand X (DFABr) is considered an 'all-in-one'\nligand ((Accounts Mater. Res. 2, 1141–1155 (2021)) with an ideal molecular structure and\nmultifunctionality, possessing three different functional groups (O, F, and NH+3) Ligand X, with its\nsmaller size and specific functional groups, can penetrate and bind preferentially at GBs, where defect\ndensities are higher, and access is more constrained. Ligand Y, typically larger and more hydrophobic,\nfavors coverage on GSs, forming a protective surface layer. These chemical differences govern their\nspatial selectivity and contribute to the enhanced stability and optoelectronic performance observed.\nQ2. Can the authors explain more in detail (a) why the steric hindrance imposed by the bulky ring\nstructure of Ligand Y limits its penetration depth, as evidenced by TOF-SIMS? (b) Can the authors\nexplain why DFABr is more efficient than TFABr, and 4APPBr than 4APBr?\nA2. (a) The bulky ring structure of Ligand Y imposes significant steric hindrance, limiting its\npenetration into the perovskite film as evidenced by TOF-SIMS (lines 132-142). The large, rigid\nmolecule framework physically restricts diffusion through GBs and bulk regions, confining Ligand Y\nprimarily to the GSs.\n(b) Although TFABr contains three fluorine atoms and DFABr has two, DFABr demonstrates higher\nefficiency among them due to the specific positioning and electronic effects of its fluorine substituents.\nThe difluoro configuration in DFABr results in an optimal balance of electron-withdrawing strength\nand molecular interactions that enhance binding affinity to defect sites and improve passivation. In\nR-1/17\ncontrast, the trifluoro substituents in TFABr can introduce steric and electronic effects that slightly\nreduce its passivation efficiency.\nRegarding 4APPBr and 4APBr, 4APPBr is non-aromatic, which provides greater molecular flexibility\nand conformational adaptability among them, facilitating more effective interaction with surface defects.\nConversely, 4APBr is aromatic and more rigid, limiting its ability to conform to and passivate defect\nsites effectively. This difference in aromaticity and molecular flexibility accounts for the superior\npassivation performance of 4APPBr.\nQ3. How does the formation of well-oriented 2D perovskite phases influence the efficiency and exciton\ndynamics? Additionally, how the two types of 2D perovskites improve the overall stability of target\nperovskite films.\nA3. We thank to reviewer comments and suggestions. Well-oriented 2D perovskite layers enhance\ncharge carrier transport by reducing trap states and grain boundaries, thereby decreasing recombination\nlosses and improving device efficiency (Adv. Energy Mater. 12, 2202704 (2022). In our TSS-DL\nmethod, Step I utilizes Ligand-X (DFABr) to reconstruct GBs and form 2D perovskite phases at these\nlocations, effectively blocking the penetration of water and humidity. In Step II, Ligand-Y (4APPBr₂)\nis introduced to form a rigid 2D perovskite layer at the GSs, which protects the perovskite surface (GSs)\nfrom oxygen and direct contact, further enhancing the overall stability of the device\nQ4. The authors are suggested to provide more detail in Fig.1a for all the cases about different defects\nand label them properly. Additionally, label different cations of perovskite.\nThank you for your valuable suggestion. We revised Fig. 1a in the revised manuscript, which includes\nclear and distinct labels for all identified defect types present in each case, such as vacancies, interstitials,\nand antisite defects, with arrows or markers pointing to their specific locations. Explicit labelling of the\nvarious perovskite cations (e.g., Pb²⁺, MA⁺, FA⁺, Cs⁺, or other relevant species used in this study) is\nclearly distinguished in the crystal structure.\nQ5. The authors shall provide more detail on how the order of ligand application (X then Y vs. Y then\nX) affects the optoelectronic properties, as special PL spectra show obvious differences for both cases.\nThank you for your comment. As discussed in the manuscript, the sequence of ligand application\nsignificantly affects the optoelectronic properties. Applying Ligand X first (X→Y) allows it to\neffectively reconstruct GBs, followed by Ligand Y forming a protective layer on GSs. This sequence\nleads to improved defect passivation, higher PL intensity, and longer exciton lifetimes. In contrast,\napplying Ligand Y first (Y→X) can hinder Ligand X’s access to GBs, resulting in less effective\npassivation and reduced PL performance.\nQ6. It is recommended to investigate a range of concentrations and assess their impact on the device\nperformance to establish optimal conditions and enhance overall efficiency.\nA6. Thank you for your comment. The influence of ligand concentration of Ligand X and Ligand Y on\ndevice performance was systematically explored, and the results have been included in the revised\nSupporting Information (Fig. S18 and Tables S1 and S2). The optimized concentrations of Ligand X\nin the first step and Ligand Y in the second step were found to be 2 mg/mL and 0.4 mg/mL, respectively.\nQ7. What is the underlying reason for the increase in dark current observed at the GBs and GSs\nfollowing treatment with ligands X and Y, respectively? Additionally, does the sequence of the two-step\nligand application influence the dark current levels?\nR-2/17\nAs discussed in the manuscript (lines 165-173):\nFor the Ctrl-Perovskite, the observed dark currents at both GBs and GSs are the same as in previous\nreports27,28 (Fig. S5a). For the Perovskite/X, when the dark current at the GSs is the same as the Ctrl-\nPerovskite, different at the GBs, almost three times that of the Ctrl-Perovskite (Fig. 2e and 2f). This\nindicates a substantial enhancement in conductivity at the GBs by Ligand X treatment, suggesting the\nsuccessful incorporation of Ligand X at the GBs29.\nRegarding the sequence of ligand application, we find that it does influence dark current levels.\nAs we discussed in the manuscript (lines 170-173):\nFor the Perovskite/X/Y and Perovskite/Y (Fig. S5b-S7) samples, the dark currents at the GSs are\nmarkedly higher than those of the Ctrl-Perovskite and Perovskite/X samples, confirming the\npredominant localization of Ligand Y at the GSs. Collectively, these findings demonstrate that the\nsequence of the two-step ligand application plays a critical role in modulating dark current levels,\nunderscoring the importance of processing order in tailoring the electrical properties of perovskite films.\nReviewer #2 (Remarks to the Author):\nIn this manuscript entitled “Sequential Organic Ligand Modifications to Dedicatedly Restructure Grain\nBoundary and Surface of Perovskites”, the authors present a two-step sequential dedicated-ligand (TSS-\nDL) strategy targeting grain boundaries (GBs) and grain surfaces (GSs) in perovskite solar cells (PSCs),\nresulting in impressive operational stability under harsh conditions. The work is timely and of potential\ninterest to the perovskite research community. The double modification strategy is utilized at\nperovskite/C60 interface, with in-depth analysis on each modification. I would like to recommend the\npublication of the paper upon the following issues are addressed properly to strengthen the scientific\nrigor and broaden the impact of the study:\nResponse to reviewer comments: We thank the reviewer for their encouragement, “Sequential Organic\nLigand Modifications to Dedicatedly Restructure Grain Boundary and Surface of Perovskites.” We are\ngrateful for the reviewer’s recognition of the timelines and potential impact of our work within the\nPerovskite Research Community. We also appreciate the constructive suggestions provided to further\nstrengthen scientific rigor and broaden the impact of our study. We carefully addressed each of the\npoints raised and revised the manuscript accordingly.\nQ1: While Ligand Y is claimed to enhance moisture resistance, the underlying mechanism is not well\nexplained. Additional discussions or experimental evidence (e.g., contact angle measurements) are\nneeded to clarify how the ligand contributes to moisture protection.\nA1: Thank you for your insightful comment regarding the moisture resistance conferred by Ligand Y.\nWe have included contact angle measurements and added the following discussion in the revised\nmanuscript and supporting information.\nIn manuscript (lines 344-348):\nTo further assess the moisture stability of the perovskite/X/Y films, contact angle measurements were\nconducted using water droplets (Fig. S22). The results showed an increase in contact angle from 42.62°\nfor the Ctrl-Perovskite to 53.77° for Perovskite/X, and up to 67.13° for Perovskite/X/Y, indicating that\nthe TSS-DL treatment effectively enhances the hydrophobicity of the surface3. We then evaluated long-\nterm moisture stability by exposing Ctrl-perovskite, Perovskite/X, and Perovskite/X/Y films to ambient\nR-3/17\nair with a relative humidity of 80–90%. After three months, the Ctrl-Perovskite film completely turned\nyellow, while Perovskite/X exhibited a slight brown coloration. In contrast, the Perovskite/X/Y films\nretained their original pure black phase under the same conditions (Fig. S23). These findings further\nhighlight the superior moisture resistance of Perovskite/X/Y films compared to the Ctrl-Perovskite and\nPerovskite/X films. This suggests that the TSS-DL treatment confers enhanced protection against\nmoisture-induced degradation in perovskite films.\nFig. S22. Snapshots of the contact angle of a water droplet on Ctrl-Perovskite, Perovskite/X,\nPerovskite/Y, Perovskite/X/Y and Perovskite/Y/X film, where X and Y represent DFABr and 4APPBr,\nrespectively.\nFig. S23: Long-term thin film stability of Ctrl-Perovskite, Perovskite/X and Perovskite/X/Y under\nambient conditions, R.H. 80-90%.\nQ2: The influence of ligand concentration on the final device performance is not explored.\nOptimization studies would help determine process sensitivity and further validate the robustness of the\nmethod.\nA2: Thank you for your comment. The influence of ligand concentration of Ligand X and Ligand Y on\nfinal device performance was systematically explored, and the results have now been included in the\nrevised supporting information and manuscript (Fig. S18 and Tables S1 and S2). The optimized\nconcentrations of Ligand X in the first step and Ligand Y in the second step were found to be 2 mg/mL\nand 0.4 mg/mL, respectively.\nIn manuscript: (lines 303-306)\nWe find that Ligand X, especially DFABr at a concentration of 2 mg/mL, exhibits the best performance\n2\nin the first step (Fig.S18a and Table S1). In the second step, Ligand Y, especially 4APPBr at 0.4 mg/mL,\ndemonstrates superior efficacy (Fig. S18b and Table S2).\nR-4/17\nFig. S14. Influence of the dosage of ligand X (a) and ligand Y (b) on the device performance in step I\nand step II, respectively.\nWe first optimized the concentration of ligand X in step I to fabricate high-performing Perovskite/X\ndevices. In step II, we used the optimized Perovskite/X and subsequently optimized the concentration\nof ligand Y (Perovskite/X/Y), where X and Y represent DFABr and 4APPBr, respectively. The\nperformance of various concentrations is summarized in Tables S1 and S2.\nTable S1: Effect of Ligand-X concentration on device performance (Step I)\nConcentration V (V) J (mA/cm2) FF PCE\noc sc\n(mg/mL) (%) (%)\n0 1.10 25.14 80.53 22.7\n1 1.14 25.56 80.02 23.22\n2 1.15 25.47 83.24 24.32\n4 1.15 25.12 81.74 23.75\n6 1.13 24.8 76.93 21.56\nTable S2: Effect of ligand-Y concentration on the device performance (step-II)\nConcentration V (V) J (mA/cm2) FF PCE\noc sc\n(mg/mL) (%) (%)\nPerovskite/X /Y 1.15 25.47 83.24 24.32\n2 0\n0.2 1.16 26.10 83.40 25.30\n0.4 1.19 26.14 83.92 26.06\n0.6 1.18 24.98 84.00 24.86\n0.8 1.17 24.79 83.16 24.24\nQ3: The description of Figure S8 (left panel) does not match the actual content shown in the figure. The\nauthors should revise either the caption or the figure itself to ensure consistency and avoid confusion.\nR-5/17\nA3: We thank the reviewer for pointing out the inconsistency between the description and content of\nFig. S8. In the revised manuscript, we have addressed this issue and split Fig. S8 into two separate\nfigures (now Fig. S10 and Fig. S11) to ensure that the figure content and captions are clear and\nconsistent.\nQ4: From the Pb⁺ signal intensity, it appears that the perovskite film thicknesses in Fig. 2a and 2b differ\nsignificantly. The authors should explain what causes this variation and whether it may influence the\nTOF-SIMS analysis results or interpretations.\nA4: Thank you for this valuable comment, reviewer. We have re-measured the top-surface profiles and\nupdated Fig.1a in the revised manuscript. We find that the difference in Pb+ signal intensity between\nthe films has been substantially reduced. However, a minor difference in plateau intensity remains. This\nresidual variation may be attributed to differences in crystallinity, roughness, and chemical composition\nbetween the perovskite/X/Y and perovskite/Y/X films, like the previous report (Science 377, 1425–\n1430 (2022). Such differences can influence TOF-SIMS signal intensities.\nQ5: Figure 2j lacks a scale bar. Please add an appropriate scale bar.\nA5: Thank you for the reviewer comments. We have added the scale bar to Fig. 2j, (now Fig.3a) as\nshown in the revised manuscript.\nQ6: In line 220, GIWAXS data is referenced as being in Figure 2b, while it is shown in Figure 2j.\nA4: Thank you for bringing this to our attention. We have addressed the issue in the revised manuscript\nby replacing Fig. 2b with Fig. 2j, which is now Fig. 3a in the revised manuscript.\nQ7: Figures 3c, 3d, and 3e lack clear legends indicating what each color represents. The authors should\nrevise these figures to include labels that clarify the meaning of the different colored traces or data\npoints.\nA7: Thank you for the reviewer's comments and suggestions. We are sorry for the confusion caused by\nthe different colors. We have now added clear legends to each subfigure throughout the revised\nmanuscript, including Figures 3c, 3d, and 3e (Now Fig. S16, Fig.3e, and Fig.3f in the revised manuscript\nand supporting information)\nQ8. Figures 4c, 4d, 4e, 4f, and 4g also omit the definition of each color used in the plots.\nA8: We have added clear legends to each subfigure throughout the revised manuscript, including\nFigures 4c, 4d, 4e, 4f, and 4g.\nQ9. In Figure 2a and 2b, the signal of Pb2+ shows quite differences between each other. This\nphenomenon is quite rare in the past. Please make sure the test of these two samples is made in single\nbatch, along with same test parameters.\nA9: Thank you for your valuable comments and suggestions. As discussed in response to comment 4,\nwe have remeasured and updated the data in the revised manuscript. We confirm that both samples in\nFig. 2a and 2b were tested in a single batch under identical test parameters.\nQ10. In Figure 3a inset, for a PL test on a perovskite film with 2D/3D mixed structure, the laser\nincidence direction may greatly affect the results. (Nat. Rev. Mater., 2023 8, 533) Therefore, please\nprovide detailed testing methods.\nA10. Thank you for your suggestion. In the original manuscript, the inset of Fig. 3a presents the\nphotoluminescence (PL) spectra measured from the front side (perovskite side), consistent with\nR-6/17\nprevious reports (Joule 2, 2706–2721 (2018). In the revised supporting information, we added details\nabout PL measurement in the characterization section.\nIn the revised manuscript, we also provide a brief discussion about the back side and have added the\nfollowing discussion.\nIn manuscript lines (266-273)\nFurthermore, the long-wavelength peak at 555 nm for Perovskite/X and at 620 nm for\nPerovskite/X/Y can also be observed from the back side, although with relatively weaker intensity than\nthe front side (Fig.S15). This observation confirms that Ligand-X predominantly penetrates at the GBs.\nIn contrast, for Perovskite/Y and Perovskite/Y/X samples, no low-wavelength peak is observed from\nthe back side, suggesting that the Ligand-Y primarily reacts at the GSs and forms a 2D perovskite layer\nat the top. These results also align with c-AFM, SEM, and GIWAXS findings.\nQ11. In Figure S11, please provide the information of the bandgap of the perovskite film.\nA11: Thanks for the reviewer's suggestion and comments. We have provided the bandgap of the\nperovskite film (E = 1.55 eV) in the revised supporting information (now Fig.3c in the manuscript). No\ng\nobvious changes were observed in the bandgap of the perovskite film after different treatments.\n12. In Figure 4c, please provide the integrated JSC.\nA12: Thanks for this; We have provided the integrated J in Fig.4c in the revised manuscript.\nsc\n13. The caption of Figure 4g is not correct.\nA13: Thanks for the reviewer comments. We have corrected the caption of Figure 4g in the revised\nmanuscript.\nReviewer #3 (Remarks to the Author):\nThe manuscript submitted by Ahmad, Choy et Al. presents a double treatment aimed to passivate grain-\nboundary defects and create a protective 2D overlayer on 3D polycrystalline Perovskite Solar Cells.\nThe main novelty of the manuscript lies in the materials chosen, since the choice of ligands has not been\nfound in other publications. After the treatment(s) the authors report very high (>26%) efficiencies and\nstabilities (>90% after 4,000 hours under high humidity >85% RH).\nThough it is possible to navigate through the manuscript and understand the message that the authors\nseek to transmit, the manuscript in its current form suffers from numerous minor errors of form and a\nfew errors of substance that deprive it of the quality and rigour expected in this journal. In particular,\nthere is an enormous lack of details regarding the experimental and characterization methods; without\nthese details, the reproducibility of this work, and hence the contribution to the wider community of\nperovskite researchers is greatly diminished and turns the exercise of reading the paper in an exercise\nof faith. Because of these reasons I cannot recommend publishing the paper in its current form. The\nfollowing is a non-exhaustive list of the issues found in the manuscript:\nResponse to reviewer comments: We thank the reviewer for their thorough evaluation of our manuscript\nand for acknowledging the novelty of our ligand selection and the significance of our reported\nefficiencies and stabilities. We greatly appreciate the time and effort dedicated to providing detailed\nfeedback, which is invaluable for improving the quality and rigor of our work. We have revised all\nR-7/17\ncharacterization methods in detail in the revised supporting information. Below is the detailed point-\nby-point response to the reviewer comments.\nQ1: The main argument of the manuscript is that ligand X passivates grain boundaries while ligand Y\npassivates GS creating a 2D layer. This is mainly supported by the ToF-SIMS data that shows a higher\nconcentration of ligand X in the bulk of the perovskite when the order of the treatment is X/Y rather\nthan Y/X, this is an indicative however not a solid proof.\nA1. We thank the reviewer for their valuable comment regarding the main argument of our manuscript\nand the interpretation of the TOF-SIMS data. Indeed, our main argument is that the ligand X primary\ntargets GBs and Ligand Y GSs sequentially.\nThe ToF-SIMS measurements were primarily used to investigate the penetration ability of the ligands,\nlike the previous report (Nature 603, 73–78 (2022), particularly the extent to which ligand X can access\nGBs, which are the main humidity penetration channel ( Nano Lett. 19, 6498–6505 (2019).\nImportantly, thank you to the reviewer for allowing us to provide a more comprehensive understanding\nand substantiate our claims regarding the location (distribution) of each ligand (GBs vs. GSs). In this\nwork, we conducted XPS c-AFM and KPAFM measurements to further experimentally validate our\nclaim as discussed in the manuscript.\nIn manuscript lines (161-191) (cAFM)\nTo confirm the dedicated distributions of Ligand X at GBs and Ligand Y at GSs, local dark\nconductivity is assessed using conducting atomic force microscopy (c-AFM) to measure Ctrl-Perovskite\nand TSS-DL-treated perovskite films at designated locations, as the AFM topographic image in Fig. 2g.\nThe GSs are denoted with square symbols, and the GBs are marked with triangular symbols. The tip\nbias is swept from 2.5 V to -2.5 V at a rate of 0.15 V/s. For the Ctrl-Perovskite, the observed dark\ncurrents at both GBs and GSs are the same as in previous reports27,28 (Fig. S5a). However, for the\nPerovskite/X, the dark current at the GSs is the same as the Ctrl-Perovskite, but at the GBs, the dark\ncurrent is significantly higher by almost three times that of the Ctrl-Perovskite (Fig. 2e and f).\nThis indicates a substantial enhancement in conductivity at the GBs by Ligand X treatment, suggesting\nthe successful incorporation of Ligand X at the GBs29. For the Perovskite/Y and Perovskite/X/Y\nsamples, the dark currents at the GSs are markedly higher than those of the Ctrl-Perovskite and\nPerovskite/X samples (Fig. S5b-S7), confirming the predominant localization of Ligand Y at the\nGSs. Statistical analysis of dark current measurements taken at 40 different points in both the GBs and\nGSs for all samples is summarized in Fig. 2h, demonstrating the robustness and reproducibility of these\nresults.\nKelvin Probe force microscopy (KPFM) is also used to analyze the effects of Ligand X and Y surface\npotential changes in perovskite films. The Ctrl-Perovskite film exhibited numerous GBs black pits\n(represented by white circles in Fig. 2g). A 1D height profile extracted from the topography indicates\nan average depth of ~70 nm between the GS and these pits (Fig. S8). In the Perovskite/X, the number\nof black pits decreased significantly, and the average depth reduced to ~40 nm. While for the\nPerovskite/X/Y, nearly all pits were eliminated, with the average depth further reduced to ~20 nm.\nThese results suggest that the TSS-DL method effectively fills the pits, indicating successful\nreconstruction of GBs and GSs sequentially. Moreover, the KPFM measurements showed a reduction\nin contact potential difference (CPD) from ~400 mV (Ctrl-perovskite) to ~150 mV for Perovskite/X\nand ~10 mV for perovskite//X/Y). This sharp decline in CPD correlates with the filling of GB pits,\nsuggesting improved surface homogeneity. As a result, the Perovskite/X/Y sample displayed a\nuniform potential profile than both the Ctrl-perovskite and Perovskite/X.\nIn manuscript (line 152-163) (XPS)\nR-8/17\nMeanwhile, we employed X-ray photoelectron spectroscopy (XPS) to verify potential chemical\ninteractions between Ligand X and Ligand Y with the perovskite structure, and to elucidate\ntheir sequential effects at GBs and GSs. When Ligand X, which contains O and F groups,\nintroduced before Ligand Y, the Pb 4f peaks exhibited a more substantial shift toward lower\nbinding energy (by 0.23 eV), suggesting increased electron density at Pb²⁺, likely due to direct\ncoordination with the oxygen and fluorine atoms in Ligand X (Fig. 2c). Additionally, the F 1s\nand O 1s peaks shifted to higher binding energies (Fig. 2d and Fig. S4), indicating that these\natoms are involved in strong chemical bonding with Pb²⁺ or other positively charged defects28.\nThis pronounced shift in the XPS spectra when Ligand X is applied first implies that Ligand X\nhas a strong and direct chemical interaction with Pb²⁺ sites, which are abundant at GBs due to\ntheir higher defect density compared to GSs16,23. In contrast, (Perovskite/Y/X), less pronounced\nbinding energy shifts for Pb 4f, O 1s, and F 1s were observed, suggesting that initial treatment\nof the perovskite film by Ligand Y at the GSs hinders subsequent interactions of Ligand X with\nthe perovskite structure, particularly at the GBs.\n(Line 191-194)\nConsequently, the comprehensive suite of TOF-SIMS, XPS, c-AFM, and KPFM\ncharacterizations collectively demonstrates that the choice and sequence of ligand treatments\nprofoundly influence the spatial distribution, chemical interaction, and electronic properties of\nthe perovskite films.\nFig.2. Ligands Distribution: (a-b) TOP-SIMS spectra of Perovskite/X/Y and Perovskite/Y/X\nthin film. (c) Pb 4f XPS spectra of Ctrl-Perovskite, Perovskite/X and Perovskite/X/Y,\nPerovskite/Y/X and O 1s (e) XPS spectra of only X and Perovskite/X and Perovskite/Y/X\nfilm. Local dark currents measured at the GBs (e) and GS (f) of Ctrl-Perovskite and\nPerovskite/X thin film. (g) Topography and KPFM images of Ctrl-Perovskite, Perovskite/X\nand Perovskite/X/Y. (g) Statistical local dark-current distribution collected from 40 points at\nR-9/17\nGBs and GSs of Ctrl-Perovskite, Perovskite/X, Perovskite/Y and Perovskite/X/Y (X= DFABr\nY= 4APPBr)\nWe have revised the manuscript to better clarify the role and limitations of each technique in supporting\nour conclusions.\nQ2: The XPS analyses are very poor in quality (images too small) and their contribution to the overall\nresults is of dubious relevance. The authors should consider that XPS gets information out of the\noutermost layers of the material, usually 2 nm and very often no more than 5, and given that the AFM\nsuggest welss between 20 and 70 nm, it is not clear where the information we are seeing is coming from.\nAppart from that, knowing exactly the history of the sample and the conditions of the analysis (before\nthe analysis and inside the chamber, transport in vacuum?, cleaning?, ion bombardment?, samples were\nanalysed the same day? Etc) is of extreme importance to judge the relevance of the results shown.\nA2. We thank the reviewer for their insightful comments. We have replaced the XPS images in the\nrevised manuscript with higher-resolution versions and have increased their size for improved visibility\nand clarity.\nWe apologize for the confusion about the explanation of XPS and AFM data. We agree with the reviewer\nthat XPS primarily probes the outermost ~2-5 nm (depth sensitivity of XPS) of the sample surface. As\ndiscussed in response 1, the XPS analysis is simply used to confirm the possible chemical interactions\n(binding) of ligands X and Y with defect sites in the perovskite films, like other previous reports\n(Science 384, 189–193 (2024). However, due to the limited information of XPS about the penetrability\nand spatial information of ligands, we performed TOP-SIM and c-AFM measurements, which allow us\nto directly assess the local electrical properties at GBs and GS and get information about the ligand’s\nlocation and penetration.\nRegarding the sample history and experimental conditions, we have now revised and provided further\ndetailed information on each characterization, including XPS in the revised supporting information:\nXPS measurements were performed in an ultra-high vacuum analysis chamber with a base\npressure of approximately 1 × 10⁻¹⁰ Torr. A monochromatic Al Kα X-ray source (photon\nenergy = 1486.6 eV) was used for excitation. The photoelectron spectra were acquired\nusing a hemispherical electron energy analyser (PHOIBOS 150, SPECS). The samples\nwere prepared on cleaned glass/ITO substrates inside an N₂-filled glovebox. Following\nfabrication of perovskite films, the samples were transferred to the XPS facility in vacuum-\nsealed plastic bags and measured on the same day to minimize exposure to air and potential\ncontamination.\nQ3: The dark-current statistical analysis of figure S5 seems one of the more conclusive results of the\nmanuscript, how many different samples of each kind have been analyzed?, the data was collected the\nsame day with the same tip?\nA3. We appreciate the reviewer’s recognition of the significance of the dark-current statistical analysis\npresented in Figure S5 (now Fig 2j in revised manuscript). We measured a total of 3 batches of samples\n(five samples for each treatment) for each treatment condition to ensure statistical reliability and\nreproducibility. To minimize variability, all data for each batch were collected on the same day using\nthe same tip. Nitrogen gas continuously flows into the chamber to reduce the moisture. This approach\nwas chosen to avoid potential inconsistencies arising from tip-to-tip variations or environmental\nfluctuations.\nR-10/17\nQ4: The navigation of the manuscript between the main text and the supporting file turns complicated,\nsince there is a lot of information and images in the supporting file that are actually very relevant to\nunderstand the main text and they should be there. Supporting files should have information that are in-\nprinciple not required in a first read of the manuscript, but the authors continuously compare results of\nthe main manuscript and the supporting files, obliging the reader to have both documents open and\ncontinuously switching from one to the other. This is another reason why the current\nA4. Thank you for your valuable comments and suggestions. In response to the reviewer’s suggestions,\nwe have made the following revisions and reorganization of the figures, including merging and\nrearranging some figures from the Supporting Information to the main manuscript:\n1. We have thoroughly revised and improved the consistency between the main manuscript and\nthe Supporting Information for example.,\n2. Figure 2j in the original manuscript has been moved to Figure 3a.\n3. Figure S5 has been moved to Fig. 2 (as Fig. 2h). Figures S11 (UV-Vis) and S12 have been\nmoved to Figures 3c and 3d, respectively.\n4. Figure 3c in the original manuscript has been moved to the Supporting Information as Fig.\nS16 in the revised version.\n5. Additionally, as requested in comment 5, we have enlarged the size of all figures for better\nclarity and judgment.\nWe appreciate the reviewer’s feedback, which has helped us to improve the quality and clarity of our\nmanuscript.\nQ5. In general, all the images are too small; the authors are strongly encouraged to reconsider the\ncurrent image sizes and distribution they have chosen. At the moment, all the images lack the size and\nquality required to make an informed judgement.\nA5. Thank you for your valuable suggestions. In the revised manuscript, we have enlarged all the images\nto enhance clarity and ensure better representation. Additionally, we have improved the image quality\nto 600 dpi to facilitate more accurate and informed interpretation. Furthermore, we have provided\nsupplementary PDF files for each figure to support detailed examination and understanding.\nQ.6: Sustainability: The authors propose to pass from a single spin-coating process to a double process.\nThis doubles the amount of solvent required, time, and energy to produce a device. Today, when\nperovskite devices are on the brink of industrial deployment, these questions are extremely relevant. Do\nthe authors have estimated how much solvents and energy these additional steps require and how it\ntranslates to the cost, CO2 impact and lyfe-cycle-assessment of potentially millions of devices\nmeasuring millions of m2?, it would be interesting to have a brief statement on this regard, since we are\nworking on creating a sustainable technology we should aim to use only the most sustainable processes\nand materials.\nA6. We thank the reviewer for highlighting the critical importance of sustainability as perovskite solar\ncells approach industrial scale. As we know, the levelized cost of electricity (LCOE) of perovskite solar\ncells/modules and other photovoltaic technologies depends on several factors, including but not limited\nto fabrication method, materials cost, particularly the performance and stability (lifetime) play a major\nrole as shown in Fig. R1 (Adv. Mater. 36, 2307357 (2024).\nR-11/17\nFig. R1; (a) Golden triangle in photovoltaic community. (b) Comparison of cost, efficiency, and lifetime\nbetween silicon modules and single-junction PSM from reference (Mater. 36, 2307357 (2024).\nIndeed, the PSC power conversion efficiency is almost suppressed by that of the Si solar cell, but their\noperation under standard conditions is hindered by poor stability and thus increasing/killing the overall\nLOCE of PSCs. Particularly, improving the stability to 25 years is a highly challenging task. It is\ndifficult to achieve through improvements in current technologies and requires technological innovation.\nFor example, the ion migration to the outside can be blocked by passivation and the use of a block layer,\nwhile the phase separation caused by ion migration at GBs and GSs is more complex and requires new\nmethods to address.\nAs suggested by the reviewer, we conducted a comprehensive analysis of the materials cost, energy\nyield, and LCOE associated with transitioning from a single-step to a double-step process, and details\nare given in the revised supporting information (Table S3-S4) and below Table R1-R2. As expected, a\nminor increase in the cost of approximately 0.14%/m2 for the large-scale production. However, the\nimproved stability and efficiency given by the TSS-DL method yield a substantial increase in energy\noutput (Energy yield) of around 47.7% compared to the single-step (Table R1 and Table S3-S4).\nFurthermore, the TSS-DL method results in a lower LCOE of approximately 32.1% and a more\nfavourable energy payback time over the device's lifetime. Given these findings, we believe that the\nenhanced operational lifetime and performance of the TSS-DL method will significantly offset the slight\nincrease in costs associated with large-scale production, ultimately leading to a more efficient and cost-\neffective solution.\nTable R1. Comparison of the cost, Energy yield, and LOCE of Ctrl perovskite, Perovskite/X and\nPerovskite/X/Y devices. We use our operational stability test (ISOS-L2) as an example here for the\ncalculation below.\nType of solar cell PCE Cost (USD) Final PCE Average Energy LCOE\n(%) (%) (after PCE (%) Yield ($)/Wh)\n3000 (Wh)\nhours)\nCtrl-Perovskite 22.7 31.63 4.54(20% 13.62 32.688 0.97\nof initial)\nPerovskite/X 24.32 31.68 9.73(40% 17.025 40.8576 0.78\nof initial)\nPerovskite/X/Y 26.06 31.723 24.24(93% 25.1479 60.355 0.53\nof initial)\nR-12/17\nCost of solvent/materials and Energy for additional step process, especially for m2 areas: We have\nestimated the solvent, material, labor, and electricity costs for 1 m² based on recent literature (Joule 4,\n822–839 (2020)) related to large-area (m²) fabrication. The estimated cost increases from 31.63 used/m2\nto 31.723 for 1 m2 production using the two-step TSS-DL method.\nTable R2. Cost estimation of step- and step-II using TSS-DL method.\nMaterials Unit Usage/m2 Price per Total Cost Other cost Total cost Ref\nUnit at of (electricity and of one\nMax. materials/m² labor cost) 15% layer\nVolume of total ($/m2)\n($/Unit) materials cost10\nIPA mL 11 0.003 Joule 4, 822–839\n(2020)\nDMSO mL 0.55 (200:1, 0.013\nIPA: DMSO)\nLigand X1 g 0.022 0.027\n(2mg/mL)\nLigand-Y2 g 0.0044 0.022\n(0.4mg/ml)\nStep-I 0.043 0.00645 0.050\nStep-II 0.038 0.0057 0.043\nCO impact and life Cycle: First, this additional step is performed under mild conditions, dynamic spin\n2\ncoating for 30 seconds, followed by annealing at 100°C for 5 minutes-without the need for high\ntemperatures or high-vacuum environments. As a result, the extra energy input is minimal, contributing\nnegligibly to the overall CO₂ impact during production. Secondly, the enhanced lifetime and\nsimultaneous movement in the PCEs of our TSS-DL-based devices will help offset any increase in CO₂\nemissions associated with large-scale production.\nBrief statement in the manuscript about this:\nBased on the above analysis and discussion, we have added the following brief statement to the main\nmanuscript (lines 369-377):\n\" Although, as expected, the additional step causes a minor increase in cost of approximately 0.14%/m2\nfor the large-scale production. However, the improved stability and efficiency given by the TSS-DL\nmethod yield a substantial increase in energy output (Energy yield) of around 47.7% compared to the\nsingle-step (Table S3-S4). Furthermore, the TSS-DL method results in a lower Levelized cost of\nelectricity (LCOE) of approximately 32.1% and a more favourable energy payback time over the\ndevice's lifetime. Given these findings, we believe that the enhanced operational lifetime and\nperformance of the TSS-DL method will significantly offset the slight increase in costs associated with\nlarge-scale production, ultimately leading to a more efficient and cost-effective solution41.\nQ7: Lines 117-122 include statements that would require citation of sources. For example is there\nany reference to the library of organic ligands mentioned? is it published? are there previous\nexperimental works on this regard?. While the statements about the adequacy of ligand X based on its\nsize also seem rather arbitrary without further experimental evidence to prove these claims.\nA7. Thank you for the reviewer comments and suggestions., We have added citations to support the\nstatements made in the original manuscript. The term 'library' refers to a collection of ligands,\nclassified as Ligand X and Ligand Y, as depicted in Figure 1c, similar to a previous report (Science\n381, 209–215 (2023). We have revised it now in the original manuscript to avoid confusion, below:\nIn the manuscript (lines 117-122):\nR-13/17\nBoth DFABr and 4APPBr were identified from the Ligand X and Ligand Y groups in Fig.1b based on\ntheir distinct chemical properties, which align with the specific stabilization needs of GBs and GSs,\nrespectively. DFABr was selected as Ligand X for its ability to effectively target GBs due to its strong\nbonding capabilities via triple functional groups and small molecular size configuration (Fig.S3), which\nallows it to penetrate and adhere to the narrow GBs more effectively (Accounts Mater. Res. 2, 1141–\n1155 (2021), Energy Environ. Mater. 7, e12696 (2024) and Chem. Commun. 56, 5006–5009 (2020)\nWe conducted preliminary studies on the effects of all ligands in Fig. S2 on device performance and\nfound that DFABr (Ligand X) and 4APPBr (Ligand Y) exhibited the best performance in Step 1 and\nStep II, respectively. Therefore, we focused on combining these two ligands for further in-depth\ninvestigation.\nWe have now provided the exact calculated sizes of Ligand X (DFABr) and Ligand Y (4APPBr₂),\nwhich\nwere found to be 1.538 Å and 2.660 Å, respectively (Fig. R2 here and Fig. S3 in the revised supporting\ninformation, using Mercury software).\nFig. R2. Size of Ligand X (a) and Ligand Y (b) (based on the distance between two consecutive C\natoms directly connected to the functional group, where X and Y represent DFABr and 4APPBr,\nrespectively.\nBased on the known characteristics of GBs as long and narrow circulation areas, we assumed that good\npenetrability and uniform spatial distribution of GB, a small size, and an 'all-in-one' ligand are required\nfor effective defect suppression, as suggested by Yang et al (Accounts Mater. Res. 2, 1141–1155 (2021)\nand Energy Environ. Mater. 7, e12696 (2024). Ligand X (DFABr) has an ideal molecular structure and\nmultifunctionality, possessing three different functional groups (O, F, and NH3 ). These features are\n+\nessential for effective passivation of deep defects at the GBs. Previous studies have shown that short-\nchain amines with alkyl chains shorter than 6 carbons tend to surround the GBs, which are vulnerable\nsites for electron-hole recombination and film degradation (Chem. Commun. 56, 5006–5009 (2020).\nFurthermore, fluorinated ligands have already been found to be more effectively adsorbed on\nperovskite surfaces than non-fluorinated ligands (Nat. Energy 8, 839–849 (2023). By combining these\nreported properties, we propose and then experimentally validate that Ligand X can act as an all-in-one\nligand for effective defect passivation at the GBs. We believe in light of these discussions and the\nliterature review, the size and chemical properties of the ligand play an important role in the defect\npassivation. We confirmed experimentally the penetrability of Ligand X using TOP-SIM data and its\npresence at the GBs using c-AFM, as discussed in the manuscript.\nQ8: The AFM images of Figure 2 have a scale bar in mm, probably it should micrometres (um), on the\nother hand the SEM images lack of scale bar at all, something recurrent in the manuscript (and\nsupporting file) with most of the SEM images.\nA8. We thank the reviewer for carefully examining our figures and bringing these issues to our attention.\nWe have corrected the scale bars in Figure 2i; the AFM images now display the scale in micrometres\nR-14/17\n(μm) instead of millimetres (mm). Additionally, scale bars have been added to all SEM images\nthroughout the manuscript and supporting information. We have thoroughly reviewed all figures to\nensure accuracy and consistency in the representation of scale.\nQ9: Line 245: “PL intensity improved by over 50%”…an increase of the PL counts does not necessarily\nmeans an improvement. In order to judge that, the structure of the device analysed should be known but\nthis is not mentioned in any part of the manuscript or SI file, is there any transport layer in these devices?\nthis is particularly important to understand the TRPL results. On the other hand, since the PL images\ndon’t have a proper scale-bar, the reader most believe the 50% statement. The authors are encouraged\nto include a proper scale-bar rather than using the easy but unprofessional resource of the arbitrary units.\nA9: Thanks for the reviewer's constructive comments. For the PL and TRPL measurements, we prepared\nall samples on Glass (Glass/Perovskites) without any interlayer, like other reports (Nat. Energy 9, 172–\n183 (2024). We have included an appropriate scale bar for PL in the revised manuscript. In addition, as\nsuggested in Comment 4, we have moved Figure S12 from the Supporting Information to Fig. 3c–d in\nthe main text to facilitate better comparison.\nQ10): The stability measurements lack a lot of information to judge their relevance, for example\nscanning frequency and scanning resolution and speed are very important factors to rule out ionic\neffects. The authors are encouraged to follow these\ndirections: https://www.nature.com/articles/s41560-019-0529-5.\nA10. Thanks for the comments and suggestions. We have revised the manuscript and given full details\nof the stability in the revised supporting information and manuscript. In the supporting information,\nwe revised the stability method (highlighted in yellow) as follows, according to the above reference\npaper.\nIn supporting information\nStability measurement: The ISOS-L2 and ISOS-D3 stability tests were conducted by storing the\ndevices in a humidity chamber equipped with precise humidity and temperature control, a calibrated\nLED light source providing one-sun illumination, and an automated device stability tracking system.\nThis system utilized custom MATLAB software integrated with a Keithley 2400 source meter for real-\ntime performance monitoring. For the ISOS-L2 test, the temperature was maintained at 65°C, with the\nchamber door left open to ambient air at a relative humidity of 85 ± 5%. The maximum power point\n(MPP) was set based on the V values measured under 1-sun illumination for fresh devices: Ctrl-\nmax\nPerovskite (0.96 V), Perovskite/X (0.98 V), Perovskite/X/Y (1.03 V), and Perovskite/Y/X (1.01 V).\nCurrent vs voltage (J-V) measurements were performed over the voltage range of -0.1 V to 1.2 V with\na step size of 0.01 V and a scan rate of 100 mV/s.\nFor the ISOS-D3 (damp heat) test, devices were exposed to a constant temperature of 85°C and relative\nhumidity of 85%, with performance measurements taken at regular intervals to assess degradation under\naccelerated aging conditions. For dark storage (ISOS-D1), unencapsulated devices were stored in the\ndark under ambient conditions with a relative humidity of 85 ± 5%. Device performance was\nperiodically monitored throughout the storage period to evaluate shelf-life stability. The J-V scanning\nparameters were kept consistent across all stability measurements. For each aging condition, three\ndevices were tested, each featuring five fingers, and the reported data represent the average performance\nof these devices.\nR-15/17\nQ11: All the experimental analysis technique lacks information (XRD, XPS, UPS, etc), just stating the\nbrand and model of the equipment used is not enough, it is advisable to state the particular conditions\nof the measurement like the energy used, ions, chamber conditions, calibration procedures, etc.\nPublishing is an act of sharing information with the rest of the scientific community, being transparent\nis the real contribution to the community, otherwise it can be seen as an exercise of\nshowcasing dubious results. Read for example https://doi.org/10.1016/j.apsadv.2023.100534.\nA.11. We appreciate the reviewer’s valuable feedback regarding the reporting of our experimental\nanalysis techniques. We agree that transparency and reproducibility are essential for advancing the\nscientific community, and we apologize for not providing sufficient detail in the original manuscript.\nIn the revised supporting information, we have included detailed descriptions of the conditions and\nparameters used for all key characterization techniques, including XRD, XPS, and UPS. Specifically,\nwe now provide information such as:\n XRD: XRD of all perovskite films was measured using Rigaku Smart Lab diffractometer with\na Cu Kα radiation (λ= 1.5406 Å) at a voltage of 40 kV and 200 mA currents. The scanning\nrange was 5.0°-60° with a scan rate of 5°/min and 0.02°/step. The perovskite samples for XRD\nwere prepared on cleaned Glass/ITO substrates and measured under ambient conditions without\nany additional protocol.\n XPS: XPS measurements were performed in an ultra-high vacuum analysis chamber with\na base pressure of approximately 1 × 10⁻¹⁰ Torr. A monochromatic Al Kα X-ray source\n(photon energy = 1486.6 eV) was used for excitation. The photoelectron spectra were\nacquired using a hemispherical electron energy analyzer (PHOIBOS 150, SPECS).\n UPS: Ultraviolet photoelectron spectroscopy (UPS) measurements were performed using\nan ultrahigh vacuum surface analysis system equipped with a ULVAC-PHI 5000\nVersaProbe III spectrometer and a helium I (He I, 21.2 eV) photon source. The samples\nwere biased at -5 V during measurement. The pass energy was set to 5.0 eV to ensure high-\nresolution spectra. Calibration of the UPS system was conducted using a clean gold (Au)\nsubstrate before measurements.\n TOF-SIMS analyses were conducted on a NanoTOF-II instrument (ULVAC-PHI, Japan)\nemploying a Bi₃⁺ primary ion beam (30 kV) for high-resolution spectral acquisition and an\nAr⁺ sputter beam (3 keV, 100 nA) for depth profiling.\n GWAXS measurements were conducted at the BL03HB beamlines of the Shanghai\nSynchrotron Radiation Facility (SSRF), utilizing an X-ray energy of 10 keV (wavelength\n1.2398 Å) with a PLATUS detector, which was positioned vertically from the sample. The\nexperiment was set with a grazing incidence angle of 0.4° and an exposure time of 10\nseconds. The perovskite films for GIWAS measurement were prepared on Glass/ITO\nsubstrates.\n The contact angles of all samples were tested by the JC2000C1 contact angle meter of\nShanghai Powereach at room temperature on the same day. 1.0 μl of deionized water droplets\nwas dropped carefully onto the perovskite films.\nQ.12: The devices analysed are extremely small, nowadays, this is less acceptable in perovskite research.\nThe authors are encouraged to either show results in bigger devices (>1 cm2) or at least state why they\nhave not done so or why they consider these are not relevant.\nR-16/17\nA.12. Thank you for your valuable feedback regarding the device area. Our initial focus on small-area\ndevices was primarily to enable systematic screening of material compositions and processing\nparameters with high reproducibility and minimal material consumption, utilizing our novel TSS-DL\nmethod.\nIn response to the reviewer's suggestion, we have now included results for larger-area devices (1 cm²)\nin the revised manuscript (Fig. R3 and Fig. S18). We fabricated three batches of devices based on\nPerovskite/X/Y, achieving a maximum PCE of 24.3% for the 1.0 cm², which is comparable to reported\nlarge-area devices (J. Am. Chem. Soc. 146, 13391–13398 (2024), Nature 633, 359–364 (2024), and Adv.\nEnergy Mater. 14, 2303941 (2024).\nIn the revised manuscript, we include the following statement about this (lines 319-323)\nIn manuscript.,\nTo evaluate the feasibility of large-scale production of PSCs, 1 cm² devices are frequently employed as\nan intermediate step toward scaling up the device area. Fig. S20 shows the J-V curve of the champion\ndevice based on Perovskite/X/Y with an active area of 1 cm². A champion PCE exceeding 24% was\nachieved using our TSS-DL method, further demonstrating its scalability.\nFig. S20. Reverse and forward J-V curves of a large-area Perovskite/X/Y-based device.\nQ13: Line 63: TSS-DS should read TSS-DL\nA.13. We thank the reviewer for pointing out the typographic error. We have corrected \"TSS-DS\" to\n\"TSS-DL\" in line 63 of the revised manuscript.\nR-17/17"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The current version of manuscript has been much improved. In my opinion, it can be published now.", "reviewer_2": "The revision has addressed all my major concerns. It is good to publish as is.", "reviewer_3": "The manuscript submitted by Ahmad, Choy et Al in its revised version presents a significantly better work. In this version\nthey are offering to the perovskite community a much better explained work, where both the methodology and the results are\nbetter described and could be better reproduced. In particular, they made a very good work on improving the supporting\ninformation document, the figures, the XPS analysis and economic impact analyses. I can recommend this revised version\nfor publication in Nature Communications.\nStill there are some minor issues that should be addressed before publication:\n- Figure S13 lacks a legend, though it would be possible for a reader to guess which results belong to which sample looking\nat other images (like S16 or S17), it would be much better if the image has its own legend.\n- The Solar Cell Reporting Summary mentions that the authors provide “A comparison between the integrated response\nunder the standard reference spectrum and the response measure under the simulator”, though this has not been found in\nFigure 4c, though I would say it is not absolutely mandatory for publication. Is better to correct the Reporting Summary.\n- I would highly advice the authors to consider depositing their (full) research data in an Open Repository (like Zenodo), this\nwould be the ultimate favour you can make to the research community and would show total transparency and would go in\naccordance with the publication in an Open Access journal as Nature Communications."}, "author_response": "RESPONSE TO REVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nThe current version of manuscript has been much improved. In my opinion, it can be\npublished now.\nResponse: Thank you for the reviewer’s positive feedback and recommendation. We\nappreciate the reviewer’s helpful comments and support.\nReviewer #2 (Remarks to the Author):\nThe revision has addressed all my major concerns. It is good to publish as is.\nResponse: Thank you very much for your thoughtful review and positive remarks regarding\nour revised manuscript. We greatly appreciate reviewer valuable feedback, which has\ncontributed to improving the quality and clarity of our work. Thank you for recommending\nour manuscript for publication.\nReviewer #3 (Remarks to the Author):\nThe manuscript submitted by Ahmad, Choy et Al in its revised version presents a significantly\nbetter work. In this version they are offering to the perovskite community a much better\nexplained work, where both the methodology and the results are better described and could be\nbetter reproduced. In particular, they made a very good work on improving the supporting\ninformation document, the figures, the XPS analysis and economic impact analyses. I can\nrecommend this revised version for publication in Nature Communications.\nStill there are some minor issues that should be addressed before publication:\nResponse: We are delighted to hear that the reviewer finds the improvements to our\nmethodology, results, supporting information, figures, XPS analysis, and economic impact\nanalyses valuable to the perovskite research community. We greatly appreciate your\nrecognition of our efforts to enhance the clarity and reproducibility of our work.\nBelow is a detailed, point-by-point response to the remaining minor comments and suggestions.\nQ1- Figure S13 lacks a legend, though it would be possible for a reader to guess which results\nbelong to which sample looking at other images (like S16 or S17), it would be much better if\nthe image has its own legend.\nA1: We thank the reviewer for their suggestions and have now provided a separate legend in\nFigure S13 (Supplementary Fig. 13 in the final version) in the revised Supplementary\nR-1/17\nInformation.\nQ2: The Solar Cell Reporting Summary mentions that the authors provide “A comparison\nbetween the integrated response under the standard reference spectrum and the response\nmeasure under the simulator”, though this has not been found in Figure 4c, though I would\nsay it is not absolutely mandatory for publication. Is better to correct the Reporting Summary.\nA2: We have updated and corrected the Solar Cell Reporting Summary as requested.\nQ3: I would highly advice the authors to consider depositing their (full) research data in an\nOpen Repository (like Zenodo), this would be the ultimate favour you can make to the\nresearch community and would show total transparency and would go in\naccordance with the publication in an Open Access journal as Nature Communications.\nA3: Thank you for reviewer suggestion regarding the deposition of our research data in an open\nrepository. We have provided a separate source data file with the final version, which will be\npublished alongside this paper. All data generated and supporting the current work are included\nin the main text, Supplementary Information, and the source data file. We believe this ensures\ntransparency and facilitates access for the research community, in accordance with the\nprinciples of open access publication in Nature Communications.\nR-2/17"}]

Decision letter