Homogeneous crystallization via sustained solvent-extraction channels for methylammonium-free all-perovskite tandem solar cells

Domain: chemistry
Published: 2026-01-05
Editorial outcome: minor_revision

Abstract
Replacing volatile methylammonium (MA+) with formamidinium (FA+) or cesium (Cs+) cations in mixed Pb-Sn perovskite composition improves thermal resilience. Nevertheless, the low-solubility Cs-based perovskite tends to preferentially crystallize into a dense Cs-rich surface layer during the AS-assisted crystallization process, which impedes the AS to extract the internal solvent. Here, we introduce a multi-Lewis-base modulator to maintain sustained solvent-extraction channels (SSC) open throughout the AS process, thereby homogenizing MA-free mixed Pb-Sn perovskite crystallization in depth. This approach yielded a PCE of 22.7% in FACs Pb-Sn perovskite solar cells. We fabricated monolithic all-perovskite tandem solar cell with a high PCE of 29.2% (certified PCE of 29.2%), which is the highest certified PCE for MA-free all-perovskite TSCs. The unencapsulated FACs-based mixed Pb-Sn perovskite solar cell remained over 80% of its initial PCE after 800 h of aging at 85 °C. This strategy enables the simultaneous realization of high-performance and thermal stability.

Introduction
All-perovskite tandem solar cells1–4, combining wide and narrow bandgap subcells5–8, demonstrate the potential to surpass the Shockley-Queisser efficiency limit of single-junction photovoltaic devices9,10. While the efficiency of all-perovskite tandem solar cells is progressing rapidly, device stability becomes a critical issue currently hindering their commercial potential11,12. Photovoltaic devices under real-world operating conditions must endure operational temperature extremes to 85°C. However, current high-efficiency Pb-Sn narrow bandgap perovskite solar cells (PSCs) are often doped with thermally unstable methylammonium (MA), whose propensity for thermal decomposition compromises device longevity13,14. Although replacing MA with thermally stable Cs enhances the perovskite thermal stability15–18, it often compromises film quality and reduces PCE17,19–22. This is mainly because low-solubility Cs-based23–27 perovskite tends to preferentially crystallize at the perovskite precursor solution film/anti-solvent (AS) solution interface during the AS-assisted crystallization process, forming a dense perovskite top-surface barrier layer (Supplementary Fig. 1). This continuous dense Cs-rich surface layer impedes the AS solvent extraction through the perovskite precursor solution, resulting in excessive solvent retention and asynchronous crystallization23–27. The retained excess solvent rapidly escapes during subsequent annealing processes, creating numerous voids at the perovskite buried interface. This ultimately induces an inhomogeneous Cs distribution and compromises the device performance.

Balancing the prior precipitation of dense perovskite at upper surface and the efficient solvent extraction during the AS-assisted crystallization process is critical for improving the photovoltaic (PV) performance of FACs-based Pb-Sn perovskites. The strong solvent extraction capability of AS hampers the effectiveness of conventional crystallization-delay strategies, such as adding dimethyl sulfoxide (DMSO)28 or Lewis acid to form intermediate-phase engineering29–35. Meanwhile, the trapped solvent, unable to be effectively removed, leads to the formation of voids and defects during thermal annealing.

Here, we deploy a surface tension modulation strategy using a strong multi-Lewis-base modulator to maintain sustained solvent-extraction channels (SSC)—a mechanism that prevents the formation of a continuous, dense thin perovskite layer at the perovskite/AS contact surface and preserves continuous solvent extraction channels throughout the AS process. Unlike conventional additive strategies that primarily regulate the intrinsic crystallization of perovskite36–39, SSC focus on maintaining efficient solvent-extraction channels during anti-solvent treatment, similar to solvent-extraction modulation concepts proposed in other material systems40. We reported a binary -COOH additive, L-Glutamic acid hydrochloride (Glu), added into perovskite precursor solutions to regulate the crystallization process of FACs-based Pb-Sn perovskites, aiming for high-performance and enhanced thermal stability of devices (Supplementary Fig. 2). The NH3+ group in Glu has a higher electrostatic potential and is likely to fill possible A-site vacancies (e.g., Cs⁺/FA⁺), while its dual -COOH groups tend to coordinate more strongly with Pb2+/Sn2+32,41,42. Such enhanced coordination effect enables Glu to modify the surface energy of the precursor solution, resulting in a higher heterogeneous nucleation barrier. As a consequence, the nucleation density is reduced, leading to fewer nucleation sites and the formation of larger perovskite grains. During the AS-assisted crystallization process, these larger grains pack less densely than smaller ones, exhibiting more channel-like structures between grains compared with the control perovskite surface. Therefore, Glu acts as the key additive that implements the SSC concept by maintaining sustained solvent-extraction channels during AS treatment. This strategy enables a delicate balance between perovskite precipitation and efficient solvent extraction during the AS-assisted crystallization to achieve high-quality FACs-based Pb-Sn perovskite films. As a result, the champion MA-free mixed Pb-Sn perovskite solar cell yields the highest PCE of 22.7%. We further fabricated an all-perovskite tandem solar cell with a PCE of 29.2% (certified 29.2%), representing the highest certified PCE for MA-free all-perovskite tandem solar cells. The FACs-based mixed Pb-Sn perovskite solar cell with indium zinc oxide (IZO) electrode remained over 80% of its initial PCE after 800 h of thermal aging at 85 °C, demonstrating the state-of-the-art thermal stability for MA-free Pb-Sn perovskite devices. As a result, this approach achieves a combination of high power conversion efficiency and thermal stability in MA-free Pb-Sn perovskite solar cells.

Results
MA-free Pb-Sn perovskite with SSC
We used a one-step anti-solvent process to fabricate the MA-free Pb-Sn mixed perovskite films with a composition of FA0.85Cs0.15Pb0.5Sn0.5I3. However, due to the low solubility of Cs-based species, the preferential formation of a dense film at the perovskite/AS solution contact interface during the AS-assisted crystallization process inhibits efficient extraction of the internal solvents by AS. To address this issue, we implemented the SSC strategy by incorporating multi-Lewis base modulators into precursor solutions with different coordination ability (see “Methods”). We investigated three representative molecules with increasing numbers of carboxyl groups—glycine hydrochloride (Gly, one -COOH), Glu (two -COOH), sodium citrate (Sc, three -COOH) (Supplementary Fig. 3). Optical microscopy revealed anti-solvent (AS) extraction channels in the discontinuous 3D perovskite surfaces. The fraction of uncovered surface domains increased with the number of carboxyl groups in the additive, rising from 1.78% (control) to 5.17%, 7.11% and 15.72% for Gly, Glu and Sc, respectively (Fig. 1a and Supplementary Figs. 4 and 5).

To better understand this process, we measured the viscosity of the precursor solution and the contact angles of the corresponding perovskite droplets on PEDOT:PSS coated substrates. The viscosity increased with the number of -COOH groups, due to enhanced coordination and steric hindrance (Supplementary Table 1). Correspondingly, the contact angle of the precursor solution on the substrate increased progressively with the number of -COOH groups (Fig. 1b and Supplementary Fig. 6). According to classical nucleation theory, the energy barrier for heterogeneous nucleation (ΔG*Het) is proportional to the geometric factor f(θ), which is a monotonic function of the contact angle θ. A larger θ corresponds to a higher f(θ), and thus a higher ΔG*Het. This thermodynamic relationship implies that the higher nucleation energy barrier induced by SSC not only increases the critical radius, but also reduces the nucleation density, allowing fewer nuclei and ultimately leading to larger grain sizes (Supplementary Note 1). These theoretical insights align with our morphological observations: the control films with smaller contact angles undergo denser nucleation and yield smaller, more tightly packed grains. From a geometric standpoint, smaller particles tend to stack more densely, whereas larger ones leave more interstitial volume, contributing to increased film porosity. It is further demonstrated that the modulator additives are able to effectively adjust the superficial energy of the precursor solution, resulting in a higher heterogeneous nucleation barrier and a more restrained nucleation process, enabling more spatially distributed and uniform crystal growth. Scanning electron microscopy (SEM) images of the resulting perovskite films revealed not only enlarged grain sizes from control to Gly, and further to Glu, but also a marked improvement in grain uniformity (Supplementary Fig. 7). However, the Sc film did not follow this trend, which instead displayed poor surface crystallinity with non-uniform and small grains (Supplementary Figs. 8 and 9). These findings underscore the importance of balancing coordination strength for designing additive molecules. The Glu with two -COOH groups facilitates the formation of better perovskite films by regulating the crystallization process. Meanwhile, the excessively strong coordination—as observed in the case of Sc—may disrupt the controlled crystallization process and compromise film quality, which is consistent with previous reports that over-coordinating additives tend to induce non-uniform nucleation and hinder crystal growth37,41,43,44. To exclude the influence of amino groups and sodium salts, we also compared Formamidine hydrochloride (FACl) and NaCl as comparative additives (Supplementary Figs. 3 and 7–9). Both resulted in poor morphology, confirming that the improved film formation in Glu-treated samples primarily originates from balanced -COOH coordination rather than amino groups and sodium salts. Accordingly, Sc was excluded from further discussion in subsequent sections.

To investigate the crystallization modulation mechanism of SSC, we systematically explored its coordination chemistry and dynamic interactions with precursor components through a suite of chemical characterization techniques. FTIR spectra revealed that the characteristic C=O stretching peak of Glu shifted from 1699.78 to 1716.17 and 1668.93 cm−1, respectively, implying the strong coordination interaction between the -COOH groups of Glu and Pb²⁺/Sn²⁺ through Lewis acid-base adduct (Supplementary Fig. 10a). Compared to the commonly used monocarboxylic acid Gly33 with a single -COOH group, Glu with dual -COOH groups demonstrated a stronger effect (Supplementary Fig. 10b). The dicarboxylic modulator Glu induced more significant shifts in the -COOH stretching vibrations when mixed with PbI₂ or SnI₂. This stronger interaction likely arises from the dual -COOH groups in Glu, which can chelate metal centers more effectively than Gly. This strong coordination increases the effect of steric hindrance in the solution, thereby raising the viscosity and further supporting the observed increase in contact angle. Based on the comparative device performance of these modulators, Glu devices with higher average PCE were ultimately selected for the SSC strategy (Supplementary Fig. 11).

To complement these coordination analyses, we further employed ¹H nuclear magnetic resonance (NMR) spectra to investigate the effects of Glu incorporation. The ammonium proton resonance of Glu at 8.49 ppm shifted upfield upon mixing with FAI/CsI, while the dual resonances of FAI (8.65/8.99 ppm) shifted downfield (Fig. 1c), suggesting that Glu interacts and coordinates with the perovskite precursor components rather than new species formation and helps regulate the cation distribution45. X-ray diffraction (XRD) patterns of Glu-PbI₂ and Glu-SnI₂ mixtures exhibited distinct diffraction peaks absent in pure components (Supplementary Fig. 12), suggesting the intermediate phase formation via Glu-perovskite coordination. X-ray photoelectron spectroscopy (XPS) revealed a shift in the binding energy of the Sn signal on both the surface and bottom sides of the Glu film compared to the control (Supplementary Fig. 13), indicating a change in the Sn chemical environment. Concurrently, Glu reduced the proportion of Sn4+ peak area in Sn element peak area, suggesting suppressed Sn²⁺ oxidation and reduced defect density. Dynamic light scattering (DLS) further revealed that the addition of Glu increased the colloidal size of the perovskite precursor by 38% compared to the control (Supplementary Fig. 14), further supporting the presence of stronger coordination and a higher heterogeneous nucleation barrier46.

Based on a systematic comparison of film morphology, crystallinity, and device performance at different concentrations (Supplementary Figs. 15–17), a 2% concentration of Glu was identified as the optimal condition to achieving the SSC strategy, and was therefore used throughout this study. To directly probe the effect of SSC on perovskite crystallization kinetics, we performed in situ optical microscopy on spin-coated precursor films without applying anti-solvent inside a nitrogen-filled glovebox. This setup enabled direct observation of the solvent evaporation-driven crystallization process. The nucleation and growth process of SSC perovskite crystallization can be significantly retarded compared to the control (Fig. 1d). These observations indicate that SSC not only delays nucleation but also significantly suppresses the crystal growth rate. X-ray diffraction spectra of wet films before AS treatment (Fig. 1e) further support this conclusion. The characteristic perovskite phase diffraction peaks appeared much earlier in the control film than in the SSC one, confirming a slower phase evolution. Collectively, these findings demonstrate that strong coordination between Glu and metal ions leads to a decelerated crystallization process, influencing final film morphology and microstructure. To further probe the crystallization process, we monitored the digital photos of the film during anti-solvent deposition (Supplementary Fig. 18). The control sample exhibited a darkening-lightening-darkening behavior, consistent with the transient formation and collapse of a dense surface layer. To delve deeper into the impact of SSC on perovskite film formation, we conducted in situ PL spectroscopy during both the spin-coating and annealing stages (Supplementary Fig. 19). The transient PL peak evolution of the control film (appearance-disappearance-reappearance) mirrors the changes in Supplementary Fig. 18, confirming the temporary formation of a dense surface layer that hinders solvent extraction. In contrast, the SSC film displayed a more continuous PL evolution without such interruptions, indicating more uniform crystallization and an unobstructed solvent-extraction pathway. Due to the PL detection fiber has an unavoidable absorption feature near ~940 nm that can produce an apparent two-peak appearance in the raw spectra (this effect and its manifestation in in-situ PL have been discussed previously47).

Gas chromatography (GC) quantification showed that SSC films retained <15% residual DMF/DMSO versus >32% in controls post-AS treatment (Fig. 1f), confirming enhanced solvent extraction capacity. The coordinated interplay between enlarged early-stage crystallites and sustained channel architecture enables complete removal of coordinated solvents, establishing a pathway for high-quality perovskite crystallization.

To elucidate the mechanism of SSC in suppressing surface preferential crystallization during the AS process, we proposed a schematic illustration comparing the control and SSC systems (Fig. 1g). In the control film (left), rapid surface crystallization forms a dense perovskite layer that obstructs efficient AS extraction. In contrast, the SSC system (right) balances the interfacial precipitation of dense perovskite and the AS deep solvent extraction during the AS-assisted crystallization through the coordination of Glu with perovskite precursor constituents. SSC can generate interstitial channels during AS-assisted crystallization. These channels prevent premature closure of extraction pathways by breaking compact perovskite crystallization, thereby preserving permeable networks for AS extraction process. This schematic illustration highlight that SSC does not solely delay crystallization but actively maintains sustained solvent-extraction during the AS stage—a key mechanistic distinction from conventional additive-assisted crystallization.

We investigated the effect of the SSC on the morphology of MA-free Pb-Sn alloyed perovskite films. Cross-sectional SEM images (Fig. 2a) and the corresponding buried interface SEM images (Supplementary Fig. 20) further revealed significant interfacial voids at the perovskite/substrate interface in the control film, indicating optimized crystallization kinetics enabled by the SSC strategy. To further distinguish our approach from conventional solvent-coordination control, we investigated the effect of DMSO content on film formation. Excessive DMSO lead to the formation of larger and more numerous voids at the perovskite interface, whereas the SSC strategy effectively alleviates this issue by preserving efficient solvent-extraction pathways during crystallization (Supplementary Figs. 21 and 22). To further examine the vertical distribution of Cs, we conducted cross-sectional EDS elemental mapping (Supplementary Figs. 23). The control film shows clear Cs accumulation near the surface and a gradual decrease toward the substrate, whereas the SSC film exhibits a much more uniform Cs distribution across the film thickness, supporting that the SSC treatment suppresses the formation of a Cs-rich surface layer. Time-of-flight secondary-ion mass spectrometry (TOF-SIMS) depth profiling unambiguously revealed the presence of a pronounced vertical composition gradient of Cs in control films (Fig. 2b and Supplementary Figs. 24 and 25), due to the preferentially surface crystallization of Cs-rich phases. Although the Cs⁺ profiles in Fig. 2b are plotted on a logarithmic scale for clarity, the corresponding linear-scale data (Supplementary Fig. 24) clearly show that the slope difference between the control and SSC films appears from the beginning of the measurement, confirming the existence of a steeper Cs gradient in the control film. Remarkably, SSC significantly flattens this gradient, homogenizing the vertical Cs distribution. This improvement can be attributed to the intrinsically low solubility of Cs-based salts, which often leads to premature surface crystallization during solution processing, particularly in FA-Cs mixed systems. By modulating the crystallization kinetics, SSC prevents the early aggregation of Cs-rich phases, thus enabling more uniform incorporation of Cs throughout the film. We further explored the carrier dynamics behavior of perovskite films to evaluate the impact of the SSC strategy on the film quality.

The steady-state photoluminescence (PL) emission of the SSC film measured from both sides were identically stronger and higher than those of the control film, indicating suppressed non-radiative recombination due to reduced defect density (Fig. 2c). Notably, the emission peaks of the control film displayed a slight blue shift between its top and bottom surfaces, while the SSC sample showed identical emission peaks, reflecting improved vertical homogeneity. These results further support that the SSC strategy effectively mitigates the preferential surface crystallization of Cs-rich phases, leading to a more homogeneous Cs distribution throughout the film. We further performed time-resolved PL (TRPL) from both the perovskite side (Supplementary Fig. 26) and the glass side (Fig. 2d). The SSC film exhibited significantly longer carrier lifetimes than the control film, suggesting suppressed non-radiative recombination and reduced carrier capture loss. Considering the top-to-bottom crystallization pathway of perovskite films48, this observation is consistent with Glu gradually migrating toward the bottom during film growth, enabling effective defect passivation at the perovskite/substrate interface (Supplementary Fig. 27). Meanwhile, TOF-SIMS analysis (Supplementary Fig. 28) shows that a small amount of Cl- remains mainly at the buried interface. Together with the FACl comparison experiment, this indicates that Cl- plays a secondary passivation role rather than being the dominant factor in crystallization regulation. It is noteworthy that this spatial distribution is conducive to the defect passivation at the buried interface of perovskite films by Glu. We measured their UV-vis absorption spectra (Supplementary Fig. 29a) and then calculated their bandgap values (Supplementary Fig. 29b), which revealed negligible differences between control and SSC-treated films. Electroluminescence quantum yield (ELQY) was further used to analyze charge-carrier recombination49. At current densities equivalent to the short-circuit current density (JSC) under 1-sun illumination, the ELQYs of the control and SSC devices were 0.12% and 1.04%, respectively (Fig. 2e). To quantify the interfacial non-radiative recombination losses50,51, the photoluminescence quantum yield (PLQY) of perovskite was measured to estimate quasi-Fermi level splitting (QFLS) and voltage losses (Fig. 2f and Supplementary Table 2). The higher PLQY and ΔQFLS values of SSC films imply a significant suppression of non-radiative recombination channels in perovskite films, which is usually accompanied by an increase in VOC to enhance device performance. These results further validated that the reduction of defect density in Pb-Sn perovskite film was achieved after SSC treatment. Therefore, in addition to regulating the solvent-extraction pathways, Glu also behaves similarly to previously reported amino acid additives, which can modulate perovskite crystallization and passivate defect states, thereby enhancing both efficiency and stability of perovskite solar cells32,41,42,44,52. Based on the above results, the SSC-treated perovskite film can passivate the acceptor vacancy defect, which can reduce the probability of non-radiative recombination of carriers and improve the optoelectronic performance.

Photovoltaic (PV) performance of perovskite solar cells with SSC
We fabricated FACs-based mixed Pb-Sn PSCs without and with SSC treatment. The perovskite solar cells employed a device structure of ITO/PEDOT:PSS/Perovskite/C60/ALD-SnO2/Cu, where ITO is indium tin oxide, PEDOT:PSS is Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate). Glu outperformed the control and other additives with a single -COOH group33,42, highlighting the superior effectiveness of the SSC strategy, which benefits from the stronger coordination achieved by dual -COOH functional groups (Fig. 3a and Supplementary Fig. 30). The SSC devices (aperture area of 0.049 cm2) delivered a champion PCE of 22.7% (stabilized at 22.5%), with an open-circuit voltage (VOC) of 0.871 V, a short-circuit current density (JSC) of 31.8 mA cm−2, and a fill factor (FF) of 81.9% (Fig. 3b and Supplementary Fig. 32). The increased JSC is ascribed to higher light absorption in the near-infrared range, as indicated by the external quantum efficiency (EQE) spectra (Supplementary Fig. 33). Furthermore, we also counted the histogram of PCE values of the SSC narrow-bandgap PSCs (Supplementary Fig. 34). The devices exhibited an average PCE of 21.76±0.36% with a narrower distribution than the control devices.

We fabricated monolithic all-perovskite tandem solar cells using the optimized narrow-bandgap (NBG) perovskite with a device aperture area of 0.049 cm2. The wide-bandgap (WBG) perovskite had a composition of FA0.8Cs0.2Pb(I0.6Br0.4)3. The tandem device consists of a ~450 nm WBG sub-cell and a ~900 nm NBG sub-cell, as shown in the cross-sectional SEM image (Fig. 3c). The WBG PSC showed a PCE of 20.1%, with a VOC of 1.348 V, a JSC of 17.8 mA cm−2, and a FF of 83.6% (Supplementary Fig. 35). Comparative analysis with control tandem devices showed improvement of all photovoltaic figures of merit for SSC tandem devices (Supplementary Fig. 36). The champion SSC tandem solar cell exhibited a high PCE of 29.2% (stabilized at 28.8%), with a VOC of 2.171 V, a JSC of 16.1 mA cm−2, and a FF of 83.72% (Fig. 3d, Supplementary Fig. 37 and Table 3). The histogram of PCE values of tandem solar cells showed a narrow distribution, with an average PCE of 28.37±0.39% (Fig. 3d). The integrated JSC values of the front and back subcells from EQE spectra (Fig. 3e) were 16.1 and 16.1 mA cm−2, respectively. Furthermore, we sent one of the best-performing tandem solar cells to an accredited independent laboratory and achieved a certified PCE 29.2% (29.2%) at reverse (forward) scan (Supplementary Fig. 39). The tandem device delivered a certified stabilized PCE of 28.8% (Supplementary Fig. 39). This represents the highest certified PCE for MA-free all-perovskite tandem solar cells.

Thermal stability of MA-free Pb-Sn perovskite solar cells
In addition to the efficiency, long-term stability is another important parameter for evaluating the practical potential of the device. Therefore, we evaluated the thermal stability potential of MA-free Pb-Sn alloyed PSCs in real applications. We conducted ageing tests on unencapsulated mixed Pb-Sn PSCs by maintaining them on a hot plate at 85 °C in a N2-filled glovebox (H2O, <0.01 ppm; O2, <0.01 ppm). The unencapsulated SSC devices exhibited better thermal stability compared to the control devices, attributed to the improved perovskite film quality. In contrast, the FAMA devices exhibited a more rapid degradation immediately (Fig. 4a), underscoring the inferior thermal stability of FAMA absorbers under thermal stress.

In addition to the perovskite absorber itself, charge transport materials also play a crucial role in affecting long-term stability and efficiency of PSCs. PEDOT:PSS is the most commonly used hole transport layer (HTL) for efficient mixed Pb-Sn PSCs, despite the fact that the deleterious interfacial reaction between the acidic PEDOT:PSS and Pb-Sn perovskites under thermal stress can lead to perovskite degradation and deteriorate the carrier extraction53. To further evaluate the impact of HTL selection, we proceeded to compare the thermal stability of mixed Pb-Sn PSCs fabricated with PEDOT:PSS and without any HTL (Fig. 4b). Although the device without HTL exhibited enhanced thermal stability, its PCE was markedly inferior to that of a device fabricated by PEDOT:PSS, indicating the urgent need for future research to develop alternative HTL materials that can balance both efficiency and stability.

Furthermore, the top electrode material has a pronounced impact on device durability. All devices discussed above employed Cu as the top metal electrode. To mitigate metal-induced degradation, we replaced the metal top contact with Indium Zinc Oxide (IZO) to eliminate its adverse impact on device stability54–56. We chose to capped the device with sputtered IZO in front of the metal electrode (Fig. 4c). This IZO interlayer effectively suppressed metal diffusion, thereby significantly enhancing the thermal stability of the devices. It was demonstrated that the thermal stability of PSCs can be significantly enhanced by sputtered IZO. In addition to using a barrier layer, replacing Cu with inherently more stable electrode materials also contributed to improved device stability. For instance, substituting Cu with Au electrodes yielded perovskite tandem solar cells with enhanced thermal stability (Supplementary Note 2 and Supplementary Fig. 38). Thermal aging tests revealed that the device with only Cu electrode degraded to 60.6% of its initial PCE within just 180 h at 85 °C, whereas those employing IZO/Au electrodes remained over 80% of its initial PCE after 800 h, demonstrating state-of-the-art thermal stability for MA-free Pb-Sn perovskite devices (Fig. 4d). The preceding discussion of thermal stability results highlights the dual importance of developing both intrinsically stable mixed Pb-Sn perovskite and robust device structures to ensure the long-term operational stability.

Methods
Materials
All materials were used as received without further purification. Lead halides (PbI2 and PbBr2, 99.99%), [2-(9|H|-Carbazol-9-yl) ethyl] phosphonic Acid (2PACz, >98%), and [2-(3,6-Dimethoxy-9H-carbazol-9-yl) ethyl] phosphonic Acid (MeO-2PACz, >98%) were obtained from TCI Chemicals. Organic halide salts, including formamidinium iodide (FAI), cesium iodide (CsI), and phenylethylammonium iodide (PEAI, >99%), were purchased from GreatCell Solar Materials (Australia). Poly(3,4-ethylenedioxythiophene) - poly (styrenesulfonate) (PEDOT:PSS) aqueous solution (Al 4083) was supplied by Heraeus Clevios (Germany). SnF2 (99%), Glycine hydrochloride (Gly, ≥99%), L-Glutamic acid hydrochloride (Glu, ≥99%), Anisole (99%), dimethylformamide (DMF, 99.8% anhydrous), and dimethyl sulfoxide (DMSO, 99.9% anhydrous) were purchased from Sigma-Aldrich. Sodium citrate (Sc, ≥98%) was purchased from Aladdin. Ethylenediammonium diiodide (EDADI, 99.5%) was purchased from Xi’an Polymer Light Technology (China). The C60 was purchased from Nano-C (USA).

Perovskite precursor solution
Narrow-bandgap (NBG) FA0.85Cs0.15Pb0.5Sn0.5I3 perovskite: A 2.0 M precursor solution was prepared by dissolving FAI, CsI, PbI2, and SnI2 in a mixed solvent of DMF and DMSO (volume ratio 3:1). The stoichiometric ratios of FAI:CsI and PbI2:SnI2 were fixed at 0.85:0.15 and 0.5:0.5, respectively, with a total A-site to B-site ratio of 1:1. SnF2 (10 mol% relative to SnI2) was added in the precursor solution. Additives (Gly, Glu, or Sc) were introduced at 1 mol% with respect to the total metal halide content. The precursor solution was stirred magnetically at room temperature for 2 h and filtered through a 0.22 μm PTFE membrane prior to use.

Wide-bandgap (WBG) FA0.8Cs0.2Pb(I0.6Br0.4)3 perovskite: The WBG precursor solution (1.25 M) was formulated in DMF/DMSO (4:1 by volume) using FAI, CsI, PbI2, and PbBr2, with molar ratios of 0.8:0.2 for FAI:CsI and 0.6:0.4 for PbI2:PbBr2. PEAI was added at 0.5 mol% relative to PbX2. The precursor solution was stirred at room temperature overnight, then filtered through a 0.22 μm PTFE membrane before making the perovskite films.

Mixed Pb-Sn narrow-bandgap perovskite solar cell fabrication
ITO-patterned glass substrates were cleaned using acetone and isopropanol and dried under a nitrogen flow. PEDOT:PSS was spin-coated on ITO substrates at 4000 r.p.m. for 30 s, followed by annealing at 150 °C for 10 min in ambient air. After cooling, the substrates were transferred into a nitrogen-filled glovebox for perovskite deposition. The NBG perovskite films were deposited using a two-step spin-coating process: an initial step at 1000 r.p.m. for 10 s with an acceleration of 200 r.p.m. s–1, followed by a second step at and 5000 r.p.m. for 80 s with a ramp-up of 1000 r.p.m. s–1. During the second spinning stage, anisole (200 μl) was introduced as an anti-solvent, which was swiftly dropped on the spinning substrate 20 s before the end of the second spin-coating step, using a pipette positioned approximately 3 cm above the film center. The anti-solvent treatment was performed under an inert N2 atmosphere to ensure consistent solvent extraction and film formation. The substrates were then transferred to a hot plate and heated at 100 °C for 10 min. For surface passivation, EDADI post-treatment solution was dynamically spin-coated onto perovskite films at 4000 r.p.m. for 20 s and subsequently annealed at 100 °C for 1 min. After cooling down to room temperature, the substrates were transferred to a thermal evaporation system. Electron-transport layers were subsequently deposited by thermal evaporation of 20 nm C60 at a rate of 0.2 Å s−1. The ALD-SnO2 layer (~15 nm) was then deposited after C60 deposition at low temperatures (typically 70 °C). Finally, Cu electrodes (150 nm) were deposited by thermal evaporation at a rate of 1.0 Å s−1. The fabrication of FA0.85Cs0.15Pb0.5Sn0.5I3 perovskite solar cells is consistent with our previous works2.

For completeness, we note that EDADI post-treatment is applied as a standard surface passivation step in this work. A comparison of devices with and without EDADI (Supplementary Fig. 40) shows the passivation treatment leads to improved device performance, confirming its beneficial effect on defect passivation.

Monolithic all-perovskite tandem solar cell fabrication
Monolithic tandem devices were fabricated with the structure glass/ITO/NiO/SAM/WBG perovskite/C60/ALD SnO2/Au/PEDOT: PSS/NBG perovskite/C60/ALD/Cu. Patterned ITO substrates were ultrasonically cleaned in acetone and isopropanol for 30 min each. NiO nanocrystals (25 mg ml−1 in deionized water) were synthesized following previous reported methods57 and spin-coated on ITO substrates at 3000 r.p.m. for 30 s, followed by annealing at 130 °C for 30 min in air. After transfer to a glovebox, SAMs (2PACz:MeO-2PACz=75:25, 1 mM in isopropanol) were spin-coated on the NiO film at 6000 r.p.m. for 30 s, and then annealed at 150 °C for 10 min. Details on the Mixed SAMs for HTL has been discussed in our previous work58. The WBG perovskite layer was then deposited using a two-step spin program (2000 r.p.m. for 10 s, acceleration 200 r.p.m. s⁻¹; followed by 6000 r.p.m. for 40 s, acceleration 2000 r.p.m. s⁻¹), with anisole (200 μL) applied 20 s before the end of the second step. Films were annealed at 100 °C for 15 min. Subsequently, 20 nm C60 was deposited on top by thermal evaporation at a rate of 0.2 Å s–1, followed by deposition of a 20 nm SnO2 by the ALD system (Veeco Savannah S200) at low temperatures (typically 100 °C) using precursors of tetrakis(dimethylamino)tin(iv) (99.9999%, Nanjing AiMouYuan Scientific Equipment) and deionized water. An ultrathin layer of Au clusters (~1 nm) was evaporated onto ALD-SnO2 prior to PEDOT: PSS deposition. PEDOT: PSS was spin-coated and annealed at 120 °C for 20 min in air. The NBG sub-cell was then fabricated on top using the same procedure as for single-junction devices, followed by deposition of 20 nm C60 film, ALD-SnO2 layer (~15 nm, 70 °C), and Cu electrodes (150 nm).

Characterization of solar cells
The J-V characteristics of single-junction solar cells were measured using a Keithley 2450 source meter under the illumination of a solar simulator (EnliTech, Class AAA) at a light intensity of 100 mW cm−2 as checked with NREL calibrated reference solar cells (KG-5 and KG-0 reference cells were used for the measurements of the WBG and NBG solar cells, respectively). Measurements were performed in a nitrogen-filled glovebox at a scan rate of 100 mV s−1 with 10 mV voltage steps and a delay time of 100 ms. An aperture mask defined the active area (0.049 cm2). External Quantum Efficiency (EQE) spectra were recorded using a QE system (EnliTech) with monochromatic light focused on the device pixel and a chopper frequency of 20 Hz. The J-V characteristics of tandem solar cells were carried out under the illumination of a two-lamp high spectral match solar simulator (SAN-EI ELECTRIC, XHS-50S1), with spectral mismatch controlled within 100±3% for each 50 nm interval in the wavelength range of 400–1000 nm. The illumination intensity of the solar simulator was adjusted to 100 mW cm−2 and verified using a calibrated crystalline silicon reference cell equipped with a quartz window (KG-0). EQE measurements were carried out in ambient conditions. During EQE acquisition, high-intensity LED bias lights with emission maxima at 850 nm and 460 nm were selectively applied to probe the front and rear sub-cells, respectively. No external electrical bias was applied throughout the EQE measurements of the tandem devices.

Thermal stability tests of solar cells
Unencapsulated devices were aged at 85 °C in a nitrogen-filled glovebox. J-V measurements were periodically recorded to evaluate performance evolution over time.

Steady-state and time-resolved photoluminescence (PL)
PL measurements were performed on a home-built wide-field microscope based on Olympus IX73. A diode laser with a wavelength of 485 nm served as the excitation light, and the laser power density incident on the film surface was maintained at 100 mW cm−2. The fluorescence of the samples was collected through a dry objective lens (Olympus LUCPlanFI 40×, Numerical Aperture (NA) = 0.6) and recorded using an Electron Multiplying Charge-Coupled Device (EMCCD) camera (iXon Ultra 888, Andor), after passing through an 850 nm long-pass filter (ET550LP, Chroma). Time-resolved photoluminescence (TRPL) measurements were carried out on a Horiba Fluorolog-3 time-correlated single photon counting system. The samples were excited from the glass side using a pulsed laser at 485 nm. The resulting PL decay curves were analyzed by fitting with a triple-exponential decay model, from which the fast and slow carrier lifetime components were extracted.

GC characterization
Pb-Sn perovskite films were deposited on glass substrates by spin coating and extracted by anti-solvent, and then put 20 those samples into 10 mL anisole to extract residual DMF and DMSO solvents. Then 0.1 mmol dodecane was added into the target mixed solvents as an internal standard. GC analyses were performed on a GC equipped with a flame ionization detector and an Rtx@-65 (30 m × 0.32 mm ID × 0.25 μm df) column. The temperature of SPL and FID is 280 °C. The injection volume of mixed solvents is 5.0 μL. The column temperature kept at 67 °C for 7 min and then rise to 250 °C at 20 °C/min retaining for 1 min. The signal peak of DMF, DMSO, dodecane appeared at 3.396 min, 3.984 min, 6.675 min, respectively.

Other characterizations
X-ray diffraction (XRD) measurements were performed on a Rigaku MiniFlex 600 diffractometer equipped with a NaI scintillation counter, employing monochromatized Cu Kα radiation with a wavelength of λ=1.5406 Å. X-ray photoelectron spectroscopy (XPS) was conducted using the Thermo Scientific Al K-Alpha XPS system, with spectra recorded at energy steps of 0.1 eV. Ultraviolet-visible absorption spectra were acquired using a Lambda 950 UV-vis spectrophotometer. Scanning electron microscopy (SEM) imaging was carried out on a TESCAN microscope operated at an accelerating voltage of 2 kV.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Guo et al. fabricated MA-free all-perovskite tandems with FA/Cs Pb-Sn NBG and WBG perovskite subcells. To address the preferential crystallization of Cs-rich domains in FA/Cs-based Pb-Sn perovskite, they developed a sustained solvent-extraction channels strategy. This approach enabled optimized FA/Cs-based Pb-Sn perovskite and MA-free all-perovskite tandem devices. This is a well-executed and timely study that addresses an issue in the development of stable, high-efficiency MA-free perovskite solar cells. The authors identify a critical issue. Premature surface crystallization occurs during anti-solvent processing. Researchers often overlook this phenomenon. Yet it compromises both film quality and device performance. This study is innovative and timely, with a record efficiency MA-free all perovskite tandem solar cell. With minor revisions, this manuscript is acceptable for publication.\n• The manuscript uses “SSC” to refer to both a strategy and a structural feature. At times, the concept can be unclear, especially for readers outside the field. I recommend clearly defining SSC as either a processing strategy, structural feature, or both, and maintaining this consistency throughout.\n• Regarding the ToF-SIMS analysis, presenting the device architecture in Fig. 2b is appropriate, as it more distinctly reveals the differences in Cs+ ion distribution between the two devices.\n• The discussion of the NMR results in the main text cites Fig. 1b, but based on the content, it seems the correct reference should be Figure 1c. Clarifying this would improve the accuracy and clarity of the manuscript.\n• The SEM images presented in Supplementary Fig. 6 only show a small part of the perovskite film morphology. It is recommended to include larger-area images, or adding a grain size distribution histogram to provide better statistical insight.\n• Supplementary Fig. 9 only presents PCE comparisons. It would be more informative to also include other key photovoltaic parameters (such as Voc, Jsc and FF) to provide a more complete performance comparison.\n• It is recommended to include the photovoltaic parameters (e.g., PCE, Voc, Jsc and FF) of the wide-bandgap subcell. This would clarify its performance contributions and current matching role in the tandem device.\n• The use of dashes in the main text appears inconsistent (e.g., “-COOH” vs “–COOH”). It is recommended to unify the formatting throughout the manuscript for clarity and typographical consistency.\n• Stability is critical for the future development of perovskite photovoltaics. The authors could propose future research directions focused on the stability of all-perovskite tandem solar cells, offering potential research strategies for investigators in this field.", "reviewer_2": "Guo et al. propose an innovative crystallization modulation strategy for FACs based Pb-Sn perovskite films, wherein the sustained solvent-extraction channels (SSC) are designed during anti-solvent (AS) processing. The authors demonstrate that (S)-2-aminopentanedioic acid hydrochloride (2AP), a dicarboxylic acid, effectively delays surface crystallization and facilitates more uniform perovskite growth. This approach yields a champion PCE of 22.7% for single-junction narrow-bandgap cells and 29.0% (certified 28.3%) for monolithic tandem devices, representing the highest certified PCE for MA-free all-perovskite TSCs to date. Notably, the thermal stability of the devices is remarkable: unencapsulated devices retaining >80% of their initial efficiency after 2 efficiency after 800 h aging at 85 °C. This manuscript presents an outstanding and meticulously designed study that addresses one of the most critical challenges in the field of perovskite photovoltaics, achieving high thermal stability in MA-free Pb-Sn perovskite systems while maintaining record-breaking efficiencies. The work aligns with the scope of the journal. The manuscript is recommended for minor revision prior to consideration for publication. The detailed comments are as follows:\n1. Authors provide cross-sectional SEM images of perovskite films treated with 2AP and Sc to illustrate the effect of -COOH group number on film morphology. However, the SEM image of Gly-treated perovskite film is missing, making the comparison among three additives incomplete. Could the authors provide the cross-sectional SEM of the Gly sample? It will offer a more complete morphological comparison and reinforce the correlation between additive structure and film formation.\n2. To more comprehensively evaluate the effects of different additives (Sc and Gly), the conclusion would be further strengthened by the morphology and crystallinity analysis of the resulting perovskite films.\n3. This manuscript focuses on the crystallization dynamics and performance of the NBG Pb–Sn perovskite subcell. The all-perovskite tandem configuration intrinsically depends on the performance of both subcells. It would enhance the completeness and clarity of the device-level discussion if the photovoltaic parameters (e.g., PCE, Jsc, Voc, FF) and J–V curves of WBG perovskite solar cells were also provided. This would help readers assess the performance contribution and current matching of each subcell in the tandem solar cell.\n4. To better demonstrate the reproducibility and statistical significance of the tandem device performance, it is recommended authors provide box plots showing the distribution of the PV performance parameters measured from J-V curves of all-perovskite tandem devices.\n5. To further validate the reported champion PCE values, it is recommended that the authors should include steady-state power output measurements for both the single-junction and tandem devices. Providing stabilized PCE data under maximum power point tracking will enhance the reliability and comparability of the reported results.\n6. The manuscript uses inconsistent unit formatting (e.g., “μmol/ml” in Figure 1f vs. “mA cm-2” in Figure 3a). Please ensure uniformity across all figures and text in accordance with journal style.\n7. The text and labels in some figures (e.g., Fig. 3b and Supplementary Fig. 11) appear too small and may not be legible when printed. Please ensure that all figure fonts meet the journal’s resolution and size requirements.", "reviewer_3": "Yijia Guo and coworkers reported on the addition of a Lewis base additive to regulate the crystallization of an MA-free Sn-Pb perovskite for all perovskite tandem solar cell application. Even though the single Sn-Pb perovskite solar cell is not the highest reported, the authors were able to achieve the highest PCE from an MA-free all perovskite tandem. The results on the tandem devices performance are impressive and certified, but I have a few concerns regarding the novelty of the additive engineering approach.\n1) The use of additives to control the kinetic of crystallization in perovskites thin films is ubiquitous. [1–4] In this work, the same concept of having an additive to slow down the crystallization rate is used. However, the mechanism is described as \"sustained solvent-extraction channels (SSC)\", which results in larger grain size and improved crystallinity. How does this SSC strategy differ from the usual additive-assisted crystallization? Also, when using the SSC terminology, citations for this mechanism should be provided even if employed on other systems.[5]\n2) The molecule used: (S)-2-Aminopentanedioic acid hydrochloride (2AP), should be referred to as an additive and not as a dopant (line 76). A dopant is an element or compound used to dope a semiconductor by changing its electrical properties, but in this case the molecule is added to regulate the crystallization process.\n3) The (S)-2-Aminopentanedioic acid hydrochloride (2AP) additive is more commonly referred to as L-Glutamic acid. This traditional nomenclature should be provided and other relevant works using this molecule and other amino acids as additive should be cited since this molecule has been explored extensively in the perovskite literature.6–8\n4) In line 81 the authors say that the NH3+ group would interact with the A-site cations. How would this group interact with FA+ / Cs+? From the NMR (Figure 2c) it looks like there are no new molecules formed when FAI+2AP are added together. I think the most plausible use of the NH3+ group is to fill any possible A-site vacancies rather than any direct interaction with FA+/Cs+.\n5) In Figure 1 the authors sometimes referred to the film with 2AP additive as “2AP” or “SSC”. Since in the SI other 3 molecules which supposedly also show SSC-regulated crystallization, I would suggest being consistent and only use the “2AP” nomenclature for the data related to those films.\n6) In Figure 2b the authors show the Cs+ fragment with ToF-SIMS depth profiling. In the discussion, they say that the data “unambiguously revealed the presence of a pronounced vertical composition gradient of Cs in control films, due to the preferentially surface crystallization of Cs-rich phases”. However, by looking at Figure 2b this interpretation is debatable. In fact, at the surface there seems to be the same amount of Cs+ for the control and the 2AP film. Only after 50s the slope of the Cs+ gradient changes. However, it looks like the Cs+ in the control is changing more gradually than the “SSC/2AP” film. This seems to be in contrast with the data discussion. Can the author comment on this? The 2 films seem to have different thickness since the slope of the In+ signal has different rise time. Correcting for the film thickness and showing a non-normalized depth-profile would help the discussion on the surface presence of Cs+, which is the one of the main pieces of evidence for the presence of the \"SSC strategy\".\nMinor points:\n1) In line 166 Fig. 1c should be cited instead of Fig. 1b\n2) Year and volume number missing from reference 40\n3) When discussing the DMSO crystallization delay strategy in line 68 the following relevant citation should be added [9].\n4) Adding a graph showing the PCE of MA-free perovskite single junction and tandem would help to better place the achievement compared to the literature.\nReferences:\n(1) Zhang, F.; Zhu, K. Additive Engineering for Efficient and Stable Perovskite Solar Cells. Advanced Energy Materials 2020, 10 (13), 1902579. https://doi.org/10.1002/aenm.201902579.\n(2) Cao, Y.; Yan, N.; Wang, M.; Qi, D.; Zhang, J.; Chen, X.; Qin, R.; Xiao, F.; Zhao, G.; Liu, Y.; Cai, X.; Zhao, K.; Liu, S. (Frank); Feng, J. Designed Additive to Regulated Crystallization for High Performance Perovskite Solar Cell. Angewandte Chemie International Edition 2024, 63 (30), e202404401. https://doi.org/10.1002/anie.202404401.\n(3) Han, L.; Ran, H.; Liu, Y.; Ai, X.; Zhou, C.; Wang, F.; Zhou, X.; Wu, J.; Han, Y.; Cao, Q.; Feng, Y.; Xia, H.; Zhou, K.; Li, J.; Hu, H.; Lu, W.-T.; Wang, S.; Lin, H. Modulation of Crystallization Dynamics via Multifunctional Additive Engineering to Achieve High-Performance Perovskite Solar Cells. J. Mater. Chem. A 2025, 13 (27), 21963–21970. https://doi.org/10.1039/D5TA02519B.\n(4) Ke, W.; Xiao, C.; Wang, C.; Saparov, B.; Duan, H.-S.; Zhao, D.; Xiao, Z.; Schulz, P.; Harvey, S. P.; Liao, W.; Meng, W.; Yu, Y.; Cimaroli, A. J.; Jiang, C.-S.; Zhu, K.; Al-Jassim, M.; Fang, G.; Mitzi, D. B.; Yan, Y. Employing Lead Thiocyanate Additive to Reduce the Hysteresis and Boost the Fill Factor of Planar Perovskite Solar Cells. Advanced Materials 2016, 28 (26), 5214–5221. https://doi.org/10.1002/adma.201600594.\n(5) Hou, J.; Lin, S.; Zhang, M. Ionic-Liquid-Enhanced Solvent Extraction Mechanism: A Novel Concept. Journal of Environmental Chemical Engineering 2022, 10 (3), 107899. https://doi.org/10.1016/j.jece.2022.107899.\n(6) Abbas, M.; Rauf, M.; Cai, B.; Guo, F.; Yuan, X.-C.; Rana, T. R.; Mackenzie, J. D.; Kyaw, A. K. K. Enhanced Open-Circuit Voltage and Improved Stability with 3-Guanidinoproponic Acid as the Passivation Agent in Blade-Coated Inverted Perovskite Solar Cells. ACS Appl. Energy Mater. 2023, 6 (12), 6485–6495. https://doi.org/10.1021/acsaem.3c00378.\n(7) Xu, P.; Xie, L.; Yang, S.; Han, B.; Liu, J.; Chen, J.; Liu, C.; Jia, R.; Yang, M.; Ge, Z. Manipulating Halide Perovskite Passivation by Controlling Amino Acid Derivative Isoelectric Point for Stable and Efficient Inverted Perovskite Solar Cells. Solar RRL 2023, 7 (2), 2200858. https://doi.org/10.1002/solr.202200858.\n(8) Zhang, L.; Cao, K.; Qian, J.; Huang, Y.; Wang, X.; Ge, M.; Shen, W.; Huang, F.; Wang, M.; Zhang, W.; Chen, S.; Qin, T. Crystallization Control and Multisite Passivation of Perovskites with Amino Acid to Boost the Efficiency and Stability of Perovskite Solar Cells. J. Mater. Chem. C 2020, 8 (48), 17482–17490. https://doi.org/10.1039/D0TC04186F.\n(9) Yang, X.; Ma, T.; Hu, H.; Ye, W.; Li, X.; Li, M.; Zhang, A.; Ge, C.; Sun, X.; Zhu, Y.; Yan, S.; Yan, J.; Zhou, Y.; Li, Z.; Chen, C.; Song, H.; Tang, J. Understanding and Manipulating the Crystallization of Sn–Pb Perovskites for Efficient All-Perovskite Tandem Solar Cells. Nat. Photon. 2025, 19 (4), 426–433. https://doi.org/10.1038/s41566-025-01616-1.", "reviewer_4": "In this work, the authors propose a novel solvent extraction channel strategy by introducing multi-Lewis-base modulators into the precursor solution to suppress the formation of dense Cs-rich perovskite film on the surface. The resulting homogenized FA-Cs mixed Pb-Sn perovskite achieves a champion efficiency of 22.7%, and the corresponding monolithic all-perovskite tandem solar cell delivers an impressive PCE of 29.0%. While the work demonstrates methodological effectiveness, the mechanistic explanation of key observations requires further elaboration to strengthen the scientific rigor. Detailed comments are given below:\n1. While the authors focus on homogenizing FA/Cs distribution to address crystallization inhomogeneity, it is well-established that the primary challenge in mixed Pb-Sn perovskites stems from the divergent crystallization kinetics of Pb²⁺ and Sn²⁺ (due to their distinct ionic radii and reduction potentials). Given that Pb/Sn stoichiometry dominates crystallization dynamics, the emphasis on FA/Cs modulation appears secondary.\n2. In addition, please elaborate on the essential difference between the additive strategy proposed in this manuscript and the commonly used DMSO in delaying crystallization?\n3. The authors claim that according to classical nucleation theory, the energy barrier for heterogeneous nucleation is proportional to the contact angle, implying that larger contact angles should yield larger grains. However, Figure S6 clearly demonstrates the opposite trend for Sc-based films, suggesting that classical nucleation theory fails to explain these observations. A revised theory is needed to reconcile these experimental findings.\n4. The authors attribute the distinct morphology of Sc-based films solely to the number of COOH groups, arguing that excessive coordination strength is the primary cause. This conclusion appears overly simplistic, as potential influences from amino groups and sodium salts remain unaddressed. Further control experiments are necessary to rule out these competing factors.\n5. Please clarify the origin of the small particles observed on the perovskite surface in Figure S6.\n6. The 1H NMR in Figure 1c demonstrates bidirectional coordination between 2AP and Cs+, suggesting that the amino group plays a pivotal role in suppressing surface Cs enrichment. This conclusion is further supported by the markedly different crystallization behavior observed in amino-free Sc-based films. Given these findings, does the exclusive focus on COOH groups in the comparative analysis remain justified?\n7. Since the additive contains Cl- ions, it is necessary to further prove the role of Cl- and whether it remains in the film.\n8. Figure 1d shows the solvent evaporation driven crystallization process, which does not represent the anti-solvent assisted crystallization process. It is recommended to supplement the in-situ PL study of the entire crystallization process from spin coating to annealing. In particular, the authors' claim of the suppression of the dense Cs-rich layer on the surface lacks strong evidence.\n9. How to ensure that the formation of interstitial channels does not destroy the density of the film?\n10. It is recommended to supplement the buried interface morphology to reveal whether there are pores at the perovskite/substrate interface.\n11. Figure S11 shows that the Sn peak shifts to higher binding energy after SSC treatment. Does this indicate an increased proportion of Sn⁴⁺? In addition, it is necessary to supplement the bottom XPS of Control.\n12. Please explain the difference in the ToF-SIMs of Figure 2b and Figure S14 on the sputtering time? And supplementary signals of Cs are necessary.\n13. The authors claim that 2AP is squeezed to the bottom for passivation, but there is no corresponding evidence. Further characterization of the bottom of the perovskite is needed, such as the back-excited TRPL, etc.\n14. The stabilized efficiency or stabilized power output is missing in the certification report of tandem devices. Please provide stabilized efficiency from certified agencies.", "reviewer_5": "This manuscript presents a strategy to improve the performance of MA-free mixed Pb-Sn perovskite films using a multi-functional additive. The authors introduce a dicarboxylic Lewis base (2AP) to modulate the crystallization kinetics during anti-solvent processing. The strategy enables the formation of films with improved uniformity and reduced void formation, resulting in high-efficiency single-junction devices and all-perovskite tandem solar cells with notable thermal stability. The reported certified PCE of 28.3% for MA-free tandem cells is particularly impressive and timely for the field. The authors not only present an innovative technical solution but also provide a deep mechanistic understanding of the problem. The impressive efficiency and stability achieved in this work represent a meaningful advance toward the practical viability of this emerging photovoltaic technology. Several points need further clarification or improvement to strengthen the manuscript. I would recommend publication of this work in Nature Communication after the authors address the following comments.\n1. For both the single-junction and tandem devices, steady-state output under maximum power point tracking should be shown to confirm stability of the reported PCE values.\n2. In the method section, “after cooling down, EDADI post-treatment solution were dynamically coated onto perovskite films at 4,000 r.p.m. for 20 s and annealed at 100°C for a minute”. The authors should comment on the influence of this post treatment on the perovskite solar cells.\n3. A key limitation is the absence of a concentration optimization study for 2AP. Without this, it is unclear whether the selected amount represents the optimal condition or whether device performance could be further improved. At a minimum, the authors should provide a concentration series to justify the choice.\n4. The optical microscopy images in Fig. 1a and 1d are crucial for supporting the discussion on anti-solvent extraction channels. However, the specific sample preparation and observation conditions (e.g., whether images were taken immediately after anti-solvent treatment, prior to annealing, or under an inert atmosphere) are not stated. For clarity and reproducibility, it is recommended that the authors include these experimental conditions in the figure captions.\n5. The authors provide a novel and in-depth understanding of the crystallization. In Supplementary Fig. 16, they present performance comparisons of several additional additives. The chemical structures or molecular formulas of these additives should be provided to offer clearer insight into structure–function design and understand how molecular choice influences crystallization behavior.\n6. Given the central importance of anti-solvent engineering to the SSC strategy, I find the description of AS application parameters in the Methods section to be relatively brief. For reproducibility and clarity, the authors should expand on the procedural details.\n7. Additional information on the Gas Chromatography (GC) procedure is needed in the Methods section to fully understand how solvent residues were quantified."}, "author_response": "Point-to-point actions in response to the Reviewers’ comments\nManuscript #: NCOMMS-25-52158A\nWe sincerely thank all reviewers for their insightful and constructive comments, which\nhave significantly helped us improve the quality and clarity of the manuscript. Below,\nwe provide a point-by-point response detailing the revisions made in accordance with\ntheir suggestions.\nREVIEWER COMMENTS:\nReviewer #1 (Remarks to the Author):\nGuo et al. fabricated MA-free all-perovskite tandems with FA/Cs Pb-Sn NBG and\nWBG perovskite subcells. To address the preferential crystallization of Cs-rich domains\nin FA/Cs-based Pb-Sn perovskite, they developed a sustained solvent-extraction\nchannels strategy. This approach enabled optimized FA/Cs-based Pb-Sn perovskite and\nMA-free all-perovskite tandem devices. This is a well-executed and timely study that\naddresses an issue in the development of stable, high-efficiency MA-free perovskite\nsolar cells. The authors identify a critical issue. Premature surface crystallization occurs\nduring anti-solvent processing. Researchers often overlook this phenomenon. Yet it\ncompromises both film quality and device performance. This study is innovative and\ntimely, with a record efficiency MA-free all perovskite tandem solar cell. With minor\nrevisions, this manuscript is acceptable for publication.\nResponse: We appreciate the reviewers' recognition of this work. We appreciate the\nreviewer’s acknowledgment that our study identifies a critical yet often overlooked\nissue —the premature formation of a dense Cs-rich surface layer during anti-solvent\nprocessing, which impedes uniform crystallization and limits the performance of MA-\nfree Pb–Sn perovskite solar cells.\nIn response to the reviewer’s constructive suggestions, we clearly define the concept of\nsustained solvent-extraction channels (SSC) and ensure consistent terminology\nthroughout the manuscript, distinguishing SSC as both a processing strategy and a\ntransient structural feature during crystallization. Larger-area SEM images and grain\nsize statistics were added to provide better morphological evidence. We added the key\nperformance metrics of the WBG sub-cell and a comparative table summarizing\nWBG/NBG/tandem performance were added. Meanwhile, we provided a brief outlook\non future stability-focused research directions in MA-free all-perovskite tandem\ntechnologies. The manuscript has been revised according to reviewers'\nrecommendations, enhancing its scientific robustness.\n• The manuscript uses “SSC” to refer to both a strategy and a structural feature. At times,\nthe concept can be unclear, especially for readers outside the field. I recommend clearly\ndefining SSC as either a processing strategy, structural feature, or both, and maintaining\nthis consistency throughout.\nResponse: We sincerely thank the reviewer for this suggestion. We fully agree that the\nterm “SSC” should be used with consistent definition throughout the manuscript. In this\nwork, sustained solvent-extraction channels (SSC) are introduced as both a processing\nstrategy and the resulting transient structural feature that collectively contribute to\nimproved film formation. Specifically, SSC strategy enables the temporary formation\nof solvent-extraction channels during the anti-solvent (AS)-assisted crystallization\nprocess. These channels facilitate solvent removal and uniform crystal growth but\nvanish after full crystallization, leaving behind a dense and uniform perovskite film.\nTo clarify this dual concept, we have revised the text in the Introduction and Results\nsections to explicitly state that SSC represents a strategy that induces transient structural\nchannels during crystallization, and we have ensured consistent terminology across the\nentire manuscript. For clarity, we now explicitly define the samples as: “control”\n(without additive) and “SSC” (with 2% Glu additive under SSC processing conditions).\nThe revised sentence now reads:\n“Here, we deploy a surface tension modulation strategy using a strong multi-Lewis-\nbase modulator to maintain sustained solvent-extraction channels (SSC) —a\nmechanism that prevents the formation of a continuous, dense thin perovskite layer at\nthe perovskite/AS contact surface and preserves continuous solvent extraction channels\nthroughout the AS process. Unlike conventional additive strategies that primarily\nregulate the intrinsic crystallization of perovskite, SSC focus on maintaining efficient\nsolvent-extraction channels during anti-solvent treatment, similar to solvent-extraction\nmodulation concepts proposed in other material systems.”\nThe following sentence has been added to the manuscript:\n“We refer to the perovskite films and devices without and with 2% concentration of Glu\nadditive as the control and SSC samples, respectively.”\n• Regarding the TOF-SIMS analysis, presenting the device architecture in Fig. 2b is\nappropriate, as it more distinctly reveals the differences in Cs+ ion distribution between\nthe two devices.\nResponse: We sincerely thank the reviewer for this suggestion. As recommended, we\nhave revised Fig. 2b to include the schematic device architecture. This modification\nimproves the clarity and interpretability of the TOF-SIMS data, especially in\ndemonstrating the suppression of Cs accumulation at the top interface by the SSC\nstrategy. The updated figure is provided in the revised manuscript.\nFig. 2 | Characterization of mixed Pb–Sn perovskite films with SSC treatment. b,\nTOF-SIMS depth profiles of Cs elements for control and SSC mixed Pb-Sn perovskite\nfilms, plotted on a logarithmic scale to show the elemental depth distribution.\n• The discussion of the NMR results in the main text cites Fig. 1b, but based on the\ncontent, it seems the correct reference should be Figure 1c. Clarifying this would\nimprove the accuracy and clarity of the manuscript.\nResponse: We sincerely thank reviewer for pointing out these errors.. We have\ncorrected the citation in the main text from “Fig. 1b” to “Fig. 1c” to accurately reflect\nthe content discussing the ¹H NMR spectra of Glu mixed with FAI/CsI. This correction\nensures consistency between the textual description and the corresponding figure,\nthereby improving the clarity and accuracy of the manuscript. The revision has been\nmade in the updated version accordingly.\n“The ammonium proton resonance of Glu at 8.49 ppm shifted upfield upon mixing with\nFAI/CsI, while the dual resonances of FAI (8.65/8.99 ppm) shifted downfield (Fig. 1c),\ndemonstrating bidirectional coordination between 2AP and FA⁺/Cs⁺ cations.”\n• The SEM images presented in Supplementary Fig. 6 only show a small part of the\nperovskite film morphology. It is recommended to include larger-area images, or adding\na grain size distribution histogram to provide better statistical insight.\nResponse: We sincerely thank the reviewer for this valuable suggestion. In response,\nwe have added larger-area SEM images and included the corresponding grain size\ndistribution histograms in the revised Supplementary Fig. 7 (originally\nSupplementary Fig. 6) to provide better statistical insight into the film morphology.\nWe have revised Supplementary Fig. 7 in the revised manuscript as follows:\nSupplementary Figure 7. a Top-view SEM image and b grain size distribution of\nmixed Pb-Sn perovskite films on PEDOT: PSS. The small surface particles observed in\nthe perovskite film are attributed to residual SnF₂, a standard additive in Sn-based\nperovskites used to suppress the oxidation of Sn2+ and decrease the background hole\nconcentration.\n• Supplementary Fig. 9 only presents PCE comparisons. It would be more informative\nto also include other key photovoltaic parameters (such as V , J and FF) to provide a\noc sc\nmore complete performance comparison.\nResponse: We sincerely thank the reviewer for this suggestion. We now have added a\ncomparison of all key photovoltaic parameters in Supplementary Fig. 11 (originally\nSupplementary Fig. 9), including V , J , and FF, in addition to PCE, for devices with\noc sc\ndifferent additives. We have revised Supplementary Fig. 11 in the revised manuscript\nas follows:\nSupplementary Figure 11. Comparison of PV parameters of devices with different\nadditives.\n• It is recommended to include the photovoltaic parameters (e.g., PCE, V , J and FF)\noc sc\nof the wide-bandgap subcell. This would clarify its performance contributions and\ncurrent matching role in the tandem device.\nResponse: We thank the reviewer for highlighting this important point. In the revised\nmanuscript, we have provided the J-V curves of the WBG sub-cell in Supplementary\nFigure 35:\nSupplementary Figure 35. J-V curves of WBG sub-cell.\nIn addition, to enable a more complete comparison, we have included a summary of\nkey photovoltaic parameters (PCE, V , J , FF) for the WBG, NBG, and tandem devices\noc sc\nin Supplementary Table 3.\nSupplementary Table 3. Photovoltaic performance of WBG sub-cell, NBG sub-cell\nand tandem solar cells.\nV J FF PCE\noc SC\nDevice performance\n(V) (mA cm-2) (%) (%)\nWBG sub-cell 1.348 17.8 83.6 20.1\nNBG sub-cell 0.871 31.8 81.9 22.7\nTandem device 2.171 16.1 83.7 29.2\n• The use of dashes in the main text appears inconsistent (e.g., “-COOH” vs “–COOH”).\nIt is recommended to unify the formatting throughout the manuscript for clarity and\ntypographical consistency.\nResponse: We thank the reviewer for pointing out this errors. We have carefully\nreviewed the manuscript and corrected all inconsistent dash formatting (e.g., “-COOH”\nvs “–COOH”) to ensure typographical consistency throughout the main text. We have\nrevised this sentence in the revised manuscript as follows:\n“FTIR spectra revealed that the characteristic C=O stretching peak of 2AP shifted from\n1699.78 to 1716.17 and 1668.93 cm-1, respectively, implying the strong coordination\ninteraction between the -COOH groups of 2AP and Pb²⁺/Sn²⁺ through Lewis acid–base\nadduct (Supplementary Fig. 10a).”\n• Stability is critical for the future development of perovskite photovoltaics. The authors\ncould propose future research directions focused on the stability of all-perovskite\ntandem solar cells, offering potential research strategies for investigators in this field.\nResponse: We thank the reviewer for the thoughtful suggestion. In response, we have\nadded a sentence to the end of the Discussion section to outline potential future research\ndirections for improving the stability of all-perovskite tandem solar cells. The following\nsentence has been added to the manuscript:\n“Future research could explore integrating more stable interfacial materials under\nthermal stress, such as transparent conductive oxides and stable tunnel recombination\nlayers, as well as coupling advanced stabilization strategies with mechanistic studies\nof degradation pathways to further enhance the long-term operational stability of all-\nperovskite tandem devices.”\nReviewer #2 (Remarks to the Author):\nGuo et al. propose an innovative crystallization modulation strategy for FACs based\nPb-Sn perovskite films, wherein the sustained solvent-extraction channels (SSC) are\ndesigned during anti-solvent (AS) processing. The authors demonstrate that (S)-2-\naminopentanedioic acid hydrochloride (2AP), a dicarboxylic acid, effectively delays\nsurface crystallization and facilitates more uniform perovskite growth. This approach\nyields a champion PCE of 22.7% for single-junction narrow-bandgap cells and 29.0%\n(certified 28.3%) for monolithic tandem devices, representing the highest certified PCE\nfor MA-free all-perovskite TSCs to date. Notably, the thermal stability of the devices\nis remarkable: unencapsulated devices retaining >80% of their initial efficiency after\n800 h aging at 85 °C. This manuscript presents an outstanding and meticulously\ndesigned study that addresses one of the most critical challenges in the field of\nperovskite photovoltaics, achieving high thermal stability in MA-free Pb-Sn perovskite\nsystems while maintaining record-breaking efficiencies. The work aligns with the scope\nof the journal. The manuscript is recommended for minor revision prior to consideration\nfor publication. The detailed comments are as follows:\nResponse: We sincerely thank the reviewer for the very positive assessment of our\nwork and for recognizing both the scientific significance and practical relevance of our\nMA-free Pb-Sn perovskite and all-perovskite tandem devices. We appreciate the\nreviewer’s acknowledgment of the novelty of the sustained solvent-extraction channels\nstrategy, the record device efficiencies, and the long-term thermal stability\ndemonstrated in this study.\nWe carefully addressed all comments raised by the reviewer. In the revised manuscript\nand Supplementary Information, we added the missing cross-sectional SEM of the Gly-\ntreated film and expanded the morphological and crystallinity comparisons across\ndifferent additive systems to support the mechanistic discussion. We also provided the\nphotovoltaic performance and J-V characteristics of the wide-bandgap sub-cell, along\nwith statistical box-plot data to demonstrate device reproducibility. Furthermore,\nstabilized power output measurements for both single-junction and tandem devices\nwere supplied to strengthen the reliability of the reported efficiencies. The manuscript\ntext and figures were reviewed for consistent unit formatting and improved figure\nreadability in accordance with journal specifications.\nWe greatly appreciate the reviewer’s constructive feedback, which has helped us\nimprove clarity, completeness, and presentation of the manuscript. We believe the\nrevisions further highlight the robustness of the SSC approach and reinforce the\nsignificance of our findings for the development of stable, high-performance MA-free\nall-perovskite tandem photovoltaics.\n1. Authors provide cross-sectional SEM images of perovskite films treated with 2AP\nand Sc to illustrate the effect of -COOH group number on film morphology. However,\nthe SEM image of Gly-treated perovskite film is missing, making the comparison\namong three additives incomplete. Could the authors provide the cross-sectional SEM\nof the Gly sample? It will offer a more complete morphological comparison and\nreinforce the correlation between additive structure and film formation.\nResponse: We thank the reviewer for this helpful comment. We have added the cross-\nsectional SEM image of the Gly-treated perovskite film, along with other representative\nadditive-treated samples, to provide a more complete comparison. These images have\nbeen included in the revised Supplementary Figure 8, which now presents the cross-\nsectional morphology of perovskite films prepared with different additives.\nThe following clarify can be found in the revised manuscript as follows:\n“Scanning electron microscopy (SEM) images of the resulting perovskite films revealed\nnot only enlarged grain sizes from control to Gly, and further to Glu, but also a marked\nimprovement in grain uniformity (Supplementary Fig. 7). However, the Sc film did not\nfollow this trend, which instead displayed poor surface crystallinity with non-uniform\nand small grains (Supplementary Fig. 8 and 9).”\nSupplementary Figure 8. Cross-sectional SEM images of mixed Pb-Sn perovskite\nfilms with different additives.\n2. To more comprehensively evaluate the effects of different additive (Sc and Gly), the\nconclusion would be further strengthened by the morphology and crystallinity analysis\nof the resulting perovskite films.\nResponse: We appreciate the reviewer’s valuable suggestion. To provide a more\ncomprehensive evaluation of the effects of different additives, we have added the\nsurface SEM images and corresponding grain size distribution of mixed Pb-Sn\nperovskite films on PEDOT:PSS. These are now presented in Supplementary Figure 7,\nproviding better statistical insight into the film morphology.\nThe following clarify can be found in the revised manuscript as follows:\n“Scanning electron microscopy (SEM) images of the resulting perovskite films revealed\nnot only enlarged grain sizes from control to Gly, and further to Glu, but also a marked\nimprovement in grain uniformity (Supplementary Fig. 7). However, the Sc film did not\nfollow this trend, which instead displayed poor surface crystallinity with non-uniform\nand small grains (Supplementary Fig. 8 and 9).”\nSupplementary Figure 7. a Top-view SEM image and b grain size distribution of\nmixed Pb-Sn perovskite films on PEDOT: PSS. The small surface particles observed in\nthe perovskite film are attributed to residual SnF₂, a standard additive in Sn-based\nperovskites used to suppress the oxidation of Sn2+ and decrease the background hole\nconcentration.\nIn addition, to further assess the crystallinity of the perovskite films, we have included\nXRD patterns of mixed Pb-Sn perovskite films prepared with different additives in\nSupplementary Figure 9.\nNACl\nFACl\nSc\n)\n.\nu\n.\na\n(\ny\nt Glu\nsi\nn\ne\nt\nn Gly\nI\n(100)\n(111)\n(110)\n* Control\n*:ITO\n10 20 30 40 50\n2q (degree)\nSupplementary Figure 9. XRD patterns of mixed Pb-Sn perovskite films with\ndifferent additives.\n3. This manuscript focuses on the crystallization dynamics and performance of the NBG\nPb-Sn perovskite subcell. The all-perovskite tandem configuration intrinsically\ndepends on the performance of both subcells. It would enhance the completeness and\nclarity of the device-level discussion if the photovoltaic parameters (e.g., PCE, J ,\nV ,\nsc oc\nFF) and J-V curves of WBG perovskite solar cells were also provided. This would help\nreaders assess the performance contribution and current matching of each subcell in the\ntandem solar cell.\nResponse: We thank the reviewer for highlighting this important point. In the revised\nmanuscript, we have provided the J-V curves of theWBG sub-cell in Supplementary\nFigure 36:\nSupplementary Figure 36. J-V curves of WBG sub-cell.\nIn addition, to enable a more complete comparison, we have included a summary of\nkey photovoltaic parameters (PCE, V , J , FF) for the WBG, NBG, and tandem devices\noc sc\nin Supplementary Table 3.\nSupplementary Table 3. Photovoltaic performance of WBG sub-cell, NBG sub-cell\nand tandem solar cells.\nV J FF PCE\noc SC\nDevice performance\n(V) (mA cm-2) (%) (%)\nWBG sub-cell 1.348 17.8 83.6 20.1\nNBG sub-cell 0.871 31.8 81.9 22.7\nTandem device 2.171 16.1 83.7 29.2\n4. To better demonstrate the reproducibility and statistical significance of the tandem\ndevice performance, it is recommended authors provide box plots showing the\ndistribution of the PV performance parameters measured from J-V curves of all-\nperovskite tandem devices.\nResponse: We sincerely thank the reviewer for this suggestion. To better demonstrate\nthe reproducibility and statistical significance of the tandem device performance, we\nhave added box plots comparing the photovoltaic parameters (PCE, J , V , FF) of the\nsc oc\ncontrol and SSC tandem solar cells (Supplementary Figure 36).\nThe following discussion can be found in the revised manuscript as follows:\n“Comparative analysis with control tandem devices showed improvement of all\nphotovoltaic figures of merit for SSC tandem devices (Supplementary Fig. 36).”\nSupplementary Figure 36. Comparison of PV performance for control and SSC\ntandem solar cells.\n5. To further validate the reported champion PCE values, it is recommended that the\nauthors should include steady-state power output measurements for both the single-\njunction and tandem devices. Providing stabilized PCE data under maximum power\npoint tracking will enhance the reliability and comparability of the reported results.\nResponse: We thank the reviewer for the valuable suggestion. To further support the\nreliability of the reported PCE values for the single-junction devices, we have included\nthe steady-state power output measurement of the champion SSC mixed Pb–Sn\nperovskite solar cell and tandem solar cell (Supplementary Figure 32 and\nSupplementary Figure 37).\nThe following discussion can be found in the revised manuscript as follows:\n“The SSC devices (aperture area of 0.049 cm2) delivered a champion PCE of 22.7%\n(stabilized at 22.5%), with an open-circuit voltage (V ) of 0.871 V, a short-circuit\nOC\ncurrent density (J ) of 31.8 mA cm-2, and a fill factor (FF) of 81.9% (Fig. 3b and\nSC\nSupplementary Fig. 32).”\n20\n)\n%\n( 15\nE\nC\nSSC mixed Pb-Sn PSC\nP\n10\n5\n0\n0 50 100 150 200\nTime (s)\nSupplementary Figure 32. The steady-state PCE of the champion SSC mixed Pb-Sn\nperovskite solar cell.\n“The champion SSC tandem solar cell exhibited a high PCE of 29.2% (stabilized at\n28.8%), with a V of 2.171 V, a J of 16.1 mA cm-2, and a FF of 83.72% (Figure 3d,\nOC SC\nSupplementary Fig. 38 and Table 3).”\n35\n25\n)\n%\n20\n(\nE\nC\nP 15 SSC tandem cell\n10\n5\n0\n0 50 100 150 200\nTime (s)\nSupplementary Figure 37. The steady-state PCE of the champion SSC tandem solar\ncells.\n6. The manuscript uses inconsistent unit formatting (e.g., “μmol/ml” in Figure 1f vs.\n“mA cm-2” in Figure 3a). Please ensure uniformity across all figures and text in\naccordance with journal style.\nResponse: We thank the reviewer for pointing out this inconsistency. In response, we\nhave carefully reviewed all figures and text and have standardized the unit formatting\nthroughout the manuscript and Supporting Information according to journal guidelines.\nSpecifically, we have standardized the unit formatting to “μmol ml-1”, including in\nrevised Fig ure 1f.\nSSC\n3.185\n)\n1\n- ml 3 2.845\n2.663\nol\nm 2.336\nμ\n(\nn\n2\no\nt\ni\na\nr\nt\nn\ne\nc\nn\n1\no\nC\nFig. 1 | Mechanism of SSC-regulated crystallization process. f, GC of the control\nand SSC treated wet perovskite films.\n7. The text and labels in some figures (e.g., Fig. 3b and Supplementary Fig. 11) appear\ntoo small and may not be legible when printed. Please ensure that all figure fonts meet\nthe journal’s resolution and size requirements.\nResponse: We appreciate the reviewer’s attention to detail. In response, we have\nadjusted the font size and resolution of the text and labels in Figure 3b and\nSupplementary Figure 13 (originally Supplementary Fig. 11), as well as other related\nfigures where necessary. All figures now comply with the journal’s resolution and size\nrequirements.\nFig. 3 | PV performance of perovskite solar cells. b, The J–V curves of the champion\ncontrol and SSC-treated device under reverse scan.\nSupplementary Figure 13. The Sn 3d XPS spectra of control, control-Bottom, SSC\n5/2\nand SSC-Bottom mixed Pb-Sn perovskite films.\nReviewer #3 (Remarks to the Author):\nYijia Guo and coworkers reported on the addition of a Lewis base additive to regulate\nthe crystallization of an MA-free Sn-Pb perovskite for all perovskite tandem solar cell\napplication. Even though the single Sn-Pb perovskite solar cell is not the highest\nreported, the authors were able to achieve the highest PCE from an MA-free all\nperovskite tandem. The results on the tandem devices performance are impressive and\ncertified, but I have a few concerns regarding the novelty of the additive engineering\napproach.\nResponse: We appreciate the reviewers' recognition of the high PCE for MA-free all-\nperovskite tandem solar cell. Reviewers' critical suggestions have been invaluable in\nenhancing the scientific rigor and innovation of this work.\nIn the revised manuscript, we have included a comparative discussion between the SSC\nstrategy and conventional crystallization-delaying additives, providing a clearer\ndistinction between the two approaches. Additionally, TOF-SIMS 3D depth profiling\ndata have been supplemented to further corroborate the distribution of the Cs\ncomponent and the role of the SSC mechanism.\nIn summary, our study provides an in-depth comparison highlighting the distinctions\nbetween conventional crystallization-delaying additives and our SSC strategy. The SSC\nstrategy not only functions as a crystallization-regulating additive for perovskites but,\nmore importantly, emphasizes the structural modulation of the top interfacial perovskite\nlayer induced by multi-Lewis acid-based surface tension tuning. The key distinction\nlies in the channel-structured regulation of solvent extraction for FACs Pb–Sn\nperovskites, which differentiates SSC from conventional crystallization-delaying\napproaches. We offer new mechanistic insights into how dense perovskite layers at the\nAS/perovskite interface influence solvent extraction. Incorporating reviewer comments\nand new experimental evidence, the revised manuscript presents an enhanced\nmechanistic understanding of the SSC strategy.\n1) The use of additives to control the kinetic of crystallization in perovskites thin films\nis ubiquitous. [1-4] In this work, the same concept of having an additive to slow down\nthe crystallization rate is used. However, the mechanism is described as \"sustained\nsolvent-extraction channels (SSC)\", which results in larger grain size and improved\ncrystallinity. How does this SSC strategy differ from the usual additive-assisted\ncrystallization? Also, when using the SSC terminology, citations for this mechanism\nshould be provided even if employed on other systems.[5]\nResponse: We thank the reviewer for this important comment. We agree that additive-\nassisted crystallization is a broad and well-established concept, and that any new\nterminology (such as SSC) should be explicitly positioned with respect to prior additive\nstrategies and supported by appropriate citations. Below we provide a more detailed\nexplanation that distinguishes SSC from typical additive-assisted mechanisms.\nAdditive engineering in perovskites encompasses multiple molecular-level\nmechanisms: coordination/chelation to Pb/Sn to form intermediate adducts;\nintermediate phases that slow conversion to the perovskite phase; modulation of\nnucleation and growth kinetics; and defect passivation at surfaces and grain boundaries\n[Advanced Energy Materials 10, 1902579 (2020), Angewandte Chemie International\nEdition 63, e202404401 (2024), Journal of Materials Chemistry A 13, 21963-21970\n(2025), Advanced Materials 28, 5214-5221 (2016)]. The SSC strategy not only\nleverages its well-known chemical effects to control nucleation and growth, but more\nimportantly, preserves macroscopic solvent extraction pathways during the antisolvent\nprocess.\nConventional additive approaches typically highlight how additives form intermediate\ncomplexes or alter nucleation kinetics to delay crystallization and improve film quality\n[Advanced Energy Materials 10, 1902579 (2020), Angewandte Chemie International\nEdition 63, e202404401 (2024), Journal of Materials Chemistry A 13, 21963-21970\n(2025), Advanced Materials 28, 5214-5221 (2016).]. In contrast, the SSC explicitly\naddresses the impacts during the antisolvent treatment process. Specifically, due to the\nrapid extraction capability of antisolvent for DMF/DMSO, a dense perovskite layer\nforms at the interface between the anti-solvent and the FACs Pb-Sn perovskite. This\ncompact perovskite layer hinders further penetration of the anti-solvent to extract\nDMF/DMSO from the perovskite ink [Journal of Colloid and Interface Science 534,\n459-468 (2019)] (Fig. 1g). By employing the surface tension modulation strategy, we\nengineered the perovskite layer at the antisolvent-perovskite interface to be\nincompletely dense, thereby sustained solvent-extraction channels (SSC, Fig. 1g). This\nenables sufficient extraction of DMF/DMSO from the FACs Pb-Sn perovskite ink by\nthe antisolvent. This effectively prevents pore formation at the buried interface while\nsimultaneously facilitating a uniform distribution of Cs throughout the perovskite layer\n(Fig. 2b).\nIn summary, the SSC strategy not only serves as a crystallization-regulating additive\nfor perovskites, but more importantly, emphasizes the top interfacial perovskite layer\nstructural induced by surface tension modulation from multiple Lewis acids. This\nstructural maintains open solvent extraction channels during AS-treatment, enabling\nmore removal of DMF/DMSO from the FACs Pb-Sn perovskite ink and reducing the\nissue of buried interfacial voids caused by residual solvents.\nWe now better clarify this in the revised introduction:\n“Here, we deploy a surface tension modulation strategy using a strong multi-Lewis-\nbase modulator to maintain sustained solvent-extraction channels (SSC)—a mechanism\nthat prevents the formation of a continuous, dense thin perovskite layer at the\nperovskite/AS contact surface and preserves continuous solvent extraction channels\nthroughout the AS process. Unlike conventional additive strategies that primarily\nregulate the intrinsic crystallization of perovskite36-39, SSC focus on maintaining\nefficient solvent-extraction channels during anti-solvent treatment, similar to solvent-\nextraction modulation concepts proposed in other material systems40.”\nIn addition, we updated the corresponding description of Fig. 1g in the revised\nmanuscript:\n“This schematic illustration highlights that SSC does not solely delay crystallization\nbut actively maintains sustained solvent-extraction during the AS stage—a key\nmechanistic distinction from conventional additive-assisted crystallization.”\nFig. 1 | Mechanism of SSC-regulated crystallization process. g, Schematic\nillustration of the SSC mechanism during AS-assisted crystallization process.\n2) The molecule used: (S)-2-Aminopentanedioic acid hydrochloride (2AP), should be\nreferred to as an additive and not as a dopant (line 76). A dopant is an element or\ncompound used to dope a semiconductor by changing its electrical properties, but in\nthis case the molecule is added to regulate the crystallization process.\nResponse: We thank the reviewer for the correction comment. In accordance with the\nsuggestion, we have revised the terminology in the manuscript, changing “dopant” to\n“additive”, which more accurately reflects its role in regulating the crystallization\nprocess.\nWe have revised this sentence in the revised manuscript as follows:\n“We reported a binary -COOH additive, L-Glutamic acid hydrochloride (Glu), added\ninto perovskite precursor solutions to regulate the crystallization process of FACs-\nbased Pb-Sn perovskites, aiming for high-performance and enhanced thermal stability\nof devices (Supplementary Fig. 2).”\n“The fraction of uncovered surface domains increased with the number of carboxyl\ngroups in the additive, rising from 1.78% (control) to 5.17%, 7.11% and 15.72% for\nGly, Glu and Sc, respectively (Fig. 1a, Supplementary Fig. 4 and 5).”\n3) The (S)-2-Aminopentanedioic acid hydrochloride (2AP) additive is more commonly\nreferred to as L-Glutamic acid. This traditional nomenclature should be provided and\nother relevant works using this molecule and other amino acids as additive should be\ncited since this molecule has been explored extensively in the perovskite literature.6–8\nResponse: We thank the reviewer for this helpful suggestion. We have revised all\nrelevant parts of the manuscript to use L-Glutamic acid hydrochloride (Glu) instead of\n2AP. To emphasize the relevance of our additive strategy and its connection to prior\nresearch, we have also cited several representative studies reporting amino acid-based\nadditives for perovskite crystallization control and defect passivation.\nWe have added the following discussion on the revised manuscript:\n“Therefore, in addition to regulating the solvent-extraction pathways, Glu also behaves\nsimilarly to previously reported amino acid additives, which can modulate perovskite\ncrystallization and passivate defect states, thereby enhancing both efficiency and\nstability of perovskite solar cells32, 41, 42, 44, 52.”\n4) In line 81 the authors say that the NH + group would interact with the A-site cations.\nthere are no new molecules formed when FAI+2AP are added together. I think the most\nplausible use of the NH + group is to fill any possible A-site vacancies rather than any\n3\ndirect interaction with FA+/Cs+.\nResponse: We thank the reviewer for the helpful comment. We clarify that our original\nintention was not to indicate a direct interaction between the NH₃⁺ group in L-Glutamic\nacid hydrochloride (Glu) and the A-site cations (FA⁺/Cs+). We agree that the NH₃⁺ group\nis more likely to fill possible A-site vacancies, thereby contributing to improved lattice\nstability. We have revised the sentence in the main text accordingly to ensure accuracy\nand consistency with the NMR results (Fig. 1c).\nThe revised sentence now reads:\n“The NH + group in Glu has a higher electrostatic potential and is likely to fill possible\nstrongly with Pb2+/Sn2+.”\nRevised NMR description (Figure 1c paragraph):\n“To complement these coordination analyses, we further employed ¹H nuclear magnetic\nresonance (NMR) spectra to investigate the effects of Glu incorporation. The\nammonium proton resonance of Glu at 8.49 ppm shifted upfield upon mixing with\nFAI/CsI, while the dual resonances of FAI (8.65/8.99 ppm) shifted downfield (Fig. 1c),\ndemonstrating bidirectional coordination between 2AP and FA⁺/Cs⁺ cations.”\n5) In Figure 1 the authors sometimes referred to the film with 2AP additive as “2AP”\nor “SSC”. Since in the SI other 3 molecules which supposedly also show SSC-regulated\ncrystallization, I would suggest being consistent and only use the “2AP” nomenclature\nfor the data related to those films.\nResponse: We thank the reviewer for this valuable comment. We agree that consistent\nterminology is essential for clarity.\nIn our work, the central focus is the SSC strategy, with Glu (originally 2AP) serving as\nthe additive that enables this mechanism. To better emphasize the conceptual\nframework of SSC in the main figures—particularly Figure 1, where the mechanistic\nnarrative is established—we use “SSC” in both figures. To avoid ambiguity, we have\nclarified in the figure captions that “SSC” refers specifically to the Glu‐added films (2%\nL-Glutamic acid hydrochloride). In Supporting Information, where additive\ncomparison is more relevant, we use “Glu” to denote films prepared with this additive.\nMeanwhile, the other additives evaluated in the Supporting Information are labeled by\ntheir respective molecular names.\nThis correction has been applied consistently throughout all figures, captions, and text\nto avoid confusion.\nThe following sentence has been added to the introduction:\n“Therefore, Glu acts as the key additive that implements the SSC concept by\nmaintaining sustained solvent-extraction channels during AS treatment.”\nThe following clarification has been added to the caption of Figure 1:\n“SSC denotes the perovskite film processed with 2% L-Glutamic acid hydrochloride\n(Glu) additive, which enables sustained solvent-extraction channels.”\n6) In Figure 2b the authors show the Cs+ fragment with TOF-SIMS depth profiling. In\nthe discussion, they say that the data “unambiguously revealed the presence of a\npronounced vertical composition gradient of Cs in control films, due to the\npreferentially surface crystallization of Cs-rich phases”. However, by looking at Figure\n2b this interpretation is debatable. In fact, at the surface there seems to be the same\namount of Cs+ for the control and the 2AP film. Only after 50s the slope of the Cs+\ngradient changes. However, it looks like the Cs+ in the control is changing more\ngradually than the “SSC/2AP” film. This seems to be in contrast with the data\ndiscussion. Can the author comment on this? The 2 films seem to have different\nthickness since the slope of the In+ signal has different rise time. Correcting for the film\nthickness and showing a non-normalized depth-profile would help the discussion on\nthe surface presence of Cs+, which is the one of the main pieces of evidence for the\npresence of the “SSC strategy”.\nResponse: We thank the reviewer for the helpful comment. Following the reviewer’s\ncomment, we re-measured the TOF-SIMS depth profiles for both the control and SSC\nfilms to ensure data accuracy and consistency. The updated results are now presented\nin Figure 2b of the revised manuscript.\nWe would like to clarify that in Figure 2b, the y-axis (Cs⁺ intensity) was plotted on a\nlogarithmic scale, which compressed the difference in slope and made it appear that the\nCs⁺ gradient only changed after approximately 50 s. In the revised version, we have\nprovided the corresponding non-logarithmic depth profiles in Supplementary Figure\n23, where it is evident that the slope difference between the control and SSC films starts\nfrom the very beginning of the measurement, indicating the presence of a pronounced\nvertical composition gradient in the control sample.\nIn addition, as suggested, we have included the non-normalized 3D depth-profiling data\nin Supplementary Figure 24 to better visualize the spatial distribution of Cs⁺. The\nresults clearly reveal that Cs⁺ accumulation is more concentrated near the surface in the\ncontrol film, consistent with the preferential surface crystallization of Cs-rich phases,\nwhereas the SSC-treated film exhibits a more homogeneous Cs⁺ distribution across the\ndepth.\nWe also note that the slightly different rise times of the In⁺ signal between the two\nsamples are attributed to minor variations in sputtering rate or film compactness, which\nare common during SIMS measurement and do not significantly affect the relative\ndepth comparison. Moreover, cross-sectional SEM images (Fig. 2a) show no noticeable\nthickness difference between the two films. Importantly, the overall trend and relative\ndepth distribution of Cs⁺ remain consistent across repeated measurements, confirming\nthe validity of our interpretation regarding the SSC effect.\n“Time-of-flight secondary-ion mass spectrometry (TOF-SIMS) depth profiling\nunambiguously revealed the presence of a pronounced vertical composition gradient of\nCs in control films (Fig. 2b, Supplementary Figure 24 and 25), due to the preferentially\nsurface crystallization of Cs-rich phases. Although the Cs⁺ profiles in Fig. 2b are plotted\non a logarithmic scale for clarity, the corresponding linear-scale data (Supplementary\nFigure 24) clearly show that the slope difference between the control and SSC films\nappears from the beginning of the measurement, confirming the existence of a steeper\nCs gradient in the control film. Remarkably, SSC significantly flattens this gradient,\nhomogenizing vertical Cs distribution. This improvement can be attributed to the\nintrinsically low solubility of Cs-based salts, which often leads to premature surface\ncrystallization during solution processing, particularly in FA-Cs mixed systems. By\nmodulating the crystallization kinetics, SSC prevents the early aggregation of Cs-rich\nphases, thus enabling more uniform incorporation of Cs throughout the film. We further\nexplored the carrier dynamics behavior of perovskite films to evaluate the impact of the\nSSC strategy on the film quality.”\n)\ns\nt\nn\nu\no\nc 106 SSC-Cs+\n(\ny\nsi\nt SSC-In+\nn Control-Cs+\ne\nn t Control-In+\nI\ng\no\nL\n8×102\n6×102\n4×102\n0 50 100 150 200 250\nSputter time(s)\nFig. 2 | Characterization of mixed Pb–Sn perovskite films with SSC treatment. b,\nTOF-SIMS depth profiles of Cs elements for control and SSC mixed Pb-Sn perovskite\nfilms, plo tted on a logarithmic scale to show the elemental depth distribution.\n6×106\nPerovskite Substrate\n5×106\nSSC-Cs+\n)4×106\ns SSC-In+\nt\nn\nu Control-Cs+\no\n(\nc3×106\nControl-In+\ny\nt\nsi\nn 2×106\ne\nt\nn\nI\n1×106\n0\n0 50 100 150 200 250\nSputter time(s)\nSupplementary Figure 24. TOF-SIMS depth profiles of Cs elements for control and\nSSC mixed Pb-Sn perovskite films, plotted on a linear (non-logarithmic) scale to show\nthe initial slope difference more clearly.\nSupplementary Figure 25. 3D TOF-SIMS depth profiles showing the distribution of\nCs2+ ions within perovskite films.\nMinor points:\n1) In line 166 Fig. 1c should be cited instead of Fig. 1b\nResponse: We thank the reviewer for highlighting this oversight. The citation has been\ncorrected from Fig. 1b to Fig. 1c in the revised manuscript, as reflected in the updated\nsentence:\n“The ammonium proton resonance of 2AP at 8.49 ppm shifted upfield upon mixing with\nFAI/CsI, while the dual resonances of FAI (8.65/8.99 ppm) shifted downfield (Fig. 1c),\ndemonstrating bidirectional coordination between 2AP and FA⁺/Cs⁺ cations.”\n2) Year and volume number missing from reference 40\nResponse: We thank the reviewer for this point. The missing year and volume\nnumber have been added to reference 48 (originally reference 40) in the revised\nmanuscript. The updated reference now reads:\n“48 Chen, S, et al. Crystallization in one-step solution deposition of perovskite films:\nUpward or downward? Science Advances 7, eabb2412 (2021).”\n3) When discussing the DMSO crystallization delay strategy in line 68 the following\nrelevant citation should be added [9].\nResponse: We thank the reviewer for the suggestion. The relevant reference has been\nadded to the sentence to support the discussion of the DMSO crystallization-delay\nstrategy.\n“The strong solvent extraction capability of AS hampers the effectiveness of\nconventional crystallization-delay strategies, such as adding dimethyl sulfoxide\n(DMSO)28 or Lewis acid to form intermediate-phase engineering.\n28. Yang, X, et al. Understanding and manipulating the crystallization of Sn–Pb\nperovskites for efficient all-perovskite tandem solar cells. Nature Photonics 19, 426-\n433 (2025).”\n4) Adding a graph showing the PCE of MA-free perovskite single junction and tandem\nwould help to better place the achievement compared to the literature.\nResponse: We thank the reviewer for the suggestion. To better contextualize the\nperformance of our devices, we have added a summary of the photovoltaic performance\nof MA-free perovskite single-junction (WBG/NBG) and tandem solar cells in\nSupplementary Table 3.\nSupplementary Table 3. Photovoltaic performance of WBG sub-cell, NBG sub-cell\nand tandem solar cells.\nV J FF PCE\noc SC\nDevice performance\n(V) (mA cm-2) (%) (%)\nWBG sub-cell 1.348 17.8 83.6 20.1\nNBG sub-cell 0.871 31.8 81.9 22.7\nTandem device 2.171 16.1 83.7 29.2\nReferences:\n(1) Zhang, F.; Zhu, K. Additive Engineering for Efficient and Stable Perovskite\nSolar Cells. Advanced Energy Materials 2020, 10 (13), 1902579. https://doi.org\n/10.1002/aenm.201902579.\n(2) Cao, Y.; Yan, N.; Wang, M.; Qi, D.; Zhang, J.; Chen, X.; Qin, R.; Xiao,\nF.; Zhao, G.; Liu, Y.; Cai, X.; Zhao, K.; Liu, S. (Frank); Feng, J. Designed A\ndditive to Regulated Crystallization for High Performance Perovskite Solar Cell.\nAngewandte Chemie International Edition 2024, 63 (30), e202404401. https://do\ni.org/10.1002/anie.202404401.\n(3) Han, L.; Ran, H.; Liu, Y.; Ai, X.; Zhou, C.; Wang, F.; Zhou, X.; Wu, J.;\nHan, Y.; Cao, Q.; Feng, Y.; Xia, H.; Zhou, K.; Li, J.; Hu, H.; Lu, W.-T.; Wan\ng, S.; Lin, H. Modulation of Crystallization Dynamics via Multifunctional Addi\ntive Engineering to Achieve High-Performance Perovskite Solar Cells. J. Mater.\nChem. A 2025, 13 (27), 21963–21970. https://doi.org/10.1039/D5TA02519B.\n(4) Ke, W.; Xiao, C.; Wang, C.; Saparov, B.; Duan, H.-S.; Zhao, D.; Xiao, Z.;\nSchulz, P.; Harvey, S. P.; Liao, W.; Meng, W.; Yu, Y.; Cimaroli, A. J.; Jiang,\nC.-S.; Zhu, K.; Al-Jassim, M.; Fang, G.; Mitzi, D. B.; Yan, Y. Employing Lea\nd Thiocyanate Additive to Reduce the Hysteresis and Boost the Fill Factor of\nPlanar Perovskite Solar Cells. Advanced Materials 2016, 28 (26), 5214-5221. htt\nps://doi.org/10.1002/adma.201600594.\n(5) Hou, J.; Lin, S.; Zhang, M. Ionic-Liquid-Enhanced Solvent Extraction Mech\nanism: A Novel Concept. Journal of Environmental Chemical Engineering 2022,\n10 (3), 107899. https://doi.org/10.1016/j.jece.2022.107899.\n(6) Abbas, M.; Rauf, M.; Cai, B.; Guo, F.; Yuan, X.-C.; Rana, T. R.; Mackenz\nie, J. D.; Kyaw, A. K. K. Enhanced Open-Circuit Voltage and Improved Stabili\nty with 3-Guanidinoproponic Acid as the Passivation Agent in Blade-Coated In\nverted Perovskite Solar Cells. ACS Appl. Energy Mater. 2023, 6 (12), 6485–64\n95. https://doi.org/10.1021/acsaem.3c00378.\n(7) Xu, P.; Xie, L.; Yang, S.; Han, B.; Liu, J.; Chen, J.; Liu, C.; Jia, R.; Yan\ng, M.; Ge, Z. Manipulating Halide Perovskite Passivation by Controlling Amin\no Acid Derivative Isoelectric Point for Stable and Efficient Inverted Perovskite\nSolar Cells. Solar RRL 2023, 7 (2), 2200858. https://doi.org/10.1002/solr.202200\n858.\n(8) Zhang, L.; Cao, K.; Qian, J.; Huang, Y.; Wang, X.; Ge, M.; Shen, W.; Hu\nang, F.; Wang, M.; Zhang, W.; Chen, S.; Qin, T. Crystallization Control and\nMultisite Passivation of Perovskites with Amino Acid to Boost the Efficiency a\nnd Stability of Perovskite Solar Cells. J. Mater. Chem. C 2020, 8 (48), 17482–\n17490. https://doi.org/10.1039/D0TC04186F.\n(9) Yang, X.; Ma, T.; Hu, H.; Ye, W.; Li, X.; Li, M.; Zhang, A.; Ge, C.; Sun,\nX.; Zhu, Y.; Yan, S.; Yan, J.; Zhou, Y.; Li, Z.; Chen, C.; Song, H.; Tang, J.\nUnderstanding and Manipulating the Crystallization of Sn–Pb Perovskites for Ef\nficient All-Perovskite Tandem Solar Cells. Nat. Photon. 2025, 19 (4), 426–43\n3. https://doi.org/10.1038/s41566-025-01616-1.\nReviewer #4 (Remarks to the Author):\nIn this work, the authors propose a novel solvent extraction channel strategy by\nintroducing multi-Lewis-base modulators into the precursor solution to suppress the\nformation of dense Cs-rich perovskite film on the surface. The resulting homogenized\nFA-Cs mixed Pb-Sn perovskite achieves a champion efficiency of 22.7%, and the\ncorresponding monolithic all-perovskite tandem solar cell delivers an impressive PCE\nof 29.0%. While the work demonstrates methodological effectiveness, the mechanistic\nexplanation of key observations requires further elaboration to strengthen the scientific\nrigor.\nResponse: We sincerely thank the reviewer for the thoughtful and constructive\nevaluation of our work. The reviewer’s comments helped us refine the mechanistic\ndiscussion and strengthen the evidence supporting the sustained solvent-extraction\nchannels (SSC) strategy.\nIn the revised manuscript, we expanded the mechanistic discussion to more clearly\ndifferentiate the SSC process from traditional strategies. We further clarified the role of\nSSC during anti-solvent processing and strengthened the mechanistic evidence through\nadditional experiments. These include control studies with FACl and NaCl to exclude\nthe influence of amino groups and sodium salts, in-situ PL tracking during spin-coating\nand annealing, bottom-excitation TRPL, buried-interface SEM, and 3D TOF-SIMS\ndepth profiling. Finally, as requested, certified stabilized power output data for the\ntandem device have been added.\nWe thank the reviewer again for the critical insights, which have led to clearer\nmechanistic rationale, more complete experimental verification, and an overall\nstrengthened manuscript. We believe the revised version presents a more rigorous and\nconvincing understanding of the SSC strategy and its role in enabling high-efficiency,\nthermal stable perovskite solar cells.\nDetailed comments are given below:\n1. While the authors focus on homogenizing FA/Cs distribution to address\ncrystallization inhomogeneity, it is well-established that the primary challenge in mixed\nPb-Sn perovskites stems from the divergent crystallization kinetics of Pb²⁺ and Sn²⁺\n(due to their distinct ionic radii and reduction potentials). Given that Pb/Sn\nstoichiometry dominates crystallization dynamics, the emphasis on FA/Cs modulation\nappears secondary.\nResponse: We thank the reviewer for this insightful comment. We agree that the\ndivergent crystallization kinetics of Pb²⁺ and Sn²⁺—arising from their different ionic\nradii and reduction potentials—is one of the key challenges in fabricating high-quality\nmixed Pb–Sn perovskite films [Journal of Materials Chemistry A 9, 17830-17840\n(2021), Advanced Energy Materials 13, 2300181 (2023)]. However, recent advances\nparticularly in additive engineering [Nature 624, 69-73 (2023), Advanced Energy\nMaterials 14, 2402171 (2024), Advanced Materials n/a, 2411677 (2024), Science 383,\n855-859 (2024)], have been shown to effectively mitigate this issue.\nInstead, our study focuses on the issue of Cs-rich perovskite phases preferentially\nsegregating at the anti-solvent/perovskite ink interface during processing, which\nthereby constrains the sufficient extraction of DMF/DMSO solvents. This inadequate\nremoval leads to pore formation at the buried interface of the FA/Cs Pb–Sn perovskite,\nwhich in turn exacerbates non-radiative recombination losses.\nIn our study, we focus on another critical yet often overlooked factor—the preferential\nsurface crystallization of Cs-rich phases in FACs-based systems, which induces the\nformation of a dense surface barrier layer during anti-solvent (AS) treatment. This layer\nimpedes solvent extraction, leaving residual solvent trapped near the buried interface.\nThis inadequate removal leads to pore formation at the buried interface of the FA/Cs\nPb-Sn perovskite. As shown by our Gas chromatography (GC) (Fig. 1f), TOF-SIMS\ndepth profiling (Fig. 2b), cross-sectional SEM (Fig. 2a), and steady-state/time-resolved\nPL measurements (Fig. 2c, d), this incomplete solvent removal leads to poorer\ninterfacial crystallinity and enhanced non-radiative recombination at the bottom\ninterface.\nWe have added the following discussion on the revised manuscript:\n“To further verify the compositional uniformity achieved by the SSC strategy, we\nadditionally analyzed the Pb and Sn distributions (Supplementary Fig. 26). The SSC-\ntreated film exhibited a much more homogeneous Pb/Sn depth profile compared with\nthe control, indicating that the SSC strategy not only homogenizes the FA/Cs\ndistribution but also harmonizes the crystallization dynamics of Pb²⁺ and Sn²⁺, thereby\nsuppressing compositional segregation across the film thickness.”\nFig. 1 | Mechanism of SSC-regulated crystallization process. f, GC of the control\nand SSC treated wet perovskite films.\nFig. 2 | Characterization of mixed Pb–Sn perovskite films with SSC treatment. a,\nCross-sectional SEM image of mixed Pb–Sn perovskite films without (control) and\nwith SSC treatment. b, TOF-SIMS depth profiles of Cs elements for control and SSC\nmixed Pb-Sn perovskite films, plotted on a logarithmic scale to show the elemental\ndepth distribution. c, Steady-state PL spectra of the control and SSC perovskite films\ndeposited on ITO-PEDOT:PSS substrates. d, time-resolved PL spectra of control and\nSSC Pb-Sn perovskite films deposited on bare glass (excitation from the glass sides).\nThe decay curves were fitted with biexponential components to obtain fast and slow\ncarrier lifetimes.\n2. In addition, please elaborate on the essential difference between the additive strategy\nproposed in this manuscript and the commonly used DMSO in delaying crystallization?\nResponse: We thank the reviewer for this insightful question. DMSO is known to\ncoordinate strongly with metal halides [Accounts of Chemical Research 49, 311-319\n(2016), Chemical Reviews 119, 3193-3295 (2019), Chemical Society Reviews 48, 2011-\n2038 (2019)], leading to the formation of PbI -DMSO complexes that retard\n2\ncrystallization of the perovskite through a stable FAI-DMSO-PbI intermediate phase.\nupon annealing [Science 348, 1234-1237 (2015)].\nTo further clarify the distinction, we experimentally investigated the effect of increasing\nDMSO content in the precursor solution. As shown in Supplementary Figure 21, a\nhigher DMSO ratio leads to the formation of a dense surface layer that impedes solvent\nremoval, resulting in pronounced voids at the buried interface after annealing. This\nincomplete solvent extraction produces films with inferior morphology and larger\ninterfacial voids. Correspondingly, Supplementary Figure 22 compares the\nphotovoltaic performance of devices with different DMF:DMSO ratios, revealing that\nexcessive DMSO results in lower efficiency, confirming the detrimental influence of\nexcessive DMSO retention.\nIn contrast, our sustained solvent-extraction channels (SSC) strategy operates at the\nperovskite/anti-solvent interface. The multi-Lewis-base additive L-Glutamic acid\nhydrochloride regulates solution viscosity and interfacial energy, and maintains\nsustained solvent-extraction channels. These combined effects effectively suppress the\npremature surface crystallization of Cs-rich phases. Therefore, The SSC strategy is\nfundamentally distinct from the DMSO-based intermediate-phase engineering\napproach.\nWe have added the following discussion on the revised manuscript:\n“To further distinguish our approach from conventional solvent-coordination control,\nwe investigated the effect of DMSO content on film formation. Excessive DMSO leads\nto the formation of larger and more numerous voids at the perovskite interface, whereas\nthe SSC strategy effectively alleviates this issue by preserving efficient solvent-\nextraction pathways during crystallization (Supplementary Figures 20 and 21).”\nSupplementary Figure 21. Cross-sectional SEM images of mixed Pb-Sn perovskite\nfilms prepared with a high DMSO solvent ratio (DMF:DMSO = 2.5:1, volume ratio).\nCompared with Fig. 2a, the high-DMSO film exhibits significantly more voids at the\nburied interface, indicating that excessive DMSO suppresses efficient solvent\nextraction and leads to poorer interfacial morphology.\nSupplementary Figure 22. Comparison of PV parameters of devices with different\nDMF:DMSO ratios.\n3. The authors claim that according to classical nucleation theory, the energy barrier for\nheterogeneous nucleation is proportional to the contact angle, implying that larger\ncontact angles should yield larger grains. However, Figure S6 clearly demonstrates the\nopposite trend for Sc-based films, suggesting that classical nucleation theory fails to\nexplain these observations. A revised theory is needed to reconcile these experimental\nfindings.\nResponse: We thank the reviewer for the helpful comment. We agree that the\nmorphology of the Sc-based film cannot be fully explained by classical heterogeneous\nnucleation theory. While classical nucleation theory predicts that larger contact angles\nincrease the heterogeneous nucleation barrier and thereby reduce nucleation density\n(favoring larger grains), this geometric description neglects the influence of chemical\ncoordination.\nIn systems containing strongly chelating additives, the coordination strength can\nsignificantly alter the crystallization pathway and override the geometric effects\npredicted by classical nucleation theory. In our study, the surface-energy modulation\noriginates from the coordination between the carboxyl groups and Pb²⁺/Sn²⁺, which\ninfluences both the wetting behavior of precursor droplets and the nucleation kinetics.\nAdditives with moderate coordination strength, such as L-Glutamic acid hydrochloride\n(Glu, two -COOH groups), tune the surface tension and moderately delay nucleation,\nleading to enlarged grains. In contrast, sodium citrate (Sc) contains three carboxyl\ngroups that provide excessively strong multidentate coordination with Pb2+/Sn2+, which\nretards or locally arrests the normal crystallization process. Such over-coordination\ndisrupts uniform nucleation and leads to heterogeneous growth with smaller, irregular\ngrains.\nThis phenomenon has been widely reported in the perovskite literature: multiple studies\nhave shown that moderate Lewis-base coordination promotes controlled crystallization\nand large grains, whereas excessive coordination or multidentate chelation leads to poor\nfilm coverage and inhomogeneous morphology [Angewandte Chemie International\nEdition 63, e202404401 (2024), ACS Applied Energy Materials 6, 6485-6495 (2023),\nSolar RRL 7, 2200858 (2023), ACS Energy Letters 10, 3931-3940 (2025)].\nTherefore, the smaller grains observed in the Sc-treated films do not contradict the\ncontact-angle argument but rather reflect a competition between thermodynamic\nwetting effects and additive-driven chemical and kinetic influences. This understanding\naligns with prior findings suggesting that achieving balanced coordination strength is\ncrucial for morphology regulation, while overly strong coordination interactions hinder\ncrystal growth and degrades film quality.\nWe have added the following discussion on the revised manuscript:\n“Meanwhile, the excessively strong coordination—as observed in the case of Sc—may\ndisrupt controlled crystallization process and compromise film quality, which is\nconsistent with previous reports that over-coordinating additives tend to induce non-\nuniform nucleation and hinder crystal growth 37, 41, 43, 44. Accordingly, Sc was excluded\nfrom further discussion in subsequent sections.”\n4. The authors attribute the distinct morphology of Sc-based films solely to the number\nof COOH groups, arguing that excessive coordination strength is the primary cause.\nThis conclusion appears overly simplistic, as potential influences from amino groups\nand sodium salts remain unaddressed. Further control experiments are necessary to rule\nout these competing factors.\nResponse: We sincerely thank the reviewer for this insightful comment. We agree that\nthe previous discussion attributing the morphological differences only to the number of\n-COOH groups did not fully consider the possible effects of amino groups or sodium\nsalts.\nTo clarify the influence of different functional groups and salt types on film morphology,\nwe performed additional control experiments using Formamidine hydrochloride (FACl)\nand NaCl as comparative additives. FACl provides the ammonium functionality without\ncarboxyl groups, while NaCl introduces Na⁺ without organic coordination groups. The\ncorresponding chemical structures are shown in Supplementary Figure 3, and the\nresulting film morphologies, crystal structures, and device performances are presented\nin Supplementary Figures 7-9 and 11.\nIn contrast to Sc, the introduction of NaCl resulted in pinholes and poorly defined grains\nin the perovskite film (Supplementary Fig. 7a). Furthermore, we prepared perovskite\nfilms by incorporating additional FACl as a source of ammonium groups. Notably, the\nFACl-added perovskite exhibited abundant continuous precipitates along the grain\nboundaries, which were not observed on the surface of the Sc film. The Sc-based\nperovskite exhibited moderately larger average grain sizes and greater surface\nroughness compared to those derived from NaCl and FACl (Supplementary Fig. 7b\nand Supplementary Fig. 8), which were attributed to the presence of the COOH group.\nWe further characterized the crystallinity of the different films. The XRD results\nrevealed that the perovskite films with NaCl and FACl additives exhibited not only less\nintense diffraction peaks than the Sc-based film but also correspondingly lower average\nPCEs in their respective devices (Supplementary Fig. 9 and 11).\nTaken together, these results support that coordination from -COOH groups plays the\nprimary role, while effects from amino groups and sodium salts are secondary. These\nfindings further validate that the abnormal morphology of Sc-based films mainly arises\nfrom over-coordination rather than the presence of amino groups and sodium salt.\nWe have updated the Supplementary Information (Supplementary Figs. 3, 7-9 and 11)\nand revised the discussion in the manuscript accordingly.\n“To exclude the influence of amino groups and sodium salts, we also compared\nFormamidine hydrochloride (FACl) and NaCl as comparative additives\n(Supplementary Fig. 3 and 7-9). Both resulted in poor morphology, confirming that\nthe improved film formation in Glu-treated samples primarily originates from balanced\n–COOH coordination rather than amino groups and sodium salts.”\n“Based on the comparative device performance of these modulators, Glu devices with\nhigher average PCE were ultimately selected for the SSC strategy (Supplementary Fig.\n11).”\nSupplementary Figure 3. Chemical structure of a glycine hydrochloride (Gly), b\nsodium citrate(Sc), c Formamidine hydrochloride(FACl).\nSupplementary Figure 7. a Top-view SEM image and b grain size distribution of\nmixed Pb-Sn perovskite films on PEDOT: PSS. The small surface particles observed in\nthe perovskite film are attributed to residual SnF₂, a standard additive in Sn-based\nperovskites used to suppress the oxidation of Sn2+ and decrease the background hole\nconcentration.\nSupplementary Figure 8. Cross-sectional SEM images of mixed Pb-Sn perovskite\nfil ms with different additives.\nNACl\nFACl\nSc\n)\n.\nu\n.\na\n(\ny\nt Glu\nsi\nn\ne\nt\nn Gly\nI\n(100)\n(111)\n(110)\n* Control\n*:ITO\n10 20 30 40 50\n2q (degree)\nSupplementary Figure 9. X-ray diffraction (XRD) patterns of mixed Pb-Sn perovskite\nfilms with different additives.\nSupplementary Figure 11. Comparison of PV parameters of devices with different\nadditives.\n5. Please clarify the origin of the small particles observed on the perovskite surface in\nFigure S6.\nResponse: We thank the reviewer for this helpful comment. The small particles\nobserved on the perovskite surface in Supplementary Figure 7 (previously referred to\nas Supplementary Figure 6) originate from SnF . SnF is a commonly used and\n2 2\nindispensable additive in Sn-containing perovskites to suppress the oxidation of Sn²⁺\nand reduce background hole concentration [Advanced Functional Materials 30,\n2005594 (2020), Advanced Energy Materials 11, 2101045 (2021).]. A small portion of\nSnF tends to segregate at the film surface during crystallization, leading to the\n2\nappearance of fine surface particles in the SEM image [Journal of the American\nChemical Society 138, 3974-3977 (2016), Advanced Materials 34, 2107729 (2022),\nAdvanced Energy Materials 8, 1800997 (2018)].\nA corresponding clarification has been added to the revised manuscript to explain this\nphenomenon:\n“Supplementary Figure 7. Top-view SEM image and grain size distribution of mixed\nPb-Sn perovskite films on PEDOT: PSS. The small surface particles observed in the\nperovskite film are attributed to residual SnF₂, a standard additive in Sn-based\nperovskites used to suppress the oxidation of Sn2+ and decrease the background hole\nconcentration.”\n6. The 1H NMR in Figure 1c demonstrates bidirectional coordination between 2AP and\nCs+, suggesting that the amino group plays a pivotal role in suppressing surface Cs\nenrichment. This conclusion is further supported by the markedly different\ncrystallization behavior observed in amino-free Sc-based films. Given these findings,\ndoes the exclusive focus on COOH groups in the comparative analysis remain justified?\nResponse: We thank the reviewer for this insightful comment. We agree that the\nNMR spectra confirm the interaction between the amino group in Glu and Cs+. To clarify\nthe influence of different functional groups and salt types on film morphology, we\nperformed additional control experiments using Formamidine hydrochloride (FACl) as\na comparative additive. FACl provides the ammonium functionality without carboxyl\ngroups. The corresponding chemical structures are shown in Supplementary Figure 3,\nand the resulting film morphologies, crystal structures, and device performances are\npresented in Supplementary Figures 7-9 and 11.\nIt is important to clarify that the multi‑carboxyl additive effectively prevents the\nformation of a dense perovskite blocking layer at the surface by modulating the size\nand packing arrangement of preferentially precipitated perovskite during solvent\nevaporation (Figure 1a). This less compact precipitate layer does not hinder the\ncontinuous extraction of DMF/DMSO by the antisolvent, thereby avoiding buried voids\ncaused by residual solvent and enabling a uniform crystallization process along the\nthickness direction. Such homogeneous crystallization ultimately leads to a\ngradient‑free distribution of the Cs component.\nIn contrast to the control, the FACl-added perovskite exhibited abundant continuous\nprecipitates along the grain boundaries, which were not observed on the surface of the\ncontrol film. The control perovskite exhibited moderately larger average grain sizes\ncompared to those derived from FACl (Supplementary Fig. 7b), which was attributed\nto the presence of the COOH group. Moreover, in contrast to the control film, the\npresence of voids at the buried interface in the FACl film persists, indicating that\nalthough the amino group can interact with Cs, it does not prevent the formation of\nvoids at the perovskite buried interface (Supplementary Fig. 8). Notably, the\nintroduction of COOH-containing additives more effectively suppresses the formation\nof such buried interfacial voids (Supplementary Fig. 8), suggesting that the multi-\ncarboxyl additive acts by influencing the size and packing of perovskite phases\npreferentially precipitated during the antisolvent process (Figure 1a), thereby\neffectively preventing the emergence of a dense surface-blocking perovskite layer.\nTo preclude ambiguity, we have added a new discussion in the revised manuscript: We\nhave added the following discussion on the revised manuscript:\n“To exclude the influence of amino groups and sodium salts, we also compared\nFormamidine hydrochloride (FACl) and NaCl as comparative additives\n(Supplementary Fig. 3 and 7-9). Both resulted in poor morphology, confirming that\nthe improved film formation in Glu-treated samples primarily originates from balanced\n–COOH coordination rather than amino groups and sodium salts.”\n“Based on the comparative device performance of these modulators, Glu devices with\nhigher average PCE were ultimately selected for the SSC strategy (Supplementary Fig.\n11).”\nSupplementary Figure 3. Chemical structure of a glycine hydrochloride (Gly), b\nsodium citrate(Sc), c Formamidine hydrochloride(FACl).\nSupplementary Figure 7. a Top-view SEM image and b grain size distribution of\nmixed Pb-Sn perovskite films on PEDOT: PSS. The small surface particles observed in\nthe perovskite film are attributed to residual SnF₂, a standard additive in Sn-based\nperovskites used to suppress the oxidation of Sn2+ and decrease the background hole\nconcentration.\nSupplementary Figure 8. Cross-sectional SEM images of mixed Pb-Sn perovskite\nfil ms with different additives.\nNACl\nFACl\nSc\n)\n.\nu\n.\na\n(\ny\nt Glu\nsi\nn\ne\nt\nn Gly\nI\n(100)\n(111)\n(110)\n* Control\n*:ITO\n10 20 30 40 50\n2q (degree)\nSupplementary Figure 9. X-ray diffraction (XRD) patterns of mixed Pb-Sn perovskite\nfilms with different additives.\nSupplementary Figure 11. Comparison of PV parameters of devices with different\nad ditives.\n7. Since the additive contains Cl- ions, it is necessary to further prove the role of Cl- and\nwhether it remains in the film.\nResponse: We thank the reviewer for the helpful comment. To further elucidate the role\nof Cl-, we introduced FACl as a comparison additive (Supplementary Figs. 7–9 and\n11). FACl contains Cl- but lacks -COOH groups. Unlike Glu, FACl does not improve\nfilm uniformity or device performance, confirming that Cl- alone is not responsible for\nthe observed crystallization regulation; rather, the dual-carboxyl coordination in Glu\nplays the dominant role.\nTo examine whether Cl- remains in the film and clarify its role, we carried out TOF-\nSIMS analysis to trace the Cl- depth distribution (Supplementary Fig. 28). The control\nsample exhibits only a background-level Cl- signal, attributable to instrument noise.\nIn contrast, the SSC film shows a slightly higher Cl- intensity predominantly located near\nthe buried interface, indicating that a small fraction of Cl- introduced from Glu remains\nand migrates toward the bottom interface during crystallization. This is consistent with\nprior reports showing that Cl- often volatilizes (or partly leaves the film) during\nannealing or crystallization, especially under elevated temperature, while small\namounts may remain (especially at interfaces or grain boundaries). This behavior\nsuggests that Cl- plays a transient but beneficial role in defect suppression during crystal\ngrowth, but is not necessarily retained in large amounts in the final bulk perovskite film\n[Nature 616, 724-730 (2023), Chemistry of Materials 34, 2231-2237 (2022)].\nWe have added the following discussion on the revised manuscript:\n“Meanwhile, TOF-SIMS analysis (Supplementary Fig. 29) shows that a small amount\nof Cl- remains mainly at the buried interface. Together with the FACl comparison\nexperiment, this indicates that Cl- plays a secondary passivation role rather than being\nthe dominant factor in crystallization regulation.”\nSupplementary Figure 7. a Top-view SEM image and b grain size distribution of\nmixed Pb-Sn perovskite films on PEDOT: PSS. The small surface particles observed in\nthe perovskite film are attributed to residual SnF₂, a standard additive in Sn-based\nperovskites used to suppress the oxidation of Sn2+ and decrease the background hole\nconcentration.\nSupplementary Figure 8. Cross-sectional SEM images of mixed Pb-Sn perovskite\nfil ms with different additives.\nNACl\nFACl\nSc\n)\n.\nu\n.\na\n(\ny\nt Glu\nsi\nn\ne\nt\nn Gly\nI\n(100)\n(111)\n(110)\n* Control\n*:ITO\n10 20 30 40 50\n2q (degree)\nSupplementary Figure 9. X-ray diffraction (XRD) patterns of mixed Pb-Sn perovskite\nfilms with different additives.\nSupplementary Figure 11. Comparison of PV parameters of devices with different\nad ditives.\n105\n)\ns104\nt\nn\nu\no\nc\n(103\ny\nt\nsi\nn\ne\nn\nt102\nSSC-Cl-\nI\nSSC-InO-\nControl-InO-\n100\n0 50\nSputter time(s)\nSupplementary Figure 28. TOF-SIMS depth profiles of Cl elements for control and\nSSC mixed Pb-Sn perovskite films.\n8. Figure 1d shows the solvent evaporation driven crystallization process, which does\nnot represent the anti-solvent assisted crystallization process. It is recommended to\nsupplement the in-situ PL study of the entire crystallization process from spin coating\nto annealing. In particular, the authors' claim of the suppression of the dense Cs-rich\nlayer on the surface lacks strong evidence.\nResponse: We thank the reviewer for the helpful comment. To monitor the\ncrystallization behavior during anti-solvent treatment, we recorded changes of the\nperovskite surface during spin coating. As shown in Supplementary Fig. 18, the\ncontrol film undergoes a characteristic darkening → lightening → darkening transition\nupon anti-solvent dropping. This behavior reflects the transient formation of a compact\nsurface perovskite layer, which initially crystallizes prematurely and restricts solvent\nescape. The layer is partially dissolved by the insufficiently extracted DMF/DMSO and\neventually crystallizes again once the excess solvent is removed.\nIn addition, we performed in-situ photoluminescence (PL) during spin coating and\nannealing and have included these data in Supplementary Figure 19. The in-situ PL\nresults show that the control film exhibits a transient PL peak that appears, disappears,\nand then reappears during crystallization, consistent with the formation and subsequent\ndissolution of a surface-initiated dense perovskite layer that temporarily impedes\nsolvent escape. This PL evolution directly correlates with the changes observed in\nSupplementary Fig. 18, confirming that these evidences all point to the same surface\ncrystallization–dissolution–recrystallization phenomenon. By contrast, the SSC-treated\nfilm displays a smoother and more continuous PL evolution, indicating more uniform\ncrystallization without the formation of a surface blocking layer. This behavior agrees\nwell with our proposed mechanism.\nWe have added the following clarification to the revised manuscript:\n“To further probe the crystallization process, we monitored the digital photos of the\nfilm during anti-solvent deposition (Supplementary Fig. 18). The control sample\nexhibited a darkening–lightening–darkening behavior, consistent with the transient\nformation and collapse of a dense surface layer. To delve deeper into the impact of SSC\non perovskite film formation, we conducted in situ PL spectroscopy during both the\nspin-coating and annealing stages (Supplementary Fig. 19). The transient PL peak\nevolution of the control film (appearance–disappearance–reappearance) mirrors the\nchanges in Supplementary Fig. 18, confirming temporary formation of a dense surface\nlayer that hinders solvent extraction. In contrast, the SSC film displayed a more\ncontinuous PL evolution without such interruptions, indicating more uniform\ncrystallization and an unobstructed solvent-extraction pathway. Due to the PL detection\nfiber has an unavoidable absorption feature near ~940 nm that can produce an\napparent two-peak appearance in the raw spectra (this effect and its manifestation in\nin-situ PL have been discussed previously47).”\nSupplementary Figure 18. Real-time digital photos of the control perovskite film\nduring anti-solvent dropping. A characteristic “darkening–lightening–darkening”\ntransition is observed, corresponding to (a) rapid surface crystallization forming a\ntransient dense layer, (b) partial dissolution by the insufficiently extracted DMF/DMSO,\nand (c) the surface after solvent removal crystallizes again.\nSupplementary Figure 19. In situ PL spectra of the a control and b SSC perovskite\nfilms during spin-coating and annealing.\nTo further verify the suppressed Cs-rich surface layer, we additionally conducted cross-\nsectional HR-STEM and EDS elemental mapping. As shown in Supplementary\nFigure 23, the control film exhibits surface-enriched Cs and compositional\ninhomogeneity along the depth direction, whereas the SSC film shows a uniform\nelemental distribution, supporting that the SSC strategy effectively prevents Cs-rich\nsurface accumulation.\nWe have added the following clarification to the revised manuscript:\n“To further examine the vertical distribution of Cs, we conducted cross-sectional EDS\nelemental mapping (Supplementary Fig. 23). The control film shows clear Cs\naccumulation near the surface and a gradual decrease toward the substrate, whereas\nthe SSC film exhibits a much more uniform Cs distribution across the film thickness,\nsupporting that the SSC treatment suppresses the formation of a Cs-rich surface layer.”\nSupplementary Figure 23. Cross-sectional HR-STEM image and the corresponding\nEDS mapping of Pb-Sn PSCs.\n9. How to ensure that the formation of interstitial channels does not destroy the density\nof the film?\nResponse: We thank the reviewer for the helpful comment. The sustained solvent-\nextraction channels (SSC) generated during anti-solvent (AS)-assisted crystallization\nare transient and play a facilitating rather than destructive role in film formation.\nSpecifically, these channels are formed only in the early stage of AS extraction,\nproviding efficient pathways for solvent removal and promoting uniform nucleation\nacross the film. During subsequent crystallization and annealing, the channels are filled\nby growing perovskite grains, resulting in a dense and continuous film. We note that\nvarious studies on perovskite annealing and grain growth show that despite initial\nporosity, full crystallization and annealing lead to films with minimal voids and high\ndensity [Nature Communications 13, 6655 (2022), Nature Communications 16, 1164\n(2025)].\nTo support this, we have included top-view SEM images and corresponding grain-size\ndistributions of our films on PEDOT:PSS (Supplementary Figure 7), which\ndemonstrate that after annealing the SSC-treated film is continuous.\nSupplementary Figure 7. a Top-view SEM image and b grain size distribution of\nmixed Pb-Sn perovskite films on PEDOT: PSS. The small surface particles observed in\nthe perovskite film are attributed to residual SnF₂, a standard additive in Sn-based\nperovskites used to suppress the oxidation of Sn2+ and decrease the background hole\nconcentration.\n10. It is recommended to supplement the buried interface morphology to reveal whether\nthere are pores at the perovskite/substrate interface.\nResponse: We thank the reviewer for the suggestion. To address this point, we have\nincluded the SEM images of the buried interface between the perovskite layer and the\nsubstrate, comparing films without (control) and with SSC treatment. This has been\nadded as Supplementary Figure 20 to reveal the interfacial morphology and assess the\npresence of pores.\nConsistent with the cross-sectional SEM results in Figure 2a, the buried-interface SEM\nclearly shows that the control film contains noticeable voids at the perovskite/substrate\ninterface, whereas the SSC-treated film exhibits a compact and continuous interface.\nThis further confirms that the SSC strategy promotes more efficient solvent extraction\nand more uniform crystallization, thereby suppressing interfacial void formation.\nWe have added the following discussion on the revised manuscript:\n“We investigated the effect of the SSC on the morphology of MA-free Pb-Sn alloyed\nperovskite films. Cross-sectional SEM images (Fig. 2a) and the corresponding buried\ninterface SEM images (Supplementary Figure 20) further revealed significant\ninterfacial voids at the perovskite/substrate interface in the control film, indicating\noptimized crystallization kinetics enabled by the SSC strategy.”\nControl SSC\n1 μm 1 μm\nSupplementary Figure 20. SEM image of the buried interface in mixed Pb–Sn\nperovskite films without (control) and with SSC treatment.\n11. Figure S11 shows that the Sn peak shifts to higher binding energy after SSC\ntreatment. Does this indicate an increased proportion of Sn⁴⁺? In addition, it is necessary\nto supplement the bottom XPS of Control.\nResponse: We thank the reviewer for this helpful comment. Below we clarify the\ninterpretation of the Sn 3d binding energy changes.\nShifts in the Sn 3d peaks can originate from two possible factors: (i) a change in\noxidation state—where an increase in the Sn4+ fraction usually causes a shift toward\nhigher binding energy—and (ii) variations in the local chemical coordination or\nelectronic environment [Applied Surface Science Advances 12, 100332 (2022),\nSpringer Handbook of Inorganic Photochemistry. Cham: Springer International\nPublishing, 249-271 (2022)]. Therefore, the binding energy shift alone cannot be\ndirectly attributed to Sn oxidation and must be interpreted together with quantitative\npeak fitting and supporting evidence.\nIn our case, the SSC-treated films exhibit a slight shift of the Sn 3d peaks toward higher\nbinding energy compared with the control. However, the fitted results show that the\nrelative area of the Sn4+ component is actually smaller in SSC films, particularly at the\nbottom interface, indicating that Sn2+ oxidation is effectively suppressed. This suggests\nthat the observed shift mainly arises from changes in the local coordination\nenvironment—such as stronger interaction between Sn and the additive ligands—rather\nthan an increase in the Sn4+ content. The higher binding energy reflects chemical\nenvironment modification rather than enhanced oxidation.\nTo address the reviewer’s suggestion, we have additionally included the bottom-side\nXPS spectra of the control film (Supplementary Fig. 13) and performed consistent\npeak fitting for both surface and bottom Sn 3d spectra of the control and SSC samples\nto support this conclusion.\nThe revised sentence now reads:\n“X-ray photoelectron spectroscopy (XPS) revealed a shift in the binding energy of Sn\nsignal on both the surface and bottom sides of the SSC film compared to the control\n(Supplementary Fig. 13), indicating a change in the Sn chemical environment.\nConcurrently, the approach of SSC reduced the proportion of Sn4+ peak area in Sn\nelement peak area, suggesting suppressed Sn²⁺ oxidation and reduced defect density.”\nSupplementary Figure 13. The Sn 3d XPS spectra of control, control-Bottom, SSC\n5/2\nand SSC-Bottom mixed Pb-Sn perovskite films.\n12. Please explain the difference in the TOF-SIMs of Figure 2b and Figure S14 on the\nsputtering time? And supplementary signals of Cs are necessary.\nResponse: We thank the reviewer for the valuable comments. The observed difference\nin sputtering time between Figure 2b and Supplementary Figure 27 (originally\nSupplementary Figure 14) arises from different sputtering rates used for those\nmeasurements, not from different interpretations of the data. Sputter rate can vary with\ninstrument settings (sputter beam energy/current, raster size, ion species) and with small\ndifferences in film compactness or surface roughness; these factors change the time\nrequired to reach a given physical depth, so identical features can appear at different\nsputtering times. Additionally, as suggested, we have supplemented the Cs⁺ signal in\nthe form of a 3D depth profile in Supplementary Figure 25 to provide a more complete\nelemental distribution.\nSupplementary Figure 25. 3D TOF-SIMS depth profiles showing the distribution of\nCs2+ ions within perovskite films.\n13. The authors claim that 2AP is squeezed to the bottom for passivation, but there is\nno corresponding evidence. Further characterization of the bottom of the perovskite is\nneeded, such as the back-excited TRPL, etc.\nResponse: We thank the reviewer for this valuable suggestion. We have replaced\nFigure 2d with time-resolved PL (TRPL) spectra measured from the glass side of the\nfilms (i.e., back-excitation), as suggested. In addition, for completeness, we have\nincluded TRPL spectra measured from the perovskite side in Supplementary Figure 26.\nThese results further support the bottom-side localization and passivation role of Glu\n(originally 2AP) in SSC-treated films.\nThe revised sentence now reads:\n“We further performed time-resolved PL (TRPL) from both the perovskite side\n(Supplementary Fig. 15) and the glass side (Fig. 2d). The SSC film exhibited\nsignificantly longer carrier lifetimes than the control film, suggesting suppressed non-\nradiative recombination and reduced carrier capture loss (Fig. 2d and Supplementary\nFigure 26).”\nSSC-Bottom, τ =931 ns, τ =3811 ns\n1 2\nControl-Bottom, τ =61 ns, τ =367 ns\n1 1 2\n)\nm.\nr\no\nn\n(\ns\nnt\nu\no\nc\nL\nP\n0.1\n0 500 1000\nDecay Time (ns)\nFig. 2d, time-resolved PL spectra of control and SSC Pb-Sn perovskite films deposited\non bare glass (excitation from the glass sides). The decay curves were fitted with\nbiexponential components to obtain fast and slow carrier lifetimes.\nControl-Top, τ =42 ns, τ =2517ns\n1 2\n)\nm.\nr\no\nn\n(\ns\nnt\nu\no\nc\nL\nP\n0 1000\nDecay Time (ns)\nSupplementary Figure 26. time-resolved PL spectra of control and SSC Pb-Sn\nperovskite films deposited on bare glass (excitation from the perovskite sides).\n14. The stabilized efficiency or stabilized power output is missing in the certification\nreport of tandem devices. Please provide stabilized efficiency from certified agencies.\nResponse: We thank the reviewer for the suggestion. Following the reviewer’s\nsuggestion, we have performed additional certified measurements of the stabilized\npower output for our tandem devices at an authorized testing institution. The new\ncertification results confirm the stabilized efficiency values and are now included in the\nrevised manuscript.\nSpecifically, we have updated the relevant data and added Supplementary Figure 40,\nwhich presents the certification results of a representative MA-free all-perovskite\ntandem solar cell, including the stabilized power output obtained under continuous\noperation. These new results further validate the accuracy and reliability of the reported\ndevice performance.\n“Furthermore, we sent one of the best-performing tandem solar cells to an accredited\nindependent laboratory and achieved a certified PCE 29.2% (29.2%) at reverse\n(forward) scan (Supplementary Fig. 39). The tandem device delivered a certified\nstabilized PCE of 28.8% (Supplementary Fig. 39).”\nSupplementary Figure 39. Certification results of a representative MA-free all-\nperovskite tandem solar cell.\nReviewer #5 (Remarks to the Author):\nThis manuscript presents a strategy to improve the performance of MA-free mixed Pb-\nSn perovskite films using a multi-functional additive. The authors introduce a\ndicarboxylic Lewis base (2AP) to modulate the crystallization kinetics during anti-\nsolvent processing. The strategy enables the formation of films with improved\nuniformity and reduced void formation, resulting in high-efficiency single-junction\ndevices and all-perovskite tandem solar cells with notable thermal stability. The\nreported certified PCE of 28.3% for MA-free tandem cells is particularly impressive\nand timely for the field. The authors not only present an innovative technical solution\nbut also provide a deep mechanistic understanding of the problem. The impressive\nefficiency and stability achieved in this work represent a meaningful advance toward\nthe practical viability of this emerging photovoltaic technology. Several points need\nfurther clarification or improvement to strengthen the manuscript. I would recommend\npublication of this work in Nature Communication after the authors address the\nfollowing comments.\nResponse: We appreciate the reviewers' recognition of this work. We are pleased that\nthe reviewer finds the technical advance and mechanistic insights compelling, as well\nas the significance of achieving simultaneously high efficiency and long-term thermal\nstability in MA-free mixed Pb-Sn perovskites. Following the reviewer’s suggestions,\nwe have carefully revised the manuscript and supplemented the necessary data to\nimprove its clarity, comprehensiveness, and reproducibility.\nIn particular, steady-state MPP tracking data for both single-junction and tandem\ndevices have now been included to confirm the stability of the reported efficiencies. We\nclarified the role of EDADI passivation by providing device comparisons before and\nafter treatment, and we performed a concentration-dependent study of the Glu\n(originally 2AP) additive to justify the chosen condition. To support the mechanistic\nunderstanding, we have also provided the chemical structure of CysHCl and expanded\nthe discussion in the Methods where appropriate.\nWith these revisions, we believe the manuscript now more clearly illustrates the SSC-\nassisted crystallization concept, the role of the multifunctional Glu (originally 2AP)\nadditive in modulating solvent extraction and defect passivation, and the reproducible\ndevice improvements enabled by this approach. We greatly appreciate the reviewer’s\ninsightful feedback and believe that the revised version is substantially strengthened.\n1. For both the single-junction and tandem devices, steady-state output under maximum\npower point tracking should be shown to confirm stability of the reported PCE values.\nResponse: We thank the reviewer for the suggestion. To validate the stability of the\nreported PCE for the single-junction device, we have added the steady-state power\noutput of the champion SSC mixed Pb-Sn perovskite solar cell under maximum power\npoint tracking. This data is now presented in Supplementary Figure 32.\nIn addition, the steady-state PCE of the champion SSC tandem solar cell has also been\nmeasured under maximum power point tracking to confirm the reliability of the\nreported efficiency. The results are provided in Supplementary Figure 37.\nWe have added the following discussion on the revised manuscript:\n“The SSC devices (aperture area of 0.049 cm2) delivered a champion PCE of 22.7%\n(stabilized at 22.5%), with an open-circuit voltage (V ) of 0.871 V, a short-circuit\nOC\ncurrent density (J ) of 31.8 mA cm-2, and a fill factor (FF) of 81.9% (Fig. 3b and\nSC\nSupplementary Fig. 32).”\n20\n)\n%\n( 15\nE\nC\nSSC mixed Pb-Sn PSC\nP\n10\n5\n0\n0 50 100 150 200\nTime (s)\nSupplementary Figure 32. The steady-state PCE of the champion SSC mixed Pb-Sn\nperovskite solar cell.\n“The champion SSC tandem solar cell exhibited a high PCE of 29.2% (stabilized at\n28.8%), with a V of 2.171 V, a J of 16.1 mA cm-2, and a FF of 83.72% (Figure 3d,\nOC SC\nSupplementary Fig. 37 and Table 3).”\n35\n25\n)\n%\n20\n(\nE\nC\nP 15 SSC tandem cell\n10\n5\n0\n0 50 100 150 200\nTime (s)\nSupplementary Figure 37. The steady-state PCE of the champion SSC tandem solar\ncells.\n2. In the method section, “after cooling down, EDADI post-treatment solution were\ndynamically coated onto perovskite films at 4,000 r.p.m. for 20 s and annealed at 100°C\nfor a minute”. The authors should comment on the influence of this post treatment on\nthe perovskite solar cells.\nResponse: We thank the reviewer for the suggestion. EDADI is a commonly used\nsurface passivation material that has been widely applied in perovskite solar cells to\nreduce interfacial defects and suppress non-radiative recombination [Nature 635, 596-\n603 (2024), Energy & Environmental Science 15, 2096-2107 (2022), Advanced Energy\nMaterials 11, 2101069 (2021), Advanced Materials 34, 2110241 (2022)]. To clarify its\ninfluence in our work, we have added a comparison of the photovoltaic performance\nbefore and after EDADI passivation in Supplementary Figure 40. The results show that\nthe passivation treatment leads to improved device performance, confirming its\nbeneficial effect on defect passivation.\nWe have added the following discussion on the revised manuscript:\n“For completeness, we note that EDADI post-treatment is applied as a standard surface\npassivation step in this work. A comparison of devices with and without EDADI\n(Supplementary Fig. 40) shows the passivation treatment leads to improved device\nperformance, confirming its beneficial effect on defect passivation.”\nSupplementary Figure 40. Comparison of PV performance for perovskite solar cells\nbefore and after EDADI passivation.\n3. A key limitation is the absence of a concentration optimization study for 2AP.\nWithout this, it is unclear whether the selected amount represents the optimal condition\nor whether device performance could be further improved. At a minimum, the authors\nshould provide a concentration series to justify the choice.\nResponse: We appreciate the reviewer’s comment regarding the need for a\nconcentration optimization study of Glu (originally 2AP). To address this, we have\nconducted a systematic investigation using three different concentrations of Glu 1%,\n2%, and 3% to evaluate its effect on film morphology, crystallinity, and device\nperformance.\nTo examine the influence of Glu (originally 2AP) concentration on film morphology,\nwe have included SEM images of perovskite films prepared with 1%, 2%, and 3% Glu\n(originally 2AP). The results, presented in Supplementary Figure 15, show significant\nmorphological differences, suggesting that additive concentration plays a key role in\nfilm formation.\nTo further support this, we have added XRD patterns of perovskite films with varying\nGlu concentrations in Supplementary Figure 16, which reveal differences in\ncrystallinity across the concentration series.\nLastly, to directly assess the impact of Glu concentration on device performance, we\nhave summarized the photovoltaic parameters (PCE, J , V , FF) for each condition in\nsc oc\na box plot, as shown in Supplementary Figure 17. These results confirm that the\nchosen concentration (2%) yields the most balanced and optimal performance.\nWe have added the following discussion on the revised manuscript:\n“Based on a systematic comparison of film morphology, crystallinity, and device\nperformance at different concentrations (Supplementary Fig. 15-17), a 2%\nconcentration of Glu was identified as the optimal condition and was therefore used\nthroughout this study.”\n1%Glu 2%Glu 3%Glu\nControl\n1%LG\n1 μm\nSupplementary Figure 15. Top-view SEM images of mixed Pb-Sn perovskite films\nprepared with different concentrations of Glu additive: 1%, 2%, and 3%.\n)\n.\nu\n.\na 3%\n(\ny\nsit\nn\ne\nt\nn\nd\ni\n2%\ne\nz\nali\nm (100)\no r (111)\nN (110) * 1%\n*: ITO\n10 15 20 25 30 35 40 45 50\n2q (degree)\nSupplementary Figure 16. X-ray diffraction (XRD) patterns of mixed Pb-Sn\nperovskite films incorporating 1%, 2%, and 3% Glu.\nSupplementary Figure 17. Comparison of PV parameters of devices with different\nconcentration.\n4. The optical microscopy images in Fig. 1a and 1d are crucial for supporting the\ndiscussion on anti-solvent extraction channels. However, the specific sample\npreparation and observation conditions (e.g., whether images were taken immediately\nafter anti-solvent treatment, prior to annealing, or under an inert atmosphere) are not\nstated. For clarity and reproducibility, it is recommended that the authors include these\nexperimental conditions in the figure captions.\nResponse: We thank the reviewer for highlighting the need to clarify experimental\nconditions related to the optical microscopy images. In response, we have revised the\ncaption of Figure 1 to include detailed information on sample preparation and\nobservation conditions:\n“Fig. 1 | Mechanism of SSC-regulated crystallization process. a, Optical microscopy\nimages (100× objective) of freshly AS-treated perovskite films, captured immediately\nafter anti-solvent treatment and prior to annealing. The fraction of discontinuous\ndomains increased from 1.78% (control) to 7.11% (SSC-Glu). d, Semi-in situ optical\nmicroscopy (50× objective) tracking the morphological evolution of perovskite\nprecursor films without (control) and with SSC over time. The images were acquired\nunder an inert atmosphere without anti-solvent treatment or annealing.”\n5. The authors provide a novel and in-depth understanding of the crystallization. In\nSupplementary Fig. 16, they present performance comparisons of several additional\nadditives. The chemical structures or molecular formulas of these additives should be\nprovided to offer clearer insight into structure–function design and understand how\nmolecular choice influences crystallization behavior.\nResponse: We thank the reviewer for the helpful suggestion. To improve clarity\nregarding the structure-function relationship of the additives, we have added the\nchemical structures of the previously missing molecules-cysteine hydrochloride\n(CysHCl)-in Supplementary Figure 32.\na b\nSupplementary Figure 32. Chemical structure of cysteine hydrochloride (CysHCl)\nmolecules.\n6. Given the central importance of anti-solvent engineering to the SSC strategy, I find\nthe description of AS application parameters in the Methods section to be relatively\nbrief. For reproducibility and clarity, the authors should expand on the procedural\ndetails.\nResponse: We thank the reviewer for this valuable comment. To improve clarity and\nensure reproducibility, we have expanded the description of the anti-solvent (AS)\napplication process in the Methods section. The revised text now provides detailed\ninformation on the timing, delivery method, and processing environment during AS\ntreatment:\n“Methods\nMixed Pb–Sn narrow-bandgap perovskite solar cell fabrication. The pre-patterned\nITO glass substrates were sequentially cleaned using acetone and isopropanol. PEDOT:\nPSS was spin-coated on ITO substrates at 4000 rpm for 30 s and annealed on a hot\nplate at 150 °C for 10 min in ambient air. After cooling, we transferred the substrates\nimmediately to a nitrogen-filled glovebox for the deposition of FA Cs Pb Sn I\n0.85 0.15 0.5 0.5 3\nperovskite films (2.0 M). The perovskite films were deposited with two-step spin-coating\nprocedures: (1) 1,000 r.p.m. for 10 s with an acceleration of 200 r.p.m. s–1 and (2) 5,000\nr.p.m. for 60 s with a ramp-up of 1,000 r.p.m. s–1. Anisole (200 μl) was dropped on the\nspinning substrate during the second spin-coating step at 20 s before the end of the\nprocedure. The substrates were then transferred to a hotplate and heated at 100 °C for\n10 min. After cooling down, EDADI post-treatment solution were dynamically coated\nonto perovskite films at 4,000 r.p.m. for 20 s and annealed at 100°C for a minute. After\ncooling down to room temperature, the substrates were transferred to the evaporation\nsystem. Then, 20 nm C film was subsequently deposited on top by thermal evaporation\n60\nat a rate of 0.2 Å s-1. The ALD-SnO layer (~15 nm) was deposited after C deposition\n2 60\nat low temperatures (typically 70 °C) to avoid any damage to the Pb-Sn perovskite\nabsorber layer. Finally, 150 nm Cu films as electrode were deposited by thermal\nevaporation at the rates of 1.0 Å s-1. The fabrication of FA Cs Pb Sn I\n0.85 0.15 0.5 0.5 3\nperovskite solar cells is consistent with our previous works2.”\n7. Additional information on the Gas Chromatography (GC) procedure is needed in the\nMethods section to fully understand how solvent residues were quantified.\nResponse: We sincerely thank the reviewer for this suggestion. We have added relevant\ninformation about GC in the Methods section.\n“Methods\nGC characterization\nThe Pb-Sn perovskite films were spin-coated on the glass substrates and extracted by\nanti-solvent and then put 20 those samples into 10 mL anisole to extract residual DMF\nand DMSO solvents. Then 0.1 mmol dodecane was added into the target mixed solvents\nas internal standard. GC analyses were performed on a GC equipped with a\nflameionization detector and an Rtx@-65 (30 m × 0.32 mm ID × 0.25 μmdf) column.\nThe temperature of SPL and FID is 280°C . The injection volume of mixed solvents is\n5.0 μL. The column temperature kept at 67 °C for 7 min and then rise to 250 °C at 20 °C\n/min retaining for 1 min. The signal peak of DMF, DMSO, dodecane appeared at 3.396\nmin, 3.984 min, 6.675 min, respectively.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have satisfactorily addressed all of my comments. I have no further questions, and I recommend the manuscript for publication.", "reviewer_2": "The author has appropriately answered my questions. I recommend to accept it as it is.", "reviewer_3": "The authors clearly explained and distinguished with references the difference between the novel SSC proposed mechanism and the slower crystallization from additive and DMSO-coordination. The comments were addressed carefully, and new experiments were performed when needed for all reviewers' comments. I recommend publishing the new version of the manuscript.", "reviewer_4": "The response letter has fully addressed my comments. I recommend it to be accepted as is.", "reviewer_5": "I've reviewed the revised manuscript and authors's responses. My initial concerns are fully addressed with supplementary data and experiments.It now meets the journal's high publication standards."}, "author_response": ""}]

Decision letter