A tin fluoride-free, efficient and durable tin-lead perovskite solar cell

Domain: physics
Published: 2026-01-12
Editorial outcome: minor_revision

Abstract
The photo-thermal stability of tin-lead perovskite solar cells remains a major challenge. SnF2 is commonly used to inhibit Sn2+ oxidation and reduce hole density, however, the stability of devices remains poor. Here, we found that the poor stability partially results from an adverse effect of SnF2, which reacts with formamidine iodide during photo-thermal treatments. This reaction leads to degradation of perovskite and release of hydrofluoric acid, which corrodes electrodes. To address this issue, we develop a strategy that combines lead powder in precursor with PbF2 post-treatment, replacing the role of SnF2 in film formation and surface defect passivation, respectively. The d-electron polarization in Pb2+ strengthens its bond with F⁻, making it react inert to perovskite. In this work, the efficiency of SnF2-free devices increases from 16.43% to 24.07%. The cells retain 60% of their initial efficiency after 550 hours operating at 85°C under maximum power point.

Introduction
Tin-lead perovskite solar cells (Sn-Pb PSCs) are recognized for their potential to achieve the highest Shockley-Queisser limit among single-junction solar cells, which has sparked considerable research interest1. The certified efficiency of Sn-Pb PSCs has indeed surpassed 24%, and it is conceivable that they may match or even exceed the performance of lead-based PSCs in the future2. However, the stability of Sn-Pb PSCs remains the most significant challenge, particularly under light and thermal conditions. This is primarily attributed to the oxidation of stannous ion (Sn2+), Sn vacancy defects, iodine (I)-related defects, phase separation, and interfacial degradation reactions3–7.

All high-efficiency Sn-containing PSCs rely on the incorporation of SnF2 as an essential additive. From the earliest studies to the latest reports, Sn-Pb devices fabricated without SnF2 have never exceeded 15% efficiency-originally below 10% and a viable alternative to SnF2 remains elusive8–13. SnF2 plays a crucial role in inhibiting the oxidation of Sn2+ and the formation of tin vacancies (VSn), thereby reducing the background hole density and mitigating p-type doping. This effect can be attributed to the Sn compensating effect and the stronger Sn-F bond compared to Sn-I bond8–17. However, the roles of SnF2 appears to be more complex. Studies have shown that a large amount of SnF2 tends to aggregate preferentially on the bottom surface, followed by the top surface, with very little present in the bulk, which is an uncontrollable process11,18. This is possibly due to the large mismatch between Sn-I and Sn-F bonds partially hinder the assimilation of SnF2 into the bulk. On the top surface, moderate F⁻ ions are particularly well-suited to bind strongly with Sn atoms, thereby increasing the formation energy of VSn defects19,20. Nevertheless, the enrichment of SnF2 at the bottom surface (the hole transport interface) would cause downward band bending and create a hole transport barrier due to the de-p-doping function of SnF2, thereby restricting hole transport in p-i-n structure of Sn-containing PSCs. Despite numerous efforts that have significantly enhanced stability, performance under bias, light, thermal, or combined stress conditions remains limited, even when cells are subjected to inert conditions or hermetically sealed to prevent moisture and oxygen ingress21–30.

Herein, we find that SnF2 can react with formamidine iodide (FAI) component in the perovskite film during thermal treatments, a process accelerated by light soaking, which disrupts the perovskite structure. Moreover, volatile byproducts like hydrofluoric acid (HF) can erode ITO and metal electrodes, and the inhibition of Sn2+ oxidation becomes ineffective once F⁻ ions are depleted (Fig. 1). To avoid the adverse effects of SnF2 on stability and hole transport, we replace SnF2 additive with lead powders—known for its antioxidant and crystallization-regulating effects as reported in our previous work—to remove Sn4+ from the precursor, combined with a PbF2 post-treatment to passivate surface defects26. The polarization of d-electrons in Pb2+ enhances its bonding with F⁻ ions, reducing its likelihood to react with FAI. Meanwhile, Pb2+ could fill VSn and F⁻ could suppress the formation of VSn by forming Sn-F bond. By employing this strategy, we increase the efficiency of SnF2-free Sn-Pb PSCs from 16.43% to 24.07% with improved photo-thermal stability, where the cells remain 60% of their initial efficiency after continuous operation at 85°C under maximum power point (MPP) conditions for 550 h.

Results
Side reaction between Sn-Pb perovskite and SnF2 additive
Halide perovskite (Cs0.2FA0.8Sn0.5Pb0.5I3 in this work) forms perfectly through the reaction between monovalent halides (CsI and FAI) and metal halides (SnI2 and PbI2) when the precursor is prepared in accordance with the stoichiometric ratio of the molecular formula. However, in the presence of 10 mol% SnF2 relative to SnI2, the spontaneous reaction between FAI with SnF2 (2FAI + SnF2 → SnI2 + 2FA + 2HF) disrupts the stoichiometry of the precursor and interferes with the formation of perfect crystals. Thermogravimetric-differential scanning calorimetry (TGA-DSC) is performed on the FAI and FAI+SnF2 powders (with a molar ratio of 2:1) in N2 atmosphere from room temperature to 600°C (Fig. 2a and Supplementary Fig. 1). FAI powder begins to decompose at 235°C31, while FAI+SnF2 powders start to decompose at temperature below 100°C, accompanied by two distinct exothermic peaks at 60 and 102°C (Supplementary Fig. 1). Meanwhile, the final weight loss of FAI+SnF2 powders is 25.9% matching the calculated weight fraction of FA and HF in FAI+SnF2 powders (25.9%). Thermogravimetric-mass spectrometry (TGA-MS) analysis of FASn0.5Pb0.5I3 powders, both with and without the addition of 10 mol% SnF2, also reveals that SnF2 significantly accelerates the decomposition process of perovskite. This is accompanied by a noticeable release of HF and FA, which is more pronounced compared to the decomposition of the perovskite without SnF2 (Fig. 2b). Nuclear Magnetic Resonance (NMR) characterizations also verify this reaction. In the ¹H NMR spectrum of FAI, the primary characteristic signals of FA+ are observed at 7.8 ppm (assigned to the CH group) and 8.7 ppm (assigned to the NH2 group) (Supplementary Fig. 2). Upon the addition of SnF2 to the FAI solution, a noticeable splitting pattern of the NH2 signal is observed, indicating a strong interaction between the F⁻ ions in SnF2 and the NH2 group in FA+. After aging at 85°C for 3 h, a new signal peak of triazine emerges at 9.3 ppm, indicating the deprotonation of FA+. Scanning electron microscopy (SEM) of perovskite film shows cracks in the presence of SnF2 after aging at 85°C with light soaking for 200 h, while nearly no change is observed in its absence, intuitively reflecting the detrimental effect of SnF2 on film stability (Fig. 2c, d and Supplementary Fig. 3a, b). This accelerated degradation effect of SnF2 is further investigated using X-ray photoelectron spectroscopy (XPS). The N 1s spectra reveals two components in both fresh and aged perovskite, corresponding to FA+ at higher binding energy and reaction by-products (FA) at lower binding energy (Fig. 2e, f and Supplementary Fig. 3c, d)32. In the presence of SnF2, the fraction of FA is 22.5% in the fresh perovskite film and increases to 32% after aging at 85°C with light soaking for 200 h. In comparison, the reaction by-products fraction of SnF2-free perovskite is only 6.7% in the fresh state and 8.6% after aging, showing little increase. Meanwhile, the disappearance of the F1s peak after this aging process is consistent with the reaction between of FAI and SnF2 (Supplementary Fig. 4).

Instability issues in Sn-Pb PSCs caused by SnF2 additive
In addition to the reaction between SnF2 and FAI, the migration of F⁻ and diffusion of HF through the whole device simultaneously corrode all the functional layers, including anode and cathode during photo-thermal degradation process. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis of a complete device after continuous operating at 85°C under MPP for 200 h shows the distribution of F⁻ from the Cu cathode to ITO anode (Fig. 3a and Supplementary Fig. 5). This result is driven by the spontaneous reactions that generate HF, which in turn damages the perovskite crystal structure and accelerates ion migration. The SnF2-free device, subjected to a PbF2 post-treatment, exhibits minimal diffusion of F⁻ ions. This observation indicates the low reactivity between PbF2 and FAI, which will be discussed in detail in the following section. The activation energy (EA) for ion migration decreases from 0.336 eV to 0.284 eV after introducing SnF2 into perovskite before aging (Supplementary Fig. 6). These processes mutually reinforce each other during thermal or light aging, ultimately leading to a vicious cycle of degradation within the device. After aging at 85°C with light soaking, the Fermi level downshifts at the surface, further increasing the electron transfer barrier (Fig. 3b and Supplementary Fig. 7a). This degradation process alters the energy levels of perovskite, particularly at the surface (Supplementary Fig. 8a), while the bulk perovskite remains largely unchanged (Supplementary Fig. 8b). In contrast, although the Fermi level in SnF2-free perovskite remains relatively stable, both fresh and aged perovskite exhibit a high electron transport barrier, which is one of the reasons for the low performance of SnF2-free devices and highlight the need for further optimization (Supplementary Figs. 7b and 8c, d). We investigated the real-time variations in the electrical conductivity of Cu electrodes under continuous light soaking at room temperature, a condition known to accelerate ion migration, as illustrated in the inset of Fig. 3c33. The conductivity of Cu electrodes in SnF2-containg device drops to zero even within 200s, while no decline is observed in SnF2-free devices. Additionally, we compare the resistance of ITO in these two types of devices by aging them at room temperature and measuring their resistance periodically as illustrated in the inset of Fig. 3d. The resistance of ITO in the SnF2-containg device increases from 3 to 13 Ohm after 60 days, while that of SnF2-free device shows no increase. After 48 h immersion in a 0.1 M SnF2/FAI mixed solution followed by DMF rinsing, the pronounced SEM transformation of ITO from a dense, continuous surface to one marked by conspicuous inter-particle gaps, provides direct evidence that the reaction products of SnF2 and FAI compromise the structural integrity of the ITO substrate (Supplementary Fig. 9). The changes of both Cu and ITO electrodes even at room temperature or without light soaking prove the strong corrodibility of migrated ion and reaction products.

Effective and stable alternative strategy to SnF2 additive
To avoid the adverse effects of SnF2 on stability and hole transport at the bottom of perovskite film, its positive role during film-forming process and on top surface defect passivation need to be strategically replaced. Toward this goal, we first investigate the use of lead powder and tin powder as a sacrificial agent to inhibit the oxidation of Sn2+ in the precursor and to regulate crystallization26. Pb powder react fully with FAI, releasing extra FA that regulate the crystallization growth (Supplementary Figs. 10 and 11) and suppress the formation of VSn (Supplementary Fig. 12) in the perovskite films. However, the device efficiency based on the addition of lead powder is lower than that of SnF2 based devices, confirming the role of SnF2 is not limited to as a sacrificial agents. Giving the distribution of SnF2 in resulted perovskite films both at the surface and the bottom of film, where the bottom SnF2 would create a hole transport barrier due to the de-p-doping function of SnF2 (Supplementary Fig. 13), we further introduce a similar fluoride, PbF2, as a passivation layer (Fig. 4a). After PbF2 post-treatment, SEM-EDS (energy-dispersive X-ray spectroscopy) shows the surface Sn/Pb ratio falls from 1.17 to 0.78 (Supplementary Fig. 14 and Supplementary Table 1). Compared with Sn2+, Pb2+ forms stronger Pb-I bonds that are harder to break, effectively suppressing VSn defects34. PbF2 is expected to have a low reactivity with FAI, as the presence of more d-electrons in Pb2+ could enhance its polarization effects and strengthen its ability to bind extra nuclear electrons and electrophilic F⁻, thus increasing the reaction barrier with FAI in perovskite structures compared to those in SnF2. This can be confirmed by the TGA results, where FAI+PbF2 powders shows higher decomposition temperature (over 100°C) than that of FAI+SnF2 powders (Fig. 4b). FAI+PbF2 powders exhibit a 3.1% weight loss at around 164°C (Supplementary Fig. 15), which correlates well with the calculated weight fraction of HF in FAI+PbF2 powders (3.4%). The ¹H NMR of FAI-PbF2 blend shows no change in the NH2 signal, or any new peaks after aging at 85°C for 3 h, confirming PbF2 is inert toward FAI (Supplementary Fig. 16). A more intuitive observation is that mixing FAI and SnF2, whether stirred at room temperature or heated at 100°C, results in the formation of black substances (FASnI3). In contrast, mixing FAI with PbF2 shows no color change even after heating at 100°C (inset in Supplementary Fig. 17). The X-ray diffraction (XRD) patterns of the films formed from these mixtures upon heating at 100°C are shown in Supplementary Fig. 17. Notably, the FAI+SnF2 film exhibits distinct peaks characteristic of the FASnI3 perovskite phase, indicating the formation of SnI2 resulting from the reaction between FAI and SnF2. In contrast, the XRD peaks of the FAI+PbF2 film do not correspond to the FAPbI3 perovskite phase but rather indicate the presence of a complex between FAI and PbF2, confirming no reaction occurs even at 100°C. SEM morphology of perovskite films with PbF2 post-treatment also show nearly no change after aging at 85°C with light soaking for 200 h (Supplementary Fig. 18). The reaction between PbF2 and FAI in actual perovskite films has also been confirmed through XPS analysis. The fraction of reaction by-products in PbF2 post-treated perovskite (target) is minimal, at only 7.0% before aging and 8.8% after aging at 85°C with light soaking for 200 h (Supplementary Fig. 19). These values are comparable to those of the SnF2-free film (control), as illustrated in Fig. 1d and Supplementary Fig. 2b. PbF2’s inertness toward FAI also suppresses FA vacancies (VFA). Both VFA and VSn are p-type defects that pin the Fermi level near the valence band edge. Dense VSn leaves the lattice iodine-rich, and VFA lowers the I– ion migration barrier (Supplementary Fig. 6), spawning more iodine-based p-type defects (ISn and IFA) that reinforce one another35. Their accumulation bends the bands upward at the perovskite/C60 interface. By mitigating these defects, PbF2 post-treatment establishes a stable, uniform energy alignment at the perovskite/C60 interface (Fig. 4c, Supplementary Figs. 20–22).

To access the quality of interface contact and the extent of non-radiative recombination in PbF2 post-treated perovskite films, we investigated their photo-luminescence quantum yield (PLQY). The PLQY of the control perovskite film is only 3.875%, which increases to 4.675% and 5.454% following SnF2 additive and PbF2 post-treatment, respectively. Upon creating a perovskite/C60 interface, the PLQY of the PbF2 post-treatment device retains 88%, while the control and SnF2 additive devices retain only 72% and 68% PLQY, respectively (Fig. 4d). These findings indicate that the PbF2 post-treatment approach is effective in reducing non-radiative recombination loss at this interface36. Femtosecond transient absorption (fs-TA) spectroscopy is conducted to probe the carrier transport dynamics. Figure 4e, f presents pseudo-color images of the TA spectra (ΔT/T) for control and target samples (glass/perovskite/(PbF2)/C60). The target sample shows faster ground-state bleach (GSB) (at 926 nm) and hot-exciton absorption (at 994 and 1170 nm) signal decay than the control sample (Supplementary Fig. 23 and Supplementary Table 2). Meanwhile, the carrier transport dynamics in target sample is comparable to those in samples with SnF2 additive (Supplementary Fig. 23 and Supplementary Table 2), demonstrating the effectiveness of this alternative method. The shorter carrier lifetime on microsecond timescale of target perovskite film (5.28 μs) compared to the control film (9.70 μs), calculated from transient photoluminescence spectroscopy (TRPL, excitation at 500 nm) in Supplementary Fig. 24 and Supplementary Table 3, elucidates the facilitated electron transfer and reduced defect recombination37.

Device performance of SnF2-free Sn-Pb PSCs
To evaluate the impact of PbF2 post-treatment on the photovoltaic performance of PSCs, we fabricate a series of devices. As shown in Fig. 5a, Sn-Pb PSCs employing effective lead powder as a reductant and ethanediamine (EDA) post-treatment exhibit remarkably low PCE of 16.43% when devoid of the SnF2 additive (control)12,13,30. This is primarily attributed to the low open-circuit voltage (VOC) of 0.736 V and fill factor (FF) of 69.56%. In contrast, when the SnF2-free perovskite film is post-treated with PbF2 (target), even without EDA post-treatment, the PCE significantly improves to 24.07% accompanied by a high VOC of 0.884 V and an FF of 81.3% (Supplementary Fig. 25 and Supplementary Table 4). The large VOC and FF difference is attributed to the misaligned energy level and large defect recombination loss in the control device (Fig. 4c, d). Especially, the VOC shortfall in the control device exceeds its PLQY deficit, a discrepancy chiefly rooted in its misaligned energy levels caused by the p-type defects at film surface38,39. The target device demonstrates a steady-state PCE of 23.70% under MPP condition with for 600 s, with minimal hysteresis (Supplementary Fig. 26 and Supplementary Table 5). The external quantum efficiency (EQE) spectrum of the target device, shown in Fig. 5b, exhibits an integrated short-circuit current density (JSC) of 32.55 mA cm−2, confirming the consistent of the integrated current density from J-V characteristic. Figure 5c and Supplementary Fig. 27 present the statistical parameters of 25 control and 25 target devices selected by random sampling. The narrow distributions and small standard deviations underscore the good reproducibility. All target devices exhibit simultaneous gains in VOC and FF, yielding a marked and consistent PCE enhancement after PbF2 post-treatment, which firmly corroborates the reliability of this interfacial strategy. The performance of devices post-treated with other lead salts is also investigated (Supplementary Fig. 28 and Supplementary Table. 6). Devices post-treated by PbCl2 and PbBr2 show only a modest increase in VOC and a minimal improvement in the ultimate PCE. This suggests the crucial role of F⁻ ions in suppressing Sn surface defects and highlights their indispensability for enhancing device performance. To map the full efficiency ceiling, we also test PbF2 & EDAI post-treatment (Supplementary Fig. 29 and Supplementary Table. 7), SnF2 additive+PbF2 post-treatment (Supplementary Fig. 30 and Supplementary Table 8), and PbF2 as an additive (Supplementary Figs. 31, 32 and Supplementary Table 9). It is noteworthy that PbF2 additive lags behind SnF2 because Pb2+ binds F⁻ too tightly, leaving fewer F⁻ to complex with Sn4+ in precursor and block its entry into the perovskite lattice40. The primary motivation for replacing the SnF2 additive is to overcome the stability bottleneck of Sn-Pb PSCs, especially under the light-thermal conditions. In this regard, the target device retains 60% of its initial efficiency after continuous operates at 85°C under MPP condition for 550 h. In comparison, devices with 10 mol% and 5 mol% SnF2 additives degrade to 60% of their initial efficiency after 19 h and 62 h, respectively (Fig. 5d). Supplementary Fig. 33 track the VOC, JSC, and FF evolution during 85°C MPP aging. Devices with SnF2 show a sharp FF drop, mirroring the energy-level shift in Fig. 3b. At 10 mol% SnF2, the VOC fall even faster because surface F⁻ is consumed, leaving VSn defects. This result aligns with recent studies showing significant improvements in light-soaking and thermal stability, which are correlated with a reduction in SnF2 content by half41. This enhancement in stability is particularly noteworthy within the realm of Sn-Pb PSCs. Photo-thermal stability data are rarely documented, with most reported operational temperatures being below 60°C (Fig. 5e and Supplementary Table 10)2,5–7,11,14–17,20,21,25,41–47. Our findings highlight the potential of the combination of PbF2 post-treatment and lead powder in precursor as a highly effective strategy to improve both the efficiency and long-term stability of Sn-Pb PSCs, addressing critical challenges in their practical application.

Methods
Materials
All the chemicals in this work were used as received from commercial without further purified. CH(NH2)2I (FAI) (99.9%), PC60BM (99.9%) were purchased from Advanced Election Technology CO. Ltd. Guanidinium thiocyanate (GASCN) (≥99.5%), PbI2 (≥99.99%) C60 (>99%) and BCP (>99%) were acquired from the Xi’an Polymer Light Technology. SnI2 (99.999%), SnF2 (>99%), isopropanol (99.5%), ethylenediamine dihydroiodide (EDAI2) (≥99%) and ethylenediamine (EDA) (≥99%) were purchased from Sigma-Aldrich. PbF2 (99.99%) was purchased from Boer. Poly[3-(4-carboxylbutyl)thiophene] (P3CT) was purchased from Rieke, America. Lead powder (99.95%) and CsOH•H2O (99.9%) were achieved from Aladdin. All solutions were filtered with 0.22 μm PTFE filter before use.

Solar cell fabrication
ITO glass substrates were cleaned by detergent, DI water, acetone and isopropanol for 20 min respectively via sonication treatment. Then the substrates were treated by UV-zone for 20 min before the deposition of P3CT-Cs HTL. P3CT-Cs solution was prepared through dissolving 5 mg P3CT and 4.62 mg CsOH•H2O in methanol. The 0.5 mg/ml P3CT-Cs solution was spin-coated on ITO substrates at 2000 rpm for 30 s. Then, the P3CT-Cs film was annealed at 100°C in air for 10 min. Then the substrates were transferred to glovebox for the deposition of perovskite film. For the Sn-Pb mixed perovskite precursor with SnF2 additive, 1.2 mmol FAI, 0.3 mmol CsI, 0.75 mmol SnI2, 0.075 mmol SnF2, 0.75 mmol PbI2 and 0.025 mmol GASCN were dissolved in 600 μl DMF and 200 μl DMSO mixed solvent. For the Sn-Pb mixed perovskite precursor without SnF2 additive, 0.075 mmol SnF2 is removed and other precursors remain unchanged. For the Sn-Pb mixed perovskite precursor adding lead powder, a slight excess of FAI is employed, as a portion is consumed in the in-situ reaction with the lead powder. After adding 20 mg lead powder, the precursor solution was stirred at room temperature overnight to prove the sufficient reaction between precursor and lead powder. The Sn-Pb mixed perovskite film was spin-coated on P3CT-Cs layer at 1000 rpm for 10 s and 4000 rpm for 50 s. 300 μl chlorobenzene was in-situ dripped onto the perovskite film after 45 s during the second step within 1 s. Afterwards, the perovskite films were immediately transferred to the hot plate and were annealed at 130°C for 7 min. The ambient temperature during film formation is controlled at 25°C to prevent excessively rapid crystallization of the perovskite. To the perovskite films with SnF2 additive, post-treatment with 0.1 mM EDA in chlorobenzene at 5000 rpm for 20 s is conducted. To the perovskite films without SnF2 additive, post-treatment with 1.0 mg/ml PbF2 in isopropanol at 5000 rpm for 20 s is conducted. The post-treatment with PbCl2 and PbBr2 is conducted by evaporating 1 nm film on perovskite. The combined post-treatment with PbF2 and EDAI2 is conducted by a mixed solution of 1.0 mg/ml PbF2 and 0.2 mg/ml EDAI2 in isopropanol at 5000 rpm for 20 s. Then, a further treatment at 100°C was carried out for 5 min. Finally, 30 nm C60, 6 nm BCP, 1 nm LiF and 100 nm Cu electrode were evaporated under high vacuum (<3*10−4 Torr). The device area defined by the mask was 0.0836 cm2.

Characterization and measurements
Photocurrent density-voltage (J-V) curves (Keithley 2400 Source-Meter) were measured under one sun illumination (AM1.5G, 100 mW/cm2) using the solar simulator (Enlitech, SS-F5-3A). The light source is a 450-W xenon lamp calibrated by a standard Si reference solar cell (Enli/SRC2020, SRC-00201). The J-V curves were measured from 1.0 V to −0.2 V with dwell time of 10 ms. The light source is a 45-W xenon lamp calibrated by a standard Si reference solar cell (Enli/SRC2020, SRC-00201). The EQE measurement was carried out in ambient air using a QE system (SOFN Instruments Co., Ltd) with monochromatic light focused on a device pixel. The MPP stability test was conducted on encapsulated devices in N2 under 1-sun equivalent illumination (white light-emitting diodes) at 85°C which was monitored by an infrared thermometer.

TOF-SIMS spectra was measured on aged samples using PHI nanoTOF II Time-of-Flight SIMS with a primary Bi ion gun (30 keV). 1 keV Cs gun was used for sputtering with an analysis area of 200 × 200 μm2. The aged perovskite devices were prepared by operating at MPP at 85°C under 1-sun equivalent illumination (white light-emitting diode) in N2.

X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements were carried out using Kratos AXIS ULTRA DLD XPS/UPS system. XRD spectra was recorded in an Empyrean Microdiffractometer with Cu Kα radiation. Scanning electron microscope (SEM) images were acquired by using a field-emission scanning electron Microscopy (Zeiss Gemini SEM 450).

TGA-MS is conducted using a Netzsch simultaneous thermal analysis (STA) instrument, under a nitrogen atmosphere created by fluxing 50 mL min−1 of N2 at the heating rate of 10°C min−1. TG-MS was conducted using a Netzsch simultaneous thermal analysis instrument coupled with a gas chromatography-mass spectrometer where the mass of any volatile fragments can be analyzed and identified. The powder samples were heated from 25°C to 300°C with a ramp rate of 10°C/min.

Activation energy measurement of ions migration: the current was extracted at 100 s after the voltage is switched on. The measurement was conducted in a Lakeshore Probe Station under vacuum (4.0 × 10−4 Pa). The samples were placed on a copper substrate with temperature control by a heater and injected liquid He. A semiconductor characterization system (Malaysia TEKBA1500) was used for the current measurement. During the measurement, we first cooled the devices to 165 K for 1 h and then heated to objective temperature. Every objective temperature was stabilized for 5 min before the current record.

The steady-state and time-resolved PL spectra were measured utilizing FLS980E Fluorescence Spectrophotometer (excited by 532 nm, Unite Kingdom) at FOMAD (Family of Master and Doctor) Corp, China. The PLQY was performed using an Edinburgh Instruments FS5 spectrofluorimeter using a plane-ruled diffraction grating monochromators with 1200 grooves/mm coupled to filter turrets to remove higher order diffraction signals and using a Hamamatsu R928P photomultiplier tube as a primary detector and two silicon photodiode detectors for reference and transmission detection. A Spectralon integrating sphere attachment was used for PLQY measurements with an excitation wavelength of 520 nm and excitation fluence of 1 mW/cm2. For fs-TAS measurements, the Titanium:Sapphire femtosecond laser (Astrella, Coherent Inc) generated femtosecond pulsed light with a central wavelength of 800 nm, a repetition rate of 1 kHz, and a pulse width of 100 fs. This 800 nm pulsed light was split into two parts by a beam splitter. One part entered the optical parametric amplifier (OPerA Solo, Coherent Inc) to produce the pump light at 600 nm (70 W). The other part passed through a delay line (Delayline) and entered the transient absorption spectrometer (Helios, Ultrafast system), where it was focused on a sapphire nonlinear to generate a supercontinuum probe light. The probe light, after being focused by an off-axis parabolic mirror, was directed onto the sample. The pump light, after being chopped to 500 Hz by a chopper, was adjusted by a half-wave plate to ensure that the pump and probe lights were focused onto the sample at an angle of 54.7°. During testing, the sample was clamped on a sample stage that can move two-dimensionally in the direction perpendicular to the beam to reduce damage to the sample from the pump light). The probe light absorbed by the sample carried information about the particle’s excited state and ground state and was ultimately incident on the optical fiber probe head. By using a time delay line to change the delay of the probe light relative to the pump light reaching the sample, spectral information of the sample particles at different time delays after excitation can be obtained.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript by Yuan et. al. reveals the reaction between SnF2 and FAI in tin-lead perovskite solar cells (Sn-Pb PSCs) during photo-thermal treatments. This degradation reaction reveals the origin of the low stability level of high-efficiency Sn-Pb PSCs and all-perovskite tandem solar cells. However, SnF2 is an indispensable additive in Sn-Pb PSCs without substitutes. Based on this, authors proposed the novel strategy that combines lead powder in precursor with PbF2 post-treatment. As a result, the PCE of SnF2-free Sn-Pb PSCs was greatly improved from 16.43% to 24.07%. More importantly, the photo-thermal stability of device is obtained, remaining at 60% initial efficiency after operating at 85°C at MPP condition for 550 h. The substance and conclusions of this work are likely to generate significant interest within the field. I recommend that Nature Communications accept this manuscript after addressing the following concerns.\n1. The authors focus on Sn-Pb perovskite devices without SnF2 additive and describe the irreplaceability of SnF2 additive. Have analogous devices fabrication been reported in other studies? Please provide the research status of SnF2-free devices\n2. Will the PbF2 post-treatment affect the Sn/Pb ratio on the film surface, and what potential impacts might it have on the device performance? Further evidence is needed to clarify this issue.\n3. In Figure 1, the authors depicted the SEM morphology of Sn-Pb perovskite films subjected to 85℃ light soaking, both with and without SnF2. However, the morphology characteristics of the perovskite films treated with PbF2 post-treatment were not addressed. Could you please provide the corresponding information regarding the morphology of the PbF2 post-treated perovskite films.\n4. In Figure 4, the statistical parameters of the perovskite solar cells only include the efficiency. The main text and supplementary information do not provide statistical data for other key performance parameters such as Voc, Jsc, and FF. Please provide this data.\n5. The manuscript utilized PLQY to characterize the non-radiative recombination loss on the film surface, but the increase in PLQY does not be consistent with the increase in the Voc of device. Please explain that.", "reviewer_2": "The poor photo-thermal stability of Sn-Pb perovskite solar cells, especially at 85°C, is dragging back the further development of Sn-Pb single-junction devices and all-perovskite tandem devices. Impressively, Yuan et al. discovered the thermal degradation process in Sn-Pb PSCs induced by SnF2. Further, the reductive and crystallization regulation effect of lead powder in combination with the energy level regulation effect of PbF2 ensured the device performance and thermal stability, simultaneously. Under continuous operation at the maximum power point (MPP) at 85°C, the devices maintained 60% of their initial efficiency after 550 hours. The manuscript discovered an important phenomenon and provided a promising solution to resolve the instability issue of Sn-Pb perovskite. Therefore, I suggest that Nature Communications accept this manuscript after minor revision.\n1. As a widely utilized antioxidant, SnF2 is capable of passivating Sn4+ via a substitution reactio n. Can PbF2 possess a comparable function?\n2. The post-treatment of PbF2 improved the surface work functon(Supplementary Figure 12), however, the uniformity of this modification has not been proven. Please provide the surface potential mapping of KPFM.\n3. Did the authors use classic surface passivators such as EDADI2 when conducting device post-processing with PbF2? Can the device performance be further improved when PbF2 and EDADI2 act together?\n4. In addition to the efficiency evolution, it is also necessary to provide the evolution of other perform nce parameters (including Voc, Jsc, and FF) during operation at the maximum power point (MPP) and 85°C. Furthermore, an interpretation of the evolution process would be highly appreciated.\n5. The performance degradation pathways of Sn-Pb PSCs are diverse. How does the authors consider the stability potential of Sn-containing PSCs at 85°C?", "reviewer_3": "This work demonstrates an impressive PCE exceeding 24% for SnF2-free tin-lead perovskite solar cells, and importantly, investigates a critical degradation pathway induced by SnF2 that has been previously overlooked. To address this issue, the authors propose a promising strategy involving the use of PbF2, which significantly enhances both the open-circuit voltage and fill factor of the devices. Notably, the devices also exhibit improved thermal stability at 85 °C under maximum power point tracking, even without encapsulation, underscoring their potential for practical applications. However, the manuscript requires revision, particularly in terms of language clarity and coherence. In addition, the experimental procedures should be described in greater detail to provide a comprehensive understanding of the methodology employed. Regarding the scientific and technical significance of the work, I support its publication in Nature Communications after the authors address the following points and provide the necessary additional information:\n1. Tin-lead perovskites are highly sensitive to the crystallization environment, and the absence of SnF2 can significantly influence the film formation process. Therefore, a detailed description of the device fabrication protocol is essential.\n2. Did the authors investigate the effect of varying PbF2 concentrations? If so, please include the relevant data and discuss the optimal concentration range.\n3. The manuscript lacks information regarding the morphology of the perovskite films after PbF2 post-treatment, both in their fresh and aged states. Please provide SEM or AFM images and related analysis.\n4. The authors claim that the by-product HF reacts with the ITO electrode. However, there is currently insufficient evidence to substantiate this. Please provide experimental data to support this assertion.\n5. While the experimental results clearly show the superiority of PbF2 in improving the performance of SnF2-free Sn-Pb PSCs, a more systematic explanation is needed. The authors should provide further clarification through theoretical analysis or defect characterization techniques.\n6. Can PbF2 be used as a dopant in the precursor solution instead of SnF2? If so, how does this affect device performance? Please include a comparative study.\n7. The use of Pb powder in the precursor solution, rather than Sn powder, is unusual and may raise questions for the reader. Please clarify the rationale behind this choice.", "reviewer_4": "The manuscript entitled “A Tin Fluoride-Free, Efficient and Durable Tin-Lead Perovskite Solar Cell” reveals that the instability of SnF₂ in Sn-Pb perovskite solar cells originates from its thermal reaction with FAl, which accelerates device degradation. The authors propose an innovative strategy by introducing lead powder in the precursor solution combined with a PbF₂ post-treatment to replace the role of SnF₂ in film formation and surface defect passivation. This approach effectively balances efficiency and stability. As a result, the power conversion efficiency of SnF₂-free devices is improved from 16.43% to 24.07%, with excellent photo-thermal stability—maintaining 60% of the initial efficiency after 550 hours of operation at 85°C under MPP conditions. The study is well-structured, the data are comprehensive, and the findings are of significant importance for the development of both single-junction and all-perovskite tandem solar cells. However, several issues should be thoroughly addressed and resolved prior to publication.\n1、The authors identified that SnF₂, when used as an additive, presents issues related to thermal stability, and they accordingly employed metallic Pb to suppress oxidation and used PbF₂ post-treatment for defect passivation. However, the current evidence supporting the instability caused by SnF₂ relies mainly on TG analysis. To strengthen the conclusion, the authors are encouraged to provide further theoretical explanations and complementary characterizations to support this claim. Additionally, it would be valuable to discuss whether similar findings have been reported in the literature.\n2、It is recommended that the authors include additional performance data for devices fabricated using PbF₂ as an additive, in order to better justify the rationale for employing PbF₂ as a post-treatment rather than incorporating it directly into the precursor. Moreover, while the authors emphasize the synergistic effect of Pb and PbF₂, data showing the individual contributions of each component are lacking. Further clarification is needed regarding the combined effect of SnF₂ and PbF₂, and whether their interaction is beneficial or detrimental to device performance.\n3、Figure 1c reveals pronounced pinholes/cracks in the perovskite film attributed to SnF₂ incorporation. Please provide a mechanistic explanation for this morphological degradation, addressing how SnF₂ influences crystallization kinetics or induces lattice strain.\n4、The textual annotations within the device architecture diagram are unclear in scheme 1 and figure 3a. It is recommended to use more distinct and contrasting labels or visual indicators to clearly differentiate the various layers and components of the solar cell structure. In figure S7 and S12, the “Aged 200 h” curves also should be clarified.\n5、On page 4, the authors report a 25.1% weight loss for the FAI + SnF₂ powders around 500 °C; however, this value is inconsistent with the 25.8% shown in Figure 1a. The authors should clarify this discrepancy to ensure the accuracy and consistency of the data presented.\n6、On page 9, Figure 3d should clearly indicate the conditions corresponding to each colored column.\n7、On page 10, the authors state, “The external quantum efficiency (EQE) spectrum of the target device, shown in Figure 4c, exhibits an integrated short-circuit current density (JSC) of 32.55 mA cm⁻², confirming the consistency of the integrated current density from J-V characteristic.” However, this statement contains two errors: the EQE data are actually shown in Figure 4b, not 4c, and Figure 4c is not referenced or described elsewhere in the manuscript. The authors should revise this sentence to correct the figure number and ensure consistency with the actual content."}, "author_response": "We would also like to thank the reviewer for the valuable comments and helpful suggestions. In the following pages, we provide point-by-point responses to all the questions raised by the reviewers. The corrections in the manuscript are shown in yellow for convenience. We also carefully read the entire manuscript and revised any errors we found. We hope that we have appropriately addressed all the mentioned issues.\n[... Author responses to all points from Reviewers 1-4 follow here, including detailed data, tables, and figures as provided in the source text ...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have fully addressed my comments. I support the publication of the revised manuscript.", "reviewer_2": "The authors have addressed my concerns. I recommend the manuscript for publication.", "reviewer_3": "The authors have addressed my concerns and made good revisions. I am pleased to recommend publishing this work in Nature Communications in its current form.", "reviewer_4": "The authors have provided a comprehensive and meticulous response to all points raised by the reviewers. The revisions made to the original manuscript are thorough and appropriate. All key changes have been clearly outlined in the point-by-point response letter and are accurately highlighted within the revised manuscript.\nThe quality of the manuscript has been significantly enhanced through this revision process. All major and minor concerns previously identified have been addressed satisfactorily. The manuscript now meets the required standards for publication in terms of scientific soundness, clarity, and completeness."}, "author_response": ""}]

Decision letter