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[Jannatul Ferdous](https://orcid.org/0009-0000-1089-5010), [Md. Emrul Kayesh](https://orcid.org/0000-0003-3737-819X), [Wipakorn Jevasuwan](https://orcid.org/0000-0001-9117-2497), [Naoki Fukata](https://orcid.org/0000-0002-0986-8485), [Ashraful Islam](https://orcid.org/0000-0002-1633-1432)

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[Solution‐Processed SnO<sub>  <i>x</i></sub> as a Hole‐Transporting Material for Stable Sn‐Based Perovskite Solar Cell](https://mdr.nims.go.jp/datasets/f40bec07-aff1-4651-a347-03b3200f4969)

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Solution-Processed SnOX as a Hole Transporting Material for Stable Sn-based Perovskite Solar CellJannatul Ferdousa,b,c, Md. Emrul Kayeshb, Wipakorn Jevasuwana, Naoki Fukataa*, Ashraful Islamb*aInternational Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanbPhotovoltaic Materials Group, Centre for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, JapancGraduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, JapanAbstract:Sn-based perovskites are considered a suitable alternative to toxic Pb-based perovskites due to their low toxicity and optimum optoelectronic properties. However, high-efficiency Sn-based perovskite solar cells (PSCs) typically use poly (3,4-ethylenedioxythiophene): polystyrene sulfonic acid (PEDOT:PSS) as a hole transport material (HTM), which limits their stability due to its acidic nature. This study introduces SnOX nanocrystals, synthesized through a synproportionation reaction of Sn4+ with Sn0 under mild conditions, as a replacement for PEDOT:PSS. X-ray photoelectron and ultraviolet photoelectron spectroscopy analyses revealed that the Sn0 reduces Sn4+ by 38% and elevates the highest occupied molecular orbital (HOMO) to –5.70 eV, close to PEDOT:PSS, enabling HTM behavior. The perovskite films on SnOX exhibit improved grain size and crystallinity compared to PEDOT:PSS. The resulting SnOX-based Sn-PSCs achieved a power conversion efficiency of 11.11%. They retained 90% of their efficiency after 1000 h of maximum power point tracking, indicating superior stability over PEDOT:PSS-based devices.Keywords: acidic nature, nanocrystal, stability, crystallinity.1. IntroductionPerovskite solar cells (PSCs) have gained a lot of attention lately because of their remarkable increase in power conversion efficiency (PCE), which has been observed in just 15 years, from 3.8% to 26.7%. [1,2] However, all reported high-performing PSCs consist of lead (Pb) in their perovskite absorber. Unfortunately, the toxicity of Pb prevents them from being used widely, which has compelled researchers to find a substitute for Pb-free PSC in the past few years.[3,4] To resolve this problem, numerous research teams are currently struggling to discover alternatives to Pb, such as tin (Sn)[4], bismuth (Bi)[5], antimony (Sb)[6], copper (Cu)[7], and germanium (Ge)[8] which are non-toxic metals. The objective is to find substitutes that will not compromise the optoelectronic properties of the perovskite absorber.Among them, Sn-based perovskites are found to be the most possible substitute materials for Pb-free perovskite out of all the other options because of their reduced exciton binding energy, increased mobility of charge carriers, and optimum bandgap (1.2-1.4 eV), which is closer to the ideal value denoted by the Shockley–Queisser Limit (1.3 eV).[9,10] However, the maximum reported PCE for Sn-PSCs is below 16%, which is far away from the Pb-PSCs.[2] One factor contributing to the limited performance is their high defect density due to facial tendency to oxidize from Sn2+ to Sn4+ and fast perovskite crystallization rate.[11,12] To resolve this problem, Mathews et al. utilized SnF2 as an additive to retard Sn2+ oxidation.[13] Subsequently, other additives such as trimethylthiourea, NH4SCN, and 4-amino-3-hydroxybenzoic acid were employed in conjunction with SnF2 to improve the morphology and optoelectronic properties of perovskite films.[12,14,15] Recently, Q. Mi et al. reported record PCE for Sn-based PSCs of 15.7% by doing a surface treatment of perovskite with 4-fluorophenethylamine hydrobromide to passivate surface defects and minimize energy mismatch of Sn-perovskite films with charge transport materials.[16] However, all the reported high-efficiency devices failed to demonstrate maximum power point tracking (MPPT) stability, which is essential for the industry standard degradation test for all solar cells. The factor related to the lower device stability is the acidic nature of poly (3,4- ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT:PSS), the most frequently employed hole transporting material (HTM) in high-efficiency PSCs. PEDOT:PSS is extensively employed as HTM in the inverted PSCs due to its energy level matching with Sn-perovskite, good conductivity, high transparency, and easy solution processing methods at low temperatures (<150 °C).[17,18] The acidic properties of PEDOT:PSS accelerate the oxidation of iodide to iodine in perovskites, which makes Sn-PSCs unstable under MPPT conditions.[19,20] Although NiOx and CuSCN have been identified as robust inorganic HTMs for the Sn-based PSC, their PCE is still not satisfactory.[21,22] Actually, most of the HTMs were designed originally for Pb-perovskites. So, the energy level of the present HTM doesn’t match well with Sn-based PSC. To make Sn-based PSCs more stable, it is necessary to substitute the organic HTMs with newly developed inorganic HTMs. Recent research has revealed that a mixed valence tin oxide (SnO1−X), produced by simultaneously sputtering a SnO target and a metallic Sn target, may be a good HTM. By adjusting the sputter power of the Sn target, the amounts of excess Sn (X) in the SnO1−X films can be controlled. To obtain a p-type pure phase of SnO film, the value of X should be 0.03, and it needs to be annealed at 300ºC in N2.[23] J. Huang et al. fabricated mixed valence SnO2−X by using atomic layer deposition (ALD) and used as HTM for Sn-Pb-based PSCs. These PSCs exhibit comparable performance to those made with PEDOT:PSS.[24] The primary benefit of employing SnO2−X as the HTM for Sn–Pb PSCs is the device stability. To reduce the cost and complexity of the ALD process, recently they reported a ternary Sn (II) alloy of SnOCl, synthesized by a low-temperature solution method, as a replacement of PEDOT:PSS for Pb-Sn-based PSCs.[25] S. Hayase et al. unexpectedly discovered that when nanoparticles of SnO2 came into contact with the Sn-perovskite layer, its surface was reduced to SnOX (X < 2), which then functioned as the HTM of the Sn-based PSCs.[26]  Therefore, the functionality of SnOX as n-type or p-type depends on the value of X in the fabricated SnOX films. A. Xu et. al. developed an innovative and straightforward method to produce stable Sn2+ self-doped SnO2-X nanocrystals with a significant surface area through a synproportionation reaction of Sn4+ with metallic tin (Sn0) under mild conditions.[27] They reported that the synproportionation reaction of Sn4+ and Sn0 pushed the valence band edges from 3.21 eV for SnO2 to 2.4 eV for SnO2-X and systematically investigated the photocatalytic performance of Sn2+ self-doped SnO2-X nanocrystals. From this report, we were inspired to search for novel tin oxides that can be produced using solution-based methods.In this study, we present SnOX as a replacement for PEDOT:PSS as the HTM in Sn-based PSCs. The SnOX was readily produced by reacting SnCl4.5H2O with Sn0 in water at room temperature. Here Sn0 partially reduced SnCl4·5H2O to form SnOX where the value of X remains in the range of 1.7 to 1.8 as observed from the X-ray photoelectron spectroscopy (XPS) measurement. In addition, this partial Sn4+ reduction in SnOX pulled the valence band energy from −7.86 eV (SnO2) to −5.70 eV (SnOX) as evidenced by the Ultraviolet Photoelectron Spectroscopy (UPS) close to the value of Sn-based perovskite. The Sn-based PSCs utilizing SnOX as HTM exhibit a superior device performance comparable to the PEDOT:PSS-based devices. More importantly, the SnOX HTM significantly improves the long-term stability of the Sn-based PSCs. The top-performing SnOX-based Sn-PSC maintained about 90% of its initial PCE under maximum power point tracking (MPPT) conditions for 1000 h.2. Results and discussionThe synthesis of SnO or SnO2 through a solution process is frequently accomplished using tin (II) chloride (SnCl2) or tin (IV) chloride as a precursor solution. The conditions of the solution process, such as temperature, presence of oxidation or reducing agents, O2, pH value, and H2O, determine whether SnO or SnO2 will form.[28] To work SnOX as the HTM for Sn-based PSCs, the value of X should be in the range of 1.7-1.8.[26] To synthesize SnOCl as HTM, Huang et. al. dissolved anhydrous SnCl2 in an ethyl alcohol/water solution and controlled the pH value using NaOH.[25] However, in this work, SnOX was prepared only by dissolving 1 mmol of SnCl4·5H2O in 30 ml of distilled water. Afterward, the mixture was vigorously stirred, and Sn0 powder was added at different ratios of Sn0: SnCl4·5H2O such as 1:2, 1:4, 1:8, and 1:16. The solution was stirred in a closed environment for the next 10 days. Finally, a clear light yellow SnOX solution was obtained. According to the findings displayed in Figure S1, the best performance of Sn-based PSCs was obtained using SnOX as HTM, fabricated from a 1:4 ratio of Sn0: SnCl4·5H2O solution. The SnOX films were fabricated through spin coating using commercially available 15 wt% colloidal SnO2 solution, along with a 1:4 ratio mixture of Sn0: SnCl4·5H2O solution, which will be abbreviated as SnO2 and SnOX hereafter.Measurements using high-resolution transmission electron microscopy (HRTEM) were used to confirm that SnOX nanocrystals were formed. HRTEM and Fast Fourier Transform (FFT) images of SnOX are shown in Figure 1 a. The SnOX nanoparticles were evenly distributed throughout the substrate surface, as seen by the HRTEM image. The micrographs demonstrate that SnOX nanoparticles had an average diameter of between 5 and 10 nm. The crystalline structure of individual nanoparticles exhibits interplanar spacing (d110) values of 0.33, which correspond to the tin oxide (110) atomic plane with a tetragonal cassiterite phase.[27] To observe the reducing effects of Sn0 powder in SnCl4·5H2O solution, the composition of SnO2 and SnOX was measured using XPS. The XPS spectra of Sn 3d5/2 orbitals of SnO2 and SnOX films are displayed in Figures 1b and 1c. The Sn 3d5/2 spectra, which overlap with the Sn0, Sn2+, and Sn4+ components, can be deconvoluted to get a quantitative analysis of the chemical compositions and oxidation states of Sn (Sn0, Sn2+, and Sn4+) in the SnO2 and SnOX films. The content of Sn2+ and Sn4+ determines the peak positions of these elements in SnO2 or SnOX. An increase in Sn2+ content in the SnOX films results in a shift of both Sn2+ and Sn4+ peak positions to lower binding energy.[26] This occurs because metallic tin (Sn0) in the SnCl4.5H2O solution reduces Sn4+ by undergoing oxidation to Sn2+.The peak at around 486 eV is attributed to Sn4+ (83.21%), and a small amount of Sn2+ (15.89%) was found from the Sn 3d5/2 peak for the SnO2. However, a comparatively considerable decrease in Sn4+ (50.20%) and an increase in Sn2+ (47.33%) were observed with the addition of Sn0 powder in SnCl4.5H2O. This indicates that the Sn0 acted as a reducing agent and partially reduced the percentage of Sn4+ from SnO2 which is necessary to act as HTM for Sn-based PSCs. Figure 1. (a) HRTEM image of the SnOX sample and the corresponding FFT pattern are inset in. XPS deconvolution spectrum of Sn 3d5/2 spectra of (b) SnO2 and (c) SnOX thin films. UPS spectrum of SnO2 and SnOX in the (d) energy onset (Eonset) region, (e) energy cutoff (Ecutoff) region. (f) Schematic representation of SnOX and SnO2 energy levels in relation to the perovskite layer.As Sn0 partially reduces Sn4+ content in SnOX film, it should also affect the energy level. To learn about the reducing effect of Sn0 on the energy levels of SnOX film, the energy levels of SnO2 and SnOX films were determined using UPS analysis (Figure 1d and1e). The SnO2 and SnOX films were found to have the same cutoff binding energy (Ecutoff) of 17.50 eV. The Fermi levels (Ef) for the SnO2 and SnOX films were calculated to be −3.72 eV, employing the equation Ef = −21.22 eV + Ecutoff. [29,30] SnO2 and SnOX have values of valence band (Ev) of −7.86, and −5.70 eV, respectively, determined by the formula Ev = Ef − Eonset. Lastly, the conduction band (Ec) was determined by Ec = Ev + Eb, where Eb is the band gap, and the band gap of SnO2 and SnOX was determined from the Tauc plot (Figure S2). The Ec of SnO2 and SnOX were determined to be −3.28 and −1.32 eV, respectively. Figure 1f schematically illustrates the energy diagram of SnO2 and SnOX according to the UPS measurement and the others such as PEDOT:PSS, FASnI3, C60, BCP, and Ag from the literature.[26] From UPS measurement, it is evident that SnOX becomes an HTM for Sn-PSCs like that of PEDOT: PSS because its valence band is closer to the perovskite (−5.20 eV) in comparison to SnO2 (−7.86 eV), which is advantageous for lowering the interface energy barrier and hence inhibiting interface recombination.[31]Figure 2.  (a) steady-state photoluminescence (SSPL)of FASnI3 films on PEDOT:PSS and SnOX. SEM images of FASnI3 films on (b) PEDOT:PSS and (c) SnOX. (d) XRD pattern of FASnI3 films on PEDOT:PSS and SnOX.The influence of the bottom layer on the optoelectronic characteristics of FASnI3 films was assessed by performing UV-vis absorbance and steady-state photoluminescence (SSPL) on both FASnI3 films deposited on PEDOT:PSS and SnOX. The absorption band edge was slightly blue-shifted for the FASnI3 film deposited on SnOX as observed by the UV-vis absorbance (Figure S3) and SSPL (Figure 2a). This might be because SnOX contains chloride ions (Cl−) ions, which diffuse to the perovskite precursor solution. The presence of Cl− ions on the SnOX surface was confirmed by the XPS measurement (Figure S4). From the SSPL, we also observed a higher PL intensity for the FASnI3 film deposited on SnOX as compared with the FASnI3 film on PEDOT:PSS. A PL quenching occurs when the absorber layer (like FASnI3) is placed on top of the charge transport layer (CTL) (either electron transport layer (ETL) or hole transport layer (HTL)) because the charge carrier is involved in the transportation process. The higher PL quenching means the more efficient charge extraction from the absorber layer to the CTL with a more favorable energy level matching between FASnI3 and HTL.[32] As the energy level of PEDOT:PSS is more aligned with FASnI3, it experienced a higher PL quenching effect. The effects of bottom layers such as PEDOT:PSS and SnOX on the growth of FASnI3 film were investigated. The scanning electron microscopy (SEM) results reveal that the FASnI3 deposited on SnOX has larger grain sizes than PEDOT:PSS. The average grain size of FASnI3 films on PEDOT:PSS was about 300 nm (Figure 2b), whereas the FASnI3 films on SnOX was about 900 nm (Figure 2c). This may be due to the presence of Cl− ions in the SnOX films (Figure S4) that form an intermediate perovskite phase. The Cl− ions are also well-known for kinetic control over perovskite formation. This phase later acts as a seed and guides the growth of a larger FASnI3 grain structure.[33] During spin coating of the perovskite film on PEDOT:PSS, we observed that the perovskite film became dark brown just after anti-solvent dripping. On the other hand, the perovskite film on SnOX remained orange in color after anti-solvent dripping (Figure S5), indicating the formation of the intermediate phase. These results suggest that the perovskite forms immediately after anti-solvent dripping on PEDOT:PSS substrate, whereas an intermediate perovskite phase forms on SnOX substrate. The formation of larger grain structures is needed for the reduction of grain boundary defects by reducing grain boundaries.[34] The X-ray diffraction (XRD) patterns of FASnI3 films deposited on PEDOT:PSS and SnOX are shown in Figure 2d. The analogous XRD peaks were predominantly identified at 14.0°, 24.4°, 28.1°, 31.6°, 40.4°, and 42.9°, corresponding to the (100), (102), (200), (122), (222), and (213) crystal planes of the orthorhombic FASnI3 perovskite phase, respectively.[35] Specifically, the FASnI3 film deposited on SnOX exhibits a noticeable enhancement in the intensity for the (100) and (200) planes compared to the FASnI3 film on PEDOT:PSS. This result suggests that the FASnI3 film on SnOX experiences a preferred orientation along the (h00) crystallographic plane direction. This may be due to the presence of Cl− ions in SnOX, which diffuses to the perovskite precursor solution and forms an intermediate template phase that dictates the growth of crystalline FASnI3 film.[33]According to the report, the SnOX with mixed valence states of Sn2+ and Sn4+ functions as HTM, where the hole from perovskite is collected by mid-gap state.[26,29] To investigate how SnOX as HTM affects the photovoltaic performance, inverted planar PSCs with a structure of ITO/PEDOT:PSS/FASnI3/C60/BCP/Ag were fabricated (Figure S6). The current density vs. voltage (J-V) curves of the highest-achieving PSC based on PEDOT:PSS and SnOX under 1 sun in a forward bias scan are shown in Figure 3a, and the relevant photovoltaic performance parameters are summarized in Table 1. In both forward and reverse scans, the J-V curves and accompanying parameters of PEDOT:PSS and SnOX-based Sn-PSCs are displayed in Figure S7 and Table S1, respectively. The optimum Sn-PSC based on PEDOT:PSS has a PCE of 12.01% with short circuit current (JSC) = 22.25 mAcm−2, open circuit voltage (VOC) = 0.71 V, and fill factor (FF) = 76 %. However, a comparable PCE of 11.11% was obtained for SnOX-based Sn-PSC with JSC = 21.46 mAcm−2, VOC = 0.69 V, and FF = 75 %. This result indicates the suitability of SnOX as HTM for Sn-based PSCs. For every batch, a total of nineteen cells were recorded. When compared to devices based on PEDOT:PSS, those based on SnOX demonstrated significantly higher reproducibility (Figure 3b).However, as compared with PEDOT:PSS-based PSCs, SnOX-based PSCs experienced a lower VOC. This may be due to an energy level mismatch between SnOX and FASnI3 (Figure 1f). From the SSPL measurement (Figure 2a), we also observed a lower PL quenching for SnOX/FASnI3 film, indicating lower charge extraction from the FASnI3 to SnOX. Consequently, the SnOX/FASnI3 interface underwent increased charge recombination, resulting in diminished photovoltaic performance. The incident photon-to-electron conversion efficiency (IPCE) spectra of SnOX and PEDOT:PSS-based Sn-PSCs and the corresponding integrated photocurrent density curves are displayed in Figure 3c. The integrated JSC values of 20.98 mAcm-2 for the SnOX device and 21.54 mAcm-2 for the PEDOT:PSS device computed from IPCE data are in good agreement with those from J-V curves. The IPCE spectra of SnOX-based Sn-PSC experienced a blue shift as compared with the PEDOT:PSS-based Sn-PSC at a higher wavelength (near 900 nm). This is due to the inclusion of Cl− ions from SnOX into the perovskite precursor solution. This result is consistent with the UV-vis (Figure S3) and SSPL measurements (Figure 2a) of FASnI3 films deposited on SnOX and PEDOT:PSS. Here we also observed a blue shift in bandgap for FASnI3 film on SnOX as compared with FASnI3 film on PEDOT:PSS. To observe the bottom layer’s (such as PEDOT:PSS and SnOX) effects at the HTL/Sn-perovskite interface, the cross-sectional images of PEDOT:PSS and SnOX-based Sn-PSCs were captured  (Figure 3d). In both devices, Sn-perovskite formed a single-grain structure across the cross-section. In the case of PEDOT:PSS-based PSCs, the Sn-perovskite film experienced some interfacial damage (marked by the red rectangle). This may be due to the reaction between acidic PEDOT:PSS and Sn-perovskite during the annealing time of the perovskite film deposition process.[20] On the other hand, no interfacial damage could be observed for the Sn-perovskite in the SnOX-based Sn-PSC. This is needed for stable Sn-PSCs.   Figure 3. (a) J-V curves, (b) Statistical distribution of PCE of the PSCs using the PEDOT:PSS and SnOX HTM and (c) IPCE spectra of PEDOT:PSS and SnOX-based Sn-PSCs, and (d) Cross-sectional SEM images of PEDOT:PSS and SnOX-based Sn-PSCs. Table 1. Photovoltaic parameters obtained from PEDOT:PSS and SnOX  HTM based PSCs. HTM VOC (V) JSC (mAcm−2) FF (%) PCE (%) PEDOT:PSS 0.71 22.25 0.76 12.01 SnOX 0.69 21.46 0.75 11.11The primary motivation for finding an appropriate inorganic HTM is to address the long-term instability, which is the most significant challenge for Sn-based PSCs. Figure 4a demonstrates the stability of PEDOT:PSS and SnOX-based Sn-PSCs under MPPT conditions. The PEDOT:PSS-based Sn-PSC experienced a fast degradation and became below 80% of its initial PCE. However, the SnOX-based Sn-PSC maintained about 90% of its initial PCE even after 1000 h of identical conditions. The factor supposed to be responsible for this performance degradation for PEDOT:PSS-based Sn-PSC was due to the acidic nature of PEDOT:PSS that causes oxidation of iodide in perovskites, resulting in the formation of iodine (I2). This process creates free Sn2+ ions, and the free Sn2+ ions are susceptible to Sn4+ oxidation. Figures 4b and 4c show the absorbance spectra of FASnI3 films deposited on PEDOT:PSS  and SnOX before and after annealing the film at 85 °C for 72 h inside the N2-filled glovebox. The Sn-perovskite film deposited on PEDOT:PSS exhibited absorbance spectra corresponding to  I2 and SnI4 before and after thermal stress (Figure 4b).[20] This indicates the presence of I2 and SnI4 in the FASnI3 film. However, from the SEM cross-sectional view, we already observed some morphological damage (marked by the red rectangle) at the PEDOT:PSS/Sn-perovskite interface (Figure 3d). On the other hand, the absorbance spectra for the FASnI3 film deposited on SnOX remain unchanged even after 72 h of heating at 85 °C inside the glovebox (Figure 4c). This result indicates the robustness of Sn-perovskite on SnOX film. That’s why the SnOX-based Sn-PSCs showed much more stable device performance under MPPT conditions.Figure 4. (a) Normalized operational stability of PEDOT:PSS and SnOX-based encapsulated devices under MPPT condition. UV-vis spectra of FASnI3 films deposited on (b) PEDOT:PSS and (c) SnOX films, measured fresh and after 72 h at 85 °C conditions, respectively.3. ConclusionIn this work, we synthesized SnOX nanocrystals by reducing SnCl4.5H2O solution through Sn0 powder to replace acidic PEDOT:PSS for Sn-PSCs. The synproportionation reaction between Sn4+ and Sn0 reduced the Sn4+ content up to 38% and elevated the HOMO level to −5.70 eV, close to the HOMO level of PEDOT:PSS. Sn-perovskite on SnOX shows a larger grain structure and high crystallinity. Moreover, the utilization of SnOX as a replacement for acidic PEDOT:PSS offers a solution for the degradation problem at the HTM/perovskite interface. As a result, the SnOX-based PSCs retained 90% of their initial efficiency under MPPT conditions for 1000 h. This finding provides a pathway for the use of robust inorganic SnOX as HTM for stable Sn-PSCs.Experimental Section4.1. Materials: All chemicals were utilized in their original state without supplementary purification procedures. The experimental materials comprised formamidinium iodide (FAI, >98%), phenethylammonium iodide (PEAI, >97%), tin(IV) chloride (SnCl4·5H2O), bathocuproine (BCP), bithiophene (>98%), and anhydrous solvents including 2-propanol, chlorobenzene (CB), and dimethyl sulfoxide (DMSO), sourced from Tokyo Chemical Industry Co., Japan. Furthermore, tin (II) fluoride (SnF2, >99%), tin (II) iodide (SnI2, 99.99%), metallic tin (Sn0) and fullerene-C60 from Sigma–Aldrich were employed. The PEDOT:PSS (Clevious PVP Al 4083) was sourced from Germany and Taiwan, while ethylenediammonium diiodide (EDAI2) was procured from Merck. For SnO2, 15 wt% SnO2 in water was brought from Alfa-Aesar.4.2. SnOX synthesis: The SnOX nanocrystals were synthesized by dissolving 1 mmol of SnCl4·5H2O in distilled water and adding Sn0 at different molar ratios of Sn0: SnCl4.5H2O such as 1:16, 1:8, 1:4, and 1:2. In this sequence, first SnCl4·5H2O was completely dissolved in distilled water. Then Sn0 was added under vigorous stirring and the bottle cap was closed. After that, the required transparent light-yellow colored SnOX nanocrystal solution was formed.4.3. Fabrication of SnOX and PEDOT:PSS film: An indium-doped tin oxide (ITO) conductive glass substrate was utilized as the front electrode in PSCs. ITO substrates were subjected to a 30 min cleaning process in a sonication system utilizing a detergent solution, deionized water, acetone, and a mixed solution of ethanol and isopropanol. Before the deposition of HTL, the ITO substrates underwent a 30 min treatment with UV-O3. SnOX was spin-coated on ITO at 3000 rpm for 20 s for HTM. After spin coating, the substrates were quickly transferred to the N2-filled glovebox and were subjected to annealing at 100 °C for 20 min to avoid oxidation. For PEDOT:PSS films, PEDOT:PSS was dropped on the substrate and spin-coated at 4000 rpm for 30 s. The PEDOT:PSS films were annealed at 150 °C for 20 min. After that, they were transferred to the glovebox for device fabrication.4.4. Fabrication of FASnI3 film: The FASnI3 precursor solution was prepared by mixing EDAI2, PEAI, SnF2, SnI2, and FAI in the following ratio: 0.01:0.05:0.1:1:0.94. After that, DMSO was added and stirred for 1 h at room temperature to make a 0.9 M precursor solution. The precursor solution was dropped and spin-coated for 12 and 48 s at 1000 and 5000 rpm, respectively. At the 39th s, 160 μL of CB was added as an antisolvent. First, the substrate was heated to 65 °C for a few seconds. Then, it was heated to 100 °C for an additional 12 min.4.5. PSCs fabrication: After depositing the HTL, the perovskite and other subsequent layers were fabricated inside the glovebox. The perovskite layer was deposited as mentioned in the ‘Fabrication of FASnI3 film’ part. The C60 was thermally evaporated at 1 Å/s for 30 nm, and BCP was 1 Å/s for 7 nm. Finally, 100 nm of Ag was thermally evaporated using a rate of 0.5 Å/s for 5 nm, 1 Å/s for 10 nm, and 1.5 Å/s for the rest of the thickness.4.6. Characterization: The X-ray diffraction (XRD) analysis was done with a Rigaku RINT-2500 powder X-ray diffractometer that used Cu Kα radiation. We used a Hitachi SU-8000 SEM with a 2 kV acceleration voltage for film morphology and device cross-section. The absorbance and transmittance were measured with a Shimadzu UV-visible 3600 spectrophotometer. An ultrahigh vacuum of 5×10-11 mbar, a Mg Kα source (hν = 1486.7 eV) was used to do X-ray photoelectron spectroscopy (XPS) in a PHI Quantera SXM (ULVAC-PHI). ULVAC-PHI (PHI VersaProbe 4) and an Al Kα radiation source were utilized to measure ultraviolet photoemission spectroscopy (UPS). PL measurement was conducted in the Hamamatsu C12132 fluorescence lifetime spectrometer. A solar simulator that produces standard 1 sun (100 mWcm−2, WXS-155S-10: Wacom Denso Co., Japan) was used to test the relationship between current and voltage. The CEP-2000BX device from Bunkoukeiki Co., LTD. was used to measure the IPCE. The dark J-V characteristics of PSCs were carried out in PAIOS.Acknowledgements:A.I. would like to thank JSPS KAKENHI grant number 22H02190 for the financial support. A.I also conveys gratitude for the funding provided by JST-Mirai Program Grant Number JPMJMI21E6 and JST-ALCA-Next Program Grant Number JPMJAN23B2 in Japan. 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