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. 2025 Feb 28;17(10):15613–15620. doi: 10.1021/acsami.4c21442

Improved Efficiency and Stability in Inverted-Structure Solar Cells with Lead-Free All-Inorganic Halide Perovskite CsSn1–xZnxBr3

Hanbo Jung †, Zihao Liu †, Masato Sotome †,‡,*, Kazuteru Nonomura §, Hiroshi Segawa §, Gaurav Kapil ∥, Shuzi Hayase ∥, Takashi Kondo †,‡,*
PMCID: PMC11913019  PMID: 40021132

Abstract

graphic file with name am4c21442_0007.jpg

We report the fabrication of an inverted structure solar cell with all-inorganic lead-free perovskite CsSn1–xZnxBr3 alloy thin films grown by physical vapor codeposition of CsBr, SnBr2, and ZnBr2. It was found that the deposited CsSn1–xZnxBr3 perovskite alloy thin films exhibited improved morphological characteristics (larger grain sizes, lower pinhole density, and improved flatness) compared to the CsSnBr3 thin film. The incorporation of 4% Zn (CsSn0.96Zn0.04Br3, abbreviated as 4Zn) resulted in a bandgap narrowing of ∼20 meV compared to CsSnBr3 with the upshift of the valence band maximum and conduction band minimum of ∼0.3 eV. The inverted-structure perovskite solar cells (PSCs) (ITO/PEDOT:PSS/4Zn/C60/BCP/Ag) exhibit improved energy level alignment with the transport layers of C60 and PEDOT:PSS. The 4Zn solar cell showed an open-circuit voltage (VOC: 0.35 V), short-circuit current density (JSC: 13.99 mA/cm2), and fill factor (FF: 54%), yielding a power conversion efficiency (PCE) of 2.59%. The Zn-alloyed PSCs were more efficient and stable than the pure CsSnBr3 PSCs under ambient air conditions. The 4% Zn device preserved 96% of VOC, 86% of JSC, 91% of FF, and 83% of the initial PCE after preservation for 6 days under 60% humidity at 25 °C. This result offers a potential strategy for the fabrication of air-stable all-inorganic lead-free PSCs.

Keywords: inverted structure, physical vapor codeposition, lead-free perovskite solar cells, Zn-alloy, air-stable

1. Introduction

Metal halide perovskites (MHP) featuring the chemical composition ABX3 (A: methylammonium (MA+), formamidinium (FA+), or Cs+; B: Pb2+ or Sn2+; and X: I–, Br–, or Cl–) have attracted considerable research interest since the advent of MAPbI3 solar cells.1−7 These materials exhibit favorable characteristics for photovoltaic applications, including tunable bandgap, extended carrier lifetime, and high photoabsorption coefficient.8−11 Nevertheless, the practical implementation of MHP solar cells has been impeded by the toxicity associated with lead (Pb).8,12,13 To address the toxicity issue, researchers have explored substituting the B-site Pb-ion with a tin (Sn)-ion.14−16 Recent studies on Sn-based MHP solar cells have demonstrated promising results. For instance, a FASnI3 normal-structure solar cell achieved a PCE of up to 15%.17,18 Given that MHPs incorporating organic cations often suffer from poor thermal stability due to the volatile nature of these cations, there has been a growing interest in exploring all-inorganic perovskites such as CsSnX3 (where X = Cl, Br, I).19 However, it is worth noting that CsSnCl3 and CsSnI3 are susceptible to degradation in ambient air due to the oxidation of the divalent (Sn2+) to tetravalent (Sn4+) cation.19 In contrast, CsSnBr3 exhibits higher stability against both moisture and heat,20 and its bandgap (Eg = 1.8 eV) renders it suitable for tandem solar cell applications alongside silicon solar cells.21

However, CsSnBr3 faces intrinsic issues due to deep-level defects like BrSn and BrCs, which have low formation energies and act as nonradiative recombination centers within the perovskite lattice.22 Compared to other Sn-based perovskites such as CsSnI3 and FASnI3, CsSnBr3 has much shorter minority carrier lifetimes.23 The combination of these defects and the reduced carrier lifetime increases recombination losses, thereby limiting the overall efficiency of CsSnBr3 solar cells. Additionally, the fabrication of CsSnBr3 thin films often results in the formation of undesirable byproducts such as Cs2SnBr6 (Cs-rich) and CsSn2Br5 (Sn-rich). Oxidation of Sn2+ to Sn4+ leads to Cs2SnBr6 with a vacancy-ordered double perovskite structure with a large bandgap of 3.2 eV, making it ineffective for solar cell applications.24 An excess of Sn ions leads to the formation of CsSn2Br5 with a nonperovskite structure. It is therefore essential to minimize the presence of these byproducts to make high-quality pure CsSnBr3 thin films.

MHP thin films can be fabricated by a variety of methods, including spin coating, chemical vapor deposition (CVD), and sequential vapor deposition or physical vapor deposition (PVD).25−27 However, spin-coated Sn MHP thin films suffer from rough surfaces and microscopic pinholes caused by their rapid crystallization.26 For CVD and sequential vapor deposition,26,27 impurity peaks corresponding to Cs2SnBr6 and Cs2SnBr5 were detected by X-ray diffraction, indicating poor crystallinity of the thin films. These results indicate that it may be challenging to achieve high-quality CsSnBr3 thin films solely under thermal equilibrium conditions. This viewpoint suggests that the coevaporation of CsBr and SnBr2 can be prevented by controlling the growth speed precisely, as demonstrated in our previous paper.28 Furthermore, codeposition has an advantage in compositional engineering. It enables the introduction of new elements into MHP films through the codeposition of its compounds.

A number of studies have demonstrated the potential for compositional engineering to enhance the morphology of Sn-based perovskites.29−34 The performance of solar cells can be enhanced through the incorporation of additional elements, improving the crystallinity of the material during the cocrystallization process. Among the metal ions, zinc has been demonstrated to enhance the environmental stability of CsSnI3 films as a reducing agent35 and to increase the grain size with fewer pinholes in MAPbI3.36 An attempt was made to add zinc (Zn) ions into CsSnI3 perovskite lattices using a galvanic displacement reaction.35 The Zn powder was found to behave as a reductant, resulting in an improvement in PCE and stability. In the case of Zn added to Pb-based perovskite solar cells, superior long-term stability was confirmed. For example, after 30 days of storage at 25 °C under ambient air conditions, the CsPb0.99Zn0.01IBr2 PSCs maintained 91% of their initial efficiency, whereas the pure CsPbIBr2 PSCs retained only 68% of their initial efficiency.37 Additionally, the PCE of Zn-added MAPbI3 devices remained at 96% of the initial values after 80 days, compared to only 60% for the pristine MAPbI3 samples.38 However, to the best of our knowledge, no study has fabricated CsSnBr3 thin films mixed with other elements.

We have reported the incorporation of Zn ions into the B-site to form a new alloy, CsSn1–xZnxBr3.28 CsSn1–xZnxBr3 MHPs have improvements in the crystal and optical properties within the range of 0 ≤ x ≤ 0.04.28 In this study, we fabricated an inverted structure solar cell (ITO/PEDOT:PSS/CsSn1–xZnxBr3/C60/BCP/Ag). Since the Zn-alloyed MHP layer has smoother interfaces than pure CsSnBr3, we expected reduced charge loss and improved charge collection. Additionally, the Zn-alloyed devices were expected to improve long-term stability in ambient air by increasing resistance to Sn2+ oxidation.28

The inverted structure is more promising for applications in tandem solar cells with silicon solar cells (SCs). However, the normal structure was used for CsSnBr3 SCs in previous studies with TiO2 (ETL) and spiro-OMeTAD (HTL). The electron transport layer should have a deep conduction band minimum (CBM) for optimal electron transport with CsSnBr3 since it has a deep CBM. However, the CBM of TiO2 is too shallow to efficiently transport electrons from spin-coated CsSnBr3, resulting in suboptimal solar cell performance.39

2. Experimental Methods

2.1. Materials

All of the reagents are used without further purification from the manufacturers: CsBr (99.0%, Tokyo Chemical Industry Co., Ltd.), SnBr2 (99.2%, Alfa Aesar), ZnBr2 (99.9%, Fujifilm Wako Pure Chemical Co., Ltd.), SnF2 (99%, Sigma-Aldrich), dimethyl sulfoxide (DMSO, 99.9%, Sigma-Aldrich), PEDOT:PSS (Clevios P VP AI 4083), C60 (99.9%, Sigma-Aldrich), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, 99.9%, Sigma-Aldrich).

2.2. Solar Cell Fabrication

Schematics of the fabrication procedure of the inverted structure PSCs are demonstrated in Figure 1a. The inverted structure with PEDOT:PSS (HTL) and C60 (ETL) was expected to achieve balanced charge carrier (electron/hole) extraction and minimize the band offset between the ETL and the perovskite (PVK), and that between the hole transport layer (HTL) and PVK. We expected that this approach would improve the performance of CsSnBr3 PSCs. The 0.7 mm-thick alkali-free glass substrates with ITO layer (150 nm-thick, surface Ra 1.2 nm, 159 μΩ·cm) were cleaned with deionized water, methanol, and acetone in an ultrasonic bath for 15 min in each liquid. The as-cleaned ITO substrates were treated by UV-oz at 1 atm for 15 min. PEDOT:PSS was spin-coated on the ITO substrate with 500 rpm for 5 s followed by 4000 rpm for 30 s, and annealed at 140 °C in the ambient atmosphere for 30 min. The 300 nm-thick CsSn1–xZnxBr3 thin films with x = 0, 0.01, 0.02, 0.04 (abbreviated as 0Zn, 1Zn, 2Zn, 4Zn, respectively), were vacuum deposited on the PEDOT:PSS surface by coevaporation of CsBr, SnBr2, and ZnBr2, and the substrate temperature during growth (Tsub) was fixed at 80 °C, as in our previous paper (Figure 1a).28 After the deposition of CsSn1–xZnxBr3 thin films, samples were annealed at 100 °C for 30 min in a nitrogen-filled glovebox (oxygen concentration <1 ppm, dew point lesser than −60 °C). Subsequently, thin layers of C60 (rate: 0.15 Å/s, thickness: 20 nm), BCP (0.15 Å/s, 7 nm), and Ag (1.7 Å/s, 100 nm) were consecutively deposited by using vacuum thermal evaporation techniques. Schematics of inverted structure solar cells with the structure of glass\ITO\PEDOT:PSS\CsSn1–xZnxBr3\C60\BCP\Ag is exhibited in Figure 1b.

Figure 1.

Figure 1

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(a) Schematic fabrication procedure of the inverted perovskite solar cell devices (ITO/PEDOT:PSS/CsSn1–xZnxBr3/C60/BCP/Ag) and (b) schematic structure of the solar cells.

2.3. Characterization

The solar cell measurements were performed using a solar simulator (PEC-L01, Peccell Technologies, AM1.5G, 100 mW/cm2) under a nitrogen atmosphere with a scanning rate of 0.1 V/s. The measured area was restricted to the area with the Ag electrodes (3 mm × 3 mm) using a nonreflective black metal mask. The EQE spectra were recorded using a monochromatic Xenon lamp (Bunkoukeiki Co., Ltd., CEP-2000SRR). The EQE spectrum was recorded with an EQE system (CEP-2000MLQ, Bunkoukeiki Co., Ltd.) in DC mode without any voltage bias. Valence band and DOS were measured by photoelectron yield spectroscopy (PYS) using a Bunkoukeiki KV 205-HK ionization energy system with photon energy ranging from 5.2 to 6.7 eV. Cross-sectional SEM images were taken by a scanning electron microscope (Hitachi, SEM5200S). X-ray photoelectron spectroscopy (XPS) sputtering was performed using a Versa Probe III (Sputter source: Ar 3 kV; Excitation source: Al Kα; pass energy: 112 eV).

3. Results and Discussion

Figure 2a shows the ionization energy spectrum of CsSn1–xZnxBr3 thin films, evaluated by PYS. The ionization energy decreases with increasing Zn ion content: pure CsSnBr3 (0Zn) (6.15 eV), 1Zn (6.10 eV), 2Zn (5.95 eV), and 4Zn (5.82 eV). The conduction band minimum (CBM) and valence band maximum (VBM) positions were confirmed using the ionization energy and the previously reported Eg from UV–vis spectroscopy: 0Zn (1.81 eV), 1Zn (1.81 eV), 2Zn (1.80 eV), and 4Zn (1.79 eV).28Figure 2b also illustrates the energy diagram of a glass/ITO/PEDOT:PSS/perovskite/C60/BCP/Ag solar cell. The energy levels of the carrier transport layers and electrodes were evaluated by PYS. In MHP-SC, the energetic positions of the HTL, perovskite layer, and ETL significantly impact device performance.39 Efficient movement of electrons (holes) across the interface is facilitated by minimizing the energy difference between the perovskite layer and the ETL (HTL). Although well-aligned energy levels promote efficient charge extraction in perovskite solar cells,40,41 a large energy gap (∼0.85 eV) was observed at the interface between pure CsSnBr3 and the HTL, indicating a poor energy level arrangement and a higher Schottky barrier at the HTL/perovskite interface. In the pure CsSnBr3 device, the band offset difference was unbalanced (0.59 eV): perovskite/ETL (0.26 eV) and perovskite/HTL (0.85 eV). In contrast, the 4Zn device showed improved energy alignment, reducing the band offset difference (0.05 eV): 4Zn/ETL (0.57 eV) and 4Zn/HTL (0.52 eV). The CBM and VBM shifted upward by 0.31 and 0.33 eV, respectively, contributing to the better alignment. Optimal energy alignment between the perovskite layer and the transport layers is essential for achieving efficient charge balance.42 The efficient band alignment in the 4Zn device was expected to contribute to better charge extraction efficiency and enhanced solar cell performance.

Figure 2.

Figure 2

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(a) PYS of the CsSnBr3 and Zn-based perovskite layers and (b) corresponding energy diagram of the solar cell devices.

In MHP-SCs, the quality of the perovskite crystals is crucial for optimal performance. The crystal grains should be large and densely packed with smooth connections with the ETL and HTL. Thus, we examined cross-sectional SEM images of each device to evaluate the bonding quality between the perovskite, ETL, and HTL layers (Figure 3a–d). We observed no degradation, cracks, or voids in all samples. However, the pure CsSnBr3 SC displayed a rough interface between the perovskite and C60/BCP layers caused by the nonuniform grain size distribution within the perovskite layer (Figure 3a). The Zn-alloyed SCs (1Zn and 2Zn) showed improved interface smoothness, although some grains did not exhibit perfect connectivity between the ETL and HTL. In contrast, the 4Zn device presented images showing flat interfaces and continuous connections of perovskite grains between the charge transport layers. The 4Zn sample exhibited superior crystallinity and flatness. The enhanced structural integrity was expected to be beneficial for the charge extraction efficiency and reduction of voltage losses. Figure S1a,b shows AFM images of pure CsSnBr3 and 4Zn thin films, revealing differences in surface topography. Figure S1c,d presents SEM surface images of pure CsSnBr3 and 4Zn films, highlighting improved grain size and reduced pinhole density in the 4Zn sample (see the Supporting Information). Although the RMS roughness increased slightly from pure (32.7 nm) to 4Zn (41.3 nm), the normalized RMS/grain size decreased sharply from pure (1.00) to 4Zn (0.429), indicating a smoother surface relative to grain size. These improvements contribute to better film crystallinity and enhanced device performance.

Figure 3.

Figure 3

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Cross-sectional SEM images of (a) 0Zn (pure CsSnBr3), (b) 1Zn, (c) 2Zn, and (d) 4Zn SCs.

Figure 4a shows the J–V characteristics of the champion perovskite solar cells for each Zn composition. The details of the photovoltaic parameters are depicted in Table 1. As the Zn content increased, there was a notable improvement in the open-circuit voltage (VOC). Although the short-circuit current density (JSC) initially decreased at 1Zn, JSC nearly doubled at 2Zn and 4Zn compared to the pure CsSnBr3 SC. The lower JSC observed in the 1Zn sample (7.31 mA/cm2) compared to the pure sample (8.96 mA/cm2) can be attributed to residual pinholes, as reported in our previous work.28 While Zn substitution improves grain size, the 1Zn sample still exhibits suboptimal film morphology, particularly the presence of pinholes, which hinders charge transport and collection efficiency. This aligns with our earlier findings, where pinhole-related defects were shown to adversely impact JSC. The 4Zn device showed improvements in VOC (0.35 V), JSC (13.99 mA/cm2), FF (54%), and PCE (2.59%). This PCE value was higher than that of the pure CsSnBr3 device (0.76%) and comparable to that of the champion spin-coated CsSnBr3 devices (3.04%).43VOC is determined by the bandgap (Eg) and nonradiative recombination losses, as described by44

3. 1

Figure 4.

Figure 4

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(a) J–V curves of the best sample in each batch, (b) EQE spectra, and (c) VOC versus light intensity of SCs.

Table 1. Photovoltaic Parameters of PSCs.

Sample VOC (V) JSC (mA/cm2) FF (%) PCE (%)
0Zn 0.17 8.96 52 0.76
1Zn 0.31 7.13 49 1.08
2Zn 0.34 13.86 50 2.29
4Zn 0.35 13.99 54 2.59
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Here, q is the elementary charge, kB is the Boltzmann constant, T is the absolute temperature, ni is the intrinsic carrier concentration, NC and NV are the effective density of states in the conduction and valence bands, respectively, and ΔVOC,loss accounts for recombination losses. The 4Zn sample exhibited higher VOC, attributed to reduced trap density and improved energy alignment with the transport layers, which lowers recombination losses.

JSC is influenced by the absorption of photons and is expressed as45

3. 2

Here, α(E) is the absorption coefficient, and Φsolar(E) represents the photon flux. The improved band alignment in the 4Zn device reduces the band offset difference (from pure: 0.59 eV to 4Zn: 0.05 eV), which facilitates efficient charge carrier extraction and reduces recombination losses. The band gap narrowing in the 4Zn sample allows for better absorption of lower-energy photons, increasing JSC. Together, the reduced trap density, improved band alignment, and band gap narrowing contribute to the significant enhancement in both VOC and JSC.

While spin-coated pure CsSnBr3 devices have been reported to exhibit PCEs as low as 0.01%,39 our pure CsSnBr3 devices achieved PCEs of 0.76%, which suggests that our vapor codeposited CsSnBr3 thin films have better crystallinities.28 In order to investigate the potential benefits of SnF2 additives in enhancing the film quality, we fabricated 10 mol % and 20 mol % SnF2-added CsSnBr3 thin films through the codeposition of CsBr, SnBr2, and SnF2. AFM images of CsSnBr3 thin films with 10 and 20 mol % SnF2 both exhibited poorer morphology than pure CsSnBr3 (Figure S2a,b). Additionally, the J–V curves of the samples showed an extremely low PCE (0.01%) due to the poor thin-film quality (Figure S2c,d). Additionally, 6Zn and 8Zn SCs under various Tsub conditions were fabricated, and their J–V curves were measured (Figure S3a,b). Among them, the 6Zn sample at Tsub = 180 °C (6Zn, 180 °C) showed the highest PCE (1.68%) with VOC (0.19 V), JSC (13.09 mA/cm2), and FF (47%). However, this performance was still lower than that of the 4Zn SCs. We selected 6Zn and 8Zn samples based on theoretical predictions that Zn can substitute Sn sites in CsSnI3 up to 10%.35 Given that CsSnBr3 may allow even higher Zn incorporation, we explored whether increasing Zn beyond 4% could enhance the performance. However, despite increasing the substrate temperature to promote further Zn incorporation, performance did not improve, highlighting the challenge of exceeding 4Zn.

We conducted EQE characterization (Figure 4b) to evaluate the device’s efficiency in converting incident photons into electrical current. The integrated JSC values closely matched those obtained from the J–V curves. The 2Zn and 4Zn devices displayed enhanced spectral responses. The superior crystal quality of the Zn-alloyed devices is reflected in the better light absorption of the 2Zn and 4Zn devices in the visible range.28 Moreover, the Zn-alloyed SCs showed improved EQE performance both at their absorption edge and in the short-wavelength range, suggesting the reduction of surface and bulk defect densities. In MHP-SC, the ideality factor (n) is a crucial parameter that indicates how closely the device follows ideal diode behavior. By examining the dependence of JSC, J0, and VOC on light intensity (L), we extracted n using the relationship:35

3. 3

An ideal diode with minimal charge carrier recombination has n = 1, while n approaches 2 in the case of significant carrier recombination process. Under the conditions of JSC ≫ J0 and JSC ∝ L, we can evaluate n from the slope of the VOC versus the natural logarithm of light intensity (lnL) as shown in Figure 4c. The experimental data were well fitted by the model for all samples with n: 0Zn (1.67) > 2Zn (1.45) > 1Zn (1.39) > 4Zn (1.28). An ideality factor of 1.28 indicates efficient charge collection with minimal nonideal losses, even in low-light environments. This result demonstrates that the introduction of Zn ions effectively suppresses trap-assisted charge recombination. Incorporating Zn ions significantly enhances the fill factor (FF), VOC, and JSC by reducing defect densities and improving the crystallinity of the perovskite layer.

Figure 5a–d shows the box plots of (a) PCE, (b) VOC, (c) JSC, and (d) FF of the MHP-SCs. Almost all the PCE data were in the range of ±0.2%. JSC values of 2Zn and 4Zn were almost twice those of 0Zn and 1Zn, in good agreement with their EQE spectra. These results suggest that the trapping of light carriers was reduced by Zn ions. FF remained constant at 55% across all compositions, indicating potential for improvement through optimization of thickness. To further improve the FF (54%) and VOC (0.35 V), optimizing the interface between the PEDOT:PSS and perovskite layers may be necessary.

Figure 5.

Figure 5

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Box plots of (a) PCE, (b) VOC, (c) JSC, and (d) FF of 0Zn, 1Zn, 2Zn, and 4Zn SCs.

To evaluate long-term stability, the unsealed devices were tested at 25 °C and 60% humidity. Figure 6a–d shows the changes over time in normalized PCE, VOC, JSC, and FF, respectively. Each J–V measurement was performed quickly in ambient air, and the devices were kept indoors between tests. After 6 days, 0Zn, 1Zn, and 2Zn devices maintained 13%, 26%, and 77% of their initial PCE, respectively. In contrast, the 4Zn device maintained 96% of VOC, 86% of JSC, 91% of FF, and 83% of PCE. This stability is better than previously reported CsSnBr3 solar cells degrading within 24 h in ambient air.43,46 These results show that adding Zn ions improves the air stability of the MHP-SC.

Figure 6.

Figure 6

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Time evolution of the normalized (a) PCE, (b) VOC, (c) JSC, and (d) FF of the SCs.

A noticeable decrease in VOC for 0Zn and 1Zn devices on the fifth and sixth days suggests the oxidation of Sn2+ ions. The stable VOC in 2Zn and 4Zn devices until the sixth day indicates that Zn incorporation effectively prevents Sn2+ oxidation. A sharp decrease in JSC was seen in the pure and 1Zn devices on the second day. This may be attributed to the migration of Br– ions toward the Ag electrode, resulting in the formation of silver bromide (AgBr). The AgBr layer, regarded as an insulator, likely hinders efficient charge extraction, thereby diminishing JSC and degrading the overall device performance.47 While Zn ions provide some stability, the 1Zn device still shows a significant drop in JSC, indicating that low levels of Zn are insufficient to fully stabilize the perovskite structure against environmental degradation. For Sn-based perovskite solar cells, thermal instability in ambient air is mainly caused by Sn vacancy defects formed by Sn2+ oxidation.39 The suppression of Sn2+ oxidation in Zn mixed crystals was observed in XPS of the bare perovskite films in our previous work.28 The high stability of the 4Zn device indicates that the Zn alloy can prevent the oxidation of Sn2+, and it improves the stability of the perovskite layer.

To understand the higher long-term stability of the 4Zn sample, XPS depth profiling was performed to analyze the material composition at different depths. This method provides insights into the distribution of Zn ions in CsSn1–xZnxBr3 perovskite solar cells. A 3 kV Ar ion beam was used for sputtering. The structure of the ITO/PEDOT:PSS/CsSn0.96Zn0.04Br3 thin film was characterized over 9 min with an etching rate of about 15 nm/min (Figure S4a). The XPS spectrum showed a peak at 1022.5 eV, corresponding to Zn 2p3/2 of CsSn1–xZnxBr3. No peak shift was observed after sputtering, indicating the uniform chemical state of Zn ions. On the depth profile of the normalized Zn 2p3/2 concentration (Figure S4b), the Zn peak intensity for the initial 0–3 min was 1.4 times higher than that from 4 to 9 min. This result indicates that Zn is more concentrated on the surface covering the Sn ions, with its concentration decreasing with depth and becoming constant from ∼40 nm (4 min) onward. These Zn ions may play a crucial role in the protection of the Sn ions from oxidation and enhancing the long-term stability.

4. Conclusions

We fabricated an ITO/PEDOT:PSS/CsSn1–xZnxBr3/C60/BCP/Ag solar cell and found improvements in their performance for the Zn-alloyed samples. Their cross-sectional SEM images confirmed that Zn-alloyed SCs have superior morphology with smoother interfaces. CsSn0.96Zn0.04Br3 (4Zn) was found to have a narrower bandgap (∼20 meV) compared to pure CsSnBr3 (0Zn) and an upshift of its VBM and CBM (∼30 meV). The energy shift minimized the band offsets: 4Zn/ETL (0.57 eV) and 4Zn/HTL (0.52 eV). The VOC dependence of the light intensity and EQE spectrum suggested improved crystallinity for Zn-alloyed SCs. An ideality factor of 1.28 demonstrates efficient charge collection with minimal losses, even under low light. This work highlights the potential of Zn-alloyed perovskites for low-power applications, such as remote controls, smart home devices, and wearables, where durability and reliability outweigh the need for ultrahigh efficiency. 4Zn-SC showed high stability in ambient air conditions, which could make it a strong candidate as an effective top cell with silicon in a tandem SC device. This research opens new avenues for the development of environmentally friendly and stable solar energy solutions.

Acknowledgments

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Part of this work was supported by the Nanotechnology Platform (Project No. 22UT0045) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. LZ and JH were supported by JST SPRING (Grant No. JPMJSP2108). TK and MS were supported by JSPS KAKENHI (Grant No. 22H01969).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.4c21442.

  • Surface morphology of pure CsSnBr3 and 4Zn thin films, AFM images and J–V curves of CsSnBr3 thin films with 10 mol % and 20 mol % SnF2, solar cell prepared with x = 0.06 and 0.08, depth distribution of Zn composition evaluated by XPS (PDF)

The authors declare no competing financial interest.

Supplementary Material

am4c21442_si_001.pdf (862.2KB, pdf)

References

  1. Kojima A.; Teshima K.; Shirai Y.; Miyasaka T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050–6051. 10.1021/ja809598r. [DOI] [PubMed] [Google Scholar]
  2. Lee M. M.; Teuscher J.; Miyasaka T.; Murakami T. N.; Snaith H. J. Efficient Hybrid Solar Cells Based on Meso-Super Structured Organometal Halide Perovskites. Science 2012, 338, 643–647. 10.1126/science.1228604. [DOI] [PubMed] [Google Scholar]
  3. Kim H.-S.; Lee C.-R.; Im J.-H.; Lee K.-B.; Moehl T.; Marchioro A.; Moon S.-J.; Humphry-Baker R.; Yum J.-H.; Moser J. E.; Grätzel M.; Park N.-G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2 (1), 591. 10.1038/srep00591. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Burschka J.; Pellet N.; Moon S.-J.; Humphry-Baker R.; Gao P.; Nazeeruddin M. K.; Grätzel M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499 (7458), 316–319. 10.1038/nature12340. [DOI] [PubMed] [Google Scholar]
  5. Jeon N.; Noh J.; Kim Y.; Yang W.; Ryu S.; Seok S. Solvent Engineering for High-Performance Inorganic–Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897–903. 10.1038/nmat4014. [DOI] [PubMed] [Google Scholar]
  6. Chen W.; Wu Y.; Yue Y.; Liu J.; Zhang W.; Yang X.; Chen H.; Bi E.; Ashraful I.; Grätzel M.; Han L. Efficient and Stable Large-Area Perovskite Solar Cells with Inorganic Charge Extraction Layers. Science 2015, 350, 944–948. 10.1126/science.aad1015. [DOI] [PubMed] [Google Scholar]
  7. Park J.; Kim J.; Yun H.; Paik M.; Noh E.; Mun H.; Kim M.; Shin T.; Seok S. Controlled Growth of Perovskite Layers with Volatile Alkylammonium Chlorides. Nature 2023, 616, 724–730. 10.1038/s41586-023-05825-y. [DOI] [PubMed] [Google Scholar]
  8. Zheng Y.; Yang S. Stabilization Techniques of Lead Halide Perovskite for Photovoltaic Applications. Sol. RRL 2022, 6 (1), 2100710. 10.1002/solr.202100710. [DOI] [Google Scholar]
  9. Grätzel M. The Light and Shade of Perovskite Solar Cells. Nat. Mater. 2014, 13, 838–842. 10.1038/nmat4065. [DOI] [PubMed] [Google Scholar]
  10. Green M. A.; Ho-Baillie A.; Snaith H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8 (7), 506–514. 10.1038/nphoton.2014.134. [DOI] [Google Scholar]
  11. Stranks S. D.; Eperon G. E.; Grancini G.; Menelaou C.; Alcocer M.; Leijtens T.; Herz L. M.; Petrozza A.; Snaith H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341–344. 10.1126/science.1243982. [DOI] [PubMed] [Google Scholar]
  12. Ren M.; Qian X.; Chen Y.; Wang T.; Zhao T. Potential Lead Toxicity and Leakage Issues on Lead Halide Perovskite Photovoltaics. J. Hazard. Mater. 2022, 426, 127848. 10.1016/j.jhazmat.2021.127848. [DOI] [PubMed] [Google Scholar]
  13. Ke W.; Kanatzidis M. Prospects for Low-Toxicity Lead-Free Perovskite Solar Cells. Nat. Commun. 2019, 10 (1), 965. 10.1038/s41467-019-08918-3. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Jiang X.; Zang Z.; Zhou Y.; Li H.; Wei Q.; Ning Z. Tin Halide Perovskite Solar Cells: An Emerging Thin-Film Photovoltaic Technology. Acc. Mater. Res. 2021, 2, 210–219. 10.1021/accountsmr.0c00111. [DOI] [Google Scholar]
  15. Kapil G.; Bessho T.; Sanehira Y.; Sahamir S. R.; Chen M.; Baranwal A. K.; Liu D.; Sono Y.; Hirotani D.; Nomura D.; Nishimura K.; Kamarudin M. A.; Shen Q.; Segawa H.; Hayase S. Tin–Lead Perovskite Solar Cells Fabricated on Hole Selective Monolayers. ACS Energy Lett. 2022, 7, 966–974. 10.1021/acsenergylett.1c02718. [DOI] [Google Scholar]
  16. Zhang Z.; Wang L.; Baranwal A. K.; Sahamir S. R.; Kapil G.; Sanehira Y.; Kamarudin M. A.; Nishimura K.; Ding C.; Liu D.; Li Y.; Li H.; Chen M.; Shen Q.; Ripolles T.; Bisquert J.; Hayase S. Enhanced Efficiency and Stability in Sn-Based Perovskite Solar Cells by Trimethylsilyl Halide Surface Passivation. J. Energy Chem. 2022, 71, 604–611. 10.1016/j.jechem.2022.03.028. [DOI] [Google Scholar]
  17. Wang L.; Chen M.; Yang S.; Uezono N.; Miao Q.; Kapil G.; Baranwal A. K.; Sanehira Y.; Wang D.; Liu D.; Ma T.; Ozawa K.; Sakurai T.; Zhang Z.; Shen Q.; Hayase S. SnOx as Bottom Hole Extraction Layer and Top in Situ Protection Layer Yields over 14% Efficiency in Sn-Based Perovskite Solar Cells. ACS Energy Lett. 2022, 7, 3703–3708. 10.1021/acsenergylett.2c01659. [DOI] [Google Scholar]
  18. Lee W.; Li H.; Wong A.; Zhang D.; Lai M.; Yu Y.; Kong Q.; Lin E.; Urban J.; Grossman J.; Yang P. Ultralow Thermal Conductivity in All-Inorganic Halide Perovskites. Proc. Natl. Acad. Sci. Unit. States Am 2017, 114, 8693–8697. 10.1073/pnas.1711744114. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Baranwal A. K.; Saini S.; Wang Z.; Hirotani D.; Yabuki T.; Iikubo S.; Miyazaki K.; Hayase S. Interface Engineering Using Y2O3 Scaffold to Enhance the Thermoelectric Performance of CsSnI3 Thin Film. Org. Electron. 2020, 76, 105488. 10.1016/j.orgel.2019.105488. [DOI] [Google Scholar]
  20. Yadav A.; Dubey C.; Singh S. K.; Singh A. K. Improving the Stability of Lead-Free CsSnBr3 Halide Perovskite by DDAB-Assisted Postpassivation Surface Engineering. ACS Appl. Opt. Mater. 2024, 2 (3), 492–500. 10.1021/acsaom.4c00020. [DOI] [Google Scholar]
  21. Li B.; Long R.; Xia Y.; Mi Q. All-Inorganic Perovskite CsSnBr3 as a Thermally Stable, Free-Carrier Semiconductor. Angew. Chem. Int. Ed. 2018, 57, 13154–13158. 10.1002/anie.201807674. [DOI] [PubMed] [Google Scholar]
  22. Liang Y.; Cui X.; Li F.; Stampfl C.; Ringer S. P.; Zheng R. First-Principles Investigation of Intrinsic Point Defects in Perovskite CsSnBr3 R. Phys. Rev. Mater. 2021, 5, 035405. 10.1103/PhysRevMaterials.5.035405. [DOI] [Google Scholar]
  23. Xu L.; Zhang C.; Feng X.; Lv W.; Huang Z.; Lv W.; Zheng C.; Xing G.; Huangad W.; Chen R. Vapor Incubation of FASnI3 Films for Efficient and Stable Lead-Free Inverted Perovskite Solar Cells. J. Mater. Chem. A 2021, 9, 16943–16951. 10.1039/D1TA04722A. [DOI] [Google Scholar]
  24. Bonomi S.; Patrini M.; Bongiovanni G.; Malavasi L. Versatile Vapor Phase Deposition Approach to Cesium Tin Bromide Materials CsSnBr3, CsSn2Br5 and Cs2SnBr6. RSC Adv. 2020, 48, 28478–28482. 10.1039/D0RA04680A. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Heo J.; Cho H.; Lee S.; Park M.; Kim J.; Kim H.; Park J.; Kim Y.; Yun H.; Yoon E.; Kim D.; Ahn S.; Kwon S.; Park C.; Lee T. Bright Lead-Free Inorganic CsSnBr3 Perovskite Light-Emitting Diodes. ACS Energy Lett. 2022, 7, 2807–2815. 10.1021/acsenergylett.2c01010. [DOI] [Google Scholar]
  26. Zhao Z.; Gu F.; Li Y.; Sun W.; Ye S.; Rao H.; Liu Z.; Bian Z.; Huang C. Mixed-Organic-Cation Tin Iodide for Lead-Free Perovskite Solar Cells with an Efficiency of 8.12%. Adv. Sci. 2017, 4 (11), 1700204. 10.1002/advs.201700204. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Xu W.; Yuan H.; Yang X.; Zhang Y.; Zheng M. Tin-Based All-Inorganic Perovskite Photodetectors Fabricated by Chemical Vapor Deposition. Physica E 2021, 134, 114843. 10.1016/j.physe.2021.114843. [DOI] [Google Scholar]
  28. Jung H.; Liu Z.; Sotome M.; Kondo T. Vapor Phase Deposition of Lead-Free Halide Perovskite Alloy CsSn1–xZnxBr3. Jpn. J. Appl. Phys. 2024, 63, 01SP24. 10.35848/1347-4065/acfdb3. [DOI] [Google Scholar]
  29. Balachandran N.; Robert T. M.; Jayalatha T.; Neema P. M.; Mathew D.; Cyriac J. Lead-free, mixed tin-copper perovskites with improved stability and optical properties. J. Alloys Compd. 2021, 879, 160325. 10.1016/j.jallcom.2021.160325. [DOI] [Google Scholar]
  30. Dawson M.; Ribeiro C.; Morelli M. R. MnCl2 Doping Increases Phase Stability of Tin Halide Perovskites. Mater. Sci. Semicond. Process 2021, 132, 105908. 10.1016/j.mssp.2021.105908. [DOI] [Google Scholar]
  31. Hasegawa H.; Kobayashi K.; Takahashi Y.; Harada J.; Inabe T. Effective Band Gap Tuning by Foreign Metal Doping in Hybrid Tin Iodide Perovskites. J. Mater. Chem. C 2017, 5, 4048–4052. 10.1039/C7TC00446J. [DOI] [Google Scholar]
  32. Yuan J.; Li B.; Hao C. Study on Cobalt Doped Tin Based Perovskite Material with Enhanced Air Stability. Mater. Sci. Semicond. Process 2017, 57, 95–98. 10.1016/j.mssp.2016.09.029. [DOI] [Google Scholar]
  33. Kobayashi K.; Hasegawa H.; Takahashi Y.; Harada J.; Inabe T. Electronic Properties of Tin Iodide Hybrid Perovskites: Effect of Indium Doping. Mater. Chem. Front. 2018, 2, 1291–1295. 10.1039/C7QM00587C. [DOI] [Google Scholar]
  34. Ito N.; Kamarudin M. A.; Hirotani D.; Zhang Y.; Shen Q.; Ogomi Y.; Iikubo S.; Minemoto T.; Yoshino K.; Hayase S. Mixed Sn–Ge Perovskite for Enhanced Perovskite Solar Cell Performance in Air. J. Phys. Chem. Lett. 2018, 9, 1682–1688. 10.1021/acs.jpclett.8b00275. [DOI] [PubMed] [Google Scholar]
  35. Zhang Z.; Sun Q.; Nakajima T.; Ban H.; Liu Z.; Yu H.; Wang Y.; Xiao Z.; Shen Y.; Wang M. Achieving Efficient and Stable Inorganic CsSnI3 Mesoporous Perovskite Solar Cells via Galvanic Displacement Reaction. J. Mater. Chem. A 2022, 10, 23204–23211. 10.1039/D2TA07017K. [DOI] [Google Scholar]
  36. Soleimanioun N.; Rani M.; Singh B.; Saini G. S.; Tripathi K. S. Potential Replacement to Lead: Alkali Metal Potassium and Transition Metal Zinc in Organo-Metal Halide Perovskite Materials. J. Alloys Compd. 2021, 861, 158207. 10.1016/j.jallcom.2020.158207. [DOI] [Google Scholar]
  37. Sun H.; Zhang J.; Gan X.; Yu L.; Yuan H.; Shang M.; Lu C.; Hou D.; Hu Z.; Zhu Y.; Han L. Pb-Reduced CsPb 0.9 Zn 0.1 I 2 Br Thin Films for Efficient Perovskite Solar Cells. Energy Mater. 2019, 9 (25), 1900896. 10.1002/aenm.201900896. [DOI] [Google Scholar]
  38. Chen R.; Hou D.; Lu C.; Zhang J.; Liu P.; Tian H.; Zeng Z.; Xiong Q.; Hu Z.; Zhu Y.; Han H. Zinc Ion as Effective Film Morphology Controller in Perovskite Solar Cells. Sustainable Energy Fuels 2018, 2, 1093–1100. 10.1039/C8SE00059J. [DOI] [Google Scholar]
  39. Wu T.; Liu X.; Luo X.; Segawa H.; Tong G.; Zhang Y.; Ono L. K.; Qi Y.; Han L. Heterogeneous FASnI3 Absorber with Enhanced Electric Field for High-Performance Lead-Free Perovskite Solar Cells. Nano-Micro Lett. 2022, 14 (1), 99. 10.1007/s40820-022-00842-4. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Chen J.; Luo J.; Hou E.; Song P.; Li Y.; Sun C.; Feng W.; Cheng S.; Zhang H.; Xie L.; Tian C.; Wei Z. Efficient Tin-Based Perovskite Solar Cells with Trans-Isomeric Fulleropyrrolidine Additives. Nat. Photonics 2024, 18, 464–470. 10.1038/s41566-024-01381-7. [DOI] [Google Scholar]
  41. Chen M.; Kapil G.; Wang L.; Sahamir S. R.; Baranwal A. K.; Nishimura K.; Sanehira Z.; Zhang Z.; Kamarudin M. K.; Shen S.; Hayase S. High Performance Wide Bandgap Lead-Free Perovskite Solar Cells by Monolayer Engineering. Chem. Eng. J. 2022, 436, 135196. 10.1016/j.cej.2022.135196. [DOI] [Google Scholar]
  42. Zhao Z.; Sun M.; Ji Y.; Mao K.; Huang Z.; Yuan C.; Yang Y.; Ding H.; Yang Y.; Li Y.; Chen W.; Zhu J.; Wei J.; Xu J.; Paritmongkol W.; Abate A.; Xiao Z.; He L.; Hu Q. Efficient Homojunction Tin Perovskite Solar Cells Enabled by Gradient Germanium Doping. Nano Lett. 2024, 24, 5513–5520. 10.1021/acs.nanolett.4c00646. [DOI] [PubMed] [Google Scholar]
  43. Song T.; Yokoyama T.; Stoumpos C. C.; Logsdon J.; Cao D. H.; Wasielewski M. R.; Aramaki S.; Kanatzidis M. G. Importance of Reducing Vapor Atmosphere in the Fabrication of Tin-Based Perovskite Solar Cells. J. Am. Chem. Soc. 2017, 139, 836–842. 10.1021/jacs.6b10734. [DOI] [PubMed] [Google Scholar]
  44. Rau U.; Blank B.; Müller T. C. M.; Kirchartz T. Efficiency Potential of Photovoltaic Materials and Devices Unveiled by Detailed-Balance Analysis Phys. Rev. Appl. 2017, 7, 044016. 10.1103/PhysRevApplied.7.044016. [DOI] [Google Scholar]
  45. Hybrid Perovskite Solar Cells: characteristics and Operation, Fujiwara H., Ed.; Wiley-VCH: Weinheim, Germany, 2022. [Google Scholar]
  46. Cao L.; Gu S.; Liu B.; Huang L.; Zhang J.; Zhu Y.; Wang J. Highly Ambient Stable CsSnBr3 Perovskite via a New Facile Room-Temperature “Coprecipitation” Strategy. ACS Appl. Mater. Interfaces 2023, 15, 30409–30416. 10.1021/acsami.3c02532. [DOI] [PubMed] [Google Scholar]
  47. Li Z.; Xiao C.; Yang Y.; Harvey S.; Kim D.; Christians J.; Schulz M.; Yang P.; Nanayakkara S.; Jiang C.; et al. Extrinsic Ion Migration in Perovskite Solar Cells. Energy Environ. Sci. 2017, 10 (5), 1234–1242. 10.1039/C7EE00358G. [DOI] [Google Scholar]

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