Lead-free tin perovskite solar cells

Abstract

Perovskite solar cells (PSCs) have attracted great attention all over the world because of the advantages of adjustable band gap, long carrier diffusion length, high light-absorption coefficient, and solution processability of organic metal halide perovskite absorbers. As the first-generation PSCs, the highest efficiency of lead PSCs has reached 25.5%, which is comparable with that of the silicon solar cells. However, the toxicity issue of lead raises the concern of environmental pollution and health problems; therefore, development of lead-free perovskite materials is desirable to produce the next-generation PSCs. In the past few years, tin PSCs have emerged as a promising candidate for eco-friendly PV technology, and their efficiency has increased rapidly from approximately 6% to more than 13%, which is realized by suppressing the oxidation process of Sn2+ to Sn4+ and slowing down the fast crystallization rate to produce pinhole-free and highly oriented tin perovskite layer. Moreover, a certified efficiency of 11.22% at the accredited test center has recently been achieved by using a template-growth technique to deposit the tin perovskite absorber, which will attract more and more researchers to participate in this field to accelerate the progress of tin PSCs. In this paper, a comprehensive review on the recent progress toward improving the efficiency of tin PSCs based on equivalent circuit modeling is provided. The improvement in a certain device parameter is discussed separately, including short-circuit current density, open-circuit voltage, and fill factor of the solar cell. Then, the stability issue of tin PSCs is also discussed briefly. Finally, the perspectives on the future development of tin PSCs with the challenges of how to reach 20% efficiency, enlarging the device area, and realizing the scalable production are given in detail. The development of efficient and stable lead-free perovskite solar cells (PSCs) is crucial for addressing the concern of environmental pollution from the toxic element lead. In recent years, tin PSCs have emerged as a promising candidate for high-performance, eco-friendly photovoltaic technology with a high certified power conversion efficiency (PCE) of more than 11%, indicating a great potential for future applications. Here, we review the recent efficiency progress of tin PSCs based on the equivalent circuit model of solar cells. We then discuss approaches toward efficiency improvement from the device viewpoint, such as optimizing the band gap, increasing the light-harvesting efficiency and carrier diffusion length, surface passivation, and regulating the interface energy-level alignment. Finally, we point out the possibility of reaching 20% PCE for tin PSCs and issues regarding enlarging the cell size and realizing scalable production in the future. We expect that these perspectives will be helpful for accelerating the commercialization of tin PSCs. The development of efficient and stable lead-free perovskite solar cells (PSCs) is crucial for addressing the concern of environmental pollution from the toxic element lead. In recent years, tin PSCs have emerged as a promising candidate for high-performance, eco-friendly photovoltaic technology with a high certified power conversion efficiency (PCE) of more than 11%, indicating a great potential for future applications. Here, we review the recent efficiency progress of tin PSCs based on the equivalent circuit model of solar cells. We then discuss approaches toward efficiency improvement from the device viewpoint, such as optimizing the band gap, increasing the light-harvesting efficiency and carrier diffusion length, surface passivation, and regulating the interface energy-level alignment. Finally, we point out the possibility of reaching 20% PCE for tin PSCs and issues regarding enlarging the cell size and realizing scalable production in the future. We expect that these perspectives will be helpful for accelerating the commercialization of tin PSCs. Development of the photovoltaic (PV) technology with high efficiency and low production cost is an urgent need. Perovskite solar cells (PSCs) are emerging as the most promising third-generation PV technology because of the adjustable band gap, long carrier diffusion length, high light-absorption coefficient, and solution processability of metallic halide perovskite absorbers.1Jeon N.J. Noh J.H. Kim Y.C. Yang W.S. Ryu S. Seok S.I. Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells.Nat. Mater. 2014; 13: 897-903Crossref PubMed Scopus (4934) Google Scholar, 2Wu Y. Yang X. Chen W. Yue Y. Cai M. Xie F. Bi E. Islam A. Han L. 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Hou X. et al.Stabilizing perovskite solar cells to IEC61215:2016 standards with over 9,000-h operational tracking.Joule. 2020; 4: 2646-2660Abstract Full Text Full Text PDF Scopus (65) Google Scholar Currently, high efficiency is usually achieved by the cells using lead perovskite absorbers. As the first-generation PSCs, they have attracted worldwide attention and been successfully developed as practical solar modules and monolithic tandem devices.9Ju M.G. Chen M. Zhou Y. Dai J. Ma L. Padture N.P. Zeng X.C. Toward eco-friendly and stable perovskite materials for photovoltaics.Joule. 2018; 2: 1231-1241Abstract Full Text Full Text PDF Scopus (114) Google Scholar However, the toxicity of Pb arouses the concerns of environmental pollution and health problems, which significantly limit their large-scale production and commercialized applications.10Abate A. Perovskite solar cells go lead free.Joule. 2017; 1: 659-664Abstract Full Text Full Text PDF Scopus (182) Google Scholar Therefore, development of eco-friendly lead-free perovskite materials is highly desired to produce the next-generation PSCs. So far, a variety of lead-free perovskite materials based on tin (Sn), bismuth (Bi), antimony (Sb), germanium (Ge), and copper (Cu) have been studied. Among them, tin perovskite is the most promising candidate for next-generation PSCs. Sn has similar outer electronic configuration (ns2 np2) and ionic radius to Pb (149 pm for Pb2+ and 135 pm for Sn2+),11Gu S. Lin R. Han Q. Gao Y. Tan H. Zhu J. Tin and mixed lead–tin halide perovskite solar cells: progress and their application in tandem solar cells.Adv. Mater. 2020; 32e1907392Crossref PubMed Scopus (84) Google Scholar and tin perovskites show an ideal band gap close to the Shockley-Queisser limit (1.3–1.4 eV) and high carrier mobility, which is associated with a high theoretical efficiency (more than 30%).12Hao F. Stoumpos C.C. Cao D.H. Chang R.P.H. Kanatzidis M.G. Lead-free solid-state organic–inorganic halide perovskite solar cells.Nat. Photonics. 2014; 8: 489-494Crossref Scopus (1918) Google Scholar Since Snaith and coworkers reported the first lead-free tin PSCs and achieved a PCE of 6.4%,13Noel N.K. Stranks S.D. Abate A. Wehrenfennig C. Guarnera S. Haghighirad A.A. Sadhanala A. Eperon G.E. Pathak S.K. Johnston M.B. et al.Lead-free organic–inorganic tin halide perovskites for photovoltaic applications.Energy Environ. Sci. 2014; 7: 3061-3068Crossref Google Scholar the efficiency of tin PSCs has remained at approximately 6% with negligible progress over time owing to the low device reproducibility (Figure 1). Because of the development of the two-dimensional (2D)-three-dimensional (3D) perovskite structure with high crystal orientation and better stability against Sn2+ oxidation, the reproducibility of tin PSCs has greatly improved, and the efficiency rapidly increased to 9%,14Shao S. Liu J. Portale G. Fang H.H. Blake G.R. ten Brink G.H. Koster L.J.A. Loi M.A. Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency.Adv. Energy Mater. 2018; 8: 1702019Crossref Scopus (507) Google Scholar which has attracted more research groups to participate in this field. Meanwhile, the architecture of tin PSCs was changed from normal (n-i-p) to inverted (p-i-n) structures for the following reasons: (1) a previous study reported that the diffusion length of hole is much shorter than that of electron in tin perovskite films, which is unfavorable for the hole extraction in n-i-p tin PSCs;15Li P. Liu X. Zhang Y. Liang C. Chen G. Li F. Su M. Xing G. Tao X. Song Y. Low-dimensional Dion-Jacobson-phase lead-free perovskites for high-performance photovoltaics with improved stability.Angew. Chem. Int. Ed. Engl. 2020; 59: 6909-6914Crossref PubMed Scopus (67) Google Scholar (2) the oxidation process from Sn2+ to Sn4+ can be accelerated by the chemical dopants of hole transport materials used in the normal structure and the oxygen vacancies on TiO2 surface, resulting in poor device stability.16Diau E.W.G. Jokar E. Rameez M. Strategies to improve performance and stability for tin-based perovskite solar cells.ACS Energy Lett. 2019; 4: 1930-1937Crossref Scopus (96) Google Scholar In contrast, a growing number of studies have aimed at minimizing the voltage loss of tin PSCs by reducing the defect density and optimizing the energy-level alignment at the perovskite/charge transport layer interface, which further boosted the PCE to 12% to 13%.17Nishimura K. Kamarudin M.A. Hirotani D. Hamada K. Shen Q. Iikubo S. Minemoto T. Yoshino K. Hayase S. Lead-free tin-halide perovskite solar cells with 13% efficiency.Nano Energy. 2020; 74: 104858Crossref Scopus (163) Google Scholar, 18Wang C. Zhang Y. Gu F. Zhao Z. Li H. Jiang H. Bian Z. Liu Z. Illumination durability and high-efficiency Sn-based perovskite solar cell under coordinated control of phenylhydrazine and halogen ions.Matter. 2021; 4: 709-721Abstract Full Text Full Text PDF Scopus (65) Google Scholar, 19Li H. Wei Q. Ning Z. Toward high efficiency tin perovskite solar cells: a perspective.Appl. Phys. Lett. 2020; 117060502Crossref Scopus (20) Google Scholar Recently, a certified PCE of 11.22% was achieved at an accredited test center (Newport Laboratory, USA) by a templated-growth technique that could fabricate a high-quality tin perovskite film with a low defect density and large increase in carrier diffusion length.20Liu X. Wu T. Chen J.Y. Meng X. He X. Noda T. Chen H. Yang X. Segawa H. Wang Y. Han L. Templated growth of FASnI3 crystals for efficient tin perovskite solar cells.Energy Environ. Sci. 2020; 13: 2896-2902Crossref Google Scholar Because of the rapidly growing efficiencies of tin PSCs, it is important to summarize these works and point out the challenges to future development. Until now, many literature reviews focusing on material design and processing have been reported in this field. However, there are few reviews from the viewpoint of the device itself, which can provide more practical guidelines for directing researchers toward further improvement of tin PSC performance. Here, we review recent progress toward improving the efficiency of tin PSCs based on analysis of the equivalent circuit model. First, we classify our discussion of the research work according to the three main PV parameters of solar cells: increasing the short-circuit current density (JSC) by optimizing the band gap, improving light harvest efficiency, and increasing the carrier extraction ability; the open-circuit voltage (VOC) by surface passivation and matching the energy-level alignment in devices; and the fill factor (FF) by optimizing the resistance. Although these parameters might be interrelated, we try to analyze the major influences on the specific device parameters to simplify the discussion and help the researcher to better understand the approach for efficiency enhancement. Next, we provide a brief discussion of the stability issues for tin PSCs. Finally, we survey possible future directions of tin PSCs with the challenges to further increasing the efficiency to 20%, enlarging the device area, and realizing large-scale production, which we expect to be helpful for researchers in accelerating the progress of developing efficient and stable tin PSCs. In general, the PCE of a solar cell can be calculated by measuring the current density-voltage (J–V) curves to derive the JSC, VOC, and FF when the solar cell is under solar light irradiation with a power intensity of Pin:PCE=JSCVOCFFPin(Equation 1) Because the Pin for a certain device under AM 1.5 G is fixed, it is important to increase JSC, VOC, and FF to improve the PCE of tin PSCs. A powerful tool for analyzing the whole PV performance of a solar cell from device viewpoint is the equivalent circuit model (Figure 2), which includes a constant current source, a diode, series resistance, and shunt resistance: (1) the constant current source indicates the photocurrent density (Jph) generated by device under constant light, (2) the diode represents the electron transfer process in solar cells, and (3) Rs and Rsh are associated with the series resistance and shunt resistance, respectively. Rs is mainly attributed to the resistance of functional layers, the cell geometry, and the sheet resistance of transparent conductive oxide. Rsh is affected by the pinholes and shunting path, minority carrier lifetime, and the non-radiative recombination rate. According to this equivalent circuit model, the output current density (J(V)) of a solar cell is defined as the Jph reduced by losses in diode and Rsh, so the J–V characteristics can be described asJ(V)=Jph−J0[exp(q(V+JRs)nkT)−1]−V+JRsRsh,(Equation 2) where n is the ideality factor describing the deviation from the ideal diode condition, k is the Boltzmann constant, T is the temperature, q is the elemental charge, and J0 is the reverse saturation current density of the diode. Based on this equation, we discuss the recent approaches to the improvement of JSC, VOC, and FF in the following section. At short-circuit condition, the photo-generated carriers are expected to flow into the external circuit, as described by the equivalent circuit (Figure 2). When a solar cell is under illumination with an incident photo flux density of Jphoton(λ), the total current density flowing into the external circuit can be expressed as follows:Jsc=q∫λminλmaxJphoton(λ)IPCE(λ)dλ,(Equation 3) where λmax and λmin represent the maximum and minimum wavelength of the absorbed photons, respectively, and IPCE(λ) is defined as the incident monochromatic photo-to-electron conversion efficiency:IPCE(λ)=α(λ)ηc(λ)[1−R(λ)],(Equation 4) where α(λ) is the light-harvesting efficiency and ηc(λ) is the charge collection efficiency. R(λ) is the total reflectivity of solar cell, which could be effectively reduced by anti-reflection film on the substrate. Generally,α(λ) is strongly affected by the band gap because a photon can be absorbed only when its energy is larger than the band gap of the absorbing materials. In the case of tin PSCs, α(λ) is also positively related to the absorbance of tin perovskite layer that is affected by the film thickness, the crystal quality of Sn perovskite, and the defects density of Sn4+ and Sn vacancy. Therefore, to achieve a high JSC, researchers should focus on optimizing the band gap and crystallization process of tin perovskite layer, as well as on improving the charge transportation and extraction processes in tin PSCs. Based on Equation 3, JSC can be enhanced by further broadening the spectral absorption edge (λmax), which can be realized by narrowing the optical band gap. However, VOC is positively correlated with the band gap. This trade-off between JSC and VOC leads to an ideal band gap of 1.3–1.4 eV for the light-absorbing layer in a single-junction solar cell.21Nasti G. Abate A. Tin halide perovskite (ASnX 3 ) solar cells: a comprehensive guide toward the highest power conversion efficiency.Adv. Energy Mater. 2020; 10: 1902467Crossref Scopus (61) Google Scholar For typical ASnX3-type perovskites, where A is a monovalent cation [CH3NH3+ (MA+), CH(NH2)2+ (FA+), Cs+, etc.] and X is a halide anion (I−, Br−), the band gap can be changed from 1.23 eV (MASnI3) to 2.4 eV (FASnBr3) by tuning the composition of A-site cations or X-site halide anions. This variation enables full coverage of the ideal band gap region for producing high-performance solar cells. Similar to the trend of lead perovskite, the band gap of tin perovskite can be greatly enlarged by replacing the iodide with bromide anions.22Ferrara C. Patrini M. Pisanu A. Quadrelli P. Milanese C. Tealdi C. Malavasi L. Wide band-gap tuning in Sn-based hybrid perovskites through cation replacement: the FA1−xMAxSnBr3 mixed system.J. Mater. Chem. A. 2017; 5: 9391-9395Crossref Google Scholar,23Sabba D. Mulmudi H.K. Prabhakar R.R. Krishnamoorthy T. Baikie T. Boix P.P. Mhaisalkar S. Mathews N. Impact of anionic Br– substitution on open circuit voltage in lead free perovskite (CsSnI3-xBrx) solar cells.J. Phys. Chem. C. 2015; 119: 1763-1767Crossref Scopus (246) Google Scholar However, the impact of A-site cations is quite different between these two kinds of perovskites. For MASnI3 and CsSnI3 perovskites, the strong s–p antibonding orbital coupling between the Sn and I atoms provides the broadest bandwidth and, thus, the narrowest band gap, which extends the absorption edge to the near-infrared region (950–1,000 nm). This results in a strong light-harvesting ability and achieves a high theoretical JSC for tin PSCs.24Kumar M.H. Dharani S. Leong W.L. Boix P.P. Prabhakar R.R. Baikie T. Shi C. Ding H. Ramesh R. Asta M. et al.Lead-free halide perovskite solar cells with high photocurrents realized through vacancy modulation.Adv. Mater. 2014; 26: 7122-7127Crossref PubMed Scopus (709) Google Scholar, 25Im J. Stoumpos C.C. Jin H. Freeman A.J. Kanatzidis M.G. Antagonism between spin–orbit coupling and steric effects causes anomalous band gap evolution in the perovskite photovoltaic materials CH3NH3Sn1–xPbxI3.J. Phys. Chem. Lett. 2015; 6: 3503-3509Crossref PubMed Scopus (155) Google Scholar, 26Marshall K.P. Walker M. Walton R.I. Hatton R.A. Enhanced stability and efficiency in hole-transport-layer-free CsSnI3 perovskite photovoltaics.Nat. Energy. 2016; 1: 16178Crossref Scopus (334) Google Scholar In contrast, FASnI3 perovskite shows an expanded orthorhombic lattice with a larger Sn–I bond length and a decreased s-p orbital overlap, resulting in a larger optical band gap of 1.4 eV with a light-absorption edge of 885 nm.27Koh T.M. Krishnamoorthy T. Yantara N. Shi C. Leong W.L. Boix P.P. Grimsdale A.C. Mhaisalkar S.G. Mathews N. Formamidinium tin-based perovskite with low E g for photovoltaic applications.J. Mater. Chem. A. 2015; 3: 14996-15000Crossref Google Scholar Despite a slight decrease in the absorption edge, FASnI3 perovskite has been reported to be more effective for solving the JSC-VOC trade-off and increasing the device efficiency because of its lower intrinsic carrier density and better stability against oxidation than other kinds of Sn perovskites.28Shi T. Zhang H.S. Meng W. Teng Q. Liu M. Yang X. Yan Y. Yip H.L. Zhao Y.J. Effects of organic cations on the defect physics of tin halide perovskites.J. Mater. Chem. A. 2017; 5: 15124-15129Crossref Google Scholar After fixing the band gap, the following discussion will focus on improving the light-harvesting efficiency and charge collection capability of tin PSCs. The narrow optical band gap of tin perovskites is beneficial for absorbing more photons in the near-infrared region compared with lead perovskites. However, the fast crystal growth rate and the spontaneous oxidation of Sn2+ will lead to poor crystal quality and an undesirable phase transition to the nonperovskite phase with a chemical formula of A2SnI6 that presents a much larger optical band gap (1.7–1.8 eV).26Marshall K.P. Walker M. Walton R.I. Hatton R.A. Enhanced stability and efficiency in hole-transport-layer-free CsSnI3 perovskite photovoltaics.Nat. Energy. 2016; 1: 16178Crossref Scopus (334) Google Scholar,29Liao W. Zhao D. Yu Y. Grice C.R. Wang C. Cimaroli A.J. Schulz P. Meng W. Zhu K. Xiong R.-G. Yan Y. Lead-free inverted planar formamidinium tin triiodide perovskite solar cells achieving power conversion efficiencies up to 6.22%.Adv. Mater. 2016; 28: 9333-9340Crossref PubMed Scopus (443) Google Scholar These effects induce a remarkable loss of light-harvesting efficiency and a blue shift of the absorption edge (Figure 3A). So far, much effort has been made to mediate the fast crystallization process and suppress the phase transition to improve the JSC of tin PSCs. (A) UV-vis absorption spectra of CsSnI3 films. The red line indicates the direction of change with increasing time in ambient air. Reprinted with permission from Hatton et al.26Marshall K.P. Walker M. Walton R.I. Hatton R.A. Enhanced stability and efficiency in hole-transport-layer-free CsSnI3 perovskite photovoltaics.Nat. Energy. 2016; 1: 16178Crossref Scopus (334) Google Scholar Copyright 2016 Springer Nature. (B) UV-vis absorption spectra of FASnI3, FASnI3-SnF2, and FASnI3-SnF2-TMA films. Reprinted with permission from Jen et al.30Zhu Z. Chueh C.C. Li N. Mao C. Jen A.K.Y. Realizing efficient lead-free formamidinium tin triiodide perovskite solar cells via a sequential deposition route.Adv. Mater. 2018; 30: 1703800Crossref Scopus (140) Google Scholar Copyright 2018 John Wiley and Sons. (C) IPCE spectra of the control and CDTA-treated devices. Reprinted with permission from Han et al.31Wu T. Liu X. He X. Wang Y. Meng X. Noda T. Yang X. Han L. Efficient and stable tin-based perovskite solar cells by introducing π-conjugated Lewis base.Sci. China Chem. 2020; 63: 107-115Crossref Scopus (90) Google Scholar Copyright 2020 Science China Press. The strong Lewis acidity of Sn2+ leads to fast and uncontrollable crystallization process of tin perovskite films, which might generate many pinholes and induce a low crystallinity with weak absorption. The key issue for obtaining compact and pinhole-free tin perovskite film is to introduce extra chemical interactions in perovskite precursors by additives, including the Lewis acid–base coordination and hydrogen bonding interaction. Introducing Lewis acid–base interaction is the most promising way to slow down the crystallization rate. Dimethyl sulfoxide (DMSO), a strong Lewis base solvent, was first reported to mediate the crystal growth process by forming a SnI2∙3DMSO intermediate adduct to suppress the fast reaction between SnI2 and organic ammonium salts,32Hao F. Stoumpos C.C. Guo P. Zhou N. Marks T.J. Chang R.P.H. Kanatzidis M.G. Solvent-mediated crystallization of CH3NH3SnI3 films for heterojunction depleted perovskite solar cells.J. Am. Chem. Soc. 2015; 137: 11445-11452Crossref PubMed Scopus (430) Google Scholar resulting in a high-quality MASnI3 perovskite film with increased a(λ) compared with the MASnI3 derived from dimethyl formamide. Similarly, Jen et al. used trimethylamine (TMA) as the Lewis base agent to fabricate FASnI3 film.30Zhu Z. Chueh C.C. Li N. Mao C. Jen A.K.Y. Realizing efficient lead-free formamidinium tin triiodide perovskite solar cells via a sequential deposition route.Adv. Mater. 2018; 30: 1703800Crossref Scopus (140) Google Scholar The formation of SnI2-TMA complexes slowed down the reaction rate between SnI2 and formamidinium iodide (FAI) and produced a homogeneous and compact FASnI3 film with increased light absorption in the wavelength region of 560–880 nm (Figure 3B). Consequently, the JSC of tin PSCs was increased from 17 to 22 mA cm–2. Moreover, increasing the electron density of the Lewis base agent can further stabilize the Lewis base-SnI2 intermediate adduct and control the crystallization process more precisely.31Wu T. Liu X. He X. Wang Y. Meng X. Noda T. Yang X. Han L. Efficient and stable tin-based perovskite solar cells by introducing π-conjugated Lewis base.Sci. China Chem. 2020; 63: 107-115Crossref Scopus (90) Google Scholar Our group introduced the π-conjugated Lewis base molecule, 2-cyano-3-[5-[4-(diphenylamino) phenyl]-2-thienyl]-propenoic acid (CDTA), with high electron density to control the growth of FASnI3 perovskite. This additive forms a stable intermediate phase with Sn–I framework, leading to a pinhole-free Sn perovskite film with an increased film coverage, improving the IPCE(λ) in the wavelength region of 400–800 nm (Figure 3C). At the same time, introducing hydrogen bonding interaction in precursor solution is another effective approach to slow down the crystallization rate of tin perovskite. The incorporation of poly(vinyl alcohol) with abundant hydroxyl groups (-OH) into the precursor solution promotes the formation of the O–H····I− hydrogen bond with iodide species (SnI2 and FAI).33Meng X. Lin J. Liu X. He X. Wang Y. Noda T. Wu T. Yang X. Han L. Highly stable and efficient FASnI3-based perovskite solar cells by introducing hydrogen bonding.Adv. Mater. 2019; 31e1903721Crossref PubMed Scopus (130) Google Scholar The hydrogen bonding interaction induces a slow crystal growth process of the FASnI3 perovskite grains, yielding a homogeneous and pinhole-free FASnI3 film with JSC increased by 12%. The oxidation of Sn2+ and the undesirable phase transition have a negative effect on JSC, which can be prevented by developing efficient antioxidant additives. Hydrazine derivatives with strong reducing ability have been successfully applied to suppress the oxidation of Sn2+ during both the fabrication and operation processes of tin PSCs. Introducing a hydrazine vapor atmosphere during the one-step deposition of ASnI3 film can reduce the oxidized A2SnI6 phase content via the possible redox reaction path of 2SnI62–+N2H4→2SnI42–+N2+4HI (Figure 4A).34Song T.B. 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: 836-842Crossref PubMed Scopus (312) Google Scholar As a consequence, the JSC of MASnI3-based PSCs significantly increased from 5 mA cm–2 for the control device to 19.9 mA cm–2 for the device with hydrazine vapor treatment. However, it is possible that the strong reducing ability of hydrazine vapor might directly reduce the Sn2+ to metallic Sn and destroy the perovskite lattice. To overcome this issue, a hydrazinium chloride (N2H5Cl) salt was applied to suppress oxidation while preventing the overreduction of Sn2+ to retain the high crystallinity of FASnI3 film (Figure 4B).35Kayesh M.E. Chowdhury T.H. Matsuishi K. Kaneko R. Kazaoui S. Lee J.J. Noda T. Islam A. Enhanced photovoltaic performance of FASnI3-based perovskite solar cells with hydrazinium chloride coadditive.ACS Energy Lett. 2018; 3: 1584-1589Crossref Scopus (112) Google Scholar The use of N2H5Cl salt reduced the amount of Sn4+ by 20% and led to an enhancement of device JSC from 14.8 to 16.6 mA cm–2. (A) Possible mechanism of the hydrazine vapor reaction in MASnI3 perovskite films. Reprinted with permission from Kanatzidis et al.34Song T.B. 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: 836-842Crossref PubMed Scopus (312) Google Scholar Copyright 2018 John Wiley and Sons. (B) Precursor solution and SEM images of FASnI3 perovskite before and after N2H5Cl treatment. Reprinted with permission from Islam et al.35Kayesh M.E. Chowdhury T.H. Matsuishi K. Kaneko R. Kazaoui S. Lee J.J. Noda T. Islam A. Enhanced photovoltaic performance of FASnI3-based perovskite solar cells with hydrazinium chloride coadditive.ACS Energy Lett. 2018; 3: 1584-1589Crossref Scopus (112) Google Scholar Copyright 2018 American Chemical Society. (C) IPCE spectra of tin PSCs prepared from precursor solutions stirred with Sn powder for 0, 1/3, 1, and 5 h, denoted as Films A (gray line), B (black line), C (red line), and D (blue line), respectively. Reprinted with permission from Liu et al.36Gu F. Ye S. Zhao Z. Rao H. Liu Z. Bian Z. Huang C. Improving performance of lead-free formamidinium tin triiodide perovskite solar cells by tin source purification.Sol. RRL. 2018; 2: 1800136Crossref Scopus (114) Google Scholar Copyright 2018 John Wiley and Sons. (D) Schematic illustration of the Sn4+ scavenging method. TM-DHP is added to the precursor. Metallic Sn nanoparticles are generated by the reaction be