Engineering Stable Lead‐Free Tin Halide Perovskite Solar Cells: Lessons from Materials Chemistry

Abstract

demonstrating a lower dependence on fossil fuels. Although crystalline silicon (c-Si) is still the market front runner for PV due to its ideal bandgap for single-junction solar cells, c-Si does not have a readily tunable bandgap. This makes c-Si less suited for indoor PV or multijunction solar cells, which require alternative bandgaps to operate at their peak. In just over a decade of research, perovskite solar cells (PSCs) have emerged as an alternative PV technology which have tuneable bandgaps, low fabrication costs, solution processability, compliancy with flexible materials, long electron-hole diffusion length, and high charge carrier mobility. [2][3][4][5][6][7][8][9] The perovskite absorber layer is based on the ABX 3 crystal structure made up of organic/inorganic monovalent cations (A), a divalent cation (B) and one or more halides (X) as shown in Figure 1a. [10] Since the first publication on PSCs in 2009, the power conversion efficiencies (PCE) have gone from 3.8% to 25.7 %, making PSCs the fastest growing PV technology to date. [11][12][13] Despite their promise, toxic heavy metal element Pb (in the structure's B-site) is still required to achieve highly competitive efficiencies, [14][15][16][17][18][19][20][21][22][23] which is a major obstacle in the field that is yet to be overcome. In contact with moisture and/or light + oxygen lead halide perovskites can easily decompose into watersoluble compounds of lead, [24][25][26][27][28] which then inevitably accumulate within the food chain and therefore have the potential to ingress into the human body. [29] While several low toxicity metal halide alternatives to Pb have been proposed, [30][31][32][33][34][35] the favorable physical and optical properties of Sn make it the most promising substitute. [29,36] The first demonstration of Sn-halide PSCs was in 2014, where Hao et al. and Noel et al. reported 5.7% and 6.4% PCE respectively using methylammonium tin halides as the absorber layers (MAS-nIBr 2 and MASnI 3 , respectively). [30,31] At present, the highest performing Sn-halide PSCs show PCEs of over 14%; Yu et al. reported certified 14.03%-efficient 2D/3D perovskite solar cells, [37] while Jiang et al. achieved a certified PCE of 14.6% via a novel film fabrication route based on SnI 2 adducts (further details will be provided below). [38] To date, most of the progress with Sn-halide perovskites has been made using formamidinium tin iodide (FASnI 3 , see Figure 1b) and closely related recipes. [39][40][41][42][43] The favored use of the FA cation can be attributed to the higher formation energies of Sn vacancies in FASnI 3 and thus a lower hole density in comparison to MASnI 3 . [44] In comparison to their Pb-based counterparts, Sn-halide perovskites Substituting toxic lead with tin (Sn) in perovskite solar cells (PSCs) is the most promising route toward the development of high-efficiency lead-free devices. Despite the encouraging efficiencies of Sn-PSCs, they are still yet to surpass 15% and suffer detrimental oxidation of Sn(II) to Sn(IV). Since their...

Mechanistic Understanding of Oxidation of Tin‐based Perovskite Solar Cells and Mitigation Strategies

“…[49,67] It is worth mentioning that SnF 2 with moderate concentration (10 mol %) was proven to be the optimal concentration to regulate the perovskite growth and thus optimize the film morphology, leading to highly uniform and compact Sn-based perovskite films without any pinholes. [67,68] In contrast, SnF 2 with low concentration (5 mol%) causes slightly improved film morphology with some pinholes, and SnF 2 with high concentration (15-30 mol%) produces undesired rod-like bright regions. [68]…”

“…These observations unveil that a small amount of SnF 2 doping is crucial in reducing the content of Sn 4+ in the perovskite precursor and resultant film, thereby decreasing the amount of Sn vacancies. Except for the reduction, the roles of SnF 2 also can be understood as compensating the Sn vacancies caused by the oxidation of Sn 2+ into Sn 4+ or creating Sn‐rich condition to lower the formation energy of Sn vacancy to reduce its defect density [49,67] . It is worth mentioning that SnF 2 with moderate concentration (10 mol %) was proven to be the optimal concentration to regulate the perovskite growth and thus optimize the film morphology, leading to highly uniform and compact Sn‐based perovskite films without any pinholes [67,68] .…”

Methods, progresses, and opportunities of materials informatics

“…Perovskites have attracted much attention due to their easy synthesis, controllable structure, and diverse composition. To date, thousands of perovskites have exhibited various properties in the laboratory, including catalysis, 158,159 thermoelectricity, 160,161 ferroelectricity, 162,163 solar cells, [164][165][166] lasers and photodetection, 167,168 and so on.…”

Molecular interaction modulating Ruddlesden–Popper tin-based perovskite crystallization

“…Recently, the eco-friendly two-dimensional (2D) Ruddlesden-Popper (RP) tin halide perovskites have attracted considerable attention for solar cell application owing to improved moisture resistance capability and unique optical properties compared to their three dimensional counterparts. 1,2 RP tin iodide perovskites have the general chemical formula R 2 A n−1 Sn n I 3n+1 , where R represents a bulky organic ammonium cation, A is CH 3 NH 3 + or HC(NH 2 ) 2 + (FA + ), and n is an integer. 3 As the choice of bulky organic ammonium cations for RP tin-based perovskites is not limited by Goldschmidt's concept of ionic tolerance factors, it offers more exibility and tunability in regulating the structure and band gaps from 1.83 eV (n = 1) to 1.43 eV (n = 4).…”