Compositional Modulation of Photoactive Layer for Sn-based Organometal Halide Perovskite Solar Cells

Abstract

The power conversion efficiency of perovskite solar cells (PeSCs) has been significantly improved from 3.8 % to 25.7 % with various composition changes, fabrication processes for high film quality, and additives to passivate the defects in the perovskite structure. Despite remarkable optoelectronic properties, the perovskite with outstanding performance is based on a lead (Pb) atom. The toxic and water-soluble lead remains a thorny concern in the commercialization process of PeSCs, therefore the Pb element must be replaced with a nontoxic element such as tin (Sn). The Sn-based perovskite has great potential for Pb-free PeSCs and has great photovoltaic properties such as a suitable band gap, long diffusion length, and high carrier mobility. Despite the great potential for photovoltaic devices, Sn-based perovskite solar cell shows limited device performance from some problems. The Sn is thermodynamically oxidized from Sn2+ to Sn4+. The facile oxidation of Sn causes the unintended defect state, which is Sn vacancies, self-p-doping, and degraded crystallinity. Another issue is lattice strain and disordering in the perovskite crystal structure. This strain contributes to ion migration and defect formation that is identified to be determinantal to the structural stability and affects the optoelectronic properties such as diffusion length and carrier mobility. In this perspective, I demonstrate several compositional modifications of ABX3 perovskite crystal lattice. First, the divalent metal cations such as Cd, Ge, and Zn can be substituted in the B-site of the perovskite crystal lattice. Among them, the Zn2+ ion has been widely used in Pb perovskite. I have demonstrated the addition of Zn metal powder (Zn0) into the perovskite precursor to reduce oxidized Sn. The Zn2+ ions easily are ionized from Zn0 through the redox reaction between Sn and Zn with standard reduction potential difference. During the ionization, the Sn is reduced, and Zn is oxidized. I doped the Zn ions in the B-site of Pb-free Sn-based perovskite for the first time and confirmed that the Zn doping is thermodynamically favorable. In addition, the lattice parameter of a perovskite unit cell decreases because the radius of the Zn2+ ions is smaller than that of the Sn2+ ions. I confirmed the reduced lattice strain of the Sn-based perovskite structure using the Williamson-Hall plot and grazing incident X-ray diffraction measurement. According to the Zn-doped in perovskite film, the reduced lattice strain contributes to the improvement of diffusion length and carrier mobility, especially the hole. As a result, the power conversion efficiency of Pb-free Sn-based perovskite solar cells with Zn doping was enhanced. And then, the anion is another perovskite molecular engineering for the enhancement of efficiency by controlling optoelectronic properties. Especially halide anions can change the band gap of perovskite, passivate the defects of X-site, and increase the grain size of perovskite film. But common halides such as I, Br, and Cl can’t sufficiently suppress the oxidation of Sn-based perovskite. On the other hand, some kinds of anions act as halides in X-site (called as pseudo halide). Among the various pseudohalide anions, azide (N3-), thiocyanate (SCN-), formate (HCOO-), and borohydride (BH4-), the formate has lone pair electrons of oxygen atoms which are specially coordinated with Sn. Because of coordinate bonding, the formate can effectively suppress the oxidation of Sn in the pure formamidinium tin triiodide (FASnI3) perovskite without using A-site cationic candidates. I confirmed the coordinate bond between the lone electron pair of the formate and Sn from changing the electron density of hydrogen in formate and Sn in perovskite. As a result, the oxidized Sn (Sn4+) was dramatically decreased, and successfully I fabricated the formate doped FASnI3 perovskite solar cells with a high-power conversion efficiency of 12.11% as pinhole-free, reduction of non-radiative recombination, and enhanced crystallinity. Finally, I introduce the fluorinated functionalized organic material, 4-(trifluoromethyl) benzyl ammonium iodide, to fabricate the 2D/3D heterostructure of Sn-based perovskite. This hierarchy structure and fluorinated functional group significantly enhance the stability of the device, particularly in high humidity with ambient air because of the high electronegativity fluorine. Also, the perovskite with the material reduces the defect density and charge recombination, resulting in improved performance of the Sn-based perovskite solar cell with a power conversion efficiency (PCE) of 12.92%. The device retains 80% of its initial efficiency for almost 100 min without encapsulation under exposure O2 atmosphere with around 60% humidity. The demonstrated strategies of composition modulation for A, B, and X-site in Sn-based perovskite enable to fabrication of the high-quality Sn-based perovskite layer with declined oxidation process and enhanced optoelectronic properties.