A-Site Cation Engineering of Tin Halide Perovskite Solar Cells

Metal halide perovskites are a class of materials that have gained significant attention in the field of thin film photovoltaics in recent years due to their exceptional optoelectronic properties. Among all, tin halide perovskites (chemical formula ASnX3, with A= methylammonium MA+, formamidinium FA+ or cesium Cs+ and X= iodide I- and/or bromide Br-) have gained much attention thanks to their low bandgap of 1.4 eV, which is close to the ideal bandgap for single-junction devices according to the Shockley-Quessier limit. Despite the promise, their power conversion efficiency has not reached levels comparable to traditional silicon and lead-based solar cells (>25%). The main reason lies in the soft nature of the tin-halide lattice and the facile oxidation of tin(II) to tin(IV), which induce relatively low defect formation energies and cause a severe intrinsic p-doping that limits the power conversion efficiencies in devices. The p-doping can severely change depending on the chemical composition of the material itself, for example, it can be reduced by changing the stoichiometry of the material enriching it with Sn or changing the X-site anion. Less is known about the effect of the A-site cation. The project aims at exploring the relationship between the A-site composition and optoelectronic, structural and morphological properties of tin halide perovskite thin films produced via solution process to further optimize them as absorber materials and integrate them into photovoltaic solar cells. We found that films with different cations exhibit high p-doping concentrations but distinct structural properties and markedly different morphologies, significantly affecting charge transport properties. More compact films with larger grains demonstrated higher effective charge carrier mobility and conductivity. Through careful optimization of the spin-coating process, we were able to achieve comparable conductivity values across different compositions.