Photovoltaic Performance of Phase-Pure Orthorhombic BiSI Thin-Films

A single-precursor solution approach is developed for depositing stoichiometric BiSI thin films featuring pure paraelectric orthorhombic (Pnam) phase. The compact and homogeneous films are composed of flake-shaped grains oriented antiplanar to the substrate and display a sharp optical transition corresponding to a bandgap of 1.57 eV. Optical and Raman signatures of the thin films are rationalized using the quasiparticle G0W0@PBE0 and density functional perturbation theory calculations. Electrochemical impedance spectroscopy revealed n-type doping with valence and conduction band edges located at 4.6 and 6.2 eV below vacuum level, respectively. Planar BiSI solar cells are fabricated with the architecture: glass/FTO/SnO2/BiSI/F8/Au, where F8 is poly(9,9-di-n-octylfluorenyl-2,7-diyl), showing record conversion efficiency of 1.32% under AM 1.5 illumination.