Crystal and Electronic Structure of Bismuth Thiophosphate, BiPS4: An Earth Abundant Solar Absorber

The opto-electronic properties of crystalline BiPS4 are described for the first time for solar energy conversion. Detailed structural-analysis is extracted from XRD refinement of powders synthesized by solid-state method. BiPS4 exhibits a rather unusual 3-dimensional orthorhombic structure with two distinctive Bi sites with octahedral coordination distorted by 6s2 lone-pairs. High-resolution TEM imaging clearly shows the two Bi-Bi interatomic distances in close agreement with the XRD analysis. BiPS4 displays a complex Raman spectrum under near-resonant conditions which is rationalized by Density Functional Perturbation Theory. Hybrid-functional-DFT calculations show significant spin-orbit-coupling effects in Bi-6p bands, affecting not only on the band dispersion but also lowering the conduction band minimum by approximately 0.5 eV. Optical properties of BiPS4 powders are dominated by a direct transition at 1.72 eV, closely matching the calculated band gap. Electrochemical experiments revealed n-type condu...