Optical properties of Cu2ZnSn(SxSe1-x)4 solar absorbers: Spectroscopic ellipsometry and ab initio calculations

Abstract

Dielectric functions of Cu2ZnSn(SxSe1-x)4 thin film absorbers with varied x were determined by spectroscopic ellipsometry and ab initio calculations. From the combination of experimental and theoretical studies, the fundamental interband transition energy E0 (∼1-1.5 eV) and the next following transition energy E1 (∼2-3 eV) were identified and found to blue-shift with increasing sulfur anion content, while keeping the energy separation E1−E0 almost constant, ∼1.4 eV from experiments, and 1 eV from theory. In addition, the average dielectric responses were found to decrease with sulfur anion content from both theoretical and experimental results. The Tauc optical bandgap value Eg determined on samples prepared on Mo and soda lime glass substrate showed a positive linear relationship between x and bandgap Eg. The bandgap bowing factor determined from the theoretical data is 0.09 eV.