Reversible order-disorder related band gap changes in Cu2ZnSn(S,Se)4 via post-annealing of solar cells measured by electroreflectance

We report on order–disorder related band gap changes in Cu2ZnSn(S,Se)4 solar cells which are induced by post-annealing. The band gap changes of the absorber are detected utilizing electroreflectance and analyzed by comparison with predictions of the stochastic Vineyard model. This yields a critical temperature of TC=195 °C above which the Cu2ZnSn(S,Se)4 absorber layer is entirely disordered within the Cu–Zn layers of the kesterite unit cell. The temporal evolution of the band gap during annealing shows that the equilibrium value is reached on a timescale in the order of hours, depending on the annealing temperature. In contrast to other experimental techniques, electroreflectance precisely measures the band gap and is not influenced by defect-mediated radiative recombination.