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 T-C = 195 degrees 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. (C) 2014 AIP Publishing LLC.