Analysis of the radiative transitions in CZTSSe solar cells with varying degree of Cu-Zn order

Abstract

Cu2ZnSn(S, Se)4 (CZTSSe) shows broad and asymmetric photoluminescence spectra situated far below the absorption edge at low temperatures. The physical recombination paths for the observed transitions could not be assigned unambiguously yet in literature. Nevertheless we show in this contribution that the peak position of the photoluminescence at low temperatures can be used as an indirect measure of the degree of order in the Cu-Zn planes of the kesterite crystal structure. The degree of order can be changed easily by thermal annealing procedures. The photoluminescence for different degrees of order comprises an additional contribution which seems not to change its energetic position with the degree of order which is in contrast to the main radiative contribution and band parameters, i.e., the band gap. We attribute this transition to a secondary phase or some deep defect level which does not follow the CZTSSe band edge as determined by electroreflectance.