Crystal structure and anti-site boundary defect characterisation of Cu2ZnSnSe4

Abstract

The crystal structure identification of the photovoltaic material Cu₂ZnSnSe₄ (CZTSe) is challenging due to the distinguishing feature between the two polymorphs, kesterite and stannite, being the arrangement of Cu and Zn ions. Here an energy dispersive X-ray (EDX) technique, based on electron beam channeling along specific crystallographic planes in a transmission electron microscope (TEM), is used to identify the structure. Regions a few 100 nm in size can be analysed using this method, unlike neutron or anomalous X-ray scattering. The parent crystal structure of CZTSe, annealed on either side of the order–disorder transition temperature, was correctly identified as being kesterite. The presence of 1/2[110] (001) and 1/4[201] (101) anti-site boundaries (ASBs) has also been investigated. The density of ASBs is higher above the transition temperature, due to a smaller energy penalty for disordering on the 2c and 2d Wyckoff sites. A nearest neighbour cation analysis predicted the 1/2[110] (001) ASB to have the lowest formation energy. From density functional theory (DFT) simulations the 1/2[110] (001) ASB energy is only 43 mJ m⁻² and furthermore it is not a recombination site or current blocking layer, so that photovoltaic device performance is not significantly degraded.