Point Defects in Cu2ZnSnSe4 (CZTSe): Resonant X-Ray Diffraction Study of the Low-Temperature Order/Disorder Transition

The interest in Cu₂ZnSn(S,Se)₄ (CZTS) for photovoltaic applications is motivated by similarities to Cu(In,Ga)Se₂ while being comprised of non-toxic and earth abundant elements. However, CZTS suffers from a V_oc deficit, where the V_oc is much lower than expected based on the band gap, which may be the result of a high concentration of point-defects in the CZTS lattice. Recently, reports have observed a low-temperature order/disorder transition by Raman and optical spectroscopies in CZTS films and is reported to describe the ordering of Cu and Zn atoms in the CZTS crystal structure. To directly determine the level of Cu/Zn ordering, we have used resonant-XRD, a site, and element specific probe of long range order. We used CZTSe films annealed just below and quenched from just above the transition temperature; based on previous work, the Cu and Zn should be ordered and highly disordered, respectively. Our data show that there is some Cu/Zn ordering near the low temperature transition but significantly less than high chemical order expected from Raman. To understand both our resonant-XRD results and the Raman results, we present a structural model that involves antiphase domain boundaries and accommodates the excess Zn within the CZTS lattice.