Modification of crystallization in Cu2ZnSn(S,Se)4 films using Al2O3 capping layers

In this paper, we reported a modified preparation process for Cu2ZnSn(S,Se)4 (CZTSSe) films. For this process, Al2O3 capping layers were deposited on precursor films before selenization. It was found the capping layer can prevent the formation of separate CuxSe phase on top of the film and facilitate a homogeneous growth of the crystalline grains during selenization. Detailed characterizations revealed the reason: the Al2O3 capping layers can inhibit the incorporation of Se into the film in low-temperature region (below 350 °C). So the reaction between Se and Cu, which is thermodynamically preferential in this temperature region, can be prevented. As the disappearance of CuxSe, the traditional top-to-down crystallization mode was substituted by a homogeneous crystallization mode. Therefore, CZTSSe films with homogeneous crystallization properties were achieved. By using the capping layers, the efficiencies of the solar cells were improved. In particular, the open-circuit voltage (VOC) of the solar cell increased by ~18% by using the capping layers.