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

In this paper, we reported a modified preparation process for Cu 2 ZnSn(S,Se) 4 (CZTSSe) films. For this process, Al 2 O 3 capping layers were deposited on precursor films before selenization. It was found the capping layer can prevent the formation of separate Cu x Se phase on top of the film and facilitate a homogeneous growth of the crystalline grains during selenization. Detailed characterizations revealed the reason: the Al 2 O 3 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 Cu x Se, 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 (V OC ) of the solar cell increased by ~18% by using the capping layers.