Zn(O,S) Buffer Layer Deposited by High Vapor Transport Deposition for Controlling Band Alignment of Cu2ZnSn(SxSe1−x)4 Thin‐Film Solar Cell Heterojunction

The replacement of the CdS buffer layer is desirable for the development of nontoxic and environmentally friendly kesterite thin‐film solar cells. Band alignment is critical to the energy conversion of solar cells and achieving optimum design of the layers requires high control of the process. Herein, the band alignment dependence of the CZTSSe/Zn(O,S) interface using different oxygen concentrations is reported, which results in a high controllability process for the energy levels of the buffer layers, which is desirable to achieve higher solar cell efficiencies. It is also demonstrated that a Zn(O,S) thin‐film layer can be deposited using a novel process: high vapor transport deposition. The main advantages observed are as follows: 1) absorber and buffer layers can be deposited in the same vacuum reactor, thus avoiding atmospheric contamination; 2) no additional etching or annealing process is needed; and 3) the process conditions have accuracy control of gas injection, which allows precise tuning of the oxygen content in the film. Optimal oxygen concentration is determined to avoid spike‐like CBO, achieving a conversion efficiency level of 6.87% for kesterite‐based thin‐film solar cells with a Cd‐free buffer layer.