Forming an Ultrathin SnS Layer on Cu₂ZnSnS₄ Surface to Achieve Highly Efficient Solar Cells with Zn(O,S) Buffer

Environmental friendliness demands the use of nontoxic elements in all types of solar cells, and Zn(O,S) thin film as an alternative buffer layer to replace CdS layer in chalcopyrite and kesterite solar cells has attracted enormous attention in the past. However, Cu₂ZnSnS₄ (CZTS) solar cells with a Zn(O,S) buffer are far inferior to those with CdS buffer despite the potentially better band alignment. Herein, by intentionally controlling the precursor composition, the surface of CZTS can be modified to improve the quality of the Zn(O,S)/CZTS junction for the chemical bath-deposited Zn(O,S) buffer. Such a CZTS solar cell reaches a high conversion efficiency of 7.28%, the highest among all Zn(O,S)-based kesterite solar cells so far. The CZTS surface that can jointly work well with the Zn(O,S) buffer is further investigated using X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The results indicate that an ultrathin SnS layer exists on the CZTS surface and effectively raises the conduction band edge of the absorber surface to form a conduction band offset barrier of ≈0.40 eV, significantly better than that without the assistance of SnS layer. A key route for fabricating highly efficient and low-cost Cd-free CZTS thin-film solar cells is described.