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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.

Kesterite Cu2ZnSnS4 (CZTS) has been investigated intensively as a promising absorber material for thin film solar cells. However, the reported best power conversion efficiency (PCE) is still low due to the intrinsic limitations of CZTS defects and the unfavorable front and back contact interfaces. In this study, an intermediate MoO3−x layer is introduced as a primary back contact prior to the Cd‐doped Cu2ZnSnS4 (CZCTS) absorber layer growth. It has been demonstrated that the insertion of the MoO3−x layer can suppress the decomposition reaction between CZCTS and Mo, reducing secondary phases and voids and improving the rear interface quality. The MoO3−x layer can also accelerate potassium diffusion into the CZCTS layer and interfaces, which facilitates the grain growth, passivates interfaces and hence yields better crystallinity. Moreover, the band alignment at the back contact is modified by the MoO3−x layer, resulting in better minority carrier collection and improvement of open‐circuit voltage (VOC). With the optimal MoO3−x layer, the PCE of CZCTS solar cells has increased from ≈5.43 to ≈7%, largely attributed to the ≈60 mV VOC increment. Through the modification of the CZCTS precursor and optimizing the anti‐reflection coating layer, the best PCE achieved is 8.98%, with an active area efficiency of 9.26%.

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