CZTSSe/Zn(O,S) heterojunction solar cells with 9.82% efficiency enabled via (NH4)2S treatment of absorber layer

The replacement of CdS buffer layer is desirable for the development of nontoxic, environmentally friendly kesterite thin-film solar cells (TFSCs). Recently, several ternary compound semiconductors have been extensively investigated as an alternative buffer layer for Cd-free TFSCs. Herein, the effectiveness of (NH4)(2)S treatment on the surface properties of the absorber, as well as the device performance of atomic layer deposited (ALD) Zn(O,S) buffer-based CZTSSe solar cells, has been investigated. X-ray photoelectron spectroscopy (XPS) results showed that the elemental compositions of CZTSSe surface were significantly influenced by (NH4)(2)S treatment, whereas the surface morphologies of the CZTSSe-absorber layers remained unaffected. The XPS results further suggested that the (NH4)(2)S solution treatment substantially removed the native oxide layer from the CTZSSe absorber surface. The fabricated CZTSSe/Zn(O,S) device without (NH4)(2)S treatment displayed an initial cell efficiency of 7.46%. The energy conversion efficiency increased significantly to 9.82% after the (NH4)(2)S treatment of absorber layers for an optimum duration of 1 min; to the best of our knowledge, this is the highest efficiency achieved to date for Zn(O,S) buffer-based kesterite solar cells. The improved device performance is predominantly attributed to the pronounced increase in the fill factor (FF) of TFSCs resulting from the removal of oxides/hydroxides from the CZTSSe surface and passivation of absorber surface with sulfur species. However, increasing the treatment duration to 3 or 5 min resulted in the deterioration of cell efficiency, primarily due to the progressive degradation in the FF of the device. This study demonstrates a plausible route to improve the performance of Zn(O,S) buffer layer-based kesterite solar cells through a simple surface treatment of the absorber layers using (NH4)(2)S solution.

Automated summary

The study demonstrates that a simple surface treatment of absorber layers using (NH4)(2)S solution is a plausible route to improve the performance of Zn(O,S) buffer layer-based kesterite solar cells.