Research Progress of Metal(I) Substitution in Cu2ZnSn(S,Se)4 Thin Film Solar Cells

Cu2ZnSn(S,Se)(4) solar cell (CZTSSe), as a new type of inorganic thin-film solar cells, has been widely studied in recent years due to the advantages of earth-abundant and environmental-friendly composition elements, high light absorption coefficient and adjustable band gap. CZTSSe solar cell is thus a highly competitive photovoltaic device with potential applications in flexibility, building integrated photovoltaics (BIPV) and so on. So far, 12.6% certified efficiency has been achieved for this kind of solar cells. Open-circuit voltage (V-OC) deficit is always the key factor to unsatisfied efficiency of CZTSSe solar cells, and band tailing, mismatch of energy band structure and deep level defects are the main causes to V-OC deficit. Typically, Cu-Zn disorder-induced defects widely exist in the bulk absorber, due to similar radius of Cu and Zn elements could lead to relatively low formation energy of Cu-Zn and Zn-Cu anti-site defects. Metal(I) substitution is an effective way to solve Cu-Zn disorder, which can well reduce VOC deficit via lowering band tailing and improving the device structure, leading to better cell performance. However, very few review papers have focused on the metal(I) substitution work. In this review, we will summarize the research progress of metal(I) substitution in Cu2ZnSn(S,Se)(4) thin film solar cells. Part I introduces the structure and problems of CZTSSe solar cells. Part II shows the origin of metal(I) substitution and theoretical research on substituted materials. Part III focuses on synthetic methods about metal(I) partial substitution devices and influence on crystal growth, band tailing, interface defects and band structure. Part IV briefly introduces metal(I) total substitution devices. Part V anticipates the prospects and bottleneck of metal(I) substitution devices, and give some possible solutions to these current issues.