Exploring New Cadmium Salts to Enhance the Photovoltaic Conversion Efficiency of Cu₂ZnSn(S,Se)₄ Solar Cells

In the prior art of CZTSSe solar cells, the traditional cadmium salt (Cd-salt) CdSO₄ is widely considered as a common raw material for preparing CdS buffer layers. However, in this study, we systematically demonstrated the phase structure and transmission spectra improvement effect of the other three types of Cd-salts, Cd(NO₃)₂, CdCl₂, and CdAc₂, on the CZTSSe/CdS heterojunction (HJT) compared to CdSO₄. CZTSSe/CdS HJT prepared by Cd(NO₃)₂ showed the strongest CdS phonon characteristic peaks at 295.9 and 597.6 cm^–1, indicating a pure crystal phase. The transmittance of CdS films prepared by Cd(NO₃)₂ is 20–35% higher than the other three Cd-salts in the light range of 350–1000 nm, which effectively improved the band gap of the films (2.41 eV) and depressed the photon absorption and scattering. Accordingly, CZTSSe solar cells based on the aforementioned Cd-salts showed that Cd(NO₃)₂ possessed the best ability to improve the photovoltaic conversion efficiency (PCE), about 5.7% more than the traditional CdSO₄ salt. After Sn²⁺ replaced by Sn⁴⁺ in the precursor solution, the PCE of CZTSSe solar cells using Cd(NO₃)₂ exceeded 10% (Total area = 0.21 cm², PCE = 10.23%, V_OC = 0.485 V, J_SC = 31.4 mA/cm², FF = 67.1%, without MgF₂ antireflective layer).