Enhance the Performance of CZTSSe Solar Cells Through Inhibiting the Cu²⁺, Tu, and (─COOH) Association Reaction

The Sol-gel precursor solution reaction mechanism has a significant impact on the Cu₂ZnSn(S, Se)₄ (CZTSSe) solar cells. It is discovered that in the Cu₂ZnSnS₄ (CZTS) precursor solution (CZTS-PS) in the preparation, there is an association reaction among Cu²⁺, thiourea (Tu), and carboxyl (-COOH), which is an important reason for the undesirable CZTSSe solar cells. The strong association reaction generates excessive Cu²⁺ ions, forming the Cu_xSe secondary phase on the surface of the CZTSSe absorber. The secondary phase causes a short circuit and deterioration of gadget performance. Following a 6-h aging period for the CZTS-PS, the average photoelectric conversion efficiency (PCE) of the device is enhanced to 8.02%, and there is also an improvement in device uniformity, as evidenced by a decrease in the standard deviation to less than 1. To inhibit the association reaction and eliminate the aging time phenomenon, a strategy is developed using hydrochloric acid to regulate the CZTS-PS environment. This strategy shifts the REDOX reaction in Cu²⁺+Sn²⁺ toward the formation of Cu¹⁺+Sn⁴⁺, leading to a decrease in the defect concentrations of V_Sn(-/0) and Cu_Sn(-/0), which increases the carrier concentration and reduces the impact of band tailing. The average power conversion efficiency (PCE) of the devices improved from 7.45% to 9.26%, the PCE of the best-performing CZTSSe solar cells increased from 9.25% to 9.83%, and the consistency among the devices is further enhanced, as indicated by a reduction in the standard deviation from 0.98 to 0.44. Ultimately, the device performance of the Cu²⁺+Sn²⁺-DMF system improved by 11.01% (without the MgF₂ layer) after optimization. This study serves as a reference for regulating the environment of the CZTS-PS to further enhance the CZTSSe devices' performance, and the photoelectric conversion efficiency is improved by ≈30%.