Highly efficient solution-processed CZTSSe solar cells based on a convenient sodium-incorporated post-treatment method

In CZTSSe solar cells, a simple sodium-incorporation post-treatment method toward solution-processed Cu 2 ZnSnS 4 precursor films is presented in this work. An ultrathin NaCl film is deposited on Cu 2 ZnSnS 4 precursor films by spin-coating NaCl solution. In subsequent selenization process, the introduction of NaCl is found to be benefacial for the formation of Cu 2- x Se, which can further facilitate the element transportation, leading to dense and smooth CZTSSe films with large grains and less impurity Cu 2 Sn(S,Se) 3 phase. SIMS depth profiles confirm the gradient distribution of the sodium element in Na-doped absorbers. Photoluminescence spectra show that the introduction of appropriate sodium into the absorber can inhibit the band tail states. As high as 11.18% of power conversion efficiency (PCE) is achieved for the device treated with 5 mg mL −1 NaCl solution, and an average efficiency of Na-doped devices is 10.71%, 13% higher than that of the control groups (9.45%). Besides, the depletion width and the charge recombination lifetime can also have regular variation with sodium treatment. This work offers an easy modification method for high-quality Na-doped CZTSSe films and high-performance devices, in the meantime, it can also help to further understand the effects of sodium in CZTSSe solar cells.