Enhancing Grain Growth for Efficient Solution-Processed (Cu,Ag)₂ZnSn(S,Se)₄ Solar Cells Based on Acetate Precursor

Material crystallinity is the overriding factor in the determination of the photoelectric properties of absorber materials and the overall performance of the photovoltaic device. Nevertheless, in the Cu₂ZnSn(S,Se)₄ (CZTSSe) photovoltaic device, the bilayer or trilayer structure for the absorber has been broadly observed, which is generally harmful to the cell performance because the probability of photogenerated carrier recombination at grain boundaries significantly increased. Herein, our experiment reveals that the application of anions to a new family of (Cu,Ag)₂ZnSn(S,Se)₄ (CAZTSSe) materials leads to an increase in grain size and crystallinity. It is inspiring that using acetate starting materials in the precursor solution, a uniform, compact, and pinhole-free CAZTS precursor film was obtained, and the smoothness of the films surpassed that of films fabricated from the oxide route. More importantly, the crystallization of the CAZTSSe film has been considerably enhanced after selenization, and large grains going through the entire absorber layer was successfully obtained. Additionally, it is observed that the V_oc accompanied by excellent crystallinity improved significantly due to the pronouncedly reduced carrier recombination loss at grain boundaries. As a consequence, the power conversion efficiency (PCE) of the CAZTSSe photovoltaic device is successfully increased from 10.35% (oxide route) to 11.32% (acetate route). Importantly, our work attests to the feasibility of tuning the crystallization of the CZTSSe film by simple chemistry.