Construction of a Low-Resistivity Carbon Layer To Improve Performance in Dimethyl Sulfoxide-Based Kesterite Solar Cells

Morphology of the absorber plays a decisive role in photoelectric conversion efficiency (PCE) of kersterite solar cells. Cu₂ZnSn(S,Se)₄ (CZTSSe) grain prepared from dimethyl sulfoxide (DMSO)-based solution easily grows into large grains, which can lead to the formation of some holes at the back of the absorber. These holes cause the recombination of photocarriers and greatly weaken the performance of CZTSSe devices. Here, trace amounts of thioglycolic acid (TGA) are introduced to the DMSO-based solution, and a combination of TGA and metal is formed in the absorber, leading to the formation of fine grains in the CZTSSe absorber. Next, post-annealing (PA) in a N₂ atmosphere is performed to promote Na diffusion, helping the transition from a fine-grain layer to a low-resistivity carbon layer at the interface between CZTSSe and Mo and avoiding the drawbacks of the DMSO-based system. Finally, the champion PCE of the CZTSSe device can be improved to 10.05% from 8.06%. The conclusions demonstrate that the construction of a carbon layer can boost the performance of CZTSSe devices.