11.5% efficient Cu2ZnSn(S,Se)4 solar cell from DMF molecular solution

Fabricating thin film absorber material from molecular solution is a very promising approach for low-cost and large area photovoltaic technology. The property of the molecular solution processed kesterite (CZTSSe) absorber highly depends on the components in the solution, which determine the composition of the precursor film and thus the grain growth and the property of the absorber. Here, we report the chemical reactions in N, N-dimethylformamide (DMF) solution using CuCl, Zn(OAc)2, SnCl4, and Thiourea (Tu) as precursors and the reaction path from solution to absorber material. Our results show that SnCl4 reacts with DMF and forms complex Sn(DMF)2Cl4 whereas CuCl and Zn(OAc)2 form complexes with Tu, which resulting in an amorphous sulfide kesterite (CZTS) structured precursor film. Monitoring of the selenization process reveals the precursor film took a direct substitution reaction path and a top-down and bottom up bi-direction grain growth, leading to a large grain composed double layer CZTSSe absorber film with clean surface. A champion CZTSSe device with an efficiency of 11.5%, an open-circuit voltage of 0.491 V, and a fill factor of 70.6% was achieved. Our results demonstrate coordination of Sn4+ with a weak ligand (like DMF or DMSO) that can easily de-coordinate during heat-treatment is the key for the formation of a kesterite structured precursor film that can facilitate direct phase transformation grain growth and thus fabrication of high quality kesterite absorber materials.