Band-gap-graded Cu₂ ZnSn(S_1-x,Se _x) ₄ solar cells fabricated by an ethanol-based, particulate precursor ink route

Solution processing of earth-abundant Cu 2 ZnSn(S 1-x,Se x) 4 (CZTSSe) absorber materials is an attractive research area in the economical and large-scale deployment of photovoltaics. Here, a band-gap-graded CZTSSe thin-film solar cell with 7.1% efficiency was developed using non-toxic solvent-based ink without the involvement of complex particle synthesis, highly toxic solvents, or organic additives. Despite the high series resistance due to the presence of a thick Mo(S,Se) x layer and Zn(S,Se) aggregates, a high short-circuit current density (J SC) was generated. In addition, there was no significant difference in open circuit voltages (V OC) between CZTS (0.517 V) and CZTSSe (0.505-0.479 V) cells, despite a significant band gap change from 1.51 eV to 1.24 eV. The high J SC and less loss of V OC are attributed to the effect of band gap grading induced by Se grading in the CZTSSe absorber layer. Our environmentally benign ink approach will enable the realization of low-cost, large-area, high-efficiency thin-film solar cells.