Over 10% Efficient Copper Zinc Tin Sulfoselenide Solar Cells from DMSO Solution Using SnCl<inf>4</inf> as Precursor

Solution processed kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorber materials have great potential as low cost photovoltaic electricity generation. Dimethyl sulfoxide (DMSO) is one of the most promising benign solvents that has achieved CZTSSe solar cell with efficiency beyond 10%. To further improve DMSO solution processed CZTSSe solar cell performance, understanding the reaction pathway from metal ion/compounds in solution to solid state film and their effects on the property and photovoltaic performance of the final absorber materials is critical. In this report, we have investigated how the precursor tin valence affects DMSO solution processed CZTSSe absorber materials property and solar cell device performance. We have fabricated CZTSSe absorber materials and solar cells by using same copper (Cu+) and zinc (Zn2+) precursors but respectively using SnCl 2 and SnCl 4 as the tin precursor. We found that Sn valence greatly affects the film morphology and composition upon thermal annealing which further determines the morphology and device performance of the selenized CZTSSe absorber materials. We found a high crystalline precursor film with CZTS and secondary SnS phase was formed from solution containing SnCl 2 precursor while a uniform and amorphous film was formed from SnCl 4 precursor solution. Both precursor films transferred to keserite CZTSSe phase after selenization, however, the film with SnCl 2 showed a non-uniform morphology with voids and impurity, resulting in solar cell with a power conversion efficiency (PCE) of only 2.29%. On the contrast, a uniform film with compact large grains was obtained from film with SnCl 4 and a power conversion efficiency (PCE) of 10.02% was achieved. A low V oc deficit of 0.596 V was observed from Sn4+ solution which was 0.809 V for Sn2+ solution. Our results indicate the use of Sn4+ as the precursor might be a solution to address the large V oc deficit issue of kesterite solar cells, which deserves further investigation.