Mitigating Sn loss via anion substitution in the Cu2+–Sn2+ precursor system for Cu2ZnSn(S, Se)4 solar cells

Abstract

Solution-processing has been a very successful fabrication route for Cu₂ZnSn(S, Se)₄ (CZTSSe) absorbers: since 2010, every world record for CZTSSe-based solar cells has been achieved by this approach. The solution formulation in terms of both cation oxidation states and choice of counterion varies between laboratories. Here, we investigate the influence of various counter anions on the element loss mechanisms, absorber formation path and the resulting PV properties. A Cu²⁺–Sn²⁺ system was used due to its advantages in terms of fabrication in ambient conditions. We found that solutions containing excess amounts of Cl⁻ anions result in pronounced Sn loss, as the Sn(DMSO)₂Cl₄ complex decomposes into volatile products at elevated temperatures. Fabricating a Sn-rich solution to accommodate for Sn loss is not a viable strategy, because it leads to SnSe₂ formation. Accumulation of SnSe₂ causes local PV performance degradation, which prevents the fabrication of uniform samples. By partially substituting OAc⁻ for Cl⁻ ions, Sn loss was mitigated, and PV performance uniformity improved. The highest efficiency achieved was 11.8% (12.5% active area). These results show the importance of precursor salt choice to mitigate Sn loss and enhance uniformity, which is a crucial aspect in view of future scale up of solution-processed CZTSSe devices.