Cu2ZnSn(S,Se)4 solar cell absorbers processed from Na-containing solutions in DMSO

Cu2ZnSn(S,Se)4 absorbers were processed from metal salt solutions in dimethyl sulfoxide (DMSO) and annealed in reactive Se atmosphere. A simple way to introduce sodium doping by adding NaCl to the initial precursor solution is presented. The additional sodium has no significant effect on the absorber morphology, but it improves the device performance from 5.01% to 6.09%. The sodium doping clearly reduces the recombination rate as manifested in a higher Jsc. and enhanced carrier collection. It is also shown that the absorber layer thickness should be optimized to ∼1.5 µm. By reducing the absorber thickness we found a facile way to extend the well crystallized top crust over the whole absorber and thus improve the solar cell performance.