Band alignment engineering to improve open-circuit voltage in Cd-free Cu(In,Ga)Se2/(Zn,In)S thin-film solar cells

This study investigates the potential of zinc indium sulfide (Zn,In)S as a substitute for the conventional zinc oxysulfide Zn(O,S) buffer layer in Cu(In,Ga)Se-2(CIGS) solar cells. The efficiency of Cu(In,Ga)Se-2/(Zn,In)S cell can potentially improve by optimizing the indium content in the (Zn,In)S buffer layer, the gallium concentration in the CIGS absorber layer, and the conduction band offset (CBO) at the absorber/buffer interface. The improvement in fill factor (FF) and open-circuit voltage (V-oc) of the CuIn0.6Ga0.4Se2/Zn0.9In0.1S junction is attributed to the well-suited conduction band alignment and the wide bandgap of the CuIn0.6Ga0.4Se2 absorber layer. Moreover, the results indicate that the interface carrier recombination rate at CuIn0.6Ga0.4Se2/Zn0.9In0.1S junction decreases in the condition of a small positive CBO (spike-like band alignment). Furthermore, this study revealed that the use of the Zn0.9In0.1S buffer layer led to higher external quantum efficiency (EQE) values in the short wavelengths (400 similar to 580 nm) when compared to the conventional Zn(O,S) buffer layer due to its wider bandgap energy. The simulation results indicate that in the proposed CuIn0.6Ga0.4Se2/Zn0.9In0.1S solar cell, the optimized CBO level is approximately + 0.1 eV, resulting in an impressive conversion efficiency of 25.34%. This represents a significant 2% increase compared to the conventional CIGS/Zn(O,S) cell (23.35%).