Synthesis of in-gap band CuGaS2:Cr absorbers and numerical assessment of their performance in solar cells

CuGaS2 thin films were obtained by sulfurization of CuGaSe2. CuGaSe2 thin films were first electrodeposited from aqueous solutions containing CuCl2, GaCl3, and H2SeO3 and subsequently annealed at 400 °C for 10 min in forming gas atmosphere and in the presence of molecular sulfur. This sulfurization process resulted in the complete conversion of CuGaSe2 into CuGaS2. The formation of CuGaS2 was proven by X-Ray diffraction and optical spectroscopy. Diffraction peaks of CuGaS2 shifted to higher angles than those observed for CuGaSe2 films, and the optical band gap shifted to blue rising from 1.66 eV for CuGaSe2 to 2.2 eV for CuGaS2. When Cr ions were added to the initial electrolyte, the final CuGaS2 films exhibited a broad in-gap absorption band centred at 1.63 eV that can be attributed to Cr atoms at the Ga sites. The performance of solar cells based on CuGaS2:Cr absorbers containing an in-gap absorption band was then estimated by numerical simulation using Solar Cell Capacitance Simulator Software. Both quantum efficiency and short circuit current of simulated Mo/CuGaS2:Cr/CdS/ZnO solar cells rose proportionally to the amount of Cr present in the CuGaS2:Cr absorbers. As a result, the photo conversion efficiency of the simulated devices changed from 14.7% for CuGaS2 to 34% for CuGaS2:Cr absorbers. Nevertheless, when neutral defects related to Cr-doping were introduced in the absorber layer, the positive effect of the enhancement of photon harvesting due to IGB was compensated by a decline in the carrier collection and the overall efficiency of the device diminished considerably.