Efficiency Enhancement of CIGS Solar Cells via Recombination Passivation

Cu(In,Ga)Se2 (CIGS) solar cells attract intense interest both in research and industry fields due to their high power conversion efficiencies, low costs, and high stabilities. The power conversion efficiency of a CIGS solar cell is limited by the deep level defects distributed on the surface and in the bulk of the CIGS solar cells. In this work, we elucidate that the deep level defects are significantly suppressed during the CdS buffer layer deposition process via a chemical bath deposition (CBD) method. The X-ray photoelectron spectroscopy results show the incorporation of cadmium (Cd) into CIGS during the CdS deposition process, which is the main reason for the reduced deep level nonradiative recombination defects. Raman spectroscopy and secondary ion mass spectroscopy further reveal the reduction of nonradiative recombination deep level defects due to Cd incorporation. Besides, the high-resistivity i-ZnO layer provides field-effect passivation, leading to the enhancement of the power conversion efficiency from 11.0 to 12.1%. By a combination of the incorporation of Cd, i-ZnO passivation, and optimization of the band gap of CIGS, the champion power conversion efficiency of the CIGS solar cell is as high as 15.5% without an antireflective layer and alkali metal postdeposition treatment. Our results provide insights into the effect of the CdS buffer layer, CBD method, and high-resistivity i-ZnO layer on the performance of CIGS solar cells, which is quite helpful to further improve the power conversion efficiency of CIGS solar cells.