Unveiling the impact of Cu content on the physical properties and photovoltaic performance of solution‐processed Cu(In,Ga)Se2solar cell absorber

Abstract

Cu/Ga + In (CGI) ratio in Cu(In,Ga)Se2 (CIGSe) semiconductor plays a decisive role in solar energy conversion. Investigations published so far relevant to CIGSe at different CGI values are solely based on the expensive vacuum process and as a result, there is a strong urge to explore a low-cost process. In this regard, we herein report the preparation of CIGSe at diverse CGI value from 0.70 to 0.97 by a facile and cost-effective solution process for the first time. From structural analysis, the blue shift is perceived by decreasing CGI value, which is related to shrinkage in the unit cell volume because of the reduction in the Cu content. As CGI value increases, crystal diameter of CIGSe vastly changed from nano to micro (50-1300 nm), whereas crystal shape reformed from irregular to hexagonal flake-like shape, which could be due to excessive influence of Cu content that alters nucleation and growth of crystals. Diverse CGI value modified Se/(Cu + In + Ga) ratio while Ga/(Ga + In) ratio remains unchanged. Bandgap energy is increased from 1.22 to 1.41 eV as Cu-deficiency increases. The morphological analysis showed a uniform, smooth and crack-free CIGSe films with a thickness of ~1 μm. The electrical properties of CIGSe are greatly changed. For instance, at higher CGI value (0.97), the carrier concentration is increased by five-order magnitude, whereas mobility improved by 109 times and resistivity decreased by 87 times as compared to lower CGI value (0.70). Moreover, photocurrent is markedly enhanced by six-order magnitude explicitly from nA to mA. As a proof of concept, solar cells are fabricated and power conversion efficiency significantly enhanced from 0.16% to 3.38% by altering CGI value. Hence, this work eminently evidences the importance of tuning CGI value by low-cost solution process and provides new perceptions into properties of CIGSe solar cell absorber as a function of CGI value.