Examination of Relationship between Urbach Energy and Open-Circuit Voltage Deficit of Flexible Cu(In,Ga)Se₂ Solar Cell for Its Improved Photovoltaic Performance

Flexible Cu(In,Ga)Se2 (CIGSe) solar cells on stainless steel (SUS) substrates are developed. The contribution concentrates on the investigation of the correlation between Urbach energy (EU) and open-circuit voltage deficit (VOC,def). The several CIGSe solar cells on soda-lime glass and SUS substrates with various VOC,def values are fabricated through the variations of the [Ga]/([Ga] + [In]) ratio (GGI), substrate temperature (TSUB), and Fe concentration of their CIGSe absorbers. The EU is determined based on external quantum efficiency in the long-wavelength edge. It is determined that the EU is influenced by the GGI, TSUB, and Fe concentration. The EU is well consistent with the carrier lifetimes and can be an indicator of the CIGSe quality. In addition, the relationship between EU and VOC,def is obviously observed, where the decrease in the EU by 1 meV reduces the VOC,def by 8.6 mV. Through the optimizations of GGI and TSUB as well as the minimization of Fe concentration, the EU is obviously reduced, which implies the improvement of the CIGSe quality. Ultimately, the high η of 17.9% for the flexible CIGSe solar cell on SUS substrate is attained.