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)Se-2 (CIGSe) solar cells on stainless steel (SUS) substrates are developed. The contribution concentrates on the investigation of the correlation between Urbach energy (E-U) and open-circuit voltage deficit (V-OC,V-def ). The several CIGSe solar cells on soda-lime glass and SUS substrates with various V-OC,V-def values are fabricated through the variations of the [Ga]/([Ga] + [In]) ratio (GGI), substrate temperature (T-SUB), and Fe concentration of their CIGSe absorbers. The E-U is determined based on external quantum efficiency in the long-wavelength edge. It is determined that the E-U is influenced by the GGI, T-SUB, and Fe concentration. The E-U is well consistent with the carrier lifetimes and can be an indicator of the CIGSe quality. In addition, the relationship between E(U )and V-OC,V-def is obviously observed, where the decrease in the E-U by 1 meV reduces the V(OC,def )by 8.6 mV. Through the optimizations of GGI and T-SUB as well as the minimization of Fe concentration, the E-U is obviously reduced, which implies the improvement of the CIGSe quality. Ultimately, the high eta of 17.9% for the flexible CIGSe solar cell on SUS substrate is attained.