Understanding ERE and iVOC Metrics for Graded CdSeTe Absorbers

PL-based external radiative efficiency (ERE) and implied open-circuit voltage (iV_OC) metrics were introduced for thin-film solar absorbers to better understand the voltage deficit and diagnose losses in solar cells. Traditionally, elevated ERE and iV_OC measurements are associated with diminished recombination within the solar device, a rationale heavily reliant on the assumption of a uniform bandgap and high carrier mobilities in the absorber. Recently, very low mobilities in CdSeTe absorbers (< 1 cm²/(V·s)) were measured using the light-induced transient grading technique. In this study, we use a detailed numerical model of iV_OC to investigate the possible reasons of elevated iV_OC in realistic CdSeTe absorbers with a graded Se profile. In particular, we examine how the bandgap nonuniformity and the reduced hole mobility in graded CdSeTe absorbers affect iV_OC measurements. We show that high iV_OC may result from inflated quasi-Fermi level splitting in the high-Se region in the front part of a CdSeTe absorber with slow hole transport. We reproduce the experimentally reported 360 mV increase in iV_OC–V_OC gap with reduced doping using a model with sub-1 cm²/(V·s) hole mobility in the high-Se region. Based on our results, we conclude that the iV_OC metric (or ERE metric) should not be used as a sole metric of CdSeTe absorber quality. We discuss possible ways to extract useful information from the iV_OC–V_OC gap by supplementing the front-side illumination measurements with back-side illumination measurements.