Understanding ERE and iVOC Metrics for Graded CdSeTe Absorbers

ABSTRACT 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 cm 2 /(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 cm 2 /(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.