The Effects of Inhomogeneous Carrier Lifetime on Time-Resolved Photoluminescence Measurements and CdSeTe Thin-Film Solar Cell Performance

Abstract

Carrier lifetime is a key parameter influencing solar cell performance and other semiconductor devices. Interpretations of carrier lifetime measurements often report a singular bulk lifetime. However, conventional lifetime measurements such as time-resolved photoluminescence (TRPL) generally excite an area that may encompass significant recombination inhomogeneity, including thousands of grains in polycrystalline films with grain-to-grain lateral carrier lifetime variation, inhomogeneity in the growth direction, and recombination at grain boundaries. Analysis and computational modeling here indicate that short lifetime regions in thin film solar cells have an overweighted impact on device performance. Consequently, common interpretations of conventional TRPL measurements on such films may significantly overestimate the effective carrier lifetime corresponding to solar cell performance. This paper reports the carrier lifetime mapping obtained using a TRPL system with confocal microscopy to provide a two-dimensional distribution of carrier lifetimes with sub-micron spatial resolution in polycrystalline CdSeTe films used in high efficiency solar cells. This technique provides a better means to assess the underlying mechanisms contributing to both longer- and shorter-lifetime regions, and thereby insight into methods and remedies to enhance carrier lifetime and thin-film solar cell performance, both by relating process parameters to lifetime inhomogeneity and enabling correlative metrology on intact, imaged films.