The role of drift, diffusion, and recombination in time-resolved photoluminescence of CdTe solar cells determined through numerical simulation

Time-resolved photoluminescence (TRPL) measurements are one of the key metrics available to determine the minority-carrier lifetime in the absorber layer of direct band gap photovoltaic devices. Direct measurement of the minority-carrier lifetime is essential to understanding the impact of changes in deposition and processing on material quality. Unfortunately, the TRPL signal is determined by a complex convolution of multiple physical factors including bulk carrier lifetime, interface recombination velocity, electric field, doping density, photo-excited carrier density, and carrier mobility. To gain clarity, we have used numerical simulations to analyze the carrier dynamics after a CdTe device is illuminated with a short light pulse. After the light pulse, the photo-generated carriers undergo complex dynamics including drift, diffusion, interface, and bulk recombination. In this work, we develop a new formalism that enables much greater insight into which factors dominate the TRPL decay dynamics. By breaking down the carrier dynamics into drift, diffusion, and recombination terms, we have developed six-factor, four-factor, and two-factor analyses that provide clear understanding of which physical factors dominate the decay dynamics under various conditions and at different times during the decay. We show that in a typical CdTe device under the typical experimental conditions used in our laboratories, the faster part of the decay is dominated by charge separation, whereas the slower part is dominated by carrier recombination. Therefore, under the conditions investigated in this study, the slower part of the decay is a better parameter to explain the defect density in the CdTe layer. Copyright (c) 2013 John Wiley & Sons, Ltd.