Increased efficiency in thin-film solar cells requires higher degree of understanding of the recombination mechanisms, and separating the various types of recombination involved. Time resolved photoluminescence (TRPL) measurement is a contactless and quick measurement method, and therefore one of the key metrics available to determine the minority-carrier lifetime in the absorber layer of photovoltaic devices. However, when a measurement is performed on a device, in addition to carrier lifetime, the TRPL signal is affected by multiple factors including drift, diffusion and interface recombination. Deconvolution of these factors and interpretation of results is often difficult.Numerical simulations are used to analyze carrier dynamics after a sample is illuminated with a short light pulse. Simulations can determine how material parameters such as doping and defect density at the interface and bulk, as well as experimental conditions such as wavelength, illumination intensity and spatial distribution of photo-generated carriers affect the measured result.A new formalism that enables greater insight into which factors dominate the TRPL decay dynamics was developed. By breaking down the carrier dynamics into drift, diffusion, and recombination terms this formalism can point which physical factors dominate the decay dynamics under various conditions and at different times during the decay. Using this mechanism, it was found that in a typical CdTe device under typical experimental conditions used in our laboratories, the faster part of the decay is dominated by drift and diffusion of carriers, while the slower part is dominated by carrier recombination.
Dr. Ana Kanevce received a Ph.D in Physics at Colorado State University doing characterization and modeling on thin-film solar cells. Since 2007 she has been a member of the Electro-Optical Characterization Group at the National Renewable Energy Laboratory. Her research is concentrated on different PV technologies, including silicon heterojunction solar cells, multijunction solar cells based on III-V materials, and thin-film polycrystalline solar cells with Cu(In,Ga)Se2, Cu2(Zn,Sn)Se4 and CdTe absorbers.
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