In this study strong evidence for an illumination-induced change in minority charge carrier diffusion length is given for Cu(In,Ga)Se2 solar cells. After annealing under illumination (light soaking) the cells show the well-known metastable increase in open circuit voltage, but also a metastable reduction in current collection efficiency (which can be reversed by annealing in the dark). Partly, this can be attributed to an increase in doping density causing a reduced space charge region width as verified by capacitance-voltage profiling.
Nevertheless, by using time-resolved photoluminescence and electron-beam-induced current measurements we found that the changes in doping density and space charge region width are not sufficient to describe the modification in current collection efficiency. Additionally there seems to be a reduction in minority carrier diffusion length and lifetime after white light soaking. This can be explained by a metastable change of electronic defects as found in temperature-dependent admittance spectroscopy. Device simulations confirm the impact of the found defects on the photocurrent.
Heise, S.J., Gerliz, V., Hammer, M.S., Ohland, J., Keller, J. and Hammer-Riedel, I., 2017. Light-induced changes in the minority carrier diffusion length of Cu (In, Ga) Se 2 absorber material. Solar Energy Materials and Solar Cells, 163, pp.270-276.
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