Diffusion length variation in photovoltaic cells with Bridgman-grown CuInSe2 substrates

In a group of CuInSe2-CdS-ZnO photovoltaic cells, where the absorber was a layer cut from Bridgman-grown p-type CuInSe2 ingots, electron diffusion lengths at room temperature (L n) were estimated by the photocurrent-capacitance method. Dark capacitance measurements were also made on the same cells against reverse bias and from Mott-Schottky plots, slope concentrations p MS were determined at a reverse bias of 1.5 V. In a plot of L n against p MS, it was found that, despite much scatter in the experimental points, there was an apparent trend of L n decreasing by about an order of magnitude with increase of p MS from 1016 to 1017 cm-3. Detailed proposals were then made to explain this trend. These were reduction of lifetime via Shockley-Read trapping with mobility decrease by impurity scattering, shunt resistance lowering by light and optical penetration depth reduction at shorter illumination wavelengths