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 (Ln) 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 pMS were determined at a reverse bias of 1.5 V. In a plot of Ln against pMS, it was found that, despite much scatter in the experimental points, there was an apparent trend of Ln decreasing by about an order of magnitude with increase of pMS 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.