Voltage dependent electromodulated photoluminescence of chalcopyrite solar cells Original Research Article

The dependence of radiative recombination of various polycrystalline and epitaxially grown CuGaSe2 solar cells on the applied voltage, VA, is investigated by electromodulated photoluminescence (EMPL). EMPL is a derivative method which is sensitive to electric field induced changes in the PL response [1]. With increasing forward bias the PL intensity increases, reaches a maximum and keeps constant or decreases again. Previously the increase of the PL intensity with the applied voltage has been attributed to the dead-layer-model. This approach leads to many contradictions, among others the decrease of PL intensity for voltages exceeding the built-in voltage, Vbi, cannot be explained. In this study, a model is developed based on minority carrier statistics and Shockley–Read–Hall recombination, that fits the experimental data obtained from different CuGaSe2 solar cells at various temperatures. The parameters are the built-in voltage, a field reducing factor for voltages exceeding Vbi and an effective mobility of minority carriers. It can be shown, that the decrease in PL intensity for VA>Vbi can be solely attributed to the sweeping out of charge carriers by the applied field.