Verification of phototransistor model for Cu(In,Ga)Se2 solar cells

Previous studies of Cu(In,Ga)Se-2 thin film solar cells showed that the long-term stability critically depends on the bias across the junction. As a result of a dark anneal the current-voltage (IV)-characteristics in the dark showed a blocking behavior with increasing anneal time. In the final stage the device exhibits an open circuit voltage (V-oc) which is independent from the illumination intensity, a crossover of the dark and illuminated IV-characteristics and Voc saturation for decreasing temperatures. These characteristics also occur in the initial state prior to the endurance test, however, at low temperature (<200 K) measurements. We suggested a phototransistor model to explain the observed characteristics. The prerequisite of this model is the existence of a Schottky barrier at the back contact. In this contribution more insights into this phototransistor model and its experimental verification will be given and discussed. Finally we suggest how to avoid the effects of the back barrier with the help of a CuGaSe2 layer at the back of the absorber and a Ga gradient through the absorber. These measures will be verified with simulations and compared to measurements on co-evaporated devices. (C) 2014 Elsevier B.V. All rights reserved.