Phototransistor effects in Cu(In,Ga)Se2 solar cells

Abstract

Cu(In,Ga)Se2 solar cells exhibit non-idealities with respect to a standard diode especially at low temperatures: a blocking behavior of the dark current in forward direction, a crossover of the dark and illuminated current–voltage-characteristics and a saturation of the open circuit voltage with respect to temperature and illumination intensity. It will be demonstrated that at low temperatures a Cu(In,Ga)Se2 solar cell with a back barrier essentially behaves as a phototransistor with a ZnO-emitter and Schottky collector. Hereby, the base current is provided by photoexcited holes in the field-free region of the absorber. Comparing simulations with measurements it will be illustrated that these non-idealities can be quantitatively explained. It will be shown that depending on the barrier height of the back diode and the doping profile these characteristics even occur at room temperature. This back barrier might be due to a Schottky contact or to a low doping at the back contact and a higher doping level close to the heterointerface.