Effect of localized states on the reciprocity between quantum efficiency and electroluminescence in Cu(In,Ga)Se2 and Si thin-film solar cells

We present the electroluminescence and quantum efficiency of three different types of thin-film solar cells based on absorbers made from Cu(In,Ga)Se-2 as well as from mu c-Si:H and a-Si:H. Simulations of our experimental results show that the main contribution to the electroluminescence spectrum originates from band-to-tail transitions in the case of Cu(In,Ga)Se-2, from tail-to-tail transitions in mu c-Si:H, and from tail-to-tail as well as from band-to-midgap-defect transitions in a-Si:H. By comparing the electroluminescence with the quantum efficiency, we analyze the effect of localized states on the optoelectronic reciprocity relation in the three material systems. The relatively steep band-tail density of states in Cu(In,Ga)Se-2 is compatible with the reciprocity relation at room temperature while the shallower band-tail density of states in mu c-Si:H and the deep mid-gap defect states in a-Si:H lead to substantial deviations from the reciprocity. (C) 2014 Elsevier B.V. All rights reserved.