Electronic properties of ZnO/CdS/Cu(In,Ga)Se-2 solar cells aspects of heterojunction formation

This contribution briefly reviews the present status of our knowledge on th e dominant recombination mechanisms in ZnO/CdS/Cu(In,Ga)Se-2 heterojunction devices. Then we discuss the question how the formation and the electronic structure of the heterointerface and the heterojunction partners can influ ence the electronic properties and even the amount of recombination in the bulk of the absorber material. We examine the role of Cd- and Cu- diffusion during junction formation and air-annealing of the completed device. Furth ermore, we explain the role of the intrinsic ZnO layer in the heterostructu re routinely used together with a chemical bath deposited CdS layer to prod uce high efficiency heterojunction solar cells. We propose that these layer s prevent electrical inhomogeneities from dominating the open circuit volta ge of the entire device. A simple parallel connection model of two diodes d emonstrates that 0.1-5% of the cell area with inferior electronic quality c ould degrade the photovoltaic performance without the buffer layer and have almost no consequences with the buffer layer. (C) 2001 Elsevier Science B. V. All rights reserved.