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

Abstract

This contribution briefly reviews the present status of our knowledge on the 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 influence 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. Furthermore, we explain the role of the intrinsic ZnO layer in the heterostructure routinely used together with a chemical bath deposited CdS layer to produce high efficiency heterojunction solar cells. We propose that these layers prevent electrical inhomogeneities from dominating the open circuit voltage of the entire device. A simple parallel connection model of two diodes demonstrates that 0.1–5% of the cell area with inferior electronic quality could degrade the photovoltaic performance without the buffer layer and have almost no consequences with buffer layer.