Optimum Electrical and Optical Properties of Transparent Conducting Oxide for Cu(In,Ga)Se2 Photovoltaic Module Applications

Abstract

Herein, the transparent conducting oxide (TCO) characteristics of the window layer as a factor in the decrease of conversion efficiency from lab-scale cells to modules are focused on. Then, an optical model and a circuit model are constructed based on measured data, and TCO parameters are investigated to optimize Cu(In,Ga)Se2 (CIGS) module characteristics. Simulations assuming ZnO:Al and ZnO:B as the TCO, which are commonly used in CIGS modules with a substrate structure, show that the range of the TCO parameters to obtain high efficiency is limited. In particular, the optimum values of module characteristics are separated into high-carrier density/thin-film region with a smooth surface and low-carrier density/thick-film region with a textured surface for the TCO layer. In addition, simulations assuming a high-mobility TCO and a wide-gap CIGS absorber to boost conversion efficiency reveal that the wide-gap CIGS absorber mitigates the effects of resistance losses and free carrier absorption due to the TCO, thereby expanding the design range of the carrier density in the TCO layer.