Textured, doped, ZnO thin films produced by a new process for a-Si and CIGS solar cell application

Abstract

The properties of a transparent conductive oxide (TCO) used as a front electrode for thin-film solar cells and modules play a major role in determining the maximum attainable conversion efficiency. Doped ZnO is an important TCO that is widely used in amorphous/nanocrystalline silicon (a-Si/nc-Si) and CIGS thin-film solar cells. In the case of a-Si/nc-Si cells, the ZnO thin film should be textured to promote light trapping to increase the short-circuit current density J_sc. In this work, textured, aluminum-doped ZnO (ZnO:Al) thin films have been directly deposited by a sputtering-based method and without the need for post-deposition etching. The morphology, optical properties and electrical properties of the films have been studied. SEM micrographs show that feature sizes around 0.2 - 0.4μm have been achieved at a film thickness of 1μm, and that the morphology can be controlled by the deposition conditions. AFM images were analyzed to extract a set of topographic parameters (amplitude, spatial, and hybrid). The optical transmission, haze, and angle-resolved light scattering of the textured ZnO:Al films were measured and compared to properties of commercially-available textured SnO₂:F thin films on glass. Higher haze and reduced absorption could be obtained with the textured ZnO:Al films. Hall effect measurements on these films yielded a carrier concentration and mobility of 2.75 x 10²⁰cm^-3 and 24.1cm²/Vs, respectively. We also report that the use of these textured ZnO:Al films as the top TCO for CIGS solar cells results in reduced cell reflectance and increased J_sc. The novel deposition method provides a potential pathway to large area and cost effective production of a textured ZnO TCO for thin-film PV manufacturing operations.