Transparent conductive thin films from ZnO doped with Al and Ga for high efficiency multilayer solar cells

Abstract

Transparent electronics are an essential ingredient in many of the new technologies which are set to emerge in the 21st century - transparent and interactive displays, solar windows, photonics for high speed communications, and high efficiency multilayer solar cells. The development of functionalised transparent conductive [oxide] materials (TCOs), which are abundant, cheap and environmentally friendly is critical for materials science, and underpins the long term future for high efficiency solar energy. Specifically, an important research goal is to find potential substitutes for existing TCO materials such as indium tin oxide (ITO), which are high performing, but require rare, expensive and/or toxic materials such as indium. TCOs based on zinc oxides doped with aluminium (AZO), are promising candidates for such a substitute but exhibit much lower conductance than ITO. Gallium doped ZnO (GZO) materials perform better than AZO, but suffer from similar concerns as ITO, in terms of the rarity and high cost of gallium. This project focusses on transparent conductive AZO thin films, incorporating seed layers doped with traces of gallium. The research goal is to enhance the conductivity of AZO materials, using minimum quantities of gallium in the process. The project employed the sol-gel 61 process, a solution based chemical process for synthesis of AZO/GZO nanoparticles, which are then deposited as thin films using a spin coating technique. Composite multilayer films were produced with around 400nm thickness, exhibiting transmittance above 90% across the visible range, and resistivity on the order of 100 Ω.cm. The electrical and optical properties and crystal structure of the films were characterised using UV- Vis spectroscopy, X-ray diffraction, scanning electron microscopy, and methods for measuring electrical conductivity in thin films. Preliminary results indicate promising performance in AZO films with a thin seed layer of co-doped AZO/GZO