Performance limitations in practical transparent conducting oxide thin films

Zinc oxide (ZnO) has long been advanced as a low cost, earth-abundant transparent conducting oxide (TCO) with potential as a replacement for high-performance, but costly, indium oxide (In2O3) based materials in a wide range of technological applications. However, despite decades of research and development efforts, ZnO-based materials have still failed to displace the incumbents in any large-scale applications. Given the compelling materials cost advantages of ZnO, it is almost certain that its poor adoption is due to deficits in its technical performance. This thesis aims to fulfill the need for systematic, fundamental work to identify and examine the factors that limit TCO performance, and in particular, those that limit ZnO relative to In2O3. Using spray pyrolysis as the primary deposition method, many different series of ZnO and In2O3 films have been prepared and examined using a range of chemical, structural, and optoelectronic characterization techniques. After essential background information on the basic physics and chemistry of TCOs, as well as a detailed discussion of the chosen deposition and characterization methods, three main classes of performance limitations will be covered: 1) those related to the intrinsic properties of electronic transport in crystalline TCO domains, 2) those arising in the course of impurity doping, and 3) those occurring due to grain boundary effects and the polycrystalline nature of thin film TCO samples. Taken together, these results will show that preparing ZnO-based TCOs with performance approaching that of the best In2O3-based materials, while very likely to be technically possible, will almost certainly involve overcoming significant engineering and process development challenges that, importantly, are not required to make high quality In2O3. Ultimately, whether ZnO will ever find significant, real-world use as a TCO will depend on whether the deep differences between ZnO and In2O3 performance limits that will be highlighted and examined in this thesis can be bridged in a practical and cost-effective manner.