Incredible advances over the past decade in computational physics as well as high-throughput materials synthesis and characterization are enabling a fundamentally new approach to new materials discovery and development. This theory driven approach to the computational design of new materials and material properties aims to directly guide the experimental materials development rather than retroactively explaining the observed properties. This new paradigm for developing and discovering materials with specific desired functionalities and of understanding the property, structure, morphology, and composition relationships requires tight coupling of theory, experiment and characterization to accelerate the rate of materials evolution and innovation. Here, we present our recent and on-going work on the development of p-type transparent conducting oxides (p-TCOs) as a case study in the application of Inverse Design approaches to materials development.
ABO spinel oxides can be classified into four Doping Types depending on the relative energy ordering and level within the gap of the cation anti-site defect acceptor and donor levels. Doping Type II (DT-2) spinels where the acceptor lies above the donor and, in addition, the donor lies in valance band, are natural host materials for p-type conduction. High-throughput theoretical screening finds CoZnO to be the prototype DT-2 material. In particular, the Co site defect level is resonant in the valence band, making it electrically neutral thus allowing the electrically active Zn acceptor to yield p-type conductivity, independent of the concentration of Co defects. Resonant elastic x-ray diffraction (REXRD) site occupancy measurements on bulk ceramic samples grown in air at 800 Â°C confirm this basic prediction. Further, intentional non-equilibrium growth to increase the Zn concentration due to either quenched-in cation-site-occupancy disorder or incorporation of excess Zn should be an effective doping strategy. Experiments using combinatorial co-sputtered CoZnO thin film “libraries” with intentional composition gradients on 2”x2” glass substrates support this. More definitive REXRD site occupancy measurements on as-deposited and annealed CoZnO and CoNiO films grown epitaxially on SrTiO by pulsed laser deposition confirm this. Finally, 17 candidate extrinsic dopants were evaluated theoretically yielding Li, Mg and Ni as the most promising. Ni doping was tested via combinatorial co-sputtering and found to be effective. In fact, the conductivity of CoZnNiO increases monotonically with increasing Ni all the way to CoNiO for which σ ≈ 100 S/cm.
This work is supported by the U.S. Department of Energy, Office of Science, BES, under Contract No. DE-AC36-08GO28308 to NREL as part of the DOE Energy Frontier Research Center "Center for Inverse Design".