Design and synthesis of novel p-type TCOs: From computational screening towards film deposition

Abstract

Over the last decades, great progress has been made in the field of n-type TCO materials for various opto-electronic applications. This already resulted in the device implementation and commercialization of many oxides. However, p-type TCO materials research is still in a fundamental stage, and materials science has yet to find a solution to the generally low carrier mobilities in these materials. Especially for the design of transparent electronics, the availability of transparent p-n junctions is critical for many electronic components. Using modern day’s widespread availability of material databases in combination with powerful computational techniques allows for high-throughput screenings of thousands of oxides in search of various desired properties. For potential TCO candidates, parameters such as the carrier effective mass, the optical band gap and the (extrinsic) dopability are having high priorities. The candidates resulting from these screenings are often complex multi-metal oxides, in which the matrix material typically requires the addition of external acceptor dopants. All in all, these requirements contribute to the fact that the synthesis of these “designer oxides” is often not straightforward. Additionally, the synthesis should be implemented in a process that ultimately leads to the fabrication of functional oxide films. By depositing these thin films using chemical solution deposition (CSD) instead of more traditional vacuum techniques, we can overcome some challenges that come along with the deposition of multi-metal oxides, such as homogeneity issues and control of the stoichiometry, including dopant addition. In this work, two recent p-type TCO candidate materials were selected for further study: Li-doped Cr2MnO4 and Na-doped Ln2SeO2 (Ln: lanthanide). Both of these materials are identified as primary candidates from their respective computational screening procedures. Aqueous precursor solutions for both materials are developed, in which the metal ions are stabilized by α-hydroxy carboxylic acids which fulfill the role of ligands. Thin films of these materials are deposited via spin-coating. Ultimately, a thermal treatment (combined with a selenization step for material 2) leads to crystalline films of the desired phase. The resulting films are characterized further via XRD (phase formation) and SEM (morphology). In addition, issues regarding phase formation are investigated and circumvented by optimizing the synthesis and processing steps.