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Chemical Control of Correlated Metals as Transparent Conductors

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Transparent conducting materials have a wide variety of applications. They are a key component of photovoltaic and display technologies, and increasing demands are being made of such materials to continue the rapid growth of these industries. Suitable TCMs are identified primarily via a high electrical conductivity and a small optical absorption in the visible part of the spectrum (1.75 – 3.2 eV). Transparent Conducting Oxides (TCOs) are the current front-running commercial materials with Tin-doped Indium Oxide (ITO) and Fluorine-doped Tin Oxide (FTO) being the most widely used. Practical limitations of doped semiconductor TCOs include low solubility limit of the dopant, toxicity of common dopants such as fluorine in FTO (with the use of HF in the production process) and the scarcity of In increasing the cost of ITO . The current alternative to TCO materials are very thin layer of conventional metals such as Ag. In this case the overall performance of the material as a TCM is limited by the large electron mean free path of conventional metals ( 50 nm) increasing the interfacial scattering and therefore reducing the coating conductivity.

Correlated metallic transition metal oxides offer a route to thin film transparent conductors that is distinct from the degenerate doping of broad band wide gap semiconductors. In a correlated metal transparent conductor, inter-electron repulsion shifts the plasma frequency out of the visible region to enhance optical transmission, while the high carrier density of a metal retains sufficient conductivity. By exploiting control of the filling, position and width of the bands derived from the B site transition metal in ABO3 perovskite oxide films, we show (Adv. Funct. Mater. 2019, 1808609) that pulsed laser deposition-grown films of cubic SrMoO3 and orthorhombic CaMoO3 based on the second transition series cation 4d2 Mo4+ have superior transparent conductor properties to the first transition series 3d1 V4+-based SrVO3. The increased carrier concentration offered by the greater bandfilling in the molybdates gives higher conductivity while retaining sufficient correlation to keep the plasma edge below the visible region. The reduced binding energy of the n = 4 frontier orbitals in the second transition series materials shifts the energies of oxide 2p to metal nd transitions into the near-ultra violet to enhance visible transparency. The A site size-driven rotation of MoO6 octahedra in CaMoO3 optimizes the balance between plasma frequency and conductivity for transparent conductor performance. We have demonstrated that by using the three chemically controllable parameters of carrier density, orbital energy and bandwidth we can tune the charge transfer band position, plasma frequency and conductivity to optimize the transparent conductor performance of non-toxic, earth abundant Mo-based correlated metal perovskite oxides to match the best-in-class wide band gap semiconductors. These strategies open new paths to chemically control the performance of correlated transparent conductors.

This talk is part of the Quantum Matter Seminar series.

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