Effect of Polaron Formation on Optical and Carrier Transport Properties of Transition Metal Oxides as Photoelectrodes from First-Principles Calculations

Transition metal oxides are promising photoelectrode materials for solar-to-fuel conversion applications.However, their performance is limited by the low carrier mobility due to theformation of small polarons. Recent experimental studies have shown improved carrier mobility andconductivity by atomic doping; however the underlying mechanism is not understood. A fundamentalatomistic-level understanding of the effects on small polaron transport is critical to future material design withhigh conductivity. In this talk, we will discuss the effect of small polaron formation on optical and carriertransport properties of transition metal oxides from first-principles calculations.First, we resolve the conflicting findings that have been reported on the optical gap of a well-known catalysis Co₃O₄as an example[1]. We confirm that the formation of small hole polarons significantly influences the optical absorption spectra and introduces extra spectroscopic signature below the intrinsic band gap, leadingto a 0.8 eV transition that is often misinterpreted as the band edge that defines the fundamental gap.<br/><br/>Then we discuss the formation of small polarons' effect on carrier concentration, by resolving the controversyof nature of "shallow" or "deep" impurities of intrinsic oxygen vacancies in BiVO₄ as an example[2], i.e. how tounify different experiments with the correct definition of ionization energy in polaronic oxides. We furtherdiscuss why certain dopants can have very low optimal concentrations (or very early doping bottleneck) inpolaronic oxides such as Fe₂O₃, through a novel "electric-multipole" clustering between dopants andpolarons[3]. These multipoles can be very stable at room temperature and are difficult to fully ionize comparedto separate dopants, and thus they are detrimental to carrier concentration improvement. This allows us to uncover mysteries of the doping bottleneck in hematite and provide guidance for optimizing doping and carrier conductivity in polaronic oxides toward highly efficient energy conversion applications. In addition, we show the importance of synthesis condition such as synthesis temperature and oxygen partial pressure on dopantand polaron concentrations, and how to optimize the synthesis condition based on theoretical predictions[4].<br/><br/>At the end, we show different theoretical models for polaron mobility calculations from a macroscopic dielectriccontinuum picture with an example of spin polarons in CuO[5] and a microscopic polaron hopping picture bycombining generalized Landau-Zener theory and kinetic Monte-Carlo samplings for doped oxides[6]. Our first-principles calculations provide important insights and suggest design principles for optimal optical andtransport properties of polaronic oxides.<br/><br/>References:<br/>[1] “Optical Absorption Induced by Small Polaron Formation in Transition Metal Oxides – The Case of Co₃O₄”,T. Smart, T. Pham, Y. Ping*, and T. Ogitsu*, <i>Physical Review Materials (Rapid Communications)</i>, <b>3</b>,102401(R), (2019).<br/>[2] “The Role of Point Defects in Enhancing the Conductivity of BiVO₄”, H. Seo, Y. Ping and G. Galli*,<i>Chemistry of Materials</i>, 30, 7793, (2018).<br/>[3] “Doping Bottleneck in Hematite: Multipole Clustering by Small Polarons”, T. Smart, V. Baltazar, M. Chen, B.Yao, K. Mayford, F. Bridges, Y. Li, and Y. Ping*, <i>Chemistry of Materials, 33, 4390</i>, (2021).<br/>[4] “The Critical Role of Synthesis Conditions on Small Polaron Carrier Concentrations in Hematite- A First-Principles Study”, Tyler Smart, Mingpeng Chen, Valentin Urena Baltazar, Frank Bridges, Yat Li, Yuan Ping*, <i>Journal of Applied Physics</i>, <b>130</b>, 245705, (2021).<br/>[5]“Mechanistic Insights of Enhanced Spin Polaron Conduction in CuO through Atomic Doping”, T. Smart, A.Cardiel, F. Wu, K. Choi and Y. Ping*, <i>npj Computational Materials</i>, 4, 61, (2018).<br/>[6] “Combining Landau-Zener Theory and Kinetic Monte Carlo Sampling for Small Polaron Mobility of DopedBiVO4 from First-principles”, F. Wu and Y. Ping*, <i>Journal of Materials Chemistry A</i>, 6, 20025, (2018).