Defect complexes and charge compensation in Ta-doped anatase TiO2 transparent conductor

The stability of point defects and defect complexes in Ta-doped anatase TiO₂ (TaTO) transparent conductive (TC) oxide and their effect on its structural, electronic, optical, and transport properties are comprehensively investigated combining first principles simulations and experiments. Depending on synthesis conditions (O-poor vs O-rich), the interactions between dopants and native defects cause diverse nonlinear charge-compensation effects that may reduce the quasi-free effective charge carriers, even at high levels of extrinsic donor doping (ND = 1020-1021 cm-3). This mitigates visible transmittance losses while sustaining and fine-tuning TC properties. Our findings indicate that: i. O-poor synthesis conditions or high donor defect concentrations are required to maintain low resistivity; ii. under O-poor conditions, oxygen vacancies (VO) primarily contribute to the transmittance losses in the visible range; iii. oxygen interstitials (OI) or O-rich conditions allow for nonlinear charge-compensation effects and restore transparency in the entire visible range; iv. different compensation mechanisms result in the modulation of the effective injected free charge that affects the position of the crossover energy, which can be compared with experiments. Overall, this study highlights defect engineering strategies to optimize TaTO compounds for applications in transparent electronics and optoelectronic devices.