Progress in Developing Highly Efficient p-Type TCOs for Transparent Electronics: A Comprehensive Review

Transparent conducting oxides (TCOs) represent a remarkable class of materials that possess both excellent electrical conductivity and high optical transparency, which are typically considered mutually exclusive in traditional materials. In conventional materials, achieving both high electrical conductivity and optical transparency is difficult. Materials with a wide optical band gap are transparent in the visible region but lack electrical conductivity, while conductive metals are opaque. Hence, the only way to induce both properties in a single material is to create non-stoichiometry and/or defects. By introducing shallow defects near the conduction band for n-type materials and the valence band for p-type materials, it is possible to enhance the conductivity of the material at room temperature. However, developing efficient p-type TCOs has been particularly difficult due to the localized nature of the valence band derived from O 2p orbitals and the challenges associated with shallow acceptors, resulting in large effective mass of holes. While commercially available TCOs are predominantly n-type, such as Sn:In2O3, Al:ZnO, and F:SnO2, the development of efficient p-type TCOs lags. In this review, we have discussed the origin of p-type conductivity in TCOs, and the difficulties encountered in developing efficient p-type materials. We have also demonstrated the fundamental materials physics of p-type TCOs, including electronic structures, doping, defect properties, and optical properties. A range of deposition techniques has been adopted to prepare TCO films, and this review provides a detailed discussion of these techniques and their relative deposition parameters. Overall, we have presented an up-to-date and comprehensive review of different p-type transparent conducting oxide thin films, providing insights into ongoing research and potential future directions in this field.