Publication date: 15th December 2025
Developing high-performance p-type transparent conducting oxides (p-TCOs) has been significantly challenged by the localized nature of oxygen 2p orbitals at the valence band maximum, which typically leads to an inherent trade-off between hole mobility, optical transparency, and work function. Here, we propose a novel material design strategy to realize highly conductive, wide-band-gap p-TCOs by engineering the polarizability of strongly correlated insulator oxides through Cu substitution.
First, in the binary Mott-Hubbard insulator NiO, the partial substitution of Ni with Cu (Ni1-xCuxO) effectively enhances d-p hybridization. This substitution reduces the activation energy for small polaron hopping from 0.58 to 0.24 eV, drastically improving electrical conductivity while successfully preserving the strong electron correlation, a wide band gap of 3.1 eV, and an antiferromagnetic transition temperature of 453 K.
Second, we expanded this concept to a lower-symmetry ternary matrix, NiWO4. The optimal Cu-substituted composition, Cu0.185Ni0.815WO4, exhibits an exceptional resistivity reduction of ~10^9 times compared to intrinsic NiWO4. Remarkably, Cu incorporation enhances the system's polarizability and weakens electron-phonon coupling without disturbing the strong electron correlation. This promotes a transition from small polaron hopping to band-like large polaron transport, achieving a high hole mobility approaching 7 cm^2 V^-1 s^-1 at 200 K, an extremely deep work function of 5.77 eV, and a wide optical band gap of 2.8 eV.
Finally, the practical feasibility of these newly developed p-type thin films was proven in advanced electronic and optoelectronic devices. When employed as a hole transport layer (HTL) in quantum dot light-emitting diodes (QD-LEDs), the Cu-doped NiWO4 enabled excellent charge balance and luminous efficiency. Furthermore, p-n heterojunction diodes fabricated with n-Si and n-IGZO exhibited highly rectifying behaviors with on/off current ratios of ~10^4 and successfully demonstrated high-speed AC-to-DC half-wave rectification up to 1 MHz.
This study provides a breakthrough in overcoming the fundamental limitations of conventional p-TCOs, offering a promising pathway for next-generation transparent optoelectronics, displays, and high-frequency oxide-based circuits.
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