Introduction of TiO₂ in CuI for Its Improved Performance as a p-Type Transparent Conductor

The challenges of making high-performance, low-temperature processed, p-type transparent conductors (TCs) have been the main bottleneck for the development of flexible transparent electronics. Though a few p-type transparent conducting oxides (TCOs) have shown promising results, they need high processing temperature to achieve the required conductivity which makes them unsuitable for organic and flexible electronic applications. Copper iodide is a wide band gap p-type semiconductor that can be heavily doped at low temperature (<100 °C) to achieve conductivity comparable or higher than many of the well-established p-type TCOs. However, as-processed CuI loses its transparency and conductivity with time in an ambient condition which makes them unsuitable for long-term applications. Herein, we propose CuI-TiO₂ composite thin films as a replacement of pure CuI. We show that the introduction of TiO₂ in CuI makes it more stable in ambient conditions while also improving its conductivity and transparency. A detailed comparative analysis between CuI and CuI-TiO₂ composite thin films has been performed to understand the reasons for improved conductivity, transparency, and stability of CuI-TiO₂ samples in comparison to pure CuI samples. The enhanced conductivity in CuI-TiO₂ stems from the highly conductive space-charge layer formation at the CuI-TiO₂ interface, whereas the improved transparency is due to reduced CuI grain growth mobility in the presence of TiO₂. The improved stability of CuI-TiO₂ in comparison to pure CuI is a result of inhibited recrystallization and grain growth, reduced loss of iodine, and limited oxidation of the CuI phase in the presence of TiO₂. For optimized fraction of TiO₂, an average transparency of ∼78% (in 450-800 nm region) and a resistivity of 14 mΩ·cm are achieved, while maintaining a relatively high mobility of ∼3.5 cm² V^-1 s^-1 with hole concentration reaching as high as 1.3 × 10²⁰ cm^-3. Most importantly, this work opens up the possibility to design a new range of p-type transparent conducting materials using the CuI/insulator composite system such as CuI/SiO₂, CuI/Al₂O₃, CuI/SiN, and so forth.