Amorphous copper iodide: a p-type semiconductor for solution processed p-channel thin-film transistors and inverters

Abstract

Until now, inorganic p-channel thin-film transistors (TFTs) have shown relatively low performance in terms of mobility, ON-current level, and on/off ratio compared to their n-channel counterparts. For inorganic p-channel TFTs, high-temperature annealed single- or poly-crystalline materials such as CuO, SnO, or 2D dichalcogenides have been the typical materials of choice. Development of amorphous semiconductor materials can provide a wide range of promising semiconductors for the TFT industry owing to their unique advantages, such as large area applicability, high device-to-device uniformity, and low temperature processing; however, the poor TFT performance using the conventional amorphous p-type semiconductors limits the use of materials in practical applications. In the present work, we demonstrate the 1st high-performance solution-processed p-channel TFT using an amorphous copper iodide (a-CuI) semiconductor, which outperforms its polycrystalline counterpart. Amorphous CuI films were formed by spin coating of precursor solutions based on co-solvents. By using a-CuI semiconductors as the channel layer, electrolyte-gated p-channel TFTs were fabricated with a vertical device structure. Measurement of the TFT characteristics reveals that the amorphous CuI channel layer leads to better device performance than devices with a polycrystalline CuI. The optimized vertical TFTs showed high current densities above 1000 mA cm⁻², ON/OFF current ratios of > 10⁴, and large normalized transconductances of about 6 S m⁻¹, which are the highest among solution-processed vertical TFTs. These results pave the way for application of amorphous p-type inorganics in high-performance complementary circuits and represent a breakthrough for p-type semiconductor materials.