Vacancy engineering of a solution processed CuI semiconductor: tuning the electrical properties of inorganic P-channel thin-film transistors

Abstract

Recently, copper iodide (CuI) has been considered as a promising inorganic p-type semiconductor material. Although significant progress using CuI has been made in materials and device applications for solar cells and thin-film transistors (TFTs), little consideration has been given to the vacancy formation mechanisms and their effect on the device performance. In this work, high performance electrolyte-gated TFTs were fabricated using CuI semiconductors with different vacancy states and the origin of the threshold voltage (V_th) shift of devices in terms of vacancy species and concentration were studied. The CuI films were fabricated via a simple solution process followed by thermal annealing at low temperature (55 °C). The vacancy species and concentration in the CuI semiconductor were regulated by changing the annealing conditions as well as by introducing a dopant material. Photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) analysis were conducted to demonstrate the correlation between the electrical properties of TFTs and the vacancy states in the CuI specimens. The CuI-TFTs exhibited typical enhancement mode operation of the p-channel transistor, with sub-1 V operation voltage, ON/OFF ratio of ∼10³, and high hole mobility in the range of 10–60 cm² V⁻¹ s⁻¹. Experimental results revealed that Cu vacancy (V_Cu) play the role of inducing hole carriers, hence, TFT with V_Cu-rich CuI exhibited increased carrier concentration and positively shifted V_th of 0.14 V. However, iodine vacancy (V_I) in CuI captures hole carriers, decreasing the concentration of charge carriers (holes), resulting to a negative V_th of −0.19 V. The TFT mobility was also affected by the vacancy concentration, owing to the carrier scattering effect. For instance, the I-rich (or V_Cu-rich) CuI exhibited decreased TFT mobility (14.70 cm² V⁻¹ s⁻¹) due to the high carrier concentration (10¹⁹ cm⁻³) and their scattering effect.