Developing high-performance inorganic p-channel thin-film transistors

Abstract

Over the past two decades, metal oxide semiconductor based thin-film transistor (TFT) technology has attracted considerable research interest and great success. Compared with silicon based and organic materials, metal oxides possess good transparency and balanced electrical performance, mechanical stress tolerance, and spatial uniformity, providing balanced suitability in many aspects. However, all commercially available oxide semiconductors are n-type (electron transporting), with few p-type (hole transporting) counterparts reported (e.g., CuxO, SnO, and NiO). The next attention has focused to development of high performance p-type semiconductors with comparable opto/electric properties to their n-type counterparts. This thesis focuses on the development of high-performance p-channel TFTs based on diverse low-temperature processed p-type semiconductors. The first part of the dissertation describes an eco-friendly polyol assisted reduction method for the fabrication of solution-processed p-type CuxO semiconductor and their application as channel layers in TFTs. The impacts of the post annealing temperature/environment and polyol type on the TFT performance and stability were investigated. Furthermore, a bilayer molecule doping approach was adopted to achieve the p-doping effect for enhancing the TFT performance. The final optimized TFTs exhibited the decent field-effect hole mobility (μFE) of ~0.2 cm2 V-1 s-1 and on/off current ratio (Ion/Ioff) of ~104. The second part of this thesis explores another interesting transparent p-type semiconductor of CuI, which enables nearly room-temperature solution processing for the use in TFTs. The effects of the post annealing temperature and the channel layer thickness on the TFT performance were investigated. To modulate the excessive hole concentration in pristine CuI channel layer, we further developed an external Zn doping method to improve the film formation and TFT performance and revealed the key roles of trace oxygen on the vacancy passivation and the subsequent p-doping effect. The optimized 5 ml% Zn-doped CuI TFTs delivered the high μFE of ~5 cm2 V-1 s-1 and high Ion/Ioff of ~107. The final part of this work introduces one novel p-type semiconductor system based on the metal halide perovskite, which enables low-temperature solution processing with ultrahigh electrical performance. We report halide perovskite TFTs that are based on CsSnI3 semiconducting channels and optimised through precursor engineering. We modulate the composition and crystallization process of the CsSnI3 films using a tin fluoride (SnF2)-modified cesium iodide (CsI)-rich precursor with a portion of the tin iodide (SnI2) substituted with lead iodide (PbI2). The engineered perovskite films exhibit a uniform morphology, high crystallinity, moderate hole concentrations, and high Hall mobilities. Using our approach, we create p-channel CsSnI3-based TFTs that exhibit μFE of over 50 cm2 V-1 s-1 and Ion/Ioff exceeding 108, as well as high reproducibility and operational stability.