Research On Flexible Optoelectronics Based Thin Film Transistors

The growing field of flexible large-area mirco electronics shows the requirement for amorphous semiconducting materials with electrical properties superior to tradional amorphous hydrogenated silicon(a-Si:H). Compared to orther silicon and other III-V semiconductors, Organic semiconductors exhibit much better flexibility but lower performance and stability. While metal oxide semiconductors show great promise in thin film transistors(TFTs) due to their high carrier mobility(μ, 1 – 100 cm2V-1s-1), excellent optical transparency, mechanical flexibility, and electrical stability. However, most metal oxide fabrication still relies on vacuum phase deposition methods, such as sputtering, plasma laser deposition, and ion assisted deposition. Theses techqunies suffer from expensive, inflexible and energy intensive. To overcome these limitations, my thesis work has focused on developing low-temperature solution processed metal oxide materials and metal oxide based TFTs, and the main work include the following four parts:1. Interfacial properties of source/drain electrodes and organic layer in OTFT were investigated. Various interifacial materials, including organic hole transport materials, and hole injection materials acting as interfacial layer were used in OTFTs. By optimizing the thickness of interfacial layers, OTFT electrical performance were improved; Meanwhile, the deposition rate of Au electrodes on the performance of OTFT were also investigated.2. Developing a synergistic approach to solution process high performance metal oxide TFTs using a bilayer metal oxide thin film transistor concept(bMO TFT) where the channel has the structure: dielectric/semiconducting indium oxide(In2O3) layer/ semiconducting indium gallium oxide(IGO) layer. Both semiconducting layers are grown from solution via a low-temperature combustion process. The TFT mobilities of bottomgate/top-contact bMO TFTs processed at T = 250 °C are ~5x larger(~2.6 cm2/Vs) than those of single-layer IGO TFTs(~0.5 cm2/Vs), reaching values comparable to single-layer combustion-processed In2O3 TFTs(~3.2 cm2/Vs). More importantly, and unlike singlelayer In2O3 TFTs, the threshold voltage of the bMO TFTs is ~ 0.0 V, and the current on/off ratio is significantly enhanced to ~108(vs ~104 for In2O3). The microstructure and morphology of the In2O3/IGO bilayers are analyzed by X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy, revealing the polycrystalline nature of the In2O3 layer and the amorphous nature of the IGO layer.3. Developing amorphous metal oxide/polymer blend semiconductor films, for the first time demonstrating ultra-flexible and transparent amorphous TFT based on metal oxide/polymer system. Indium oxide-polymer semiconductor blends for transistor applications were developed by doping In2O3 films with the insulating polymer poly(vinylphenole)(PVP). By adjusting the In2O3:PVP weight ratio, crystallization is frustrated, the carrier concentration in the In2O3 channel is controlled, and conducting pathways for efficient charge transport are maintained. In2O3:5%PVP-based TFTs exhibit electron mobilities approaching 11 cm2V-1s-1 before mechanical stress, and retain up to ~90 % of the performance after 100 bending/relaxing cycles at a radius of 10 mm.4. Developing high quality metal oxide films via spray-combustion synthesis(SCS). Here we report a new low-temperature thickness-controlled coating process to create high-performance solution-processed MO electronics, spray-combustion synthesis(SCS), demonstrating for the first time indium-gallium-zinc oxide( IGZO) transistors having densification, nanoporosity, electron mobility, trap densities, bias stability, and film transport approaching that of sputtered films, and compatible with conventional FAB operations. Moreover, SCS enables the processing time 10 times less than conventional solution processed metal oxide films.