Investigations on Organic Vertical Transistors and Defect Minimization in Hybrid Perovskites
The investigations on organic semiconductors and organometal halide perovskite (OHP) semiconductors have enabled tremendous research achievements for low cost, high performance, flexible optoelectronic applications such as light emitting diodes (LED), photovoltaics (PV) and field effect transistors (FET). In this thesis, we discuss two individual projects. The first project is device architecture design for achieving fast organic memory. The second project is targeting on achieving general observation of clean field effect operation in OHP transistor which is currently the major problem in the community. In the first project, we developed a new memory device based on vertical transistor configuration which scales down the effective gate modulation length below 2 micrometer (m) and incorporates a two charge transport layer structure where the bottom is an ambipolar semiconductor layer for fast charge accumulation and the top is a unipolar semiconductor layer for maintaining device transfer curve in unipolar characteristic. By this configuration, memory function can be achieved as reliable writing between ON/OFF states with ratio ~ 100, in fast speed with programming ~ 150 ns and erasing ~ 50 ns. These speed values are nearly seven orders of magnitude higher than those reported in lateral organic transistor memory based on unipolar semiconductor. In the second project, we investigated solution processed defect-removal technique for pursing high performance OHP transistors. We proposed a three-step solution treatment as cleaning-healing-cleaning to reduce defects in OHP surface and gain boundaries. We investigated ion migrations (magnitude, mobility, activation energy) on OHP samples as-deposited and post-treated by cleaning or healing using different solutions. We optimized the three-step treatment based on ion migration results and applied the optimal treatment on transistor manufacturing, achieving clean field effect modulations with mobility improved two to three orders of magnitude compared with transistors of pristine OHP films. We explored the optimal treatment on OHP films with grain size up to 1 m, achieving unprecedented high transistor mobility averaging at 3.0 cm2/Vs at 300 K and 9.2 cm2/Vs at 80 K. Additionally, we show this method is in line with high quality transformation from PbI2 single crystals into thin OHP micro-plates.
Thin film transistors, Perovskites, Organic memory, Ion migration
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