Enhancement of injection and transport in organic field-effect transistors and light-emitting transistors with multilayers

Abstract

The organic electronics have drawn great attention for their potential of application in advanced electronic device. The intrinsic advantages of organic materials, such as flexibility, light-weight, and low-cost and large-area processability, give organic electronics superior merit compared to the silicon-based materials. Moreover, the chemical versatility of organic materials opens new fields of light-emission, charge-transport, photovoltaicity, and sensing properties in organic electronics. In this regards, the detailed investigation to physical mechanism of organic electronics enables the successful introduction of organic semiconductors to various opto-electronic devices including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic photovoltics (OPVs). Among the organic electronic devices, the OFETs have been considered as a main candidate for the various fields of flexible electronics, integrated circuits, organic-based sensors, and radio frequency identification tags. In recent years, the extensive researches in material level have led to the development of high performance organic materials having field-effect mobility (up to 10 cm2/Vs). For realization of OFETs as a practical application of advanced electronics, however, the enhancement of charge injection and transport in device level is strongly required. In order to fully utilize the capability of improved electric performance of organic material, the multi-layers structures of OFETs have been adopted to improve electrical characteristics. For example, the interlayer between the source/drain electrode and the organic semiconductor (OSC) was placed to improve the injection characteristics, and the ambipolar-type OFETs was introduced to balance the transport of holes and electrons in same device. Moreover, the light-emitting transistors was extensively studied for its light-emitting property together with the switching capability of transistor in single device. The main purpose of this thesis is to demonstrate the enhancement of charge injection and transport of OFETs and OLETs with multi-layers by the optimization of device structures. At first, an ambipolar-type OFET with two-stacked OSCs in dual gate configuration was proposed. Two stacked OSCs, directly contacted with the source/drain electrodes, form the separated channels for holes and electrons, respectively. These individual channels can be effectively and independently controlled by corresponding gate bias voltage, so the ambipolar-type transport in a single device can be obtained. Next, the introduction of semiconducting organic buffer layer between the source/drain electrodes and the OSC layer is described. The semiconducting organic buffer layer greatly reduces the potential loss at the contact region (interface of source/OSC) so the injection properties of OFETs can be improved. And lastly, the vertical configuration of OLETs (VOLETs) having high on/off ratio is demonstrated by dielectric encapsulation of source electrode. The dielectric encapsulation of source electrode governs the effective charge pathway in VOLETs by blocking excessive electric fields from the drain voltage. As a result, the gate voltage can successfully control the accumulation and transport of charges, which results in the high on/off ratio of VOLETs. The novel device architectures of OFETs with multi-layers demonstrated in this thesis would pave the way towards the enhancement of electrical characteristics in OFETs, the precise control of effective charge flow in OFETs, the ambipolar-type operation in single device, the integration of light-emitting capability in switching OFETs, and further application of advanced, flexible and multi-functional organic electronics.