Solution-grown Large-area Single- Crystals Of Organic Semiconductors For Flexible Field- Effect Transistors

Organic semiconducting materials hold great promise for electronic applications in various fields, due to their advantages of lightweight, low cost and designability. As people’s need for flexible electronic devices grows, organic field-effect transistors based on flexible substrates have developed a new application field. Organic single-crystals, comparing with their amorphous and polycrystalline counterparts, are ideal for both investigating charge transport in organic materials and fabricating high-performing photoelectric devices, as they have long-range order, thus fewer structure defects and higher mobility. However, their use has been restricted to simple and basic devices in fundamental studies. To realize their wide-ranging technological applications, efforts have to be made on large-area single-crystal growth methods. Solution-processing, especially in-situ growth from solution, is simple, high-throughput and low-cost, thus promising for large-area fabrication for arrays of devices. Therefore, developing simple and efficient methods for growing large-scale aligned single-crystal arrays from solution, and studying their application in flexible devices are issues of great importance in both exploring intrinsic characteristics and potential technological application of organic single-crystals.In this thesis, we focus on the promotion for organic single-crystal field-effect transistors’potential technological applications and report a series of studies, including a facile yet efficient solution method to prepare organic semiconducting single-crystals in large scale, and FETs based on the crystal arrays.In Chapter 1, development process of OFETs is reviewed, including preparation approaches of organic semiconducting single-crystals. The applications of single crystal-based OFETs are also summarized.In Chapter 2, we reported large-scale well-aligned single crystals of four p-type and two n-type organic small molecule grown from solutions using a modified droplet-pinned crystallization (DPC) method. The surface coverage of crystals on the substrate and the uniformity of the crystal alignment were improved. Large-scale FET arrays based on the well-aligned single crystals were fabricated and field-effect charge transport characteristics were investigated. A hole mobility as high as 6.64 cm2 V-1 s-1 was obtained.In Chapter 3, the modified DPC method was adopted to grow large-scale well-aligned single crystals on flexible PET substrates. Bottom gate, top contact FETs based on these crystals were fabricated, with Au or ITO as gate electrodes and PMMA as dielectric layer. Further investigation and characterization of the bending effect on the single-crystal morphology and FET device performance were made to provide feasibility analysis for optimization of flexible organic single-crystal FETs.