Structure-Property Relationship Studies for High-Performance Organic Field-Effect Transistors and Flexible Photosensors

Abstract

Recently, organic field-effect transistors (OFETs) and OFET-based sensors have attracted great interest for their potential for use in low-cost, large-area, lightweight, flexible, and wearable electronic devices. Various researches for high-performance OFET devices have been reported to achieve high mobility, air stability, and flexibility through molecular design and optimized device configuration. In addition, OFET-based sensors based on various flexible substrates and nanostructured organic semiconducting materials have been studied for use in wearable sensor devices with high sensitivity and excellent mechanical stability under severely bent condition. In chapter 1, we studied the relationship between molecular structure in a film state and electrical performance of OFET devices. We fabricated high-performance n-channel OFETs based on semiconducting copolymers containing strong electron-withdrawing unit, naphthalene diimide (NDI). NDI-based copolymers with various donor moieties, acene- (benzene (Bz), naphthalene (Np), and pyrene (Py)) and heteroacene-type components (selenophene (Se) and thiophene (Th)) were designed to enhance efficient intramolecular charge transfer (ICT). The OFET performance of NDI-based copolymers was optimized in bottom-gate top-contact (BGTC) device configuration by tuning the solution-deposition methods and thermal annealing at various temperatures. The electrical characteristics of the devices fabricated with PNDI-Np, acene-based centrosymmetric copolymer, showed the best electron mobility of 5.63×10-2 cm2V-1s-1 among the developed polymers including axisymmetric copolymers with electron-rich donor groups. This result reveals the stronger influence of the geometric feature on the OFET performance rather than electron-donating strength of the donors in the molecular backbone. In chapter 2, we report on highly flexible OFET-based photosensors based on a textile substrate and organic semiconducting nanofibers. Textile-based OFET devices with bottom-gate bottom-contact (BGBC) configuration consist of poly(ethylene terephthalate) (PET) textile substrate buffered with polydimethylsiloxane (PDMS), gate electrode, PDMS dielectric layer and source-drain electrode. Electrospun organic semiconducting nanofibers based on a p-type semiconducting polymer, poly(3,3‴-didodecylquarterthiophene) (PQT-12) were used as the active layer. The field-effect mobility of textile-based OFETs was as high as 2.96×10-3 cm2V-1s-1 in N2 atmosphere. Under the bending test with extremely low bending radius of 0.75 mm, the device performance showed superior mechanical stability compared with the PET film-type substrates or PDMS-only substrates. Furthermore, they exhibited highly stable electrical characteristics after ~1000 cycles of bending test. The electrical responses of textile-based OFETs with photo-responsive organic semiconducting nanofibers were also investigated under the irradiation of light with various wavelengths. The developed method may pave a viable way for the fabrication of wearable photosensing or photoswitching devices.