Organic single crystals and devices : physics and applications

Abstract

Organic electronics gradually becomes a fascinating candidate for the next generation of flexible electronics owing to their low-cost, low-weight, flexible properties and compatibility to high-throughput productions such as roll-to-roll deposition. High-performance organic field-effect transistors (OFETs) exhibit enormous potential in flexible display, integrated circuits and various chemical, physical and biological sensors. This thesis aims to develop novel fabrication methods of crystalline organic semiconductors and electrodes, optimize the structure of OFETs and therefore explore the physics and applications of the organic electronic devices. Firstly, large-area, high-crystalline and monolayer organic semiconductor films have been deposited by a novel dual solution shearing (DSS) method. The monolayer C10-DNTT thin films show a highest saturation mobility of 9.0 cm2V-1s-1 with good uniformity and molecular alignment in hundreds of micrometers scale. Higher mobility, smaller threshold voltage and higher on-off ratio are observed along the shearing direction. Compared to solution shearing and ultra-slow shearing, DSS has strengths in fast depositing process, uniform orientation and ultra-thin thickness. Following this, ambipolar OFETs based on DSS C10-DNTT films have been developed. The high-crystalline and ultra-thin C10-DNTT films serve as a growth template for the top n-type semiconductor. F16CuPc films with higher crystallinity, larger grains and fewer grain boundaries are obtained. The on currents in the p-channel and n-channel of DSS C10-DNTT/F16CuPc devices show threefold and tenfold enhancement compared to the unipolar devices due to effective charge transfer. Secondly, rubrene single crystals have been deposited by physical vapor transport (PVT) method with dimension in milliliter scale. OFET devices based on rubrene single crystals have been fabricated by electrodes-transferring method. Because the interface between the semiconductor and the electrodes is free from the thermal damage caused by the thermal irradiation in the conventional metal evaporation process, a low contact resistance of 284.2 Ω·cm has been achieved, which is 30 times smaller than the devices with thermal evaporated electrodes. Besides, the increased contact resistance in thicker single crystals is detected. The ultra-low contact resistance is then attributed to the small thickness and undamaged electrode/organic semiconductor interface. Finally, dual-gate OFET (DGOFET) structure has been applied to control the threshold voltage and on-off ratio of the p-type and n-type OFETs. The shift of threshold voltage linearly depends on the capacitance coupling factor of the bottom and top dielectric layers. For DNTT DGOFET in bottom gate sweeping mode, the threshold voltage can be modulated from -48 V to +15.3V as the top gate bias changes from +80 V to -80 V. Besides, the corresponding on-off ratio is increased by two orders of magnitude. Complementary metal-oxide-semiconductor transistor (CMOS) inverters based on DNTT and F16CuPc DGOFETs have been demonstrated. The voltage transfer characteristics and gain can be precisely tuned by the bias applied on the top control gate, suggesting a feasible approach in controlling the behavior of inverter-based circuits. A largest inversion voltage shift of 11.9 V is realized at VDD = 20 V. And a highest gain of 50 is achieved, which is three times of the inverters without top gate bias.