Temperature gradient approach to grow preferentially-oriented tips pentacene crystals for organic thin film transistors

Abstract

As a functionalized pentacene, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) is a p-type organic semiconductor with remarkable intrinsic charge carrier transport and stability in ambient conditions. TIPS pentacene is soluble in most organic solvents, making it solution processable. TIPS pentacene, nonetheless, inherently forms acutely anisotropic crystals with large gaps in between the crystals, limiting charge transport and leading to vast variations in organic thin film transistor (OTFT) performance. Described in this dissertation are crystal growth techniques implemented to overcome these challenges. The presented temperature gradient technique, achieves highly aligned crystal arrays with excellent areal coverage which essentially results in an enhanced OTFT performance. The technique is firstly utilized to guide the TIPS pentacene crystal growth. An application of a temperature gradient to a TIPS pentacene solution controls the crystallization process to alleviate the intrinsic crystal misorientation and considerably improve film morphology. Employing this method resulted in TIPS pentacene films with uniform crystal orientations and extensive areal coverage. The favorable crystal morphology gave rise to a significant enhancement in OTFT average mobility compared to OTFTs without the temperature gradient. Employing the temperature gradient technique, however, simultaneously introduced thermal cracks in the films due to the occurrence of thermally induced stress during crystallization, which reduced the device performance of the TIPS pentacene OTFTs. To further improve the performance of TIPS pentacene based OTFTs, TIPS pentacene was blended with polymers to relieve the thermal stress and effectively prevent the generation of thermal cracks. Structural examination of, specifically, TIPS pentacene/Poly(α-methyl styrene) (PαMS) blend films at an optimal weight ratio, revealed a vertical phase segregation with elevated concentrations of TIPS pentacene molecules at the active layer/gate dielectric interface, facilitating charge transport. Thus, OTFTs based on TIPS pentacene/PαMS blends exhibited a dramatic increase in average hole mobility compared to those of pristine TIPS pentacene. In addition, an improved thin film uniformity directly enhanced the device performance consistency. Following the success of employing the temperature gradient technique concurrently with the insulating polymer, PαMS, studies were extended to build OTFTs on flexible substrates, indium tin oxide (ITO) coated polyethylene terephthalate (PET), to dramatically improve TIPS pentacene/PαMS system. Ultimately, TIPS pentacene/PαMS OTFTs on ITO/PET substrates demonstrated the highest achieved mobility from utilizing the temperature gradient system.