Conjugated Polymer Controlled Morphology and Charge Transport of Small-Molecule Organic Semiconductors

Abstract

In this study, we report an effective approach to tune the crystallization, microstructure and charge transport of solution-processed organic semiconductors by blending with a conjugated polymer additive poly(3-hexylthiophene) (P3HT). When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was used as a model semiconductor material to mix with different amount of P3HT, their intermolecular interactions led to distinctive TIPS pentacene film morphologies, including randomly-oriented crystal ribbons, elongated needles with enhanced long-range order, and grasslike curved microwires with interlinkages. Each type of morphology was found to further correlate to considerably different charge transport and device performance. As compared to pristine TIPS pentacene devices, bottom-gate, top-contact OTFTs with 2% in weight P3HT additive showed a 2-fold and 5-fold improvement of average field-effect mobility and performance consistency (defined as the ratio of average mobility to the standard deviation), respectively. The improvement in transistor electrical performance can be attributed to the combined effect of enhanced crystal orientation and uniformity, as well as increased areal coverage. This work can be applied beyond the particular example demonstrated in this study and to tune the charge transport of other small-molecule organic semiconductors in general. In recent years, the research of high-performance, solution-processed semiconductor based organic thin film transistors (OTFTs) has attracted great attention 1-6. In particular, substantial progress has been reported in the study of film morphology and charge transport of various small-molecule organic semiconductors, such as N,N'-1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIF-CN 2) 7,8 , 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS pentacene) 9,10 , and 5,11-bis(triethylgermylethynyl) anthradithiophene (diF-TEG-ADT) 11,12 , which enables the field-effect mobility to reach or even surpass that of amorphous silicon 13. Notwithstanding these advances, the fabrication of high-mobility organic semiconductor crystals typically requires the method of slow crystallization in solution, which inevitably results in randomly-oriented crystals 14,15 , and further leads to anisotropic charge transport and significant device-to-device performance variations of OTFTs 16. Various external alignment techniques, such as air flow navigation 17 , temperature gradient 18 and substrate tilting 19 , have been demonstrated to control the growth and align crystal orientation of small-molecule organic semiconductors. Besides, the approach of blending with polymer additives, which does not require the complex setup of external alignment techniques, has been reported as a simple and effective pathway to modulate the semiconductor crystallization, tune thin film morphology and enhance charge transport 20. For example, isotactic poly(methyl methacrylate) (i-PMMA) was blended with TIPS pentacene, and in conjunction with a vertical flow approach, the i-PMMA additive i...

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Polyferrocenylsilane Semicrystalline Polymer Additive for Solution-Processed p-Channel Organic Thin Film Transistors

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