Influence of Alkyl Side Chain on the Crystallinity and Trap Density of States in Thiophene and Thiazole Semiconducting Copolymer Based Inkjet-Printed Field-Effect Transistors

Abstract

The influence of alkyl side chains on the crystallinity of semiconducting copolymer films and their sub-bandgap density-of-states (DOS), the latter being closely related to the stability and the device performance of organic field-effect transistors (OFETs), is investigated Three different poly(hexathiophene-alt-bithiazole) (PFITBTz) based polymer semiconductors, with identical backbones but different side chain positions and lengths, were synthesized The crystallinity examined by grazing incidence X-ray diffraction (GIXRD) strongly depends on the number, position, and length of each type of alkyl side chain attached to the thiophene and thiazole copolymer backbones Also, the sub-bandgap trap DOS distributions were extracted by performing multiple frequency capacitance voltage (MF-CV) spectroscopy on the field effect devices. The relationship between film crystallinity and trap DOS in the field-effect transistors can be interpreted in terms of the complex interplay between the number, position, and length of each alkyl side chain for efficient pi-pi stacking. In particular, the number and position of the alkyl side chain attached to the polymer backbone significantly affects the device performance. Poly(tetryloctylhexathiophene-alt-dioctylbithiazole) (PHTBTz-C8) exhibits the best electrical performance among the different semiconductors synthesized, with a relatively low bulk trap density of similar to 2.0 x 10(20) cm(-2) eV(-1) as well as reasonable hole mobility of similar to 0.25 cm(2) s(-1). The microstructural analyses of this organic material strongly suggest that the short pi-pi stacking distance induces strong interaction between adjacent polymer backbones, which in turn results in enhanced electrical properties.