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) (PHTBTz) 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 π–π 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 ∼2.0 × 10 20 cm –3 eV –1 as well as reasonable hole mobility of ∼0.25 cm 2 V –1 s –1 . The microstructural analyses of this organic material strongly suggest that the short π–π stacking distance induces strong interaction between adjacent polymer backbones, which in turn results in enhanced electrical properties.