Cyclopenta[c]thiophene oligomers based solution processable D–A copolymers and their application as FET materials

Abstract

Two new solution processable, low band gap donor–acceptor (D–A) copolymers (P1 and P2) comprising a cyclopenta[c]thiophene (CPT) based oligomers as donors and benzo[c][1,2,5]selenadiazole (BDS) and 2-dodecyl[1,2,3]-benzotriazole (BTAz) as acceptors were synthesized and characterized and their field effect transistor properties were studied. The internal charge transfer interaction between the electron-donating CPT based oligothiophene and the electron-accepting BDS or BTAz unit effectively reduces the band gap in polymers to 1.3 and 1.66 eV with low lying highest occupied molecular orbital (HOMO). The absorption spectrum of P1 was found to be more red shifted than that of P2 because of incorporation of the more electron-withdrawing BDS unit. The color of neutral P1 was found to be green in both solution and film states with two major bands in the absorption spectra; however, neutral P2 revealed one dominant absorption exhibiting red color in both solution and film state which could be attributed to the less electron-withdrawing effect of the BTAz unit. The polymers were further characterized by GPC, TGA, DSC and cyclic voltammetry. P1 and P2 exhibited charge carrier mobilities as high as 9 × 10⁻³ cm² V⁻¹ s⁻¹ and 2.56 × 10⁻³ cm² V⁻¹ s⁻¹, respectively with the current on/off ratio (I_on/I_off) in the order of 10².