Copolymers of perylene diimide with dithienothiophene and dithienopyrrole as electron-transport materials for all-polymer solar cells and field-effect transistors

Abstract

Electron-accepting solution-processable conjugated polymers consisting of perylene diimide moieties alternating with dithienothiophene, oligo(dithienothiophene), or N-dodecyl dithienopyrrole units have been synthesized. All these polymers possess excellent thermal stability with decomposition temperatures over 400 °C. The glass-transition temperatures vary from 155 to 263 °C. These polymers show broad absorption extending from 250 to 900 nm with electrochemical and optical bandgaps as low as 1.4 eV; the maximum absorbance increases and the bandgap decreases with increasing the conjugation length of oligo(dithienothiophene), while the bandgap can also be decreased by the replacement of dithienothiophene by dithienopyrrole. The electrochemical onset reduction potentials range from −0.8 to −1.0 V vs. ferrocenium/ferrocene, suggesting that the electron affinities are essentially unaffected by the specific choice of donor moiety, while the onset oxidation potentials (+0.6 to +1.0 V) are a little more sensitive to the choice of donor. The mono dithienothiophene and the dithienopyrrole polymers were found to exhibit electron mobilities as high as 1.3 × 10⁻² and 1.2 × 10⁻³ cm²V⁻¹s⁻¹, respectively, in top-contact organic field-effect transistors. Power conversion efficiencies in the range 0.77–1.1% were obtained under simulated AM 1.5, 100 mW/cm² irradiation for all-polymer solar cells using the dithienothiophene-based polymers as acceptors in a 1 : 1 ratio with a polythiophene derivative as a donor. The device performance varies with the conjugation length of oligo(dithienothiophene) in the polymer acceptors, and for the best-performing material it can be further optimized to give a power conversion efficiency of 1.5% by increasing the donor/acceptor weight ratio to 3 : 1.