Conjugated polymers with deep LUMO levels for field-effect transistors and polymer–polymer solar cells

Abstract

Three thiazole-bridged DPP polymers with deep lowest unoccupied molecular orbital (LUMO) levels were designed for field-effect transistors (FETs) and polymer–polymer solar cells. By introducing thiazole–thiazole coupled segments, perfluoroalkyl side chains or strong electron-deficient naphthalenediimide units into the conjugated backbone the three thiazole-bridged DPP polymers have LUMO levels of −4.0 to −4.4 eV. The three DPP polymers exhibit optical absorption in the near-infrared region, crystallinity and an electron mobility of around 0.01 cm² V⁻¹ s⁻¹ in bottom contact FETs. The polymers were applied as electron acceptors in polymer–polymer solar cells to provide PCEs of around 0.4%. The low PCEs are mainly due to low short-circuit currents (J_sc) and attributed to large phase separation. Our results demonstrate several efficient strategies to lower the energy levels of conjugated polymers in order to be used as universal acceptors for photovoltaic cells.