Novel ladder-type heteroheptacene-based copolymers for bulk heterojunction solar cells

Abstract

Four semiconducting copolymers were designed and synthesized based on a novel ladder-type heteroheptacene building block, which was prepared in four steps from 3,7-dibromodibenzo[b,d]thiophene. The copolymers were characterized by ¹H NMR spectroscopy, UV-Vis absorption spectroscopy, cyclic voltammetry and their molecular weights were estimated using gel permeation chromatography. Field effect transistors (FETs) fabricated from these four polymers exhibit semiconducting behavior in air. The highest mobility of 2.21 × 10⁻⁴ cm² V⁻¹ s⁻¹ was found in the polymer copolymerized from the ladder-type heteroheptacene and terthiophene. Low band-gap donor–acceptor copolymers with a deep-lying HOMO energy level were synthesized by a copolymerization between the ladder-type heteroheptacene donor and the 2,1,3-benzothiadiazole acceptor. Polymer solar cells made from one of these polymers gave a power conversion efficiency of 4.17%, a current density of 9.2 mA cm⁻², an open-circuit voltage of 0.79 V, and a fill factor of ∼57.4% under 100 mW cm⁻² AM 1.5 G sunlight illumination.