Efficient Bulk Heterojunction Solar Cells Based on a Broadly Absorbing Phenylenevinylene Copolymer Containing Thiophene and Pyrrole Rings

Abstract

A novel low band gap alternating phenylenevinylene copolymer, P, containing thiophene and pyrrole rings was synthesized by Heck polycondensation. It was soluble in common organic solvents and showed a long-wavelength absorption maximum at 596−635 nm and an optical band gap of 1.65 eV. The electrochemical properties and electronic energy levels of P were studied by cyclic voltammetry. The highest occupied molecular orbital (HOMO) level of P is deeper than that of poly(3-hexylthiophene) (P3HT). The polymer solar cells (PSCs) based on P with a modified 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-C-61 (PCBM) (i.e., F) as an electron acceptor shows higher open circuit voltage (Voc) than that for the device based on P3HT. The power conversion efficiency (PCE) for the devices P−F and P−PCBM processed from tetrahydrofuran (THF) solvent is approximately 2.42% and 1.50%, respectively. The enhanced PCE is attributed to the higher value of both Voc and short circuit current (Jsc). The PCE of the devices based on P−F processed from mixed solvents has been further improved. The P−F blend film cast from mixed solvents shows higher absorption and incident photons to current efficiency (IPCE) with respect to the THF-cast film. The overall PCE for the PSC processed from 1-chloronaphthalene (CN)/THF (thermally annealed) is 4.14%. The improved PCE has been attributed to the increased crystallite size and hole mobility in the blend, resulting in balanced charge transport.