π -conjugated donor–acceptor porphyrin copolymers for organic photovoltaics

Abstract

Conjugated donor–acceptor (D-A) molecular structures play a very important role in the significant progress of organic photovotaics. However, the reports on conjugated D-A porphyrin polymers for organic solar cells are very limited. In this work, five conjugated D-A porphyrin copolymers PEZPEBTA(C12), PEZPEBT, PEZPEBTff, PEZPETPD(O), and PEZPETDPPT(O) were synthesized by Sonagashira coupling of a porphyrin donor unit with five typical acceptor units 2-dodecyl-2H-benzotriazole, benzo[1,2,5]thiadiazole, 5,6-difluoro-benzo[1,2,5]thiadiazole, 5-octyl-thieno[3,4-c]pyrrole-4,6-dione, and 3,6-bis-(thiophen-2-yl)-2,5-dioctyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione linked by ethynylene linkages, respectively. They possess excellent thermal stability with a decomposition temperature of around 400°C. All absorption spectra of the copolymers were significantly red shifted with enhanced Q bands at the near-infrared region both in solutions and in films due to the simultaneous introduction of ethynylene linkages and acceptor units, which make the polymer main chains coplanar and π-conjugated and enhance the intramolecular charge transfer. PEZPEBT and PEZPEBTff are electrochemically active in both the oxidation and reduction regions, while PEZPEBTA(C12), PEZPETPD(O), and PEZPETDPPT(O) show only oxidation peaks. Power conversion efficiencies of 0.12%, 0.41%, 0.26%, 0.19%, and 0.41% were achieved for the polymer solar cells based on PEZPEBTA(C12), PEZPEBT, PEZPEBTff, PEZPETPD(O), and PEZPETDPPT(O), respectively, under AM 1.5, 100 mW/cm2 with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) (1:2, w/w) as the active layer in the presence of 3% pyridine.