Ladder-type Diindenopyrazine Based Conjugated Copolymers for Organic Solar Cells with High Open-circuit Voltages

A ladder-type diindenopyrazine (IPY) was synthesized and used as a building block for constructing conjugated copolymers. Three copolymers based on the IPY moiety were obtained via the Suzuki coupling reaction with different monomers, including 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DBT), 5,8-dithien-2-yl-2,3-diphenylquinoxaline (DTQ), and 5,8-dithien-2-yl-2,3-di(4-fluorophenyl)quinoxaline (DFTQ). The obtained polymers were characterized by H-1 NMR spectroscopy, UV-Vis absorption spectroscopy, cyclic voltammetry, and gel permeation chromatography (GPC). Owing to the four solubilizing alkyl chains on the IPY unit, all the three copolymers have good solubility in common solvents. These polymers have deep-lying HOMO energy levels in the range of -5.55--5.60 eV, and exhibit ?eld-effect mobilities as high as 0.006 cm(2)V(-1)s(-1). Photovoltaic applications of these polymers as light-harvesting and hole-conducting materials were investigated in conjunction with [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM). Both conventional and inverted devices were fabricated based on these three polymers. A power conversion efficiency (PCE) of 2.53% and a high open-circuit voltage of 1.00 V were obtained under simulated solar light AM 1.5 G (100 mW/cm(2)) from an inverted solar cell with an active layer containing 25 wt% ladder-type IPY containing copolymer (PIPYDTQ) and 75 wt% PC61BM. Moreover, a high open-circuit voltage of 1.02 V and a PCE of 2.40% were achieved from a conventional solar cell based on PIPYDTQ.