Improved Photovoltaic Performance of a Semicrystalline Narrow Bandgap Copolymer Based on 4H-Cyclopenta2,1-b:3,4-b′dithiophene Donor and Thiazolo5,4-dthiazole Acceptor Units

A solution processable narrow bandgap polymer composed of alternating 2,5-dithienylthiazolo5,4-dthiazole and asymmetrically alkyl-substituted 4H-cyclopenta2,1-b:3,4-b′dithiophene units (PCPDT-DTTzTz) was synthesized by Suzuki polycondensation and the donor−acceptor copolymer was thoroughly characterized. Thermal analysis and X-ray diffraction studies disclosed the semicrystalline nature of the material. When blended with PC71BM and integrated in bulk heterojunction organic solar cells, a moderate power conversion efficiency of 2.43% under AM 1.5 G (100 mW/cm2) conditions was obtained. However, upon purification of the semiconducting copolymer by preparative size exclusion chromatography, a noticeable rise in power conversion efficiency to 4.03% was achieved. The purified polymer exhibited a relatively high field-effect carrier mobility of 1.0 × 10−3 cm2/(V s). The active layer morphology was explored by atomic force microscopy and transmission electron microscopy studies, showing phase segregation on the nanometer scale.