Crystalline Random Conjugated Copolymers with Multiple Side Chains: Tunable Intermolecular Interactions and Enhanced Charge Transport and Photovoltaic Properties

Random poly(3-butylthiophene-co-3-octylthiophene) of several compositions were synthesized and found to be highly crystalline in spite of the statistical arrangement of the different-sized side chains. The melting (T-m) and recrystallization (T-c) temperatures of the random copolythiophenes varied continuously with composition. The interlayer d(100) stacking distance in the lamellar crystalline structure of the random copolymers, determined from X-ray diffraction of films, varied linearly with copolymer composition over the entire range. Fullerene-based bulk heterojunction solar cells made from the random copolythiophenes had power conversion efficiencies flip to 3.0% under 100 mW/cm(2) AM 1.5 illumination in air, which is significantly enhanced compared to the homopolymers and their physical blends. The space charge limited current mobility of holes in the random copolythiophenes was enhanced compared to poly(3-butylthiophene) and physical blends of the homopolymers. Transmission electron microscopy images of the bulk heterojunction thin films showed that the random copolythiophenes had a worm-like nanoscale phase-separated morphology beneficial to solar cells. These results demonstrate that random copolymerization and multiple-sized side chains provide a facile means of fine-tuning the structural and electronic/optoelectronic properties of conjugated polymers. The results also provided new insights on the lamellar crystalline packing of side-chain bearing conjugated polymers.