Enhanced Performance of Bulk Heterojunction Solar Cells Using Block Copoly(3-alkylthiophene)s

The photovoltaic properties, charge transport, and morphology of a series of diblock conjugated copolymers, poly(3-butylthiophene)-b-poly(3-octylthiophene) (P3BT-b-P3OT), were investigated as a function of block composition. Bulk heterojunction solar cells comprising blends of P3BT-b-P3OT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were found to have power conversion efficiencies as high as 3.0%, which represents factors of 1.6−9 enhancements compared to those of the homopolymers made under similar conditions. Imaging of P3BT-b-P3OT/PC71BM blends by atomic force microscopy and transmission electron microscopy revealed interpenetrating network with 11−18 nm crystalline polymer domains. The zero-field space charge limited current mobility of holes (∼(1−3) × 10−4 cm2 V−1 s−1) was similarly enhanced in the diblock copolymer solar cells compared to the homopolymers. These results demonstrate that block conjugated copolymers offer a promising approach to advanced materials for polymer solar cells and that the block composition is an attractive means to optimize the materials.