[2,2'-Bithiophene]-4,4'-dicarboxamide: a novel building block for semiconducting polymers

A novel electron deficient building block [2,2'-bithiophene]-4,4'-dicarboxamide (BTDCA) was designed to lower the highest occupied molecular orbital (HOMO) energy level of polythiophenes in order to achieve a higher open circuit voltage (V _oc) and thus a higher power conversion efficiency in polymer solar cells (PSCs). BTDCA dibromo monomers were conveniently synthesized in four steps, and were used to prepare three thiophene-based D-A polymers, P(BTDCA66-BT) (66BT), P(BTDCA44-BT) (44BT) and P(BTDCA44-TT) (44TT). All the polymers exhibited unipolar hole transport properties, exhibiting mobilities in the range of ∼10^-4 to 10^-2 cm² V^-1 s^-1 with the highest hole mobility of up to 1.43 × 10^-2 cm² V^-1 s^-1 achieved for 44BT in bottom-gate bottom-contact organic thin film transistors (OTFTs). In PSCs, these polymers achieved high V _oc's of 0.81-0.87 V when PCBM or ITIC was used as acceptor. When 44TT was used as donor and ITIC was used as acceptor, a power conversion efficiency (PCE) of up to 4.5% was obtained, a significant improvement when compared with the poly(3-hexylthiophene) (P3HT):ITIC devices, which showed the highest PCE of merely 0.92%.