Photovoltaic performance enhancement of P3HT/PCBM solar cells driven by incorporation of conjugated liquid crystalline rod-coil block copolymers

The potential application of poly-3-hexylthiophene (P3HT) based liquid crystalline rod-coil block copolymers in polymer solar cells has been investigated. The two liquid crystalline copolymers bear a rodlike liquid crystal block poly(4-(dodecyloxy)-4 ''-(oct-7-en-1-yloxy)-1,1 ': 4 ',1 ''-terphenyl), (P3HT-b-Pterph), and a discotic liquid crystal block poly(2,3,6,7,10-pentakis(hexyloxy)-11-(oct-7-en-1-yloxy) triphenylene), (P3HT-b-PTP), respectively. Solar cells based on the two self-assembled liquid crystalline block copolymers blended with [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) show poor photovoltaic performance due to the introduction of the low conductive non-conjugated liquid crystalline block. The device performance was improved after thermal treatment at the liquid crystalline temperature originating from the self-orientation of the liquid crystalline block copolymers and the formation of well-organized domains in the blend. However, for utilization of the liquid crystalline block copolymers as compatibilizers in P3HT: PCBM blends, the morphology combined with the photovoltaic performance of P3HT: PCBM solar cells can be significantly improved after annealing from the liquid crystalline states. It is demonstrated that the self-assembly of the liquid crystalline block at the donor and acceptor interface can enhance the crystallization and ordering of P3HT chains and guarantee the formation of interpenetrating networks, subsequently resulting in the improvement of efficient exciton separation of the active layer. The copolymer with the discotic liquid crystal block is more favorable than the one with rodlike liquid crystal block, due to the greater compatibilitytwith the fullerene acceptors and the more efficient charge transport caused by the self-assembled columnar phase from the discotic liquid crystals. Therefore, the optimized morphology and promoted charge mobility improved the short-circuit current density and fill factor to give power conversion efficiency up to 4.03%.