Synthesis of Hyperbranched Polythiophenes Containing Tetrachloroperylene Bisimide as Bridging Moiety for Polymer Solar Cells

Hyperbranched polythiophene derivatives (P1 and P4) containing tetrachloroperylene bisimide as bridging moiety were synthesized for investigation of thermal, electrochemical, and opto-electrical properties of these derivatives. The polymers (P2 and P3) containing soft alkyl spacer as bridging moiety and linear poly(3-hexylthiophene) (P3HT) were also synthesized for comparison in this study. Polymers with high regioregularity were synthesized via the Universal Grignard metathesis polymerization. The GPC results showed that molecular weights of hyperbranched polythiophenes are higher than that of P3HT. The TGA experiments revealed a first-stage weight loss at about 300 oC for all polymers; besides, polymers containing rigid tetrachloroperylene bisimide groups possess less weight loss than P3HT after heating, indicative of enhanced thermal stabilities. The UV-vis absorption maxima of hyperbranched polymers are similar to that of P3HT in film state, while their absorption shoulder bands are stronger than that of P3HT, indicating stronger interchain interaction and shorter distance between backbones by the introduction of bridge architecture. Moreover, an attenuation of fluorescent intensity was found for those hyperbranched polymers, implying reduced recombination of excitons to emit light and more opportunity for carriers to migrate to both electrodes. Electrochemical analysis showed that introducing hyperbranched structure resulted in decreasing both LUMO and HOMO levels of polymers. All polymers were used for fabrication of polymer solar cells with the configuration of ITO/PEDOT/polymer:PC60BM (1:2 w/w)/LiF/Al to evaluate their performance. The power conversion efficiency (PCE) of the P3HT:PC60BM-based device is 0.54%, while devices based on hyperbranched polymers showed PCE values in the range of 0.45–0.84%. The morphological study of polymer:PC60BM blend films was performed by AFM for interpretation of efficiency trend of devices.