Theoretical and Experimental Study of Photophysical Characteristics between Poly(9,9-dioctylfluorene) and Poly(9,9-dioctylfluorene-cobenzothiadiazole)

The photo-physical characteristics of semiconductor polymer are systematically studied through comparing poly (9,9-dioctylfluorene) (PFO) and poly (9,9-dioctylfluorene-cobenzothiadiazole) (F8BT). The quantum chemical calculation shows that the introduction of benzothiadiazole unit facilitates the intrachain charge transfer (ICT) and modulates the electronic transition mechanism of polymer. The transient absorption measurement exhibits that intrachain exciton relaxation is dominant in the decay of excited PFO in a monodisperse system and intrachain exciton interaction could appear at high excitation intensity. In F8BT solution, the ICT state exists and participates in the relaxation of excited state. The relaxation processes of PFO and F8BT in the condensed phase both accelerate and show obvious exciton-exciton annihilation behavior at high excitation intensity. At the same excitation intensity, the mean lifetime of F8BT is longer than that of PFO, which may be assigned to the excellent delocalization of charge.