Exciton and Hole-Transfer Dynamics in Polymer: Fullerene Blends

Abstract

Abstract. Ultrafast hole transfer dynamics from fullerene derivative to polymer in bulk heterojunction blends are studied with visible-pump -IR-probe spectroscopy. The hole transfer process is found to occur in 50/300 fs next to the interface, while a longer 15-ps time is attributed to exciton diffusion towards interface in PC 71 BM domains. High polaron generation efficiency in P3HT blends indicates excellent intercalation between the polymer and the fullerene even at highest PC 71 BM concentration thereby yielding a valuable information on the blend morphology.A heterojunction of two organic materials is widely used in photosynthetic systems, photocatalytic reactions, organic optoelectronics, and plastic photovoltaics. In the latter devices, the role of the donor is played by a great variety of conjugated molecules, including the more recent narrowbandgap polymers, while the overwhelming majority of cases utilize the same class of molecules as acceptor: the (soluble) fullerene derivatives, e.g. PCBM. In describing the functionality of these photovoltaic systems one usually considers the process in which an exciton in the donor material dissociates by transfer of an electron to the acceptor material. This is, however, only half the story. It has been shown that the mirror process in which charges are generated by dissociation of an acceptor exciton is very efficient as well [1]. Although the importance of hole transfer was recognised only recently, the first devices functioning primarily on hole transfer have already been demonstrated [2].Charge generation processes are but one of the processes which determine the eventual efficiency of devices. For the efficiency, one of the most important parameters is the morphology of the bulk heterojunction architecture. In an ideal morphology, the interfacial region is maximized for the most efficient exciton dissociation at the interface. At the same time, the ideal morphology would optimize the percolation pathways of the generated charges toward the contacts.Here we report on an ultrafast photophysics study of PC 71 BM based photovoltaic blends addressing ultrafast hole transfer dynamics and the associated material morphology. Our results demonstrate that visible pump -IR probe spectroscopy is not only a powerful tool for studying charge dynamics, but also allows extracting important information on the morphology of the blends, which otherwise is notoriously difficult to obtain.We used PC 71 BM as hole donor material, chosen for its substantial absorption in the visible range. As hole acceptor materials MDMO-PPV and rr-P3HT were used, where the former has a purely amorphous structure and the latter typically possesses both disordered as well as crystalline domains. In our experiments we photo-excite the fullerene derivative at the red absorption flank where the fullerene absorbs significantly stronger than the polymer (630 nm for MDMO-PPV:PC 71 BM and 680 nm for P3HT:PC 71 BM based blends), while monitoring the arrival of EPJ Web of Conferences