Main-chain alternating fullerene and dye oligomers for organic photovoltaics.
Résumé
This work demonstrates for the first time that it is possible to prep. alternating oligomers, contg. both dyes and fullerenes in repeating structures, that act as electron acceptors in bulk heterojunction devices. A sterically controlled azomethine ylide cycloaddn. polymn. is employed with either C60 or phenyl-C61-butyric acid Me ester (PCBM) and the dye diketopyrrolopyrrole (DPP). The former results in low mol. wts. of around 5600 g mol-1, whereas the latter, PCBM, enables the formation of more sol. chains with higher mol. wts. of ca 11 200 g mol-1. Remarkably, cyclic voltammetry shows that the incorporation of PCBM into the main-chain raises the LUMO by ca 380 meV due to the in-chain bis-addns. The observation of the complete quenching of DPP fluorescence by the fullerene moiety, combined with computer modeling studies, indicates both electron and energy transfers between intra-chain moieties. Proof-of-concept devices show low efficiencies most likely due to as-yet-unoptimized prepn. and structures, but hint at the possibilities of these novel bi-functionalized, in-chain fullerenes due to their high Voc of 0.89 V with an example low-bandgap polymer, KP115, and reasonable charge mobilities of ca 1 × 10-4 cm2 V-1/s, making this new class of materials of strong interest for applications. Furthermore, their good thermal stability to above 300° and their stabilization of photovoltaic devices against thermal degrdn. confirm that this new pathway to a wide range of dye/fullerene structures is extremely promising.