Fluorescence Quenching in Conjugated Polymers Blended with Reduced Graphitic Oxide

Conjugated polymers blended with graphene represent a possible approach for making organic bulk heterojunction solar cells. In this paper, the time-resolved fluorescence dynamics of poly(3-hexylthiophene-2,5-diyl) (P3HT) and polu[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) blended with graphene microsheets derived from chemically reduced graphitic oxide are studied. Both polymers exhibit strong quenching and shortened fluorescence lifetimes when mixed with graphene. The fluorescence quenching function takes the form of e(-kQt1/2), where k(Q) is linearly proportional to the weight fraction of graphene in the blend. We consider two physical models to explain the origin of the fluorescence quenching. The first assumes that energy transfer Occurs within a three-dimensional space to molecular scale defects within the graphene according to the standard Forster model with an energy transfer rate proportional to the donor-acceptor separation R-6. The second model assumes a quasi-two-dimensional environment where the energy transfer rate between the donor and graphene sheets is proportional to R-4. Using the second model, an estimate of similar to 5 nin is obtained for the critical energy transfer radius for energy transfer between P3HT chains and graphene sheets. This Value is in reasonable agreement with theory. Differences between the quenching behavior of graphene in MEH-PPV and P3HT blends are also discussed.