A Shift from Diffusion Assisted to Energy Transfer Controlled Fluorescence Quenching in Polymer-Fullerene Photovoltaic Blends

Singlet exciton diffusion and decay dynamics were investigated in two conjugated polymers: the high performance photovoltaic material, PCDTBT, and the substituted poly(p-phenylenevinylene), MEH-PPV, blended with fullerenes. Polymer fluorescence dynamics were measured on a planar quenching interface and in blends with varying concentrations of fullerene acceptor. At very low concentrations of filllerene, fluorescence quenching is assisted by exciton diffusion in the polymer, whereas direct Forster-type energy transfer to the fullerene is found to be the dominant mechanism at higher fullerene concentrations. The rate of quenching in the diffusion-assisted regime is found to be consistent with exciton diffusion coefficients calculated from the fluorescence quenching on a planar interface. The results also show that in unannealed films the onset of phase segregation occurs at concentrations of 4 wt % of C-61-PCBM in MEH-PPV and 7 wt % of C-71-PCBM in PCDTBT and that fullerenes remain dispersed as single molecules within polymer films below this concentration threshold.