Effect of PCBM concentration on photoluminescence properties of composite MEH-PPV/PCBM nanoparticles investigated by a Franck-Condon analysis of single-particle emission spectra.

The emission of composite conjugated polymer (MEH-PPV)/fullerene (PCBM) nanoparticles is investigated by single particle spectroscopy (SPS), and changes in vibronic structure with nanoparticle composition are evaluated by means of a detailed Franck-Condon analysis. Consistent with previous reports we find that the emission spectra can be modeled as the superposition of two types of emitters, one with aggregate character and one with molecular character. Major findings from the fitting of the SPS data to a Franck-Condon model are that 1) the occurrence of each of the two types of emitters changes with nanoparticle composition to the point that no aggregate emitters are detected (at 50 wt% PCBM), 2) at the highest PCBM doping levels (75 wt% PCBM) aggregate emitters reappear due to nanoscale phase separation in the composite nanoparticles, 3) the molecular emitters show small Huang-Rhys factors that increase with PCBM doping, indicative of extensive delocalization and exciton migration that is reduced by the disorder introduced in the polymer material by PCBM doping and 4) the aggregate emitters show large Huang-Rhys factors, indicative of the localized nature of these energy trap sites, with a broad distribution of values of these Huang-Rhys factors. The latter observation suggests a broad heterogeneous distribution of aggregate morphologies in blended conducting polymer materials, which can be attributed to variations in polymer chain folding and stacking at the aggregate sites. The reported results obtained by the SPS approach show how blending conjugated polymers with fullerenes at various doping levels induces changes in interchain interactions and aggregate site density even at length scales below a few tens of nanometers that affect conjugated polymer material properties, an observation that has gone unnoticed in bulk studies of blended conjugated polymer films.