Interplay between fluorescence and morphology in composite MEH-PPV/PCBM nanoparticles studied at the single particle level

We report the study of the photoluminescence properties of composite conjugated polymer (MEH-PPV)/fullerene (PCBM) nanoparticles as a function of PCBM doping level. The emission properties of individual nanoparticles were studied by Single Particle Spectroscopy (SPS), and distinct changes in vibronic structure with nanoparticle composition were observed. These changes are found to be due to the presence of domains in the nanoparticles with two distinct types of optical signatures, one with molecular and one with aggregate character, for which the abundance and morphology is found to change with PCBM doping levels. Interestingly, highly delocalized structures with a large extent of exciton migration are formed at low PCBM doping levels, while at high PCBM doping levels the exciton collapses into highly localized structures. Furthermore, at very high doping levels phase separation within the MEH-PPV/PCBM nanoparticles is found, even though the reported nanoparticles are only a few tens of nanometers in diameter.