Photoinduced charge transfer in composites of conjugated polymers and semiconductor nanocrystals

Photoinduced electron transfer in composites of CdSe and InP nanocrystals (NC) with a conjugated polymer [2-methoxy-5-(3-,7-dimethyl-octyloxy)-1, 4-phenylene vinylene] (OC1C10-PPV) is studied by means of light-induced electron spin resonance (LESR), photoluminescence (PL) as well as quasi steady-state photoinduced absorption (PIA) spectroscopy. The quenching of PL, the occurrence of two new optical absorption bands, and the formation of light-induced paramagnetic species on the polymer chain are altogether interpreted as an electron transfer between a donor polymer and acceptor NC in the excited state. In particular the LESR provides evidence of an electron transfer rather than energy transfer due to an overlap of absorption and emission bands of NC and the conjugated polymer. The g-factor of the signal is consistent with the cationic origin of OC1C10-PPV radicals. Additionally, the LESR signal is sensitive to NC surface coating being smaller in blends with the tri-n-octylphosphine oxide-tri-n-octylphosphine (TOPO-TOP) coated CdSe surface and completely absent in blends with TOPO-TOP-InP. LESR probes predominantly positive polarons on the conjugated polymer chain generated at room temperature in the course of an electron transfer to NC. This is very different from the situation in blends of OC1C10-PPV with fullerenes, another promising photovoltaic acceptor, where the photogenerated cation and anion were both observed in the LESR. In PIA we found contributions of both polymer polarons and electrons on the NC. The recombination kinetics shows a broad distribution of lifetimes, which is characteristic for dispersive (diffusive) recombination processes with fractional power frequency dependence.