Photoinduced charge transfer in composites of conjugated polymers and semiconductornanocrystals

Abstract

Photoinduced electron transfer in composites of CdSe and InP nanocrystals (NC) with aconjugated 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. Thequenching of PL, the occurrence of two new optical absorption bands, and the formation oflight-induced paramagnetic species on the polymer chain are altogether interpreted as anelectron transfer between a donor polymer and acceptor NC in the excited state. In particularthe LESR provides evidence of an electron transfer rather than energy transfer due to anoverlap of absorption and emission bands of NC and the conjugated polymer. Theg-factor of the signal is consistent with the cationic origin ofOC1C10-PPV radicals. Additionally, the LESR signal is sensitive to NC surface coating being smaller in blends with thetri-n-octylphosphineoxide–tri-n-octylphosphine (TOPO–TOP) coated CdSe surface and completely absent in blendswith TOPO–TOP–InP. LESR probes predominantly positive polarons on theconjugated polymer chain generated at room temperature in the course of anelectron transfer to NC. This is very different from the situation in blends ofOC1C10-PPV with fullerenes, another promising photovoltaic acceptor, where the photogenerated cationand anion were both observed in the LESR. In PIA we found contributions of both polymerpolarons and electrons on the NC. The recombination kinetics shows a broad distributionof lifetimes, which is characteristic for dispersive (diffusive) recombination processes withfractional power frequency dependence.