Charge recombination in a poly(para-phenylene vinylene)-fullerene derivative composite film studied by transient, nonresonant, hole-burning spectroscopy

Transient, nonresonant, hole-burning spectroscopy has been used to study the charge recombination process in poly[2-methoxy-5-(3^′,7^′-dimethyloctyloxy)-1-4-phenylene vinylene] (MDMO-PPV): methanofullerene (PCBM) composite films. The position and intensity of the spectral hole in the absorption band of MDMO-PPV have been monitored as a function of time in the 10 ns–10 μs time range. A time-dependent redshift is observed. The intensity of the spectral hole decays with time according to a power law ${\mathrm{(\propto t}}^{\mathrm{-\alpha }}\mathrm{).}$ The exponent α≈0.5 is found to be nearly independent of the excitation fluence in the range 0.05–2 mJ/cm². The depth of the spectral hole depends sublinearly on the excitation fluence (I) and can be described by ${\mathrm{(\propto \Gamma }}^{\mathrm{-\beta }})$ with β∼0.5. The time-dependent redshift and the power-law type time decay can be reproduced by numerical simulations. The Monte Carlo method is used to simulate the hopping dynamics of the photoinduced charges in a lattice of energetically disordered sites before they eventually recombine at the MDMO-PPV:PCBM interface. The results indicate that charge separation is assisted by disorder and that, in the 10 ns–10 μs time range, the recombination rate is limited by the detrapping of the cationic charge carriers in MDMO-PPV.