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

Transient, nonresonant, hole-burning spectroscopy was 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 were monitored as a function of time in the 10 ns-10 ms time range. A time-dependent red shift was obsd. The intensity of the spectral hole decayed with time according to a power law (~t-a). The exponent a ~ 0.5 is 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 (~G-b) with b.apprx.0.5. The time-dependent red shift 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. Charge sepn. is assisted by disorder and, in the 10 ns-10 ms time range, the recombination rate is limited by the detrapping of the cationic charge carriers in MDMO-PPV