Low-temperature recombination kinetics of photoexcited persistent charge carriers in conjugated polymer/fullerene composite films

The recombination kinetics of long-lived photoexcited charge carriers in a composite of $\mathrm{poly}[2-\mathrm{methoxy}{-5-(3\mathrm{}}^{\prime }{,7\mathrm{}}^{\prime }-\mathrm{dimethyloctyloxy})-1,4-\mathrm{phenylene}$ vinylene] (MDMO-PPV) and $1-(3-\mathrm{methoxycarbonyl})-\mathrm{propyl}-1-\mathrm{phenyl}-(6,6)\mathrm{}{\mathrm{C}}_{61}$ (PCBM) at low temperatures $(T=40\mathrm{}\mathrm{K})$ are investigated by light induced electron-spin resonance (LESR). These long-lived (persistent) photoinduced charge carriers exhibit recombination times that extend over several hours after cessation of the photoexcitation. These long relaxation times can be explained by nongeminate recombination of randomly distributed carriers assuming charge neutrality. The decay curves fit well to a model in which the recombination mechanism of photoexcited carriers consists of tunneling processes and in which the recombination rate only depends on the intrapair distance between the photoexcited carriers. It is shown that the residual photoexcited carrier concentration after long times tends to be independent of the generation rate. The presented model has already been successful in describing the recombination kinetics of photoexcited carriers in inorganic, amorphous semiconductors, which indicates that the presented recombination mechanism is common to disordered organic and inorganic materials.