Transient optical studies of charge recombination dynamics in a polymer/fullerene composite at room temperature

The recombination kinetics of photogenerated charge carriers in a composite of poly[2-methoxy-5- ${\text{(3}}^{\prime }{\text{,7}}^{\prime }$ -dimethyloctyloxy)-1-4-phenylene vinylene], (MDMO–PPV) and the functionalised fullerene 1-(3-methoxycarbonyl)-propyl-1-phenyl-$\text{(6,6)}{\mathrm{C}}_{61}$ are investigated at room temperature by transient absorption spectroscopy. The decay dynamics of positively charged MDMO–PPV polarons were found to be either monophasic or biphasic, depending upon the laser excitation density employed. The slower, power law, decay phase (100 ns–10 ms) is attributed to recombination dynamics of localized polarons, while the fast decay component (<20 ns) is attributed to recombination of relatively mobile polarons observed when the density of localized states is exceeded by the density of photogenerated polarons ${\text{(∼10}}^{17}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}\mathrm{).}$ The implications of these observations are discussed in relation to polymer/${\mathrm{C}}_{60}$ photovoltaic cells.