Time-dependent mobility and recombination of the photoinduced charge carriers in conjugated polymer/fullerene bulk heterojunction solar cells

Time-dependent mobility and recombination in the blend of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-${\mathrm{C}}_{61}$(PCBM) is studied simultaneously using the photoinduced charge carrier extraction by linearly increasing voltage technique. The charge carriers are photogenerated by a strongly absorbed, 3 ns laser flash, and extracted by the application of a reverse bias voltage pulse after an adjustable delay time $({t}_{\mathrm{del}})$. It is found that the mobility of the extracted charge carriers decreases with increasing delay time, especially shortly after photoexcitation. The time-dependent mobility $\ensuremath{\mu}(t)$ is attributed to the energy relaxation of the charge carriers towards the tail states of the density of states distribution. A model based on a dispersive bimolecular recombination is formulated, which properly describes the concentration decay of the extracted charge carriers at all measured temperatures and concentrations. The calculated bimolecular recombination coefficient $\ensuremath{\beta}(t)$ is also found to be time-dependent exhibiting a power law dependence as $\ensuremath{\beta}(t)={\ensuremath{\beta}}_{0}{t}^{\ensuremath{-}(1\ensuremath{-}\ensuremath{\gamma})}$ with increasing slope $(1\ensuremath{-}\ensuremath{\gamma})$ with decreasing temperatures. The temperature dependence study reveals that both the mobility and recombination of the photogenerated charge carriers are thermally activated processes with activation energy in the range of 0.1 eV. Finally, the direct comparison of $\ensuremath{\mu}(t)$ and $\ensuremath{\beta}(t)$ shows that the recombination of the long-lived charge carriers is controlled by diffusion.