Bimolecular charge carrier recombination has been clarified in bulk-heterojunction solar cells based on a blend of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene using the time-of-flight method. We show how bimolecular recombination influences the charge carrier transport, how it limits the efficiency of low-mobility solar cells, and how to estimate the bimolecular recombination coefficient. We found that bimolecular recombination in these efficient photovoltaic materials is orders of magnitude slower as compared to Langevin recombination expected for low-mobility materials. This effect is inherent to the nanomorphology of the bicontinuous interpenetrating network creating separate pathways for electrons and holes, and paves the way for the fabrication of bulk-heterojunction solar cells where bimolecular recombination is not the limiting factor.

The temperature and electric-field dependence of charge carrier mobility has been studied by a conventional time-of-flight technique in chemically purified, low dark conductivity samples of regioregular poly(3-hexylthiophene). Subsequently, the mobility of doping-induced charge carriers has been determined using the technique of charge carrier extraction by linearly increasing voltage in the same samples exposed to air. The charge carrier mobility determined by both experimental techniques correspond well to each other at temperatures above $130\mathrm{K}$, indicating that these experimental techniques are mutually consistent. The study clearly shows that the typical $\log \mu \propto \beta E^{1∕2}$, $\beta >0$ Poole–Frenkel-like electric-field dependence of the charge carrier mobility diminishes at temperatures around $250–270\mathrm{K}$, and $\beta$ becomes negative at higher temperatures. Such negative electric-field dependence of mobility observed by both experimental techniques is attributed to positional disorder in a random-organic dielectric and analyzed in the framework of the disorder formalism. Finally, the overall agreement indicates that the mode of charge generation has negligible effect on the temperature- and electric-field dependence of mobility except at the lowest temperatures $(<110\mathrm{K})$, where transit time dispersion of the photogenerated charge carriers probed by the ToF technique is more pronounced.

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 $\left(t_{\mathrm{del}}\right)$. It is found that the mobility of the extracted charge carriers decreases with increasing delay time, especially shortly after photoexcitation. The time-dependent mobility $\mu \left(t\right)$ 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 $\beta \left(t\right)$ is also found to be time-dependent exhibiting a power law dependence as $\beta \left(t\right)={\beta }_0{t}^{-(1-\gamma )}$ with increasing slope $(1-\gamma )$ 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 $\mu \left(t\right)$ and $\beta \left(t\right)$ shows that the recombination of the long-lived charge carriers is controlled by diffusion.