Origin of reduced bimolecular recombination in blends of conjugated polymers and fullerenes

Bimolecular charge carrier recombination in blends of a conjugated copolymer based on a thiophene and quinoxaline (TQ1) with a fullerene derivative ((6,6)-phenyl-C₇₁-butyric acidmethyl ester, PC₇₁BM) is studied by two complementary techniques. TRMC (time-resolved microwave conductance) monitors the conductance of photogenerated mobile charge carriers locally on a timescale of nanoseconds, while using photo-CELIV (charge extraction by linearly increasing voltage) charge carrier dynamics are monitored on a macroscopic scale and over tens of microseconds. Despite these significant differences in the length and time scales, both techniques show a reduced Langevin recombination with a prefactor ζ close to 0.05. For TQ1:PC ₇₁BM blends, the ζ value is independent of temperature. On comparing TRMC data with electroluminescence measurements it is concluded that the encounter complex and the charge transfer state have very similar energetic properties. The ζ value for annealed poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) is approximately 10^-4, while for blend systems containing an amorphous polymer ζ values are close to 1. These large differences can be related to the extent of charge delocalization of opposite charges in an encounter complex. Insight is provided into factors governing the bimolecular recombination process, which forms a major loss mechanism limiting the efficiency of polymer solar cells. The measured bimolecular recombination rate of electrons and holes in a polymer:fullerene blend is 20 times smaller than the Langevin rate. The reduced rate is explained in terms of dissociation of electron-hole encounter complexes into free charge carriers rather than decay to the ground state.