  The slow decay of charge carriers in polymer-fullerene blends measured in
transient studies has raised a number of questions about the mechanisms of
nongeminate recombination in these systems. In an attempt to understand this
behavior, we have applied a combination of steady-state and transient
photoinduced absorption measurements to compare nongeminate recombination
behavior in films of neat poly(3-hexyl thiophene) (P3HT) and P3HT blended with
[6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Transient measurements show
that carrier recombination in the neat P3HT film exhibits second-order decay
with a recombination rate coefficient that is similar to that predicted by
Langevin theory. In addition, temperature dependent measurements indicate that
neat films exhibit recombination behavior consistent with the Gaussian disorder
model. In contrast, the P3HT:PCBM blend films are characterized by a strongly
reduced recombination rate and an apparent recombination order greater than
two. We then assess a number of previously proposed explanations for this
behavior, including phase separation, carrier concentration dependent mobility,
non-encounter limited recombination, and interfacial states. In the end, we
propose a model in which pure domains with a Gaussian density of states are
separated by a mixed phase with an exponential density of states. We find that
such a model can explain both the reduced magnitude of the recombination rate
and the high order recombination kinetics and, based on the current state of
knowledge, is the most consistent with experimental observations.
