  Understanding how the complex intermolecular- and nano-structure present in
organic semiconductor donor-acceptor blends impacts charge carrier motion,
interactions, and recombination behavior is a critical fundamental issue with a
particularly major impact on organic photovoltaic applications. In this study,
kinetic Monte Carlo (KMC) simulations are used to numerically quantify the
complex bimolecular charge carrier recombination behavior in idealized
phase-separated blends. Recent KMC simulations have identified how the
encounter-limited bimolecular recombination rate in these blends deviates from
the often used Langevin model and have been used to construct the new power
mean mobility model. Here, we make a challenging but crucial expansion to this
work by determining the charge carrier concentration dependence of the
encounter-limited bimolecular recombination coefficient. In doing so, we find
that an accurate treatment of the long-range electrostatic interactions between
charge carriers is critical, and we further argue that many previous KMC
simulation studies have used a Coulomb cutoff radius that is too small, which
causes a significant overestimation of the recombination rate. To shed more
light on this issue, we determine the minimum cutoff radius required to reach
an accuracy of less than $\pm10\%$ as a function of the domain size and the
charge carrier concentration and then use this knowledge to accurately quantify
the charge carrier concentration dependence of the recombination rate. Using
these rigorous methods, we finally show that the parameters of the power mean
mobility model are determined by a newly identified dimensionless ratio of the
domain size to the average charge carrier separation distance.
