Charge Carrier Concentration Dependence of Encounter-Limited Bimolecular Recombination in Phase-Separated Organic Semiconductor Blends

TL;DR

This study uses kinetic Monte Carlo simulations to analyze how charge carrier concentration affects encounter-limited bimolecular recombination in phase-separated organic semiconductor blends, emphasizing the importance of long-range electrostatic interactions.

Contribution

It extends previous work by accurately quantifying the concentration dependence of recombination rates and identifying the influence of domain size and charge separation on the power mean mobility model.

Findings

Accurate electrostatic interaction treatment is crucial for reliable recombination rate predictions.

Many prior simulations underestimated the cutoff radius, leading to overestimated recombination rates.

Recombination parameters depend on the ratio of domain size to average charge separation.

Abstract

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.