Charge carrier concentration dependence of encounter-limited bimolecular recombination in phase-separated organic semiconductor blends

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 ±10% 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.

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

“…Deviations from the Langevin model have been used as evidence for an equilibrium between CT states and free charge carriers, where charges form CT states and dissociate many times before they are finally extracted or lost to recombination . Furthermore, the Langevin model was originally derived to describe recombination in a single‐phase system, and deviations from the Langevin rate coefficient are sometimes attributed to the presence of separate phases for hole and electron transport in BHJ solar cells …”

Thermal stability analysis of buffered layer P3HT/P3HT:PCBM organic solar cells

“…where J G-REC is geminate recombination current density and J NG-REC is non-geminate recombination current density. Generally, incident photon gets absorbed in photoactive layer which generates the bounded electron-hole pair known as exciton [34][35][36]. The exciton diffuses to the donor-acceptor (D-A) interface and dissociated into free electrons and holes.…”

“…These free electrons and holes further drifts to the corresponding electrodes which determines the current conduction. In OSCs, non-geminate recombination is of bimolecular mechanism: when a free electron combines with free hole presents in its mutual columbic field [36]. The bimolecular recombination is mainly depends on mobility, mean free path and charge carrier densities [35,36].…”

Impact of Active Layer Morphology, Density of States, Charge Carrier Concentration, and Local Charge Density Fluctuations on Bimolecular Recombination of Bulk Heterojunction Solar Cells: A Theoretical Perspective

“…Previously, a number of experimental data show that, depending on the mobility of charge carriers [14], recombination evolves from trap-assisted first-order (monomolecular) dynamics under short-circuit current conditions to second-order (bimolecular) Langevin dynamics under open-circuit voltage conditions [1,[15][16][17][18][19]. Though, many recent studies [20][21][22][23][24][25][26][27][28][29] indicate that BMR in organic solar cells significantly deviates from a traditional Langevin description.…”