Interfaces between different organic materials can play a key role in determining organic semiconductor device characteristics. Here, we present a physics-based one-dimensional model with the goal of exploring critical processes at organic/organic interfaces. Specifically, we envision a simple bilayer structure consisting of an electron transport layer (ETL), a hole transport layer (HTL), and the interface between them. The model calculations focus on the following aspects: (1) the microscopic physical processes at the interface, such as exciton formation/dissociation, exciplex formation/ dissociation, and geminate/nongeminate recombination; (2) the treatment of the interface parameters and the discretization method; and (3) the application of this model to different devices, such as organic light emitting diodes and photovoltaic cells. At the interface, an electron on an ETL molecule can interact with a hole on an adjacent HTL molecule and form an intermolecular excited state (exciplex). If either the electron or the hole transfers across the interface, an exciton can be formed. The exciton may subsequently diffuse into the relevant layer and relax to the ground state. A strong effective electric field at the interface can cause excitons or exciplexes to dissociate into electrons in the ETL and holes in the HTL. Geminate recombination may occur when the Coulomb interaction between the electron and the hole generated at the interface by exciton dissociation causes the formation of a correlated state that then relaxes to the ground state. The relative impacts of the different processes on measurable macroscopic device characteristics are explored in our calculations by varying the corresponding kinetic coefficients. As it is the aim of this work to investigate effects associated with the organic/organic interface, its treatment in the numerical calculations is of critical importance. We model the interface as a continuous but rather sharp transition from the ETL to the HTL. The model is applied to different devices where different microscopic processes dominate. We discuss the results for an organic light emitting device with exciton or exciplex emission and for a photovoltaic device with or without geminate recombination. In the examples, C 60 and tetracene parameters are used for the ETL and HTL materials, respectively.
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This work was supported in part by a University of Minnesota Graduate School Fellowship. Additional support was provided by the MRSEC Program of the National Science Foundation under Award No. DMR-0819885. Work at Los Alamos National Laboratory was supported by the LDRD program. Access to the facilities of the Minnesota Supercomputing Institute is gratefully acknowledged.