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This software tool simulates the exciton dynamics measured using pump-probe spectroscopy experiments on donor-acceptor blend films used in organic photovoltaic devices. The tool uses a kinetic Monte Carlo method to simulate all dominant exciton mechanisms including exciton creation, diffusion, dissociation, and relaxation. The morphology of the blend film can be set to a bilayer, ordered heterojunction, or bulk heterojunction. This tool can be used to model experimental data and extract reactions rates for each mechanism. In addition, the dynamics of exciton dissociation can be investigated in detail by adjusting the bulk heterojunction domain size and the inverse exciton localization parameter.
The system is defined as a thin donor-acceptor blend film (~100 nm). When light is directed at the film, some photons with energy greater than the donor or acceptor bandgap are absorbed by the film. Each absorbed photon creates an excited electronic state consisting of a bound electron-hole pair called an exciton. Once created, excitons in the film can undergo several different processes. Excitons can undergo diffusion and move around in the material through energy transfer. If they reach the donor-acceptor interface, excitons can also dissociate into an unbound electron and hole. If they cannot reach the interface quick enough though, eventually the excitons will relax back to the ground state. Exciton-exciton annihilation and exciton-polaron annihilation mechanisms are also possible. However, under normal operating conditions of an organic photovoltaic device where there is low illumination intensity, the exciton and polaron concentrations are low enough such that these mechanisms are negligible.
The timescales of each of these mechanisms can be investigated using pump-probe spectroscopy. In a pump-probe experiment, the film is first excited by a short laser pulse, and then the system is measured at set time intervals in order to determine how the system changes over time. In transient absorption spectroscopy, the state of the system at a particular time is measured using a short pulse of multi-wavelength light typically covering the near-infrared range. Excitons in semiconducting polymer films have a characteristic absorption region, and the near-IR absorption spectrum can be analyzed to estimate the exciton concentration in the film at the time the spectrum is taken. Immediately after the pump pulse a number of excitons are created in the film, but as time passes these excitons are removed from the system due to either dissociation or relaxation. As a result, an exciton decay curve is obtained, where the exciton concentration in the film decays over time. This tool simulates the dominant exciton mechanisms and constructs a simulated exciton decay curve that can be fit to experimental data.
This tool uses a kinetic Monte Carlo simulation method with a cubic lattice to simulate the dominant exciton mechanisms occurring in a donor-acceptor blend film. A modified Ising method is used to generate model bulk heterojunction morphologies for this tool. More details about the methodology and application of this tool are published in JPCC.
Note To Users
If you have any questions or would like to see new features added, please let me know. If you are interested in customizing this tool for a unique system, I am open to collaboration.
Bulk Heterojunction Morphology Simulator. https://nanohub.org/resources/bhjmorphology
Thank you to Prof. Ali Dhinojwala for supporting this work.
The University of Akron
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