OPV tool is an excellent educational resource for the understanding and analysis of the operation of organic solar cells. The tool simulates the current-voltage (I-V) characteristics of organic solar cells with bilayer or planar heterojunction morphology. The following procedure is followed to get the I-V characteristics:
1) Optical absorption inside the device is calculated by the transfer matrix method. The absorption profile depends on the complex refractive indices and the thickness of various layers in the cell.
2) Exciton diffusion equation is then solved in both donor and acceptor layers. Exciton concentration at the donor-acceptor interface is set to zero with the assumption that dissociation probability is high and field independent.
3) Charged carrier transport is simulated by the self consistent solution of drift-diffusion and Poisson equation. Generation term in the carrier continuity equation is calculated from exciton diffusion profile and the recombination at the D-A interface is implemented by bi-molecular mechanism. We ignore geminate recombination at the interface.
The tool not only simulates the I-V characteristics but also calculates the solar cell performance matrices such as efficiency, short circuit current, open circuit voltage, and fill factor. The tool also plots the photon absorption profile, charged carrier distribution, electric field, and energy bands inside the active layer for different terminal voltages. Thus, the tool can be used to analyze the effects of the device geometry (e.g. thickness of various layers) and the transport parameters (e.g. exciton diffusion length, mobilities, etc.) on the performance of organic solar cells. The tool can also be used for the optimum device design for a given set of transport parameters.
Development Team: Biswajit Ray, Camila González Williamson, Mohammad Ryyan Khan, and Muhammad A. Alam
Center for Re-Defining Photovoltaic Efficiency through Molecule Scale Control, an Energy Frontier Research Center funded by the U.S. Department of Energy.
Network for Computational Nanotechnology (NCN).
1) B. Ray, P.R. Nair, and M.A. Alam, “Annealing Dependent Performance of Organic Bulk-Heterojunction Solar Cells: A Theoretical Perspective,” Solar Energy Materials and Solar Cells, 2011.
2) B. Ray and M.A. Alam, “A compact physical model for morphology induced intrinsic degradation of organic bulk heterojunction solar cell,” Applied Physics Letters, vol. 99, Jul. 2011, pp. 033303-3.
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