Establishing an Interface between Kinetic Monte Carlo and Drift Diffusion Simulations of Organic Bulk-Heterojunction Solar Cells to Investigate the Effect of the Effective Medium Approach

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Oral, Tim Albes, presentation 240
Publication date: 5th February 2015

Organic photovoltaic (OPV) devices are considered to be a promising alternative technology to inorganic solar cells because they offer the possibility to be fabricated on large scales at a low production cost. Especially the concept of an intermixed Bulk-Heterojunction (BHJ) between organic donor and acceptor materials forms the basis for devices with high power conversion efficiencies (PCE) [1] because it allows to handle the separation of photo-generated excitons efficiently. Since it is a challenging task to investigate the morphology and the internal processes experimentally [2], simulations on different scales can be viable tools to guide the optimization of BHJ cells. One way to model organic solar cells is to solve the Drift-Diffusion (DD) equations. They offer an approach at a macroscopic continuum level, low computational effort, and with good agreement to experimental data [3]. A common approximation of DD simulations is the effective medium approach (EMA) [4], i.e. the lack of incorporation of the real blend morphology. In the EMA, donor and acceptor material are treated as one effective material. The donor/acceptor interfaces to split excitons into polaronic charges are assumed to be everywhere across the photoactive layer and no real exciton dynamics is considered. For this purpose, kinetic Monte Carlo (kMC) simulations offer a suitable tool to implement the desired morphology [5,6]. We have developed a fully functional kMC simulator for organic BHJ devices [7] that is able to generate intermixed morphologies after a spin-exchange algorithm and includes exciton and charge dynamics. Despite good reproduction of experimental measurements, the kMC method comes at very high computational cost and is not suitable to simulate large device structures or long virtual time periods. The morphology generated for the kMC simulations gives the possibility to be passed to the DD simulations to overcome the EMA and to establish a common basis for both simulations (Fig. 1). This allows to adequately compare the two methods and test how much of an approximation is made with the EMA.
A model of the BHJ intermixing generated with the kMC method (left) is used as an input to DD simulations. The latter allow to easily evaluate macroscopic quantities such as the electrostatic potential across the morphology: here depicted across a 2D slice (center) and along a 1D line (right) through the blend between the electrodes.
[1] You, J. et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nature Communications 2013, 4, 1446. [2] Moon J. S. et al. ’Columnlike’ structure of the cross-sectional morphology of bulk heterojunction materials. Nano Letters 2009, 9.1, 230–4. [3] Koster L. J. A. et al. Device model for the operation of polymer/fullerene bulk heterojunction solar cells. Physical Review B 2005, 72.8, 085205. [4] Fallahpour, A. H. et al. Journal of Computational Electronics 2014, 1-10. [5] Watkins, P. K. et al. Dynamical Monte Carlo modelling of organic solar cells: the dependence of internal quantum efficiency on morphology. Nano Letters 2005, 5.9, 1814–8. [6] Meng, L. et al. Dynamic Monte Carlo simulation for highly efficient polymer blend photovoltaics. The Journal of Physical Chemistry B 2009, 114.1, 36–41. [7] Albes, T. et al. 40th IEEE Photovoltaic Specialists Conference, conference proceeding (2014).

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