Broadband Finite‐Difference Time‐Domain Modeling of Plasmonic Organic Photovoltaics

Abstract

We develop accurate finite-difference time-domain (FDTD) modeling of polymer bulk heterojunction solar cells containing Ag nanoparticles between the holetransporting layer and the transparent conducting oxidecoated glass substrate in the wavelength range of 300 nm to 800 nm. The Drude dispersion modeling technique is used to model the frequency dispersion behavior of Ag nanoparticles, the hole-transporting layer, and indium tin oxide. The perfectly matched layer boundary condition is used for the top and bottom regions of the computational domain, and the periodic boundary condition is used for the lateral regions of the same domain. The developed FDTD modeling is employed to investigate the effect of geometrical parameters of Ag nanospheres on electromagnetic fields in devices. Although negative plasmonic effects are observed in the considered device, absorption enhancement can be achieved when favorable geometrical parameters are obtained. Keywords: FDTD, organic photovoltaics, plasmonics.Manuscript received Aug. 6, 2013; revised Nov. 27, 2013; accepted Dec. 16, 2013 [17] has been widely employed because of its accuracy, robustness, and matrix-free characteristics. Moreover, a single FDTD simulation can compute a wideband response by using a Fourier transform, since it is a time-domain method. In FDTD, the frequency-dependent permittivity of materials in plasmonic OPVs should be incorporated by an appropriate dispersion model. However, the previous FDTD analyses have not considered dispersive properties of OPV materials due to the difficulty involved in doing so. In fact, because the real part of the relative permittivity of Ag is negative, a dispersive FDTD algorithm should be applied to Ag so that the resulting FDTD algorithm does not suffer from instability. dispersive FDTD for Ag and a non-dispersive FDTD for other OPV materials (with the corresponding permittivity and conductivity at a specific wavelength), which leads to overwhelming computational costs. Therefore, it is of great interest to develop accurate FDTD dispersive modeling for the optical analysis of plasmonic OPVs. In this work, we develop -based on the Drude dispersion model -FDTD dispersive modeling for plasmonic OPVs. The perfectly matched layer (PML) [18]-[19] and the periodic boundary condition (PBC) [14] are used for the termination and lateral regions of the computational domain, respectively. We also employ the proposed FDTD algorithm to investigate the effect of the geometrical parameters of Ag nanospheres on electromagnetic fields in the photoactive layer. It is worth noting that the purpose of this paper is to develop FDTD dispersive modeling suitable for plasmonic OPVs, not to optimize plasmonic OPVs for improved performance. II. FDTD ModelingWe consider polymer:fullerene bulk heterojunction (BHJ) solar cells. For the plasmonic structure, the self-assembled Ag nanospheres are formed between the indium tin oxide (ITO)-coated glass substrate and the poly(3, 4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) -the latter ac...

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