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Journal of Photonics for Energy • Open Access

Modeling approach to derive the anisotropic complex refractive index of polymer:fullerene blends for organic solar cells utilizing spectroscopic ellipsometry
Author(s): Michael F. G. Klein; Gustavo Q. Glasner de Medeiros; Panagiota Kapetana; Uli Lemmer; Alexander Colsmann

Paper Abstract

The knowledge of the complex refractive indices of all thin layers in organic solar cells (OSCs) is a prerequisite for comprehensive optical device simulations that are particularly important for sophisticated device architectures, such as tandem OSCs. Therefore, refractive indices are often determined via spectroscopic ellipsometry and subsequent time-consuming modeling. Here, we investigate a modeling approach that allows for the determination of complex refractive indices of bulk-heterojunctions by superimposing the optical models of the respective fullerenes and polymers. The optical constants of neat [6,6]-phenyl C71-butyric acid methyl ester (PC71BM), poly{[4,4’-bis(2-ethylhexyl)dithieno(3,2-b;2’,3’-d)silole]-2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} (PSBTBT) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) (PCPDTBT) are determined, covering the OSC relevant spectral region from 250 to 1,000 nm. Then the blends PSBTBT : PC71BM and PCPDTBT : PC71BM are described within an effective medium approximation. From this approximation, the mass density ratio of polymer and fullerene can be derived. This approach furthermore allows for a uniaxial anisotropic optical description of the polymers and provides insight into thin-film morphology. In contrast to x-ray diffraction experiments, this method also allows for probing amorphous materials. Spectroscopic ellipsometry can be a valuable tool for the investigation of bulk-heterojunction morphologies of the latest high-performance OSC materials that exhibit a low degree of crystallinity.

Paper Details

Date Published: 6 January 2015
PDF: 16 pages
J. Photon. Energy. 5(1) 057204 doi: 10.1117/1.JPE.5.057204
Published in: Journal of Photonics for Energy Volume 5, Issue 1
Show Author Affiliations
Michael F. G. Klein, Karlsruher Institut für Technologie (Germany)
Gustavo Q. Glasner de Medeiros, Karlsruher Institut für Technologie (Germany)
Panagiota Kapetana, Karlsruher Institut für Technologie (Germany)
Uli Lemmer, Karlsruher Institut für Technologie (Germany)
Alexander Colsmann, Karlsruher Institut für Technologie (Germany)

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