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Optical optimization of polyfluorene-fullerene blend photodiodes
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Applied Optics . Linköping University, The Institute of Technology.ORCID iD: 0000-0001-9229-2028
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
2005 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 97, no 3, 034503-1-034503-8 p.Article in journal (Refereed) Published
Abstract [en]

Blends of polyfluorene-fullerenes are promising materials for polymer-based photovoltaic devices (PPVD). Using spectroscopic ellipsometry we deduce the dielectric function for the blend of the fullerene derivative [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) and the alternating polyfluorene copolymer, poly [2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1', 3'-benzothiadiazole)] DiO-PFDTBT (4:1 by weight), for the wavelength interval 250-1300 nm. n reaches above 2 and saturates to 1.9 for high wavelengths. Absorption starts at 720 nm (1.72 eV) and reaches a crest around 550 nm (2.25 eV). The spin coating introduces anisotropy in the blend, manifested in birefringence as well as in dichroism. The dielectric function for the blend versus its constituents is not additive. There are indications that the constituents lost their dielectric identity, as screening cannot explain the experimental data. Simulations of optical absorption inside a PPVD are performed for both monochromatic and polychromatic light, using an air mass 1.5 distributed solar irradiation. The model allows calculation of absorbed energies in absolute values in all layers within the device. An optimization is carried out with respect to the layer thicknesses. From a purely optical perspective there is no gain of optical absorbance in including an additional layer of acceptor. Spatially resolved energy dissipation within the device is presented for polychromatic light. Estimates for quantum efficiencies are derived. Experimental and theoretical results for reflectance are compared.

Place, publisher, year, edition, pages
2005. Vol. 97, no 3, 034503-1-034503-8 p.
National Category
Natural Sciences
URN: urn:nbn:se:liu:diva-28476DOI: 10.1063/1.1836005Local ID: 13623OAI: diva2:249285
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-10-14
In thesis
1. Optical modelling of conjugated polymers: from materials to devices
Open this publication in new window or tab >>Optical modelling of conjugated polymers: from materials to devices
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Measurements and modelling of theoretical properties of polymer-based photovoltaic devices, PPVDs, are the subjects of this thesis. Modelling refers both to modelling of materials, based on extraction of the dielectric function, and to modelling of devices by computer simulation of the optical electrical field inside PPVDs. PPVDs include polymer-based solar cells, a promising technology for energy generation and the theme for the this thesis.

The conjugated polymers studied here arc built from fluorencs, PF, or thiophenes, PT, and combinations thereof in the form of different derivatives and copolymers, such as DAD blocks with alternating donor, acceptor, donor moieties. The latter are referred to as low-band gap materials and have absorption spectra that match the solar spectrum better than earlier generations of polymers.

PPVDs operate according to the principle of transforming incoming photons to useful current: i.e. there is an optical side and an electrical side to the performance of PPVDs.

This work is an effort taking a holistic perspective of the optical side and shows that simulation can save both materials and labour.

It is demonstrated that variable-angle spectroscopic ellipsometry, SE, is a valuable tool for the characterisation of the optical linear response of this kind of materials. Using SE, the fully complex-valued index of refraction for wavelengths spanning from ultraviolet to infrared has been determined for a number of pure conjugated polymers as well as blends with polymer and acceptor-acting fullerenes. SE was also used for morphological studies, such as confirming spin-introduced uniaxiality - more pronounced for longer pure chains, somewhat suppressed for blends with fullerenes - and it was shown that traditional: effective mean field approximations fail in composing the material from its constituents indicating a more complicated morphology than expected. Methodological developments include a "sneaking method" suitable for band gap materials by which no assumptions about an underlying parameterisation are necessary. Another development is the introduction of quantum chemistry as a valuable tool for ellipsometric modelling. The position and relative magnitude of Lorentz peaks can be predicted and hence the dielectric function of the studied low-band gap DAD copolymer can be reconstructed.

A tool for calculating the optical electrical field in these sandwich-like structures has been developed which includes polychromatic, solar-light distributed irradiation, and fully account for reflection and transmission at all interior interfaces, giving rise to interference not obeying the often assumed Beer-Lambert decay. The model enables calculation of spatially and wavelength resolved absorption profiles, of integrated absorbed energy, energy redistribution charts, upper estimates of quantum efficiencies, and the possibility of performing sensitivity analysis. The simulation also allows for optimisation by finding the set of layer thicknesses giving the highest absorption. The optical simulation has also been merged with electrical calculations in order both to give a more complete understanding of the device and also to de-couple the optical and electrical phenomena. The latter allows bottlenecks to be identified. For example, mobilities arc too low and have to be increased in coming generation of materials. In one study the coherent situation is expanded to the more general including both coherent and incoherent light addition. From this, tandem structures have been analysed. This tool is also valuable for optics in general.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2005. 98 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 984
Spectroscopic ellipsomctry, optical modelling, organic solar cell, conjugated polymer, polyfluorene, polythiophene, copolymer, fullerene blend, photovoltaic device, dielectric function
National Category
Natural Sciences
urn:nbn:se:liu:diva-30216 (URN)15711 (Local ID)91-85457-62-0 (ISBN)15711 (Archive number)15711 (OAI)
Public defence
2005-11-11, Planck, Linköpings universitet, Linköping, 10:15 (Swedish)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2012-11-22

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