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Optical properties of low band gap alternating copolyfluorenes for photovoltaic devices
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
Chemical Physics, Lund University, Lund, Sweden .
Chemical Physics, Lund University, Lund, Sweden .
Chemical Physics, Lund University, Lund, Sweden .
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2005 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 123, no 20, 204718- p.Article in journal (Refereed) Published
Abstract [en]

In a joint experimental and theoretical work the optical response and excited-state character of two novel conjugated polymers for photovoltaic applications are studied. The polymers, alternating polyfluorene (APFO) Green 1 and APFO Green 2, are both copolymers of fluorene, thiophene, and electron accepting groups. The band gaps are extended into the red and near infrared with onsets of 780 and 1000 nm, respectively, due to alternating donor and acceptor moieties along the polymer chain. Spectroscopic ellipsometry and subsequent modeling made it possible to extract the dielectric function in the range of 260-1200 nm. Semiempirical quantum chemical calculations (ZINDO) revealed the character of the main electronic transitions in the studied spectral region. The spectral band just above 400 nm was assigned to a delocalized π - π* transition for both polymers. The red band lying at 622 and 767 nm in the two polymers corresponds to an electronic state mainly occupying the acceptor units and having a strong charge-transfer character. We show that the ZINDO transition energies are valuable input to the application of Lorentz oscillators in modeling of the dielectric function of the polymer material.

Place, publisher, year, edition, pages
2005. Vol. 123, no 20, 204718- p.
National Category
Engineering and Technology
URN: urn:nbn:se:liu:diva-50343DOI: 10.1063/1.2087367OAI: diva2:271239
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2012-11-22
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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