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Optical modelling of conjugated polymers: from materials to devices
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
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.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 984
Keyword [en]
Spectroscopic ellipsomctry, optical modelling, organic solar cell, conjugated polymer, polyfluorene, polythiophene, copolymer, fullerene blend, photovoltaic device, dielectric function
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-30216Local ID: 15711ISBN: 91-85457-62-0 (print)OAI: oai:DiVA.org:liu-30216DiVA: diva2:251038
Public defence
2005-11-11, Planck, Linköpings universitet, Linköping, 10:15 (Swedish)
Opponent
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2012-11-22
List of papers
1. Optical modelling of a layered photovoltaic device with a polyfluorene derivative/fullerene as the active layer
Open this publication in new window or tab >>Optical modelling of a layered photovoltaic device with a polyfluorene derivative/fullerene as the active layer
2004 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 83, no 2-3, 169-186 p.Article in journal (Refereed) Published
Abstract [en]

Here we report on optical modelling of organic photovoltaic devices having a layered geometry, with polyfluorene-copolymer as the active material and C60 as the acceptor. Thin film theory in a matrix formalism enables analysis of the impact of reflection and interference on the optical electric field. The model allows us to predict an optimal C60 thickness where concern has been taken for light being both polychromatic and distributed according to solar irradiation. Fundamental for light–matter interaction is the dielectric function. We have extracted it for two variants of a new polyfluorene copolymer, PFDTBT, from UV via visible to the nearest infrared, using spectroscopic ellipsometry (SE). n is found to be relatively high with a max-value above 2.1. The process of spin coating induces anisotropy in the polymer film.

Keyword
Ellipsometry, Fullerene, Modelling, Photovoltaic, Polyfluorene
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-45709 (URN)10.1016/j.solmat.2004.02.023 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-13
2. Optical optimization of polyfluorene-fullerene blend photodiodes
Open this publication in new window or tab >>Optical optimization of polyfluorene-fullerene blend photodiodes
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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-28476 (URN)10.1063/1.1836005 (DOI)13623 (Local ID)13623 (Archive number)13623 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13
3. Optical properties of low band gap alternating copolyfluorenes for photovoltaic devices
Open this publication in new window or tab >>Optical properties of low band gap alternating copolyfluorenes for photovoltaic devices
Show others...
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.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-50343 (URN)10.1063/1.2087367 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-12
4. Organic tandem solar cells - modelling and predictions
Open this publication in new window or tab >>Organic tandem solar cells - modelling and predictions
2006 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 90, no 20, 3491-3507 p.Article in journal (Refereed) Published
Abstract [en]

Tandem combinations of organic photovoltaic devices are studied from an optical point of view. We unify coherent (wave-based) as well as incoherent (irradiance-based) light addition in our treatment of the incoming and reflected electromagnetic waves, and calculate the spatially resolved absorption profile in the cells. The model allows for any number and any order of thin and thick layers to be analysed. Irradiation is monochromatic or polychromatic, AM 1.5 or AM 1.0, and therefore applicable for solar cell simulation. The optical modelling is unified with electrical models of charge generation and transport in the solar cells. Through this, de-coupling of optical and electrical processes is possible. Moreover, the model allows identification of limiting processes in the devices. The model is applied to a tandem cell with copolymers of polyfluorene combined in bulk heterojunctions with fullerene acceptors, one device for high energy absorption and one for lower, where anodes and cathodes for the cells are semi-transparent metallic polymer layers. It is concluded that these materials do not at present have an electrical performance, which can be enhanced by the tandem cell combination.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-35453 (URN)10.1016/j.solmat.2006.05.009 (DOI)26905 (Local ID)26905 (Archive number)26905 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2017-12-13
5. Optical limitations in thin-film low-band-gap polymer/fullerene bulk heterojunction devices
Open this publication in new window or tab >>Optical limitations in thin-film low-band-gap polymer/fullerene bulk heterojunction devices
2007 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 91, no 8, 083503- p.Article in journal (Refereed) Published
Abstract [en]

Photovoltaic devices from the low-band-gap alternating copolymer APFO-Green1, blended with the fullerene derivative BTPF70 as electron acceptor, show a pronounced variation of the external quantum efficiency with varying thickness. Device simulation, based on ellipsometric characterization, reveals that this behavior is to be expected and valid also for most low-band-gap polymers and that it can be explained by optical interference. Requirements for materials suitable for wide spectral coverage in thin-film organic solar cells are delineated. Furthermore, the internal quantum efficiency is calculated to be ≈ 0.4.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-39410 (URN)10.1063/1.2736280 (DOI)48250 (Local ID)48250 (Archive number)48250 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2017-12-13

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