| 1. |
- Andersson, Viktor, et al.
(författare)
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Comparative study of organic thin film tandem solar cells in alternative geometries
- 2008
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 104:12, s. 124508-
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Tidskriftsartikel (refereegranskat)abstract
- Optical modeling of one folded tandem solar cell and four types of stacked tandem solar cells has been performed using the finite element method and the transfer matrix method for the folded cell and the stacked cells, respectively. The results are analyzed by comparing upper limits for short circuit currents and power conversion efficiencies. In the case of serial connected tandems all of the five cell types may be compared, and we find that the folded cells are comparable to stacked tandem cells in terms of currents and power conversion efficiencies.
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| 2. |
- Björk, Per, et al.
(författare)
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Dynamics of complex formation between biological and luminescent conjugated polyelectrolytes - a surface plasmon resonance study
- 2005
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Ingår i: Biosensors and Bioelectronics. - : Elsevier BV. - 0956-5663. ; 20:9, s. 1764-1771
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Tidskriftsartikel (refereegranskat)abstract
- A water-soluble polythiophene, POWT, with zwitterionic peptide like side chains possess good characteristics for biosensor applications. The zwitterionic side chains of the polymer can couple to biomolecules via electrostatic and hydrogen bonding. This creates possibilities to imprint biomolecules to spin-coated polymer films with maintained functionality, and use the resulting matrix as a biosensor. Polymer-biomolecular interaction studies done with surface plasmon resonance (SPR) reveal a well performing sensor matrix with high affinity for DNA hybridizations as well as for protein detection. The responses are distinct and very specific. A directional dependence of antibodies binding to POWT layer has also been observed. The polymer films have also been characterized by optical methods. Emission and absorption measurements in different buffer systems confirm that the polymer matrix can undergo structural and conformational changes on surfaces. The dielectric function in the interval 300–800 nm of POWT is reported, based on variable angle spectroscopic ellipsometry. This modeling reveals that a considerable amount of water is included in the material. The polymer layer possesses the characteristics needed for biochip applications and micro array techniques.
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| 3. |
- Campoy-Quiles, M., et al.
(författare)
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On the determination of anistropy in polymer thin films : A comparative study of optical techniques
- 2008
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Ingår i: Physica Status Solidi. C: Current Topics in Solid State Physics. - Weinheim, Germany : Wiley-VCH Verlagsgesellschaft. - 1862-6351. ; 5:5, s. 1270-1273
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Tidskriftsartikel (refereegranskat)abstract
- We have used seven different techniques to measure the anisotropic refractive index of poly(vinylcarbazole) films. These techniques are: two types of variable angle spectroscopic ellipsometry (VASE) with multiple sample analysis, Interference enhanced VASE, Transmittance combined with VASE, Polarised Reflectance, beta-scan VASE, and prism coupling. We have found the average ordinary and extraordinary indices at 633 nm to be no = nTE = 1.675 ± 0.008, and ne = nTM = 1.722 ± 0.018, respectively, consistent amongst methods and conclusive on the magnitude of Δn in polymer films.
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| 4. |
- Inganäs, Olle, et al.
(författare)
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Low bandgap alternating polyfluorene copolymers in plastic photodiodes and solar cells
- 2004
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Ingår i: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 79:1, s. 31-35
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Tidskriftsartikel (refereegranskat)abstract
- We report a comparative study of plastic photodiodes using four different copolymers of fluorene, with a variation of alkyl side chain length and chemical structure. Photodiode materials are formed by blending the polymers with a fullerene derivative and spincoating the blend solution. A photovoltage of 1 V is obtained in devices, where the anode is a doped polymer and the cathode is LiF/Al. Monochromatic quantum efficiencies are better than 40% over most of the absorption range, and under solar light AM 1.5 simulation, we reach energy efficiencies beyond 2%. The high fill factors obtained in some of the devices indicate that these are of interest for more elaborate optimisation. Reasons for the benign electrical transport are discussed. © Springer-Verlag 2004.
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| 5. |
- Johansson, Tomas, et al.
(författare)
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Moving Redox Fronts in Conjugated Polymers Studies from Lateral Electrochemistry in Polythiophenes
- 2004
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 151:4
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Tidskriftsartikel (refereegranskat)abstract
- The propagation speed of the front of electrochemical conversion, from semiconductor to highly doped polymer, in films of regioregular poly(3-hexylthiophene) spin cast on insulating substrates was analyzed. Propagation of the p-doped zone in polymer electrochromic devices was imaged simultaneously with recording of electrochemical data. The current is proportional to the propagation speed and has a Tafel-like behavior when taking the resistive drop in the film into account. The resistivity in the film, which gradually lowers the propagation speed, was used for determination of the conductivity of the p-doped polymer. By combining these values with the doping charge injected into the film during front migration we estimated the hole carrier mobility for different doping levels. © 2004 The Electrochemical Society.
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| 6. |
- Persson, Nils-Krister, et al.
(författare)
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Optical limitations in thin-film low-band-gap polymer/fullerene bulk heterojunction devices
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 91:8, s. 083503-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Persson, Nils-Krister, et al.
(författare)
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Optical modelling of a layered photovoltaic device with a polyfluorene derivative/fullerene as the active layer
- 2004
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 83:2-3, s. 169-186
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Persson, Nils-Krister, 1966-, et al.
(författare)
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Optical optimization of polyfluorene-fullerene blend photodiodes
- 2005
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 97:3, s. 034503-1-034503-8
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Persson, Nils-Krister, et al.
(författare)
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Optical properties of low band gap alternating copolyfluorenes for photovoltaic devices
- 2005
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 123:20, s. 204718-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Persson, Nils-Krister, 1966-, et al.
(författare)
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Organic tandem solar cells - modelling and predictions
- 2006
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 90:20, s. 3491-3507
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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