| 1. |
- Andersson, Lars Mattias, et al.
(författare)
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Acceptor influence on hole mobility in fullerene blends with alternating copolymers of fluorene
- 2006
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88, s. 082103-
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Tidskriftsartikel (refereegranskat)abstract
- Hole mobility in polyfluorene/fullerene blends has been studied with field effect transistors. Two different C60 derivatives and one C70 derivative have been investigated together with two different polyfluorenes. Mobility is presented as a function of acceptor loading at ratios suitable for photovoltaics and varies between 10–3 and 10–5 cm2 V–1 s–1 depending on the polymer/acceptor combination. The hole mobility is increased in blends with the commonly used acceptor [6-6]-phenyl-C61-butyric acid methylester (PCBM). With related C60 and C70 derivatives the hole mobility is decreased under the same circumstances.
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| 2. |
- Andersson, Lars Mattias, et al.
(författare)
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Bipolar transport observed through extraction currents on organic photovoltaic blend materials
- 2006
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 89, s. 142111-
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Tidskriftsartikel (refereegranskat)abstract
- Both electron and hole mobilities have been simultaneously measured through charge extraction by linearly increasing voltage on polymer heterojunction solar cells with varying stoichiometry of polymer and acceptor. The polymer is a low band gap copolymer of fluorene, thiophene, and electron accepting groups named APFO-Green 5, and the acceptor is [6,6]-phenyl-C61-butyric acid methylester. Results are correlated to field effect transistor measurements on the same material system. A monotonous increase in mobility for both carrier types is observed with increased acceptor loading.
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| 3. |
- Andersson, Lars Mattias, et al.
(författare)
-
Non-equilibrium effects on electronic transport in organic field effect transistors
- 2007
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Ingår i: Organic electronics. - : Elsevier BV. - 1566-1199 .- 1878-5530. ; 8:4, s. 423-430
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Tidskriftsartikel (refereegranskat)abstract
- Non-ideal behavior in organic field effect transistors, in particular threshold voltage drift and light sensitivity, is argued to be due to intrinsic carrier dynamics. The discussion is based on the theory for hopping transport within a Gaussian density of states. Carrier concentration is shown to be of fundamental importance, and the time required to reach equilibrium at different bias is responsible for device behavior, with implications for mobility evaluation. Experimental results from various conjugated polymers in a field effect transistor illustrate the theory.
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| 4. |
- Andersson, Lars Mattias, et al.
(författare)
-
Stoichiometry, mobility, and performance in bulk heterojunction solar cells
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 91:7, s. 071108-
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Tidskriftsartikel (refereegranskat)abstract
- Bipolar transport in blends of a copolymer of fluorene, thiophene and electron accepting groups, and the substituted fullerene [6,6]-phenyl-C61-butyric acid methylester have been studied through charge extraction by linearly increasing voltage on solar cells and with field effect transistors. Between 10% and 90% polymer has been used and the results show a clear correlation to solar cell performance. Optimal solar cells comprise 20% polymer and have a power conversion efficiency of 3.5%. The electron mobility is increasing strongly with fullerene content, but is always lower than the hole mobility, thus explaining the low amount of polymer in optimized devices.
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| 5. |
- Andersson, Mattias, et al.
(författare)
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Approximate distance oracles for graphs with dense clusters
- 2007
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Ingår i: Computational Geometry. - : Elsevier BV. - 0925-7721. ; 37:3, s. 142-154
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Tidskriftsartikel (refereegranskat)abstract
- Let H1=(V,E1) be a collection of N pairwise vertex disjoint O(1)-spanners where the weight of an edge is equal to the Euclidean distance between its endpoints. Let H2=(V,E2) be the graph on V with M edges of non-negative weight. The union of the two graphs is denoted G=(V,E1 u E2). We present a data structure of size O(M^2 + nlogn) that answers (1+ε)-approximate shortest path queries in G in constant time, where ε>0 is constant.
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| 6. |
- Andersson, Mattias, et al.
(författare)
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Chips on wafers, or packing rectangles into grids
- 2005
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Ingår i: Computational Geometry. - : Elsevier BV. - 0925-7721. ; 30:2, s. 95-111
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Tidskriftsartikel (refereegranskat)abstract
- A set of rectangles S is said to be gridpacked if there exists a rectangular grid (not necessarily regular) such that every rectangle lies in the grid and there is at most one rectangle of S in each cell. The area of a grid packing is the area of a minimal bounding box that contains all the rectangles in the grid packing. We present an approximation algorithm that given a set S of rectangles and a real epsilon constant epsilon > 0 produces a grid packing of S whose area is at most (1 + epsilon) times larger than an optimal grid packing in polynomial time. If epsilon is chosen large enough the running time of the algorithm will be linear. We also study several interesting variants, for example the smallest area grid packing containing at least k less than or equal to n rectangles, and given a region A grid pack as many rectangles as possible within A Apart from the approximation algorithms we present several hardness results.
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| 7. |
- Andersson, Mattias, et al.
(författare)
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Color measurements on prints containing fluorescent whitening agents
- 2007
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE - International Society for Optical Engineering. - 9780819466068 ; , s. Q4930-Q4950
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Konferensbidrag (refereegranskat)abstract
- Papers with a slightly blue shade are, at least among a majority of observers, perceived as whiter than papers having a more neutral color. Therefore, practically all commercially available printing papers contain bluish dyes and fluorescent whitening agents (FWA) to give the paper a whiter appearance. Furthermore, in the Paper Industry, the most frequently used measure for paper whiteness is the CIE-whiteness. The CIE Whiteness formula, does in turn, also favor slightly bluish papers. Excessive examples of high CIE-whiteness values can be observed in the office-paper segment where a high CIE-whiteness value is an important sales argument. As an effect of the FWA, spectrophotometer measurements of optical properties such as paper whiteness are sensitive to the ultraviolet (UV) content of the light source used in the instrument. To address this, the standard spectrophotometers used in the Paper Industry are equipped with an adjustable filter for calibrating the UV-content of the illumination. In the Paper Industry, spectrophotometers with d/0 measurement geometry and a light source of type C are used. The Graphic Arts Industry on the other hand, typically measures with spectrophotometers having 45/0 geometry and a light source of type A. Moreover, these instruments have only limited possibilities to adjust the UV-content by the use of different weighting filters. The standard for color measurements in the Paper Industry governs that measurements should be carried out using D65 standard illumination and the 10 degrees standard observer. The corresponding standard for the Graphic Arts Industry specify D50 standard illumination and the 2 degrees standard observer. In both cases, the standard illuminants are simulated from the original light source by spectral weighting functions. However, the activation of FWA, which will impact the measured spectral reflectance, depends on the actual UV-content of the illumination used. Therefore, comparisons between measurements on substrates containing FWA from two instruments having light sources with different UV-content are complicated. In this study, the effect of FWA content in paper on color reproduction has been quantified for an office-type paper. Furthermore, examples are given on how color measurement instruments give different readings when FWA is present. For the purpose of this study and in order to ensure that only the effect of FWA was observed, a set of papers with varying additions of FWA but otherwise identical, were produced on a small-scale experimental paper machine. The pilot papers were printed in three different printers. Two spectrophotometers representative to the instruments used in the Graphic Arts Industry and the Paper Industry respectively where used to measure the printed papers. The results demonstrate how the use of spectral weighting functions for simulating standard illuminants works properly on non-fluorescent material. However, when FWA is present, disparities in UV content between the light source and the simulated illuminant will result in color differences. Finally, in many printing processes, some of the used inks are UV-blocking, which further complicates the effect of FWA in printed material. An example is shown on how different color differences are obtained for different process ink combinations when the amount of FWA added to the paper is varied.
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| 8. |
- Andersson, Mattias
(författare)
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Digital camera calibration for color measurements on prints
- 2007
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE - International Society for Optical Engineering. - 9780819466068
-
Konferensbidrag (refereegranskat)abstract
- Papers with a slightly blue shade are, at least among a majority of observers, perceived as whiter than papers having a more neutral color. Therefore, practically all commercially available printing papers contain bluish dyes and fluorescent whitening agents (FWA) to give the paper a whiter appearance. Furthermore, in the Paper Industry, the most frequently used measure for paper whiteness is the CIE-whiteness. The CIE Whiteness formula, does in turn, also favor slightly bluish papers. Excessive examples of high CIE-whiteness values can be observed in the office-paper segment where a high CIE-whiteness value is an important sales argument. As an effect of the FWA, spectrophotometer measurements of optical properties such as paper whiteness are sensitive to the ultraviolet (UV) content of the light source used in the instrument. To address this, the standard spectrophotometers used in the Paper Industry are equipped with an adjustable filter for calibrating the UV-content of the illumination. In the Paper Industry, spectrophotometers with d/0 measurement geometry and a light source of type C are used. The Graphic Arts Industry on the other hand, typically measures with spectrophotometers having 45/0 geometry and a light source of type A. Moreover, these instruments have only limited possibilities to adjust the UV-content by the use of different weighting filters. The standard for color measurements in the Paper Industry governs that measurements should be carried out using D65 standard illumination and the 10 degrees standard observer. The corresponding standard for the Graphic Arts Industry specify D50 standard illumination and the 2 degrees standard observer. In both cases, the standard illuminants are simulated from the original light source by spectral weighting functions. However, the activation of FWA, which will impact the measured spectral reflectance, depends on the actual UV-content of the illumination used. Therefore, comparisons between measurements on substrates containing FWA from two instruments having light sources with different UV-content are complicated. In this study, the effect of FWA content in paper on color reproduction has been quantified for an office-type paper. Furthermore, examples are given on how color measurement instruments give different readings when FWA is present. For the purpose of this study and in order to ensure that only the effect of FWA was observed, a set of papers with varying additions of FWA but otherwise identical, were produced on a small-scale experimental paper machine. The pilot papers were printed in three different printers. Two spectrophotometers representative to the instruments used in the Graphic Arts Industry and the Paper Industry respectively where used to measure the printed papers. The results demonstrate how the use of spectral weighting functions for simulating standard illuminants works properly on non-fluorescent material. However, when FWA is present, disparities in UV content between the light source and the simulated illuminant will result in color differences. Finally, in many printing processes, some of the used inks are UV-blocking, which further complicates the effect of FWA in printed material. An example is shown on how different color differences are obtained for different process ink combinations when the amount of FWA added to the paper is varied.
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| 9. |
- Andersson, Mattias, et al.
(författare)
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From short to long - Optical and electrical transients in photovoltaic bulk heterojunctions of polyfluorene/fullerenes
- 2009
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448 .- 0301-0104. ; 357:1-3, s. 120-123
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Tidskriftsartikel (refereegranskat)abstract
- We combine results from transient optical absorption in a bulk heterojunction polymer donor/fullerene acceptor material, obtained in the optical range as well as in the THz range, with results from electrical transients after a short light pulse, to present a unified interpretation of the transport of charge after the very first act of photoinduced charge transfer. We find that the mobility of charges is initially very high, but dramatically reduced with time, to arrive at values three orders of magnitude lower. We show that this can be understood as a consequence of the transport of hot charges by hopping through the density of states, from higher to lower energies.
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| 10. |
- Andersson, Mattias, et al.
(författare)
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Intrinsic and extrinsic influences on the temperature dependence of mobility in conjugated polymers
- 2008
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Ingår i: Organic electronics. - : Elsevier. - 1566-1199 .- 1878-5530. ; 9:5, s. 569-574
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of charge carrier mobility in conjugated polymers and their blends with fullerenes is investigated with different electrical methods, through field effect transistor (FET), space charge limited current (SCLC) and charge extraction (CELIV) measurements. Simple models, such as the Gaussian disorder model (GDM), are shown to accurately predict the temperature behavior, and a good correlation between the different measurement methods is obtained. Inconsistent charge carrier concentrations in the modeling are explained through intrinsic non-equilibrium effects, and are responsible for the limited applicability of existing numerical models. A severe extrinsic influence from water in FETs with a hydrophilic insulator interface is also demonstrated. The presence of water leads to a significant overestimate of the disorder in the materials from measurements close to room temperature and erratic behavior in the 150-350 K range. To circumvent this problem it is shown to be necessary to measure under ultra high vacuum (UHV) conditions. © 2008 Elsevier B.V. All rights reserved.
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