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  • Admassie, Shimelis, et al. (författare)
  • A polymer photodiode using vapour-phase polymerized PEDOT as an anode
  • 2005
  • Ingår i: Solar Energy Materials and Solar Cells. - 0927-0248. ; 90, s. 133-141
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the photovoltaic properties of devices made using a highly conducting polymer electrode, from vapor-phase polymd. poly (3,4-ethylenedioxy) thiophene (VPP PEDOT) on glass substrate as an anode and a polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2thienyl-2',1'3'-benzothiadiazole)] (APFO-3) mixed with [6,6]-phenyl-C61-butyric acid methylester (PCBM) in the ratio of 1:4 as the active layer. The device performance was compared with that of devices made with PEDOT-PSS on glass substrates. The surfaces of VPP PEDOT were imaged using at. force microscopy (AFM). [on SciFinder (R)]
  • Admassie, Shimelis, et al. (författare)
  • Electrochemical and optical studies of the band gaps of alternating polyfluorene copolymers
  • 2006
  • Ingår i: Synthetic Metals. ; 156:7-8, s. 614-623
  • Tidskriftsartikel (refereegranskat)abstract
    • The electrochemical and optical properties of a series of alternating polyfluorene copolymers with low band gaps were determined. These polymers incorporated fluorene units alternating with groups including electron-withdrawing (A) and electron-donating (D) groups in donor-acceptor-donor (DAD) sequence to achieve the lowering of band gaps. The polymers were solvent-casted on platinum disk electrode and the band gaps were estimated from cyclic voltammetry (CV). These values were compared with values obtained from optical absorption measurements. Although the electrochemically determined band gaps were found to be slightly higher than the optical band gap in most cases, values are well correlated. The values of the band gaps determined range from 2.1 to 1.3 eV. © 2006 Elsevier B.V. All rights reserved.
  • Amarasinghe Vithanage, Dimali, et al. (författare)
  • Charge Carrier Dynamics of Polymer : Fullerene Blends: From Geminate to Non-Geminate Recombination
  • 2014
  • Ingår i: Advanced Energy Materials. - Wiley-VCH Verlagsgesellschaft. - 1614-6832, EISSN 1614-6840. ; 4:8
  • Tidskriftsartikel (refereegranskat)abstract
    • The charge carrier dynamics of a new polymer-fullerene blend are examined on the femtosecond to the millisecond time scale. The full time range is globally fitted using a chemical reaction rate model that includes all key processes, charge generation, energy transfer, charge separation, and recombination, over the full 12 orders of magnitude in time and a factor of 33 in light intensity. Particular attention is paid to the charge recombination processes and it is found that they are highly material specific. Comparison of the dynamics to those of a previously studied polymer: fullerene blend reveals that while for one blend the recombination dynamics are mainly controlled by geminate recombination, the charge recombination in the presently studied polymer: fullerene blend are entirely controlled by non-geminate electron-hole recombination. Carrier density dependence of the non-geminate recombination rate is analyzed and a correlated disorder model of site energies is proposed to explain the observed dependency.
  • Andersson, Mattias, et al. (författare)
  • Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements
  • 2013
  • Ingår i: Journal of Physical Chemistry Letters. - American Chemical Society. - 1948-7185. ; 4:12, s. 2069-2072
  • Tidskriftsartikel (refereegranskat)abstract
    • Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump-probe transient absorption and charge extraction of photo-generated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 x 10(-12) cm(-3) s(-1). This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed.
  • Bjoerstroem, Cecilia M., et al. (författare)
  • Influence of solvents and substrates on the morphology and the performance of low-bandgap polyfluorene: PCBM photovoltaic devices
  • 2006
  • Ingår i: Proceedings of SPIE-The International Society for Optical Engineering. ; 6192
  • Konferensbidrag (refereegranskat)abstract
    • Spin-coated thin films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) blended with [6,6]-phenyl-C61-butyric acid Me ester (PCBM) are used as the active material in polymer photovoltaic cells. Such blends are known for their tendency to phase sep. during film formation. Tuning the morphol. of the blend in a controlled way is one possible road towards higher efficiency. We studied the effect of adding chlorobenzene to chloroform-based blend solns. before spin-coating on the conversion efficiency of APFO-3:PCBM photodiodes, and related that to the lateral and vertical morphol. of thin films of the blend. The lateral morphol. is imaged by at. force microscopy (AFM) and the vertical compositional profile is obtained by dynamic secondary ion mass spectrometry (SIMS). The profiles reveal compositional variations consisting of multilayers of alternating polymer-rich and PCBM-rich domains in the blend film spin-coated from chloroform. The vertical compositional variations are caused by surface-directed spinodal waves and are frozen in during the rapid evapn. of a highly volatile solvent. With addn. of the low-vapor pressure solvent chlorobenzene, a more homogeneous vertical compn. is found. The conversion efficiency for solar cells of this blend was found to be optimal for chloroform:chlorobenzene mixts. with a vol.-ratio of 80:1. We have also investigated the role of the substrate on the morphol. We found that blend films spin-coated from chloroform solns. on PEDOT:PSS-coated ITO show a similar compositional structure as the films on silicon, and that changing the substrate from silicon to gold only affects the vertical phase sepn. in a region close to the substrate interface. [on SciFinder (R)]
  • Bjork, Per, et al. (författare)
  • Oligothiophene Assemblies Defined by DNA Interaction : From Single Chains to Disordered Clusters
  • 2009
  • Ingår i: SMALL. - 1613-6810. ; 5:1, s. 96-103
  • Tidskriftsartikel (refereegranskat)abstract
    • The organization of conjugated polyelectrolytes (CPEs) interacting with biomolecules sets conditions for the biodetection of biological processes and identity, through the use of optical emission from the CPE. Herein, a well-defined CPE and its binding to DNA is studied. By using dynamic light scattering and circular dichroism spectroscopy, it is shown that the CPE forms a multimolecule ensemble in aqueous solution that is more than doubled it? size when interacting with a small DNA chain, while single chains are evident in ethanol. The related changes in the fluorescence spectra upon polymer aggregation are assigned to oscillator strength redistribution between vibronic transitions in weakly coupled H-aggregates. An enhanced single-molecule spectroscopy technique that allows full control of excitation and emission light polarization is applied to combed and decorated;,DNA chains. It is found that the organization of combed CPE-lambda DNA complexes (when dry on the surface) allows considerable variation of CPE distances and direction relative to the DNA chain. By analysis of the polarization data. energy transfer between the polymer chains in individual complexes is confirmed and their sizes estimated.
  • Borgani, Riccardo, et al. (författare)
  • Intermodulation electrostatic force microscopy for imaging surface photo-voltage
  • 2014
  • Ingår i: Applied Physics Letters. - American Institute of Physics (AIP). - 0003-6951. ; 105:14, s. 143113
  • Tidskriftsartikel (refereegranskat)abstract
    • We demonstrate an alternative to Kelvin Probe Force Microscopy for imaging surface potential. The open-loop, single-pass technique applies a low-frequency AC voltage to the atomic force microscopy tip while driving the cantilever near its resonance frequency. Frequency mixing due to the nonlinear capacitance gives intermodulation products of the two drive frequencies near the cantilever resonance, where they are measured with high signal to noise ratio. Analysis of this intermodulation response allows for quantitative reconstruction of the contact potential difference. We derive the theory of the method, validate it with numerical simulation and a control experiment, and we demonstrate its utility for fast imaging of the surface photo-voltage on an organic photovoltaic material.
  • Camacho Dejay, Rafael, et al. (författare)
  • Polarization Imaging of Emissive Charge Transfer States in Polymer/Fullerene Blends
  • 2014
  • Ingår i: Chemistry of Materials. - American Chemical Society. - 0897-4756. ; 26:23, s. 6695-6704
  • Tidskriftsartikel (refereegranskat)abstract
    • Photoexcitation of conjugated polymerfullerene blends results in population of a local charge transfer (CT) state at the interface between the two materials. The competition between recombination and dissociation of this interfacial state limits the generation of fully separated free charges. Therefore, a detailed understanding of the CT states is critical for building a comprehensive picture of the organic solar cells operation. We applied a new fluorescence microscopy method called two-dimensional polarization imaging to gain insight into the orientation of the transition dipole moments of the CT states, and the associated excitation energy transfer processes in TQ1:PCBM blend films. The polymer phase was oriented mechanically to relate the polymer dipole moment orientation to that of the CT states. CT state formation was observed to be much faster than energy transfer in the polymer phase. However, after being formed an emissive CT state does not exchange excitation energy with other CT states, suggesting that they are spatially and/or energetically isolated. We found that the quantum yield of the CT emission is smaller for CT states spatially located in the highly oriented polymer domains, which is interpreted as the result of enhanced CT state dissociation in highly ordered structures.
  • Chen, Miaoxiang, et al. (författare)
  • High carrier mobility in low band gap polymer-based field-effect transistors
  • 2005
  • Ingår i: Applied Physics Letters. - 0003-6951. ; 87:25, s. 252105-1-252105-3
  • Tidskriftsartikel (refereegranskat)abstract
    • A conjugated polymer with a low band gap of 1.21 eV, i.e., absorbing infrared light, is demonstrated as active material in field-effect transistors (FETs). The material consists of alternating fluorene units and low band gap segments with electron donor-acceptor-donor units composed of two electron-donating thiophene rings attached on both sides of a thiadiazolo-quinoxaline electron-acceptor group. The polymer is solution-processable and air-stable; the resulting FETs exhibit typical p-channel characteristics and field-effect mobility of 0.03?cm2?V?1?s?1.
  • De, Swati, et al. (författare)
  • Exciton Dynamics in Alternating Polyfluorene/Fullerene Blends
  • 2008
  • Ingår i: Journal of Chemical Physics. - College Park, MD, United States : American Institute of Physics (AIP). - 0021-9606. ; 350:1-3, s. 14-22
  • Tidskriftsartikel (refereegranskat)abstract
    • Exciton dynamics in alternating copolymer/fullerene solar cell blends have been investigated using femtosecond transient absorption spectroscopy. The acceptor concentrations have been varied over a wide range. Experimental data, kinetic modeling and simulations, all indicate that the efficiency of exciton conversion to charges is 100% even at acceptor concentrations as low as 20 wt%. The reported dependence of solar cell efficiency on fullerene concentration may thus arise from other factors. However, there exists an acceptor concentration threshold (5 wt%) below which a substantial fraction of the excitations remain unquenched. The results, we believe are very relevant to optimization of performance efficiency by clever manipulation of morphology. We have also observed exciton–exciton energy transfer in these blends at low acceptor concentrations.
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