| 1. |
- Fredin, Kristofer, et al.
(author)
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Brownian dynamics simulations of electrons and ions in mesoporous films
- 2005
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In: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 86:2, s. 283-297
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Journal article (peer-reviewed)abstract
- This paper presents a simulation model to study charge transport processes in mesoporous films for dye-sensitized solar cells. By simulating electron and ion transport by Brownian dynamics in these films, we achieve a direct relation between the grain connectivity and the effective diffusion coefficients. By comparing the macroscopic properties of a simple cubic and a diamond structured unit cell, we conclude that the latter better resembles the properties of the mesoporous oxide films in comparison with experimental results. The model has been used to optimize the size of the contact area between the interconnected particles in the mesoporous film with respect to the photocurrent.
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| 2. |
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| 3. |
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| 4. |
- Fredin, Kristofer, et al.
(author)
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Effect on Cell Efficiency following Thermal Degradation of Dye-Sensitized Mesoporous Electrodes Using N719 and D5 Sensitizers
- 2009
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 113:43, s. 18902-18906
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Journal article (peer-reviewed)abstract
- This work examines the comparative durability of two common dyes at temperatures that may be experienced during fabrication of dye-sensitized solar cells (DSCs) such as through the application of thermoplastics for encapsulation or the use of a molten solid-state hole conductor. Dye-sensitized electrodes were heated in an atmosphere of air or nitrogen and thereafter used as working electrodes in DSCs. Electrodes sensitized with N719 appeared more sensitive to thermal degradation than electrodes sensitized with D5, although absorbance measurements suggest similar first-order degradation rates for the two dyes. Intensity modulated photovoltage spectroscopy and intensity modulated photocurrent spectroscopy were used to measure the effect of heating on electron lifetime and transport. It was found that the electron diffusion length may.. be as low as 10% for heated samples, compared to that of the unheated counterpart, and therefore, we assess recombination as an additional efficiency limiting process in our experiments.
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| 5. |
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| 6. |
- Fredin, Kristofer, et al.
(author)
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On the influence of anions in binary ionic liquid electrolytes for monolithic dye-sensitized solar cells
- 2007
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 111:35, s. 13261-13266
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Journal article (peer-reviewed)abstract
- Five ion c liquids (ILs) of the general formula Im(+)A(-), where Im(+) = I -methyl-3-n-butyl-imidazolium, A(-) = I- (1), BF4- (2), SCN- (3), CF3CO2- (4), and CF(3)S0(3)(-) (5), were used in electrolytes for dye-sensitized monolithic solar cells. The properties of the electrolytes and various characteristics of the solar cell performance, such as electron transport and electron lifetime, were studied. The composition of the binary electrolytes, i.e., the different anions, have a significant effect on the viscosity, but only a modest effect of the measured diffusior. coefficient for triiodide. No significant effect of the electrolyte composition on the electron transport time in the mesoporous TiO2 film was found, while there was a pronounced effect on the electron lifetime. Monolithic solar cells with thiocyanate, IL 3, showed overall light-to-electricity conversion efficiency up to 5.6% in 250 W m(-2) simulated sunlight and have promising stability.
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| 7. |
- Fredin, Kristofer, et al.
(author)
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Solid state dye-sensitized solar cells prepared by infiltrating a molten hole conductor into a mesoporous film at a temperature below 150 degrees C
- 2011
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In: Synthetic metals. - : Elsevier BV. - 0379-6779 .- 1879-3290. ; 161:21-22, s. 2280-2283
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Journal article (peer-reviewed)abstract
- Infiltration of a molten hole conductor in a mesoporous film at an elevated temperature exhibits good wetting performance and the procedure is therefore suitable as part of the preparation method for solid state dye-sensitized solar cells. Herein, we present a system prepared by infiltrating 4-(diethylamino)benzaldehyde-1,1)-diphenyl-hydrazone in its molten form at a temperature below 150 degrees C. The system displays a maximum photon-to-current conversion efficiency of about 35%, a value corresponding to an increase of about 5 times in comparison with a previously published system prepared by infiltrating a molten hole-conductor at a temperature exceeding 250 degrees C. By means of comparing charge transport and recombination with the results measured for a liquid analogue, we conclude that whereas the transport rates are similar, recombination is significantly more rapid in the solid-state device.
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| 8. |
- Fredin, Kristofer, 1975-
(author)
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Studies of Charge Transport Processes in Dye-sensitized Solar Cells
- 2007
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Doctoral thesis (other academic/artistic)abstract
- Dye-sensitized solar cells (DSCs) have attained considerable attention during the last decade because of the potential of becoming a low cost alternative to silicon based solar cells. Although efficiencies exceeding 10% in full sunlight have been presented, major improvements of the system are however limited. Electron transport is one of the processes in the cell and is of major importance for the overall performance. It is further a complex process because the transport medium is a mesoporous film and the pores are completely filled by an electrolyte with high ionic strength, resulting in electron-ion interactions. Therefore, present models describing electron transport include simplifications, which limit the practical use, in terms of improving the DSC, because the included model parameters usually have an effective nature. This thesis focuses in particular on the influence of the mesoporous film on electron transport and also on the influence of electron-ion interactions. In order to model diffusion, which is assumed to be the transport process for electrons in the DSC, Brownian motion simulations were performed and spatial restrictions, representing the influence of the mesoporous film, were introduced by using representative models for the structure. The simulations revealed that the diffusion coefficient is approximately half the value for electrons and ions in mesoporous systems. To study the influence of ions, a simulation model was constructed in where electric fields were calculated with respect to the net charge densities, resulting from the different charge carrier distributions. The simulations showed that electron transport is highly dependent on the nature of the ions, supporting an ambipolar diffusion transport model. Experimentally, it was found that the transport process is dependent on the wavelength of the incident light; we found that the extracted current was composed of two components for green light illumination, one fast and one slow. The slow component showed similar trends as the normal current. Also we found that the transport coefficient scaled linearly with film thickness for a fixed current, which questions diffusion as transport process. Other experiments, investigating various effects in the DSC, such as the effect of different cations in the electrolyte, are also presented.
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| 9. |
- Fredin, Kristofer, et al.
(author)
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Studies of coupled charge transport in dye-sensitized solar cells using a numerical simulation tool
- 2006
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In: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 90:13, s. 1915-1927
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Journal article (peer-reviewed)abstract
- In this paper, we present a simulation platform designed to study coupled charge transport in dye-sensitized solar cell (DSC) devices. The platform, SLICE, is used to study the influence of ions in the electrolyte on electron transport in the nanoporous medium. The simulations indicate that both cationic and anionic properties should be considered when modelling DSCs and similar systems. Additionally, it was found that the effective permittivity coefficient, epsilon, has no influence on the electron transport when the ionic concentration is sufficiently high due to the strong coupling between the respective charged species.
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| 10. |
- Fredin, Kristofer, et al.
(author)
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The influence of cations on charge accumulation in dye-sensitized solar cell
- 2007
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In: Journal of Electroanalytical Chemistry. - : Elsevier BV. - 0022-0728 .- 1873-2569 .- 1572-6657. ; 609:2, s. 55-60
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Journal article (peer-reviewed)abstract
- The relation between open-circuit voltage, VOC, light intensity, , and accumulated charge, Q, has been studied for dye-sensitized solar cells (DSCs) containing different counterions to the iodide/triiodide redox couple. At higher light intensities, VOC scaled in the order Cs+ > K+ > Na+ > Li+, which was caused in part by shifts in the conduction band edge. The relation between VOC and Q was fitted to an exponential trap model. It was found that inclusion of a capacitive term improved the fit significantly. The determined values of C were found to be relatively large, up to 75 μF cm−2, and dependent of cation. Physically, the largest fraction of C could be ascribed to the TiO2 bulk or TiO2/dye/electrolyte interface. The interpretation of the trap distribution broadening parameter, β, was found to be dependent of fitting model. Using the model including the linear CVOC term, β was independent of cation and could be viewed as a TiO2 material parameter, while in the model excluding CVOC, β was dependent of cation. Voltage decay experiments were performed to study the cationic influence on recombination. Electron lifetimes were calculated from the voltage decay curves and it was found that the DSC containing Li+ yielded by far the shortest lifetime followed by the DSCs containing Na+, K+ and Cs+. Voltage decay curves include the effect of TiO2 conduction band shifts in the comparison of electron lifetimes with different cations. We therefore suggest that the electron lifetimes should be calculated from the corresponding charge decay curves. From such a comparison, it was found that the DSC containing Li+ yielded the shortest lifetime whereas the DSCs containing Na+, K+ or Cs+ showed approximately identical lifetimes.
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| 11. |
- Fredin, Kristofer, et al.
(author)
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Using a molten organic conducting material to infiltrate a nanoporous semiconductor film and its use in solid-state dye-sensitized solar cells
- 2009
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In: Synthetic metals. - : Elsevier BV. - 0379-6779 .- 1879-3290. ; 159:1-2, s. 166-170
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Journal article (other academic/artistic)abstract
- We describe a method to fill thin films of nanoporous TiO2 with solid organic hole-conducting materials and demonstrate the procedure specifically for use in the preparation of dye-sensitized solar cells. Cross-sections of the films were investigated by scanning electron microscopy and it was observed that a hot molten organic material fills pores that are 10 mu m below the surface of the film. We characterized the incident photon to current conversion efficiency properties of the solid TiO2/organic dye/organic hole-conductor heterojunctions and the spectra show that the dye is still active after the melting process.
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| 12. |
- Lindblad, Rebecka, et al.
(author)
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Controlling energy level positions in hole conducting molecular films by additives
- 2018
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In: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier. - 0368-2048 .- 1873-2526. ; 224, s. 100-106
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Journal article (peer-reviewed)abstract
- Hard X-ray photoelectron spectroscopy (HAXPES) has been used to study the bulk electronic structure of thin molecular films of the organic compounds 2,2',7,7'-tetrakis (N,N'-di-p-methoxyphenyl-amine)-9,9'-spiro-bifluorene (spiro-OMeTAD), 4-(diethylamino)-benzaldehyde-1,1-diphenyl-hydrazone (DEH) and poly(3-hexylthiophene) (P3HT). Molecular layers of these compounds are hole conducting, a property that for example has been used in different solar cell configurations. The function of such a device benefits from the inclusion of additives such as Li-TFSI, or dopants such as Co-complexes, into the molecular layer. Here we report on effects of adding Li-TFSI to DEH and P3HT as observed by photoelectron spectroscopy and we compare with results on the spiro-OMeTAD hole conductor. It can be concluded that the Li-salt causes a shift of the Fermi level in DEH and P3HT towards the HOMO resulting in a p-doping of the molecular material. Similar shifts of the Fermi level could also be observed when adding different Co(+III) complexes to the Spiro-OMeTAD hole conductor, indicating means for more controlled doping.
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| 13. |
- Nissfolk, Jarl, et al.
(author)
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Interpretation of small-modulation photocurrent transients in dye-sensitized solar cells : A film thickness study
- 2010
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In: Journal of Electroanalytical Chemistry. - : Elsevier BV. - 1572-6657. ; 646:1-2, s. 91-99
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Journal article (peer-reviewed)abstract
- Electron transport in dye-sensitized solar cells with varying mesoporous TiO2 film thicknesses was investigated using experimental and computational methods. More specifically, photocurrent transients resulting from small-amplitude square-wave modulation of the incident light were recorded for a series of solar cells, whereby the dependence of the wavelength and direction of the illumination was investigated. The responses were compared to simulations using different models for diffusional charge transport and analyzed in detail. The photocurrent transients are composed of two components: an initial fast response in case of illumination from the working electrode side, or an initial apparent delay of photocurrent decay for illumination from the counter electrode side, followed by a single exponential decay at longer times, with a time constant that is identified as the electron transport time. The initial response depends on the thickness and the absorption coefficient of the film. Transport times for different films were compared at equal short-circuit current density, rather than at equal light intensity. Experimentally, the transport time showed a power-law dependence on the film thickness with an exponent of about 1.5. Analysis using the quasi-static multiple trapping (MT) formulation demonstrates that this behavior originates from differences in quasi-Fermi level in the TiO2 films of different thickness when equal photocurrents are generated. The Fokker-Planck relation was used to derive expressions for the electrons flux in porous TiO2 films with a position-dependent diffusion coefficient.
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| 14. |
- Nissfolk, Jarl, et al.
(author)
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Recombination and Transport Processes in Dye-Sensitized Solar Cells Investigated under Working Conditions
- 2006
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 110:36, s. 17715-17718
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Journal article (peer-reviewed)abstract
- The transport and recombination of electrons in dye-sensitized TiO2 solar cells were studied by analysis of the current and voltage response to a small square-wave light-intensity modulation. Solar cells were studied under working conditions by using potentiostatic and galvanostatic conditions. An increase in applied voltage, that is, from 0 V toward open-circuit voltage, was found to lead to faster electron transport at low light intensities, while it slowed transport at higher light intensities. This observation seems to be conflicting with the multiple trapping model with diffusive transport. An effective diffusion length at the maximum power point was calculated, and it was shown that it decreases with increasing light intensity.
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| 15. |
- Oscarsson, Johan, 1984-, et al.
(author)
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Molecular degradation of D35 and K77 sensitizers when exposed to temperatures exceeding 100 °C investigated by photoelectron spectroscopy
- 2016
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In: Physical Chemistry, Chemical Physics - PCCP. - 1463-9076 .- 1463-9084. ; 18:12, s. 8598-8607
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Journal article (peer-reviewed)abstract
- Degradation of the materials in dye-sensitized solar cells at elevated temperatures is critical for use in real applications. Both during fabrication of the solar cell and under real working conditions the solar cells will be exposed to heat. In this work, mesoporous TiO2 electrodes sensitized with the dyes D35 and K77 were subject to heat-treatment and the effects of this were thereafter investigated by photoelectron spectroscopy. For D35 it was found that heat-treatment changes the binding configuration inducing an increased interaction between the sulfur of the linker unit and the TiO2 surface. The interaction resulting from the change in binding configuration also affects the position of the HOMO level, where a shift of + 0.2 eV is observed when heated to 200 degrees C. For K77, parts of the thiocyanate units are detached and the nitrogen atom leaves the electrode whereas sulfur remains on the surface in various forms of sulfurous oxides. The total dye coverage of K77 gets reduced by heat-treatment. The HOMO level gets progressively less pronounced due to a loss of HOMO level electrons as a consequence of the lower dye coverage when heat-treated, which leads to a lower excitation rate and lower efficiency. The results are discussed in the context of performance for dye-sensitized solar cells.
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| 16. |
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