| 1. |
- Aitola, Kerttu, et al.
(författare)
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Highly catalytic carbon nanotube counter electrode on plastic for dye solar cells utilizing cobalt-based redox mediator
- 2013
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 111, s. 206-209
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Tidskriftsartikel (refereegranskat)abstract
- A flexible, slightly transparent and metal-free random network of single-walled carbon nanotubes (SWCNTs) on plain polyethylene terephthalate (PET) plastic substrate outperformed platinum on conductive glass and on plastic as the counter electrode (CE) of a dye solar cell employing a Co(II/III)tris(2,2'-bipyridyl) complex redox mediator in 3-methoxypropionitrile solvent. The CE charge-transfer resistance of the SWCNT film was 0.60 Omega cm(2), 4.0 Omega cm(2) for sputtered platinum on indium tin oxide-PET substrate and 1.7 Omega cm(2) for thermally deposited Pt on fluorine-doped tin oxide glass, respectively. The solar cell efficiencies were in the same range, thus proving that an entirely carbon-based SWCNT film on plastic is as good CE candidate for the Co electrolyte. (C) 2013 Elsevier Ltd. All rights reserved.
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| 2. |
- Cappel, Ute B., et al.
(författare)
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The influence of local electric fields on photoinduced absorption in dye-sensitized solar cells.
- 2010
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 132:26, s. 9096-9101
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Tidskriftsartikel (refereegranskat)abstract
- The dye-sensitized solar cell (DSC) challenges conventional photovoltaics with its potential for low-cost production and its flexibility in terms of color and design Transient absorption spectroscopy is widely used to unravel the working mechanism of DSCs A surprising, unexplained feature observed in these studies is an apparent bleach of the ground-state absorption of the dye, under conditions where the dye is in the ground state. Here, we demonstrate that this feature can be attributed to a change of the local electric field affecting the absorption spectrum of the dye, an effect related to the Stark effect first reported in 1913 We present a method for measuring the effect of an externally applied electric field on the absorption of dye monolayers adsorbed on flat TiO2 substrates. The measured signal has the shape of the first derivative of the absorption spectra of the dyes and reverses sign along with the reversion of the direction of the change in dipole moment upon excitation relative to the TiO2 surface A very similar signal is observed in photoinduced absorption spectra of dye-sensitized TiO2 electrodes under solar cell conditions, demonstrating that the electric field across the dye molecules changes upon illumination This result has important implications for the analysis of transient absorption spectra of DSCs and other molecular optoelectronic devices and challenges the interpretation of many previously published results.
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| 3. |
- Ellis, Hanna, et al.
(författare)
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Linker Unit Modification of Triphenylamine-Based Organic Dyes for Efficient Cobalt Mediated Dye-Sensitized Solar Cells
- 2013
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:41, s. 21029-21036
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Tidskriftsartikel (refereegranskat)abstract
- Linker unit modification of donor-linker-acceptor-based organic dyes was investigated with respect to the spectral and physicochemical properties of the dyes. The spectral response for a series of triphenylamine (TPA)-based organic dyes, called LEG1-4, was shifted into the red wavelength region, and the extinction coefficient of the dyes was increased by introducing different substituted dithiophene units on the pi-conjugated linker. The photovoltaic performance of dye-sensitized solar cells (DSCs) incorporating the different dyes in combination with cobalt-based electrolytes was found to be dependent on dye binding. The binding morphology of the dyes on the TiO2 was studied using photoelectron spectroscopy, which demonstrated that the introduction of alkyl chains and different substituents on the dithiophene linker unit resulted in a larger tilt angle of the dyes with respect to the normal of the TiO2-surface, and thereby a lower surface coverage. The good photovoltaic performance for cobalt electrolyte-based DSCs found here and by other groups using TPA-based organic dyes with a cyclopentadithiophene linker unit substituted with alkyl chains was mainly attributed to the extended spectral response of the dye, whereas the larger tilt angle of the dye with respect to the TiO2-surface resulted in less efficient packing of the dye molecules and enhanced recombination between electrons in TiO2 and Co(III) species in the electrolyte.
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| 4. |
- Feldt, Sandra, 1983-
(författare)
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Alternative Redox Couples for Dye-Sensitized Solar Cells
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dye-sensitized solar cells (DSCs) convert sunlight to electricity at a low cost. In the DSC, a dye anchored to a mesoporous TiO2 semiconductor is responsible for capturing the sunlight. The resulting excited dye injects an electron into the conduction band of the TiO2 and is in turn regenerated by a redox mediator, normally iodide/triiodide, in a surrounding electrolyte. The success of the iodide/triiodide redox couple is mainly attributed to its slow interception of electrons at the TiO2 surface, which suppresses recombination losses in the DSC.One of the main limitations with the iodide/triiodide redox couple is, however, the large driving force needed for regeneration, which minimizes the open circuit voltage and thus the energy conversion efficiency. In this thesis, alternative redox couples to the iodide/triiodide redox couple have been investigated. These redox couples include the one-electron transition metal complexes, ferrocene and cobalt polypyridine complexes. The use of one-electron redox couples in the DSC has previously been shown to lead to poor photovoltaic performances, because of increased recombination.Cobalt redox couples were here found to give surprisingly high efficiencies in combination with the triphenylamine-based organic dye, D35. The success of the D35 dye, in combination with cobalt redox couples, was mainly attributed to the introduction of steric alkoxy chains on the dye, which supress recombination losses. By introducing steric substituents on the dye, rather than on the redox couple, mass transport limitations could in addition be avoided, which previously has been suggested to limit the performance of cobalt complexes in the DSC. The result of this study formed the basis for the world record efficiency of DSCs of 12.3 % using cobalt redox couples.Interfacial electron-transfer processes in cobalt-based DSCs were investigated to gain information of advantages and limitations using cobalt redox couples in the DSC. The redox potentials of cobalt redox couples are easily tuned by changing the coordination sphere of the complexes, and regeneration and recombination kinetics were systematically investigated by increasing the redox potential of the cobalt complexes. Our hope is that this thesis can be a guideline for future design of new redox systems in DSCs.
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| 5. |
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| 6. |
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| 7. |
- Feldt, Sandra M., et al.
(författare)
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Characterization of surface passivation by poly(methylsiloxane) for dye-sensitized solar cells employing the ferrocene redox couple.
- 2010
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:23, s. 10551-10558
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Tidskriftsartikel (refereegranskat)abstract
- One-electron outer-sphere redox couples, such as ferrocene/ferrocenium, are an interesting alternative to the iodide/triiodide redox couple that is normally employed in dye-sensitized solar cells (DSCs) because they should reduce the driving force needed to regenerate the dye. Unfortunately, one-electron redox couples also show enhanced recombination with photoinjected electrons, and methods to inhibit this recombination are needed for functioning DSCs. In this study, dye-sensitized titanium dioxide surfaces were passivated by a trichloromethylsilane reaction in order to decrease the fast recombination rates when using the ferrocene redox couple. The formation and binding of poly(methylsiloxane) on the dye-sensitized TiO2 surface was verified with infrared spectroscopy and photoelectron spectroscopy. Photoelectrochemical characterization of the silanization method showed that the treatment decreased the recombination rate of photoinjected electrons with ferrocenium and thereby improved the efficiency of the DSC. Transient absorption spectroscopy revealed, however, that the poly(methylsiloxane) coatings slowed down the regeneration of the oxidized dye by the ferrocene and prevented the regeneration of some of the dye molecules.
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| 8. |
- Feldt, Sandra M., et al.
(författare)
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Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells
- 2010
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 132:46, s. 16714-16724
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Tidskriftsartikel (refereegranskat)abstract
- Dye-sensitized solar cells (DSCs) with cobalt-based mediators with efficiencies surpassing the record for DSCs with iodide-free electrolytes were developed by selecting a suitable combination of a cobalt polypyridine complex and an organic sensitizer. The effect of the steric properties of two triphenylamine-based organic sensitizers and a series of cobalt polypyridine redox mediators on the overall device performance in DSCs as well as on transport and recombination processes in these devices was compared. The recombination and mass-transport limitations that, previously, have been found to limit the performance of these mediators were avoided by matching the properties of the dye and the cobalt redox mediator. Organic dyes with higher extinction coefficients than the standard ruthenium sensitizers were employed in DSCs in combination with outer-sphere redox mediators, enabling thinner TiO2 films to be used. Recombination was reduced further by introducing insulating butoxyl chains on the dye rather than on the cobalt redox mediator, enabling redox couples with higher diffusion coefficients and more suitable redox potential to be used, simultaneously improving the photocurrent and photovoltage of the device. Optimization of DSCs sensitized with a triphenylamine-based organic dye in combination with tris(2,2'-bipyridyl)cobalt(II/III) yielded solar cells with overall conversion efficiencies of 6.7% and open-circuit potentials of more than 0.9 V under 1000 W m(-2) AM1.5 G illumination. Excellent performance was also found under low light intensity indoor conditions.
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| 9. |
- Feldt, Sandra M., et al.
(författare)
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Effects of Driving Forces for Recombination and Regeneration on the Photovoltaic Performance of Dye-Sensitized Solar Cells using Cobalt Polypyridine Redox Couples
- 2011
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 115:43, s. 21500-21507
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Tidskriftsartikel (refereegranskat)abstract
- Dye-sensitized solar cells (DSCs) with open-circuit potentials above 1 V were obtained by employing the triphenylamine based organic dye D35 in combination with cobalt phenanthroline redox couples. A series of cobalt bipyridine and cobalt phenanthroline complexes with different redox potentials were investigated to examine the dependence of the driving force for recombination and dye regeneration on the photovoltaic performance. The photovoltage of the devices was found to increase and the photocurrent to decrease with increasing redox potential of the complexes. The halftime for regeneration of the oxidized dye by cobalt trisbipyrine was about 20 mu s, similar to that found for the iodide/triiodide redox couple, whereas regeneration kinetics became slower for cobalt complexes with less driving force for regeneration. A driving force for dye regeneration of 390 mV for cobalt(II/III) tris(5-chloro-1,10-phenanthroline) was found sufficient to regenerate more than 80% of the D35 dye molecules, resulting in a conversion of incident photons to electric current of above 80%. The photocurrent of the D35 sensitized DSCs using cobalt phenanthroline complexes decreased, however, with increasing Nernst potential of the redox couples, due to the increased recombination and the decreased regeneration rate constants.
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| 10. |
- Feldt, Sandra, 1983-, et al.
(författare)
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Regeneration and Recombination kinetics in Cobalt Polypyridine based Dye-Sensitized Solar Cells, explained using Marcus theory
- 2013
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 15:19, s. 7087-7097
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Tidskriftsartikel (refereegranskat)abstract
- Regeneration and recombination kinetics was investigated for dye-sensitized solar cells (DSCs) using a series of different cobalt polypyridine redox couples, ranging in redox potential in between 0.34 and 1.20 V vs. NHE. Marcus theory was applied to explain the rate of electron transfer. The regeneration kinetics for a number of different dyes (L0, D35, Y123, Z907) by most of the cobalt redox shuttles investigated occurred in the Marcus normal region. The calculated reorganization energies for the regeneration reaction ranged between 0.59 and 0.69 eV for the different organic and organometallic dyes investigated. Under the experimental conditions employed, the regeneration efficiency decreased when cobalt complexes with a driving force for regeneration of 0.4 eV and less were employed. The regeneration efficiency was found to depend on the structure of the dye and the concentration of the redox couples. [Co(bpy-pz)2]2+, which has a driving force for regeneration of 0.25 eV for the triphenylamine based organic dye, D35, was found to regenerate 84 % of the dye molecules, when a high concentration of the cobalt complex was used. Recombination kinetics between electrons in TiO2 and cobalt (III) species in the electrolyte was also studied using steady state dark current measurements. This reaction occurred in the Marcus inverted region for most of the cobalt complexes, and recombination losses are thus not expected to be problematic for D35-sensitized DSCs employing cobalt complexes with high redox potentials. Recombination mediated by surface states was, however, found to significantly influence the result for the cobalt complexes with most positive redox potentials. The calculated system reorganization energies using Marcus theory from the regeneration kinetics and steady state current measurements were very similar, indicating that they are mostly determined by the cobalt mediator.
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