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Studies of Charge Transport Processes in Dye-sensitized Solar Cells
KTH, School of Chemical Science and Engineering (CHE), Chemistry.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , ix, 54 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:12
Keyword [en]
solar cell, mesoporous, dye-sensitized, model, simulation, electron transport, trap distribution
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-4430ISBN: 978-91-7178-602-9 (print)OAI: oai:DiVA.org:kth-4430DiVA: diva2:12288
Public defence
2007-06-15, D1, KTH, Lindstedtsvägen 5, Stockholm, 14:00
Opponent
Supervisors
Note
QC 20100708Available from: 2007-06-07 Created: 2007-06-07 Last updated: 2010-07-09Bibliographically approved
List of papers
1. Brownian dynamics simulations of electrons and ions in mesoporous films
Open this publication in new window or tab >>Brownian dynamics simulations of electrons and ions in mesoporous films
2005 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 86, no 2, 283-297 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Brownian dynamics; mesoporous; solar cells; nanostructured; ambipolar diffusion; dye sensitized
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-7310 (URN)10.1016/j.solmat.2004.07.010 (DOI)000226523900011 ()
Note
QC 20100708Available from: 2007-06-07 Created: 2007-06-07 Last updated: 2017-12-14Bibliographically approved
2. Studies of coupled charge transport in dye-sensitized solar cells using a numerical simulation tool
Open this publication in new window or tab >>Studies of coupled charge transport in dye-sensitized solar cells using a numerical simulation tool
2006 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 90, no 13, 1915-1927 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
dye sensitized; solar cell; simulation; charge transport; model
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-7311 (URN)10.1016/j.solmat.2005.12.004 (DOI)000238594000005 ()2-s2.0-33646848779 (Scopus ID)
Note
QC 20100708Available from: 2007-06-07 Created: 2007-06-07 Last updated: 2017-12-14Bibliographically approved
3. Recombination and Transport Processes in Dye-Sensitized Solar Cells Investigated under Working Conditions
Open this publication in new window or tab >>Recombination and Transport Processes in Dye-Sensitized Solar Cells Investigated under Working Conditions
2006 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 110, no 36, 17715-17718 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Dye-sensitized solar cells; Open-circuit voltage; Square-wave light-intensity modulation
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-7312 (URN)10.1021/jp064046b (DOI)000240340600005 ()2-s2.0-33749677248 (Scopus ID)
Note
QC 20100708Available from: 2007-06-07 Created: 2007-06-07 Last updated: 2017-12-14Bibliographically approved
4. The influence of cations on charge accumulation in dye-sensitized solar cell
Open this publication in new window or tab >>The influence of cations on charge accumulation in dye-sensitized solar cell
2007 (English)In: Journal of Electroanalytical Chemistry, ISSN 0022-0728, E-ISSN 1873-2569, Vol. 609, no 2, 55-60 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
dye-sensitized solar cells; mesoporous; nanostructured; photoelectrochemistry; charge transport; trap distribution
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-9856 (URN)10.1016/j.jelechem.2007.05.013 (DOI)000251038500001 ()2-s2.0-35348948423 (Scopus ID)
Note
QC 20100708Available from: 2009-01-19 Created: 2009-01-19 Last updated: 2017-12-14Bibliographically approved
5. On the influence of anions in binary ionic liquid electrolytes for monolithic dye-sensitized solar cells
Open this publication in new window or tab >>On the influence of anions in binary ionic liquid electrolytes for monolithic dye-sensitized solar cells
Show others...
2007 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, no 35, 13261-13266 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Binary mixtures; Conversion efficiency; Electrolytes; Electron transport properties; Solar cells; Viscosity
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-7314 (URN)10.1021/jp072514r (DOI)000249169900054 ()2-s2.0-34648822936 (Scopus ID)
Note
QC 20100708. Uppdaterad från Submitted till Published 20100708.Available from: 2007-06-07 Created: 2007-06-07 Last updated: 2017-12-14Bibliographically approved
6. Influence of TiOs film thickness and illumination wavelength on electron transport in dye-sensitized solar cells
Open this publication in new window or tab >>Influence of TiOs film thickness and illumination wavelength on electron transport in dye-sensitized solar cells
(English)Manuscript (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-7315 (URN)
Note
QC 20100709Available from: 2007-06-07 Created: 2007-06-07 Last updated: 2010-07-09Bibliographically approved

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