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Charge Transport Processes in Mesoporous Photoelectrochemical Systems
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry. (Organisk kemi)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

During the last decade, the dye sensitised solar cell (DSC) has attracted much attention. The technology has a potential to act as a new generation of photovoltaic device, it has also increased our knowledge within the field of photoelectrochemistry. The materials used in the DSC have been used in other technologies, such as electrochromic displays. This thesis examines how such systems can be analysed to understand their properties from their components. Both of the considered device technologies consist of a thin mesoporous semiconductor film immersed in an electrolyte. The study starts by investigating some of the fundamental properties of the mesoporous semiconductor and its interface with the electrolyte. This gives rise to the charge-voltage relationship for the devices, which is related to the chemical capacitance and electronic energy levels for the materials. In particular,special attention is given to the DSC and the properties of the charge carriers in the semiconductor. For the DSC, several techniques have been developed in order to understand the processes of transport and recombination for the charge carriers in the semiconductor film, which are vitally important for performance. In this thesis, particular focus is given to light modulation techniques and electrical analysis with impedance spectroscopy. The transportproperties show for both techniques a nonlinear behaviour, which is explained with the trapping model. The DSC solar cell is analysed in order to interpret the transport measurements for film thickness optimisation. DSC cells with new semiconductor materials, such as ZnO, were analysed with impedance measurements to provide new insights into the optimisation of the performance of the photoelectrochemical solar cell technology.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , x, 60 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2009:1
Keyword [en]
solar cell, dye-sensitized, impedance, electron transport, mesoporous
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-9849ISBN: 978-91-7415-209-8 (print)OAI: oai:DiVA.org:kth-9849DiVA: diva2:133731
Public defence
2009-02-06, F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100804Available from: 2009-01-19 Created: 2009-01-14 Last updated: 2010-08-04Bibliographically approved
List of papers
1. Electrochemical aspects of display technology based on nanostructured titanium dioxide with attached viologen chromophores
Open this publication in new window or tab >>Electrochemical aspects of display technology based on nanostructured titanium dioxide with attached viologen chromophores
2008 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 53, no 11, 4065-4071 p.Article in journal (Refereed) Published
Abstract [en]

Progress in recent years in the field of electrochromic displays based on viologen modified high-surface area TiO2 electrodes (Vio2+/TiO2) has moved the technology towards commercialisation. Viologen molecules (Vio2+), derivatised with phosphonic acid attachment groups can be chemisorbed on nanostructured TiO2 layers of thickness 2–10 μm. Characterisation by cyclic voltammetry, spectroelectrochemistry and impedance spectroscopy demonstrates that colourless Vio2+/TiO2 is reversibly reduced to the strongly coloured cation radical species Vio+/TiO2. This system can constitute the working electrode of an electrochromic display with a capacitive doped SnO2 electrode as counter electrode, the latter coated by an electrochemically inert white light-reflecting layer. Such a device is stable upon repeated colouration–bleaching cycles with a bleached-to-coloured state contrast ratio exceeding 5. Multicolour displays can be achieved by patterning different electrochromophores onto different areas of one working electrode.

Keyword
electrochromic display; nanostructured electrode; viologen; titanium dioxide; Nanochromics (TM)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-9855 (URN)10.1016/j.electacta.2007.10.011 (DOI)000254728900043 ()2-s2.0-39749134732 (Scopus ID)
Note
QC 20100804Available from: 2009-01-19 Created: 2009-01-19 Last updated: 2017-12-14Bibliographically approved
2. 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
3. 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
4. Revisiting the electron transport models in dye sensitized solar cells: film thickness study
Open this publication in new window or tab >>Revisiting the electron transport models in dye sensitized solar cells: film thickness study
(English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455Article in journal (Other academic) Submitted
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-9858 (URN)
Note
QS 20120328Available from: 2009-01-19 Created: 2009-01-19 Last updated: 2017-12-14Bibliographically approved
5. 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
6. Identification of factors giving low fill factor in ZnO based dye sensitized solar cells
Open this publication in new window or tab >>Identification of factors giving low fill factor in ZnO based dye sensitized solar cells
Show others...
(English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455Article in journal (Other academic) Submitted
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-9859 (URN)
Note
QS 2011 QS 20120328Available from: 2009-01-19 Created: 2009-01-19 Last updated: 2017-12-14Bibliographically approved

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