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The influence of cations on charge accumulation in dye-sensitized solar cell
KTH, School of Chemical Science and Engineering (CHE), Centres, Centre of Molecular Devices, CMD.
KTH, School of Chemical Science and Engineering (CHE), Centres, Centre of Molecular Devices, CMD.
KTH, School of Chemical Science and Engineering (CHE), Centres, Centre of Molecular Devices, CMD.
KTH, School of Chemical Science and Engineering (CHE), Centres, Centre of Molecular Devices, CMD.
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.

Place, publisher, year, edition, pages
2007. Vol. 609, no 2, 55-60 p.
Keyword [en]
dye-sensitized solar cells; mesoporous; nanostructured; photoelectrochemistry; charge transport; trap distribution
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-9856DOI: 10.1016/j.jelechem.2007.05.013ISI: 000251038500001Scopus ID: 2-s2.0-35348948423OAI: oai:DiVA.org:kth-9856DiVA: diva2:134145
Note
QC 20100708Available from: 2009-01-19 Created: 2009-01-19 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Charge Transport Processes in Mesoporous Photoelectrochemical Systems
Open this publication in new window or tab >>Charge Transport Processes in Mesoporous Photoelectrochemical Systems
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
solar cell, dye-sensitized, impedance, electron transport, mesoporous
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-9849 (URN)978-91-7415-209-8 (ISBN)
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
2. Studies of Charge Transport Processes in Dye-sensitized Solar Cells
Open this publication in new window or tab >>Studies of Charge Transport Processes in Dye-sensitized Solar Cells
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
solar cell, mesoporous, dye-sensitized, model, simulation, electron transport, trap distribution
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-4430 (URN)978-91-7178-602-9 (ISBN)
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

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