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Isomerization and Aggregation of the Solar Cell Dye D149
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 50, 26144-26153 p.Article in journal (Refereed) Published
Abstract [en]

D149, a metal-free indoline dye, is one of the most promising sensitizers for dye-sensitized solar cells (DSSCs) and has shown very high solar energy conversion efficiencies of 9%. Effective electron injection from the excited state is a prerequisite for high efficiencies and is lowered by competing deactivation pathways. Previous investigations have shown surprisingly short-lived excited states for this dye, with maximum lifetime components of 100-720 ps in different solvents and less than 120 ps for surface-adsorbed D149. Using steady-state and time-resolved fluorescence, we have investigated the photochemical properties of D149 in nonpolar and polar solvents, polymer matrices, and adsorbed on ZrO2, partially including a coadsorbent. In solution, excitation to the S-2 state yields a product that is identified as a photoisomer. The reaction is reversible, and the involved double-bond is identified by NMR spectroscopy. Our results further show that lifetimes of 100-330 ps in the solvents used are increased to more than 2 ns for D149 in polymer matrices and on ZrO2. This is in part attributed to blocked internal motion due to steric constraint. Conversely, concentration-dependent aggregation leads to a dramatic reduction in lifetimes that can affect solar cell performance. Our results explain the unexpectedly short lifetimes observed previously. We also show that photochemical properties such as lifetimes determined in solution are different from the ones determined on semiconductor surfaces used in solar cells. The obtained mechanistic understanding should help develop design strategies for further improvement of solar cell dyes.

Place, publisher, year, edition, pages
2012. Vol. 116, no 50, 26144-26153 p.
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-193622DOI: 10.1021/jp306636wISI: 000312519600005OAI: oai:DiVA.org:uu-193622DiVA: diva2:603455
Available from: 2013-02-06 Created: 2013-02-05 Last updated: 2015-10-27Bibliographically approved
In thesis
1. Exploring Organic Dyes for Grätzel Cells Using Time-Resolved Spectroscopy
Open this publication in new window or tab >>Exploring Organic Dyes for Grätzel Cells Using Time-Resolved Spectroscopy
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Grätzel cells or Dye-Sensitized Solar Cells (DSSCs) are considered one of the most promising methods to convert the sun's energy into electricity due to their low cost and simple technology of production. The Grätzel cell is based on a photosensitizer adsorbed on a low band gap semiconductor. The photosensitizer can be a metal complex or an organic dye. Organic dyes can be produced on a large scale resulting in cheaper dyes than complexes based on rare elements. However, the performance of Grätzel cells based on metal-free, organic dyes is not high enough yet. The dye's performance depends primarily on the electron dynamics. The electron dynamics in Grätzel cells includes electron injection, recombination, and regeneration. Different deactivation processes affect the electron dynamics and the cells’ performance.

In this thesis, the electron dynamics was explored by various time-resolved spectroscopic techniques, namely time-correlated single photon counting, streak camera, and femtosecond transient absorption. Using these techniques, new deactivation processes for organic dyes used in DSSCs were uncovered. These processes include photoisomerization, and quenching through complexation with the electrolyte. These deactivation processes affect the performance of organic dyes in Grätzel cells, and should be avoided. For instance, the photoisomerization can compete with the electron injection and produce isomers with unknown performance. Photoisomerization as a general phenomenon in DSSC dyes has not been shown before, but is shown to occur in several organic dyes, among them D149, D102, L0 and L0Br. In addition, D149 forms ground state complexes with the standard iodide/triiodide electrolyte, which directly affect the electron dynamics on TiO2. Also, new dyes were designed with the aim of using ferrocene(s) as intramolecular regenerators, and their dynamics was studied by transient absorption.

This thesis provides deeper insights into some deactivation processes of organic dyes used in DSSCs. New rules for the design of organic dyes, based on these insights, can further improve the efficiency of DSSCs. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 84 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1294
Laser spectroscopy, DSSCs, DSC, Electron dynamics, Deactivation processes, Isomerization, Twisting, TICT, Quenching by protons, Semiconductor, Electrolyte, Electron injection, Regeneration, Recombination
National Category
Natural Sciences
Research subject
Chemistry with specialization in Physical Chemistry; Chemistry with specialization in Chemical Physics
urn:nbn:se:uu:diva-263143 (URN)978-91-554-9349-3 (ISBN)
Public defence
2015-11-19, Häggsalen, Ångströmlaboratoriet, Uppsala, 10:15 (English)
Available from: 2015-10-22 Created: 2015-09-27 Last updated: 2015-10-27

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