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Exploring Organic Dyes for Grätzel Cells Using Time-Resolved Spectroscopy
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1294
Keyword [en]
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
Identifiers
URN: urn:nbn:se:uu:diva-263143ISBN: 978-91-554-9349-3 (print)OAI: oai:DiVA.org:uu-263143DiVA: diva2:857014
Public defence
2015-11-19, Häggsalen, Ångströmlaboratoriet, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2015-10-22 Created: 2015-09-27 Last updated: 2015-10-27
List of papers
1. Isomerization and Aggregation of the Solar Cell Dye D149
Open this publication in new window or tab >>Isomerization and Aggregation of the Solar Cell Dye D149
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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-193622 (URN)10.1021/jp306636w (DOI)000312519600005 ()
Available from: 2013-02-06 Created: 2013-02-05 Last updated: 2017-12-06Bibliographically approved
2. Photoisomerization of the cyanoacrylic acid acceptor group - a potential problem for organic dyes in solar cells
Open this publication in new window or tab >>Photoisomerization of the cyanoacrylic acid acceptor group - a potential problem for organic dyes in solar cells
Show others...
2014 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 6, 2251-2255 p.Article in journal (Refereed) Published
Abstract [en]

Organic solar cell dyes containing the most common anchoring group, cyanoacrylic acid, are shown to be photolabile and undergo photoisomerization. This may have significant consequences for dye-sensitized solar cells, as isomerisation competes with electron injection and leads to modifications of the dye and surface arrangement.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-218969 (URN)10.1039/c3cp54048k (DOI)000329926700005 ()
Available from: 2014-02-20 Created: 2014-02-20 Last updated: 2017-12-06Bibliographically approved
3. Ultrafast Twisting of the Indoline Donor Unit Utilized in Solar Cell Dyes: Experimental and Theoretical Studies
Open this publication in new window or tab >>Ultrafast Twisting of the Indoline Donor Unit Utilized in Solar Cell Dyes: Experimental and Theoretical Studies
2015 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 5, 2249-2259 p.Article in journal (Refereed) Published
Abstract [en]

Previous time-resolved measurements on D149, the most-studied dye of the indoline family, had shown a fast time-component of 20–40 ps that had tentatively been attributed to structural relaxation. Using femtosecond transient absorption, we have investigated the isolated indoline donor unit (i.e., without acceptor group) and found an ultrafast decay characterized by two lifetimes of 3.5 and 23 ps. Density functional theory calculations show π-bonding and π*-antibonding character of the central ethylene group for the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), respectively. The LUMO is localized on the flexible vinyl-diphenyl region of the donor unit and a twisting process is assumed to occur as a deactivation process for the excited molecule. This is confirmed by multireference second-order perturbation theory (CASSCF/CASPT2) calculations of the lowest-lying excited state, in which it is shown that torsion of the ethylene bond to 96° and pyramidalization to ca. 100° lead to a conical intersection with the ground state. Embedded in a plastic matrix, where double bond rotation is hindered, the decay is slowed down to nanoseconds. We have also investigated the dyes D102, D131, and D149, possessing the same indoline donor unit, by femtosecond transient absorption and found a similar decay component. The ca. 20 ps deactivation channel in D-family dyes is thus attributed to a twisting process of the donor unit. The fluorescence quantum yields of this unit and D149 were measured, and from comparison, the competition of the discovered twisting deactivation channel to the radiative decay of the excited indoline dyes could be confirmed. Blocking this deactivation channel is expected to further increase efficiency for the indoline dyes.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-263141 (URN)10.1021/jp505649s (DOI)
Available from: 2015-09-27 Created: 2015-09-27 Last updated: 2017-12-01Bibliographically approved
4. Fine-Tuning of the Twisted Intramolecular Charge Transfer (TICT) Energy Level by Dimerisation − An Overlooked Piece of the TICT Puzzle
Open this publication in new window or tab >>Fine-Tuning of the Twisted Intramolecular Charge Transfer (TICT) Energy Level by Dimerisation − An Overlooked Piece of the TICT Puzzle
(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-263147 (URN)
Available from: 2015-09-27 Created: 2015-09-27 Last updated: 2015-10-27
5. Concentration and Solvent Effects on the Excited State Dynamics of the Solar Cell Dye D149: The Special Role of Protons
Open this publication in new window or tab >>Concentration and Solvent Effects on the Excited State Dynamics of the Solar Cell Dye D149: The Special Role of Protons
2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 13, 6544-6553 p.Article in journal (Refereed) Published
Abstract [en]

D149 is one of the best-performing metal-free, organic dyes for dye-sensitized solar cells. Excited state lifetimes strongly depend on the solvent used and have previously been reported to be between 100 and 700 ps, without any mechanistic explanation being given. We have earlier shown that photo-isomerization is one of several deactivation processes. Here, we report that lifetimes in certain solvents depend on concentration, even in very dilute (nanomolar) solutions. A detailed investigation of the concentration dependence enables us to assign a second, faster deactivation channel besides isomerization that reduces lifetimes further: a ground-state, hydrogen-bonded 1:1 complex of D149 with acids or interaction with protic solvents leads to excited state quenching, most probably through excited state proton transfer. This includes self-quenching caused by D149's own carboxylic group through intermolecular interaction, accounting for the concentration-dependent lifetimes. We are now able to dissect the complex excited state behavior into its components, allowing us to attribute rate constants to the isomerization and the excited-state proton transfer process. We are also able to explain the excited state of D149 in a wide range of environmental conditions, in the presence of acids/bases, at different concentrations as well as with varying temperatures. Furthermore, we determine the barrier for isomerization, a thermally activated process. The consequences of these effects on solar cells are discussed. Also we show that ultrafast techniques like femtosecond pump probe and upconversion inherently do not provide the required responsiveness for work with the concentration ranges required here, whereas single photon counting with its ultimate sensitivity is able to resolve the underlying processes.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-200071 (URN)10.1021/jp400782g (DOI)000317317600007 ()
Available from: 2013-05-23 Created: 2013-05-20 Last updated: 2017-12-06Bibliographically approved
6. Dimer formation for Indoline Dyes in Solutions and Proton Impact on Surfaces
Open this publication in new window or tab >>Dimer formation for Indoline Dyes in Solutions and Proton Impact on Surfaces
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-263148 (URN)
Available from: 2015-09-27 Created: 2015-09-27 Last updated: 2015-10-27
7. Interactions with Iodide Electrolyte Affect the Electrons Dynamics in Grätzel Cells
Open this publication in new window or tab >>Interactions with Iodide Electrolyte Affect the Electrons Dynamics in Grätzel Cells
(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-263150 (URN)
Available from: 2015-09-27 Created: 2015-09-27 Last updated: 2015-10-27
8. Ferrocene as a Rapid Charge Regenerator in Grätzel Cells
Open this publication in new window or tab >>Ferrocene as a Rapid Charge Regenerator in Grätzel Cells
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-263151 (URN)
Available from: 2015-09-27 Created: 2015-09-27 Last updated: 2015-10-27

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