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Self-quenching and Slow Hole Injection May Limit the Efficiency in NiO-based Dye-Sensitized Solar Cells
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. (Hammarström)ORCID iD: 0000-0002-7964-8090
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Molecular Biomimetics.
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, Molecular Biomimetics.
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2018 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 25, p. 13902-13910Article in journal (Refereed) Published
Abstract [en]

A series of bis-tridentate ruthenium complexes was designed to feature opposite localizations of their lowest metal-to-ligand charge transfer (MLCT) excited states, relative to a carboxylic acid that served as a binding group to mesoporous NiO. The purpose was to study the effect of MLCT direction on the rates of hole injection into NiO and subsequent charge recombination. Surprisingly, femtosecond-transient absorption spectroscopy showed that the two heteroleptic, cyclometalated complexes of this series did not inject holes into NiO, but their excited states were nevertheless quenched in a rapid process (on the time scale of hundreds of picoseconds). An identical result was obtained for the dyes on nonreactive ZrO2 and we therefore attribute the short MLCT lifetime to self-quenching, due the high surface concentrations of the dyes. We further show that self-quenching on this time scale can potentially compete with hole injection also for functional NiO sensitizers. A ruthenium polypyridine complex, which has previously been used for NiO-based solar cells, was shown to inject holes only very slowly (τ ≈ 5 ns), in contrast to the common notion that hole injection in dye-NiO systems is ultrafast (predominantly subpicosecond time scale). The hole injection yield was estimated to only ca. 20%, which matches the reported APCE value of the corresponding device [Freys, J. C.; Gardner, J. M.; D’Amario, L.; Brown, A. M.; Hammarström, L. Dalton Trans. 2012, 41, 13105]. Therefore, we suggest that slow injection and self-quenching might be a reason for the low photovoltaic performance of some p-type dye-sensitized solar cells.

Place, publisher, year, edition, pages
2018. Vol. 122, no 25, p. 13902-13910
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-343442DOI: 10.1021/acs.jpcc.8b01016ISI: 000437811500078OAI: oai:DiVA.org:uu-343442DiVA, id: diva2:1190199
Funder
Swedish Research Council, 2014-5921Swedish Energy Agency, 11674-5Knut and Alice Wallenberg Foundation, 2011.0067Available from: 2018-03-14 Created: 2018-03-14 Last updated: 2018-09-21Bibliographically approved
In thesis
1. Shining Light on Molecules: Electron Transfer Processes in Model Systems for Solar Energy Conversion Investigated by Transient Absorption Spectroscopy
Open this publication in new window or tab >>Shining Light on Molecules: Electron Transfer Processes in Model Systems for Solar Energy Conversion Investigated by Transient Absorption Spectroscopy
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the recent years, solar energy conversion has attracted a huge research interest due to the potential application for limiting the greenhouse effect. In many solar cells and solar fuel cells, understanding of charge transfer (CT) and recombination is important for future improvement of the overall efficiency. One important tool for that is transient absorption spectroscopy (TAS).

Mesoporous nickel oxide films were investigated due to their potential application in p-type dye-sensitized solar cells (DSSCs), tandem DSSCs and dye sensitized solar fuel cells (DSSFC:s). Firstly, it was found that the hole generated by band gap excitation is trapped on an ultrafast time scale by Ni3+ states. It was possible to observe a direct signal from the holes by transient mid-IR absorption spectroscopy allowing for direct detection of hole injection and trapping. On a ns time scale, the trapped holes relaxed to much less reactive holes which favored long lived NiO-dye charge separation (CS).

A series of perylene monoimide (PMI) dyes with different anchoring groups was studied. Differences in binding affinity and stability were found. Nevertheless, all PMIs showed ultrafast charge separation and similar recombination kinetics. Furthermore, the effect of MLCT localization of ruthenium polypyridyl complexes was investigated. All those dyes showed slow or no hole injection. At the same time, a self-quenching process was found for all compounds that limited the photoconversion efficiency.

Furthermore, a new core-shell structure of p-type DSSCs was proposed and investigated. Here, the liquid electrolyte was replaced by a layer of TiO2. That system was found to undergo both injection and regeneration of the dye on an ultrafast time scale (below 1 ps). Furthermore, the CS state did not show any decay within 2 ns making this structure interesting for application in DSSCs.

A pentad consisting of a known Ru-based (electro)chemical water oxidation catalyst (WOC) linked to two zinc-porphyrin-fullerene dyads (ZnP-C60) was investigated. The charge transfer processes leading to the first oxidation of the WOC were understood. Low levels of water oxidation were detected in presence of a sacrificial electron acceptor.

The gained understanding of the CT dynamics and recombination processes thus allows new strategies to improve the efficiency in molecular systems for solar energy conversion.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 74
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1645
Keywords
photophysics, photoinduced electron transfer, transient absorption spectroscopy, laser spectroscopy, solar energy conversion, p-type DSSCs, Charge separation, recombination, mesoporous NiO
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-343443 (URN)978-91-513-0273-7 (ISBN)
Public defence
2018-05-04, Siegbahnsalen, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2018-04-13 Created: 2018-03-14 Last updated: 2018-04-24

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Maji, SomnathBrown, Allison M.Mijangos, EdgarOtt, SaschaHammarström, Leif

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