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Multichromophoric Sensitizers Based on Squaraine for NiO Based Dye-Sensitized Solar Cells
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.
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2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 1, 103-113 p.Article in journal (Refereed) Published
Abstract [en]

Three sensitizers were synthesized and utilized as panchromatic dyes for nanocrystalline NiO films: an iodo-squaraine (SQ), a squaraine-perylene monoimide (SQ-PMI) dyad, and a squaraine-perylene monoimide-naphthalene diimide (SQ-PMI-NDI) triad. Photophysical and photovoltaic studies showed that hole injection into the NiO valence band from the SQ excited state is ultrafast, but also that subsequent recombination is very rapid, preventing SQ from being an efficient sensitizer for photovoltaic purposes. The introduction of a second light harvesting unit (PMI) and a terminal electron acceptor (NDI) significantly improved the photovoltaic performances of the system. Irrespective of which light harvesting unit was photoexcited in NiO/SQ-PMI and NiO/SQ-PMI-NDI, intramolecular charge separation leading to SQ+ and PMI or NDI is the main excited state deactivation process. Intramolecular charge separation occurred in spite of the favorable conditions for energy transfer to the SQ unit. Subsequent hole injection from SQ+ into NiO was in competition with intramolecular recombination, which may have significantly decreased the overall photovoltaic performances. The control of this side-reaction is therefore crucial for the successful design of multichromophoric systems for dye-sensitized solar cells (DSSCs). The quantum yield of NiO(+)/SQ-PMI-NDI after 50 ns is higher than that of NiO(+)/SQ-PMI, and much higher than that of NiO(+)/SQ; intramolecular recombination was slowed down by the localization of the electron further away from the SQ+ hole and consequently from NiO+. The three sensitizers were tested in NiO based DSSC devices using either the conventional triiodide/iodide electrolyte or a cobaltIII/II(4,4′-di-tert-butyl-2,2′-bipyridine)3 electrolyte. The photoconversion efficiencies steadily increased in the following order: SQ < SQ-PMI < PMI-NDI SQ-PMI-NDI. The multichromophoric sensitizers had broader absorption spectra, more long-lived charge separation, and better photovoltaic performance than single unit chromophores. This indicates that bichromophoric systems, ones in which the antenna serves both as electron acceptor and photon harvester, are realistic sensitizers to boost photovoltaic performances. These findings are important for engineering new panchromatic and more efficient sensitizers for p-type DSSCs.

Place, publisher, year, edition, pages
2014. Vol. 118, no 1, 103-113 p.
National Category
Engineering and Technology
URN: urn:nbn:se:uu:diva-218951DOI: 10.1021/jp408900xISI: 000329678200014OAI: oai:DiVA.org:uu-218951DiVA: diva2:699287
Available from: 2014-02-27 Created: 2014-02-20 Last updated: 2015-02-03Bibliographically approved
In thesis
1. Ultrafast, Non-Equilibrium Electron Transfer Reactions of Molecular Complexes in Solution
Open this publication in new window or tab >>Ultrafast, Non-Equilibrium Electron Transfer Reactions of Molecular Complexes in Solution
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Photoinduced electron transfer is a fundamentally interesting process; it occurs everywhere in the natural world. Studies on electron transfer shed light on questions about the interaction between molecules and how the dynamics of these can be utilized to steer the electron transfer processes to achieve a desired goal. The goal may be to get electrons to the electrode of a solar cell, or to make the electrons form an energy rich fuel such as hydrogen, and it may also be an input or output for molecular switches. The importance of electron transfer reactions will be highlighted in this thesis, however, the main motivation is to gain a better understanding of the fundamental processes that affect the rate and direction of the electron transfer.

A study of photoinduced electron transfer (ET) in a series of metallophorphyrin/bipyridinium complexes in aqueous solution provided fresh insight concerning the intimate relationship between vibrational relaxation and electron transfer. The forward electron transfer from porphyrin to bipyridinium as well as the following back electron transfer to the ground state could be observed by femtosecond transient absorption spectroscopy. Both the reactant and the product states of the ET processes were vibrationally unrelaxed, in contrary to what is assumed for most expressions of the ET rates. This could be understood from the observation of unrelaxed ground states. The excess energy given by the initial excitation of the porphyrin does not relax completely during the two steps of electron transfer. This is an unusual observation, not reported in the literature prior the studies presented in this thesis. This study also gave the first clear evidence of electronically excited radical pairs formed as products of intramolecular electron transfer. Signs of electronically excited radical pairs were seen in transient spectra, and were further verified by the observation that the rates followed a Marcus normal region behavior for all excitation wavelengths, despite the relatively large excess energy of the second excited state.

This thesis also concerns electron transfer in solar cell dyes and mixed valence complexes. In the ruthenium polypyridyl complex Ru(dcb)2(NCS)2, where dcb = 4,4’-dicarboxy-2,2’-bipyridine, inter-ligand electron transfer (ILET) in the 3MLCT state was followed by means of femtosecond transient absorption anisotropy that was probed in the mid-IR region. Unexpectedly, ILET was not observed because electron density was localized on the same bpy during the time-window allowed by the rotational lifetime.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 90 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1205
electron transfer, laser, spectroscopy, transient absorption, anisortopy, inter ligand electron transfer, dye sensitized solar cell, DSSC, vibrational relaxation, ultrafast dynamics, fs spectroscopy
National Category
Chemical Sciences
Research subject
Chemistry with specialization in Chemical Physics
urn:nbn:se:uu:diva-235461 (URN)978-91-554-9107-9 (ISBN)
Public defence
2014-12-19, Häggsalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Available from: 2014-11-26 Created: 2014-11-03 Last updated: 2015-02-03

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Petersson, JonasHammarström, Leif
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