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Ultrafast and slow charge recombination dynamics of diketopyrrolopyrrole–NiO dye sensitized solar cells
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. (Prof Hammarstrom Leif's group)
(Odobel's group)
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
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2016 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, 18515-18527 p.Article in journal (Refereed) Published
Abstract [en]

In a photophysical study, two diketopyrrolopyrrole (DPP)-based sensitizers functionalized with4-thiophenecarboxylic acid as an anchoring group and a bromo (DPPBr) or dicyanovinyl (DPPCN2)group, and a dyad consisting of a DPP unit linked to a naphthalenediimide group (DPP–NDI), wereinvestigated both in solution and grafted on mesoporous NiO films. Femtosecond transient absorptionmeasurements indicate that ultrafast hole injection occurred predominantly on a timescale of B200 fs,whereas the subsequent charge recombination occurred on a surprisingly wide range of timescales,from tens of ps to tens of ms; this kinetic heterogeneity is much greater than is typically observed fordye-sensitized TiO2 or ZnO. Also, in contrast to what is typically observed for dye-sensitized TiO2, therewas no significant dependence on the excitation power of the recombination kinetics, which can beexplained by the hole density being comparatively higher near the valence band of NiO beforeexcitation. The additional acceptor group in DPP–NDI provided a rapid electron shift and stabilizedcharge separation up to the ms timescale. This enabled efficient (B95%) regeneration of NDI bya CoIII(dtb)3 electrolyte (dtb = 4,40-di-tert-butyl-2,20-bipyridine), according to transient absorptionmeasurements. The regeneration of DPPBr and DPPCN2 by CoIII(dtb)3 was instead inefficient, as mostrecombination for these dyes occurred on the sub-ns timescale. The transient spectroscopy data thuscorroborated the trend of the published photovoltaic properties of dye-sensitized solar cells (DSSCs)based on these dyes on mesoporous NiO, and show the potential of a design strategy with a secondaryacceptor bound to the dye. The study identifies rapid initial recombination between the dye and NiO asthe main obstacle to obtaining high efficiencies in NiO-based DSSCs; these recombination componentsmay be overlooked when studies are conducted using only methods with ns resolution or slower.

Place, publisher, year, edition, pages
2016. Vol. 18, 18515-18527 p.
Keyword [en]
Ultrafast, pDSSCs, Charge recombination
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Physical Chemistry
URN: urn:nbn:se:uu:diva-300147DOI: 10.1039/c6cp01762bISI: 000379486200081PubMedID: 27338174OAI: oai:DiVA.org:uu-300147DiVA: diva2:950875
Swedish Energy AgencyKnut and Alice Wallenberg Foundation
Available from: 2016-08-03 Created: 2016-08-03 Last updated: 2016-08-31Bibliographically approved
In thesis
1. Exploring Electron Transfer Dynamics of Novel Dye Sensitized Photocathodes: Towards Solar Cells and Solar Fuels
Open this publication in new window or tab >>Exploring Electron Transfer Dynamics of Novel Dye Sensitized Photocathodes: Towards Solar Cells and Solar Fuels
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The design of dyes for NiO-based dye-sensitized solar cells (DSSCs) has drawn attention owing to their potential applications in photocatalysis and because they are indispensable for the development of tandem dye-sensitized solar cells. The understanding of the electron transfer mechanisms and dynamics is beneficial to guide further dye design and further improve the performance of photocathode in solar cells and solar fuel devices.

Time-resolved spectroscopy techniques, especially femtosecond and nanosecond transient absorption spectroscopy, supply sufficient resolution to get insights into the charge transfer processes in p-type dye sensitized solar cell and solar fuel devices. In paper I-V, several kinds of novel organic “push-pull” and inorganic charge transfer dyes for sensitization of p-type NiO, were systematically investigated by time-resolved spectroscopy, and photo-induced charge transfer dynamics of the organic/inorganic dyes were summarized. The excited state and reduced state intermediates were investigated in solution phase as references to confirm the charge injection and recombination on the NiO surface. The charge recombination kinetics is remarkably heterogeneous in some cases occurring on time scales spanning at least six orders of magnitude even for the same dye.

In this thesis, we also proposed a novel concept of solid state p-type dye sensitized solar cells (p-ssDSSCs) for the first time (paper VI), using an organic dye P1 as sensitizer on mesoporous NiO and phenyl-C61-butyric acid methyl ester (PCBM) as electron conductor. Femtosecond and nanosecond transient absorption spectroscopy gave evidence for sub-ps hole injection from excited P1 to NiO, followed by electron transfer from P1●- to PCBM. The p-ssDSSCs device showed an impressive 620 mV open circuit photovoltage.

Chapter 6 (paper VII) covers the study of electron transfer mechanisms in a covalently linked dye-catalyst (PB-2) sensitized NiO photocathode, towards hydrogen producing solar fuel devices. Hole injection from excited dye (PB-2*) into NiO VB takes place on dual time scales, and the reduced PB-2 (PB-2●-) formed then donates an electron to the catalyst unit.  The subsequent regeneration efficiency of PB-2 by the catalyst unit (the efficiency of catalyst reduction) is determined to ca. 70%.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 80 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1418
Electron transfer, Laser spectroscopy, Femtosecond spectroscopy, Transient absorption, NiO, DSSCs, DSSFDs, Charge separation, Solar energy conversion, Nanosecond photolysis, Photophysics
National Category
Chemical Sciences
Research subject
Chemistry with specialization in Chemical Physics
urn:nbn:se:uu:diva-302263 (URN)978-91-554-9678-4 (ISBN)
Public defence
2016-10-21, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Available from: 2016-09-27 Created: 2016-08-31 Last updated: 2016-09-27

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