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Exploring Dye-Sensitized Mesoporous NiO Photocathodes: from Mechanism to Applications
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Increasing attention has been paid on solar energy conversion since the abundant solar energy possesses the potential to solve the problems on energy crisis and climate change. Dye-sensitized mesoporous NiO film was developed as one of the attractive photocathodes to fabricate p-type dye-sensitized solar cells (p-DSCs) and dye-sensitized photoelectrosynthetic cells (p-DSPECs) for electricity and chemical fuels generation, respectively. In this thesis, we designed a well-structured NiO-dye-TiO2 configuration by an atomic layer deposition (ALD) technique, with an organic dye PB6 as the photosensitizer. From kinetic studies of charge separation, ultrafast hole injection (< 200 fs) was observed from the excited state of PB6 dye into the valence band of NiO; dye regeneration (electron injection) was in t1/2 ≤ 500 fs, which is the fastest reported in any DSCs. On the basis of NiO-dye-TiO2 configuration, we successfully fabricated solid-state p-type DSCs (p-ssDSCs). Insertion of an Al2O3 layer was adopted to reduce charge recombination, i.e. NiO-dye-Al2O3-TiO2. Theoretically, such a configuration is possible to maintain efficient charge separation and depressed charge recombination. Based on NiO-dye-Al2O3-TiO2 configuration, the open-circuit voltage was improved to 0.48 V. Replacing electron conductor TiO2 with ZnO, short-circuit current density was increased to 680 μA·cm-2. The photocatalytic current density for H2 evolution was improve to 100 μA·cm-2 with a near unity of Faraday efficiency in p-DSPECs.

However, to further improve the performance of p-DSCs is very challenging. In p-ssDSCs, the limitation was confirmed from the poor electronically connection of the electron conductor (TiO2 or ZnO) inside the NiO-dye films. We further investigated the electronic property of surface states on mesoporous NiO film. We found that the surface sates, not the bulk, on NiO determined the conductivity of the mesoporous NiO films. The dye regeneration in liquid p-DSCs with I-/I3- as redox couples was significantly affected by surface states. A more complete mechanism is suggested to understand a particular hole transport behavior reported in p-DSCs, where hole transport time is independent on light intensity. The independence of charge transport is ascribed to the percolation effect in the hole hopping on the surface states.

 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. , p. 73
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1896
Keywords [en]
NiO, p-type, dye-sensitized solar cells, solid state, charge separation, charge recombination, charge diffusion, transport time, surface states, electron conductor, dye regeneration
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Chemical Physics
Identifiers
URN: urn:nbn:se:uu:diva-402415ISBN: 978-91-513-0852-4 (print)OAI: oai:DiVA.org:uu-402415DiVA, id: diva2:1385629
Public defence
2020-03-03, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2020-02-11 Created: 2020-01-15 Last updated: 2020-02-14
List of papers
1. Ultrafast dye regeneration in a core-shell NiO-dye-TiO2 mesoporous film
Open this publication in new window or tab >>Ultrafast dye regeneration in a core-shell NiO-dye-TiO2 mesoporous film
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2018 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 1, p. 36-40Article in journal (Refereed) Published
Abstract [en]

In this study, a core-shell NiO-dye-TiO2 mesoporous film was fabricated for the first time, utilizing atomic layer deposition technique and a newly designed triphenylamine dye. The structure of the film was confirmed by SEM, TEM, and EDX. Excitation of the dye led to efficient and fast charge separation, by hole injection into NiO, followed by an unprecedentedly fast dye regeneration (t1/2 [less-than-or-equal] 500 fs) by electron transfer to TiO2. The resulting charge separated state showed a pronounced transient absorption spectrum caused by the Stark effect, and no significant decay was found within 1.9 ns. This indicates that charge recombination between NiO and TiO2 is much slower than that between the NiO and the reduced dye in the absence of the TiO2 layer (t1/2 [approximate] 100 ps).

Place, publisher, year, edition, pages
The Royal Society of Chemistry, 2018
National Category
Natural Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-335974 (URN)10.1039/C7CP07088H (DOI)000418374800002 ()
Funder
Knut and Alice Wallenberg Foundation, 2011.0067Swedish Energy Agency, 43599-1
Note

Correction in: PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Volume: 20, Issue: 46, Pages: 29566-29566, DOI: 10.1039/c8cp91912g

Available from: 2017-12-11 Created: 2017-12-11 Last updated: 2020-01-15Bibliographically approved
2. Solid state p-type dye sensitized NiO-dye-TiO2 core-shell solar cells
Open this publication in new window or tab >>Solid state p-type dye sensitized NiO-dye-TiO2 core-shell solar cells
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2018 (English)In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, Vol. 54, no 30, p. 3739-3742Article in journal (Refereed) Published
Abstract [en]

Solid state p-type dye sensitized NiO-dye-TiO2 core-shell solar cells with an organic dye PB6 were successfully fabricated for the first time. With Al2O3 as an inner barrier layer, the recombination process between injected holes in NiO and injected electrons in TiO2 was significantly suppressed and the charge transport time was also improved.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY, 2018
National Category
Physical Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-352467 (URN)10.1039/c8cc00505b (DOI)000429592700013 ()29589009 (PubMedID)
Funder
Swedish Energy Agency, 43599-1
Available from: 2018-06-08 Created: 2018-06-08 Last updated: 2020-01-15Bibliographically approved
3. Solution-processed nanoporous NiO-dye-ZnO photocathodes: Toward efficient and stable solid-state p-type dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells
Open this publication in new window or tab >>Solution-processed nanoporous NiO-dye-ZnO photocathodes: Toward efficient and stable solid-state p-type dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells
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2019 (English)In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 55, p. 59-64Article in journal (Refereed) Published
Abstract [en]

A solution-processed NiO-dye-ZnO photocathode was developed for applications in both solid-state p-type dye-sensitized solar cells (p-ssDSCs) and p-type dye-sensitized photoelectrosynthesis cells (p-DSPECs). In p-ssDSCs, the solar cell using ZnO as electron transport material showed a short circuit current, up to 680 mu A cm(-2), which is 60-fold larger than that previously reported device using TiO2 as electron transport material with similar architecture. In the p-DSPECs, a remarkable photocurrent of 100 mu A cm(-2) was achieved in a pH = 5.0 acetate buffer solution under a bias potential at 0.05 V vs RHE with platinum as the proton reduction catalyst. A Faradaic efficiency approaching 100% for the H-2 evolution reaction was obtained after photoelectrolysis for 9 h. Importantly, the solution-processed NiO-dye-ZnO photocathode exhibited excellent long-term stability in both p-ssDSCs and p-DSPECs. To the best of our knowledge, this is the first study where a solution-processable, nanoporous NiO-dye-ZnO photocathode is used for both p-ssDSCs and p-DSPECs having both excellent device performance and stability.

Keywords
Dye-sensitized photocathode, P-type, Solution-processed, Solid-state, Solar cell, Solar fuel
National Category
Materials Chemistry Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-374114 (URN)10.1016/j.nanoen.2018.10.054 (DOI)000454636200007 ()
Funder
Swedish Energy Agency, 43599-1Swedish Energy Agency, 11674-8
Note

Bo Xu and Lei Tian contributed equally to this work.

Available from: 2019-01-23 Created: 2019-01-23 Last updated: 2020-01-15Bibliographically approved
4. Mechanistic Insights into Solid-State p-Type Dye-Sensitized Solar Cells
Open this publication in new window or tab >>Mechanistic Insights into Solid-State p-Type Dye-Sensitized Solar Cells
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2019 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, no 43, p. 26151-26160Article in journal (Refereed) Published
Abstract [en]

The study of p-type dye sensitized solar cells (p-DSCs) is appealing but challenging. Although the devices have been studied for 20 years, the light conversion efficiency lags far behind those of n-DSCs. Very recently, on the basis of a core-shell structure, a novel solid-state p-DSC (p-ssDSCs) has been fabricated, which showed great enhancement in open-circuit voltage and dye regeneration rate. To further improve the performance of such devices, charge diffusion, recombination process, and the main limiting factors have to be understood. In the present paper, core-shell p-ssDSCs with ZnO as an electron conductor were fabricated by atomic layer deposition. The charge transport time was determined to be ca. 0.1 ms, which is about 2 orders of magnitude faster than those of typical liquid devices with I-/I-3(-) as a redox mediator. As a consequence, the devices exhibit the highest reported charge diffusion coefficient (D-d)' among p-DSCs. It is ascribed to an electron-limiting diffusion process by the ambipolar diffusion model, suggesting a different charge-transport-determining mechanism in contrast to liquid p-DSCs. The charge recombination rate is 1-2 orders of magnitude slower than its charge transport time, mandating that the estimated charge collection efficiency is near unity. Detailed analysis of the incident photon-to-electron conversion efficiency suggests that the energy conversion efficiency in these p-ssDSCs is currently limited by a large fraction of dyes that is not fully electrically connected in the device.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2019
National Category
Physical Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-397591 (URN)10.1021/acs.jpcc.9b08251 (DOI)000493865700013 ()
Available from: 2019-11-25 Created: 2019-11-25 Last updated: 2020-01-15Bibliographically approved
5. Rethinking the function of surface states on mesoporous NiO films
Open this publication in new window or tab >>Rethinking the function of surface states on mesoporous NiO films
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-402255 (URN)
Available from: 2020-01-13 Created: 2020-01-13 Last updated: 2020-01-22

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