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Chemical and Physical Reduction of High Valence Ni States in Mesoporous NiO Film for Solar Cell Application.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström. (Leif Hammarström)ORCID iD: 0000-0003-0510-5541
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
Newcastle University.
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2017 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252Article in journal (Refereed) Published
Abstract [en]

The most common material for dye-sensitized photocathodes is mesoporous NiO. We transformed the usual brownish NiO to be more transparent by reducing high valence Ni impurities. Two pretreatment methods have been used: chemical reduction by NaBH4 and thermal reduction by heating. The power conversion efficiency of the cell was increased by 33% through chemical treatment, and an increase in open-circuit voltage from 105 to 225 mV was obtained upon heat treatment. By optical spectroelectrochemistry, we could identify two species with characteristically different spectra assigned to Ni3+ and Ni4+. We suggest that the reduction of surface Ni3+ and Ni4+ to Ni2+ decreases the recombination reaction between holes on the NiO surface with the electrolyte. It also keeps the dye firmly on the surface, building a barrier for electrolyte recombination. This causes an increase in open-circuit photovoltage for the treated film.

Place, publisher, year, edition, pages
2017.
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-320184DOI: 10.1021/acsami.7b01532OAI: oai:DiVA.org:uu-320184DiVA: diva2:1088877
Available from: 2017-04-17 Created: 2017-04-17 Last updated: 2017-04-17
In thesis
1. Discovering Hidden Traps: in Nickel Oxide Nanoparticles for Dye-Sensitised Photocathodes
Open this publication in new window or tab >>Discovering Hidden Traps: in Nickel Oxide Nanoparticles for Dye-Sensitised Photocathodes
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The finite nature of fossil fuels and their effect on the global climate, raised the need to find an alternative source of energy. This source should be environment compatible, cheap and abundant. The light coming from the Sun is a promising alternative. To be fruitful, the solar energy needs to be transformed in storable and transportable energy forms like electricityor fuels. Amongst the most studied techniques dye sensitised devices offer the possibility to be designed for both the scopes: solar-to-electricity and solar-to-fuel conversions. In these applications a photocathode and a photoanode, constructed by mesoporous semisconductor films sensitised with dyes, are placed in series with one another.It follows that the photocurrent generated by one electrode should be sustained by the photocurrent produced by the other electrode. At the moment there is a substantial difference between the conversion efficiencies and the photocurrent produced by photoanodes and photocathodes. In this thesis the reasons for this discrepancy are investigated. The main responsible of the bad performance is identified in the semiconductor normally used in photocathodes, Nickel Oxide (NiO). Electrochemical impedance spectroscopy was used to elucidate the electrical properties of mesoporous NiO films. The study revealed that NiO films are able to carry a large enough current to establish that conductivity is not a limiting factor. The recombination reactions were then accused as the cause of the power losses. A time resolved spectroscopic study revealed that NiO can host two kinds of holes. One of these holes is responsible for a fast dye-NiO recombination (100 ns) and the other one for a slow recombination (10 ms). A cell featuring only the slow dye-NiO recombination would possibly reach high efficiency. The characterisation of the species associated with these two holes was performed by density-of-state assisted spectroelectrochemistry. The holes were found to be trapped by Ni2+ and Ni3+ sites located on the NiO surface forming respectively Ni3+ and Ni4+ states. A study by fs and ns transient absorption spectroscopy revealed that Ni3+ sites can trap a hole in subpicosecond time scale and this hole relaxes into a Ni2+ trap in ns timescale. The control of the Ni2+/Ni3+ratio on the NiO surface was found  to be crucial for a high cell photovoltage. In the thesis these results are discussed and used to propose an explanation and some solutions to the poor performance of NiO-based dye sensitised cells.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 95 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1515
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-320187 (URN)978-91-554-9911-2 (ISBN)
Public defence
2017-06-07, Häggsalen, Ångströmlab, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2017-05-16 Created: 2017-04-17 Last updated: 2017-06-07

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