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Chemical and Physical Reduction of High Valence Ni States in Mesoporous NiO Film for Solar Cell Application.
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Kemiska sektionen, Institutionen för kemi - Ångström, Fysikalisk kemi. (Leif Hammarström)ORCID-id: 0000-0003-0510-5541
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Kemiska sektionen, Institutionen för kemi - Ångström, Fysikalisk kemi.
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Molekyl- och kondenserade materiens fysik.
Newcastle Univ, Sch Chem, Newcastle Upon Tyne NE1 7RU, Tyne & Wear, England.
Vise andre og tillknytning
2017 (engelsk)Inngår i: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 9, nr 39, s. 33470-33477Artikkel i tidsskrift (Fagfellevurdert) 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.

sted, utgiver, år, opplag, sider
2017. Vol. 9, nr 39, s. 33470-33477
HSV kategori
Identifikatorer
URN: urn:nbn:se:uu:diva-320184DOI: 10.1021/acsami.7b01532ISI: 000412717600014PubMedID: 28368109OAI: oai:DiVA.org:uu-320184DiVA, id: diva2:1088877
Forskningsfinansiär
Swedish Energy Agency, 43599-1Knut and Alice Wallenberg Foundation, 2011.0067Tilgjengelig fra: 2017-04-17 Laget: 2017-04-17 Sist oppdatert: 2018-01-10bibliografisk kontrollert
Inngår i avhandling
1. Discovering Hidden Traps: in Nickel Oxide Nanoparticles for Dye-Sensitised Photocathodes
Åpne denne publikasjonen i ny fane eller vindu >>Discovering Hidden Traps: in Nickel Oxide Nanoparticles for Dye-Sensitised Photocathodes
2017 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
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.

sted, utgiver, år, opplag, sider
Uppsala: Acta Universitatis Upsaliensis, 2017. s. 95
Serie
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1515
HSV kategori
Identifikatorer
urn:nbn:se:uu:diva-320187 (URN)978-91-554-9911-2 (ISBN)
Disputas
2017-06-07, Häggsalen, Ångströmlab, Uppsala, 13:15 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2017-05-16 Laget: 2017-04-17 Sist oppdatert: 2017-10-13

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