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Discovering Hidden Traps: in Nickel Oxide Nanoparticles for Dye-Sensitised Photocathodes
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. (Leif Hammarström)ORCID iD: 0000-0003-0510-5541
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: urn:nbn:se:uu:diva-320187ISBN: 978-91-554-9911-2 (print)OAI: oai:DiVA.org:uu-320187DiVA: diva2:1088887
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
List of papers
1. Tuning of Conductivity and Density of States of NiO Mesoporous Films Used in p-Type DSSCs
Open this publication in new window or tab >>Tuning of Conductivity and Density of States of NiO Mesoporous Films Used in p-Type DSSCs
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 34, 19556-19564 p.Article in journal (Refereed) Published
Abstract [en]

Nickel oxide has been used as the mesoporous electrode material for p-type dye sensitized solar cell (DSSC) for many years, but no high efficiency cells have been obtained yet. The poor results are commonly attributed to the lack of conductivity of the NiO film. In this paper we studied the electrical conduction of NiO mesoporous film with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). We used unsensitized NiO on FTO as an electrode with no dye adsorbed on the surface. Tests made with a DSSC device-like cell (FTO-Pt-I-/I-3(-)-NiO-FTO) showed a surprisingly high Faradaic current (20 mA/cm(-2) at 1 V), proving a good electrical conductivity of mesoporous NiO. We also used lithium as dopant to improve the electrical properties of the film. The Li-doping resulted in widening the inert (not conductive) window in the CV plot. The EIS analysis clarified that this behavior is due to a strong dependence of the valence band shape and position with respect to the Li-doping concentration. Our results show that DSSC performance does not need to be limited by the conductivity of mesoporous NiO, which encourages more effort in p-type DSSC research based on this material.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-233013 (URN)10.1021/jp504551v (DOI)000341122200006 ()
Available from: 2014-10-08 Created: 2014-09-29 Last updated: 2017-04-17Bibliographically approved
2. Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells
Open this publication in new window or tab >>Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells
Show others...
2015 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 6, no 5, 779-783 p.Article in journal (Refereed) Published
Abstract [en]

Mesoporous nickel oxide has been used as electrode material for p-type dye-sensitized solar cells (DSCs) for many years but no high efficiency cells have yet been obtained. One of the main issues that lowers the efficiency is the poor fill factor, for which a clear reason is still missing. In this paper we present the first evidence for a relation between applied potential and the charge recombination rate of the NiO electrode. In particular, we find biphasic recombination kinetics: a fast (15 ns) pathway attributed to the reaction with the holes in the valence band and a slow (1 ms) pathway assigned to the holes in the trap states. The fast component is the most relevant at positive potentials, while the slow component becomes more important at negative potentials. This means that at the working condition of the cell, the fast recombination is the most important. This could explain the low fill factor of NiO-based DSCs.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-251431 (URN)10.1021/acs.jpclett.5b00048 (DOI)000350843400005 ()
Available from: 2015-04-23 Created: 2015-04-17 Last updated: 2017-04-17Bibliographically approved
3. Chemical and Physical Reduction of High Valence Ni States in Mesoporous NiO Film for Solar Cell Application.
Open this publication in new window or tab >>Chemical and Physical Reduction of High Valence Ni States in Mesoporous NiO Film for Solar Cell Application.
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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.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-320184 (URN)10.1021/acsami.7b01532 (DOI)
Available from: 2017-04-17 Created: 2017-04-17 Last updated: 2017-04-17
4. Unveiling hole trapping and surface dynamics of NiO nanoparticles.
Open this publication in new window or tab >>Unveiling hole trapping and surface dynamics of NiO nanoparticles.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The research effort in mesoporous p-type semiconductors is increasing due to their potential application in photoelectrochemical energy conversion devices. In this paper an electron-hole pair is created by band-gap excitation of NiO nanoparticles and the dynamics of the electron and the hole is followed until their recombination. By spectroscopic characterization it was found that surface Ni3+ states work as traps for both electrons and holes. The trapped electron was assigned to a N2+ state and the trapped hole to a Ni4+ state. The recombination kinetics of these traps was studied and related with the concept of hole relaxation suggested before.The timescale of the hole relaxation was foundto be in the order of tens of ns. Finally the spectrosc opic evidence of this relaxation is presented in a sensitized film.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-320185 (URN)
Available from: 2017-04-17 Created: 2017-04-17 Last updated: 2017-04-17

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