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Reactive ZnO/Ti/ZnO interfaces studied by hard x-ray photoelectron spectroscopy
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
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2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, no 4, 043714-043714-7 p.Article in journal (Refereed) Published
Abstract [en]

The chemistry and intermixing at buried interfaces in sputter deposited ZnO/Ti/ZnO thin layers were studied by hard x-ray photoelectron spectroscopy. The long mean free path of the photoelectrons allowed for detailed studies of the oxidation state, band bending effects, and intrinsic doping of the buried interfaces. Oxidation of the Ti layer was observed when ZnO was deposited on top. When Ti is deposited onto ZnO, Zn Auger peaks acquire a metallic character indicating a strong reduction of ZnO at the interface. Annealing of the stack at 200 °C results in further reduction of ZnO and oxidation of Ti. Above 300 °C, oxygen transport from the bulk of the ZnO layer takes place, leading to re-oxidation of ZnO at the interface and further oxidation of Ti layer. Heating above 500 °C leads to an intermixing of the layers and the formation of a ZnxTiOy compound.

Place, publisher, year, edition, pages
2014. Vol. 115, no 4, 043714-043714-7 p.
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:uu:diva-220603DOI: 10.1063/1.4854636ISI: 000331210800061OAI: oai:DiVA.org:uu-220603DiVA: diva2:705748
Available from: 2014-03-17 Created: 2014-03-17 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Electronic Structures and Energy Level Alignment in Mesoscopic Solar Cells: A Hard and Soft X-ray Photoelectron Spectroscopy Study
Open this publication in new window or tab >>Electronic Structures and Energy Level Alignment in Mesoscopic Solar Cells: A Hard and Soft X-ray Photoelectron Spectroscopy Study
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Photoelectron spectroscopy is an experimental method to study the electronic structure in matter. In this thesis, a combination of soft and hard X-ray based photoelectron spectroscopy has been used to obtain atomic level understanding of electronic structures and energy level alignments in mesoscopic solar cells. The thesis describes how the method can be varied between being surface and bulk sensitive and how to follow the structure linked to particular elements. The results were discussed with respect to the material function in mesoscopic solar cell configurations.

The heart of a solar cell is the charge separation of photoexcited electrons and holes, and in a mesoscopic solar cell, this occurs at interfaces between different materials. Understanding the energy level alignment between the materials is important for developing the function of the device. In this work, it is shown that photoelectron spectroscopy can be used to experimentally follow the energy level alignment at interfaces such as TiO2/metal sulfide/polymer, as well as TiO2/perovskite.

The electronic structures of two perovskite materials, CH3NH3PbI3 and CH3NH3PbBr3 were characterized by photoelectron spectroscopy and the results were discussed with support from quantum chemical calculations. The outermost levels consisted mainly of lead and halide orbitals and due to a relatively higher cross section for heavier elements, hard X-ray excitation was shown useful to study the position as well as the orbital character of the valence band edge.

Modifications of the energy level positions can be followed by core level shifts. Such studies showed that a commonly used additive in mesoscopic solar cells, Li-TFSI, affected molecular hole conductors in the same way as a p-dopant. A more controlled doping can also be achieved by redox active dopants such as Co(+III) complexes and can be studied quantitatively with photoelectron spectroscopy methods.

Hard X-rays allow studies of hidden interfaces, which were used to follow the oxidation of Ti in stacks of thin films for conducting glass. By the use of soft X-rays, the interface structure and bonding of dye molecules to mesoporous TiO2 or ZnO could be studied in detail. A combination of the two methods can be used to obtain a depth profiling of the sample. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 87 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1135
Keyword
Photoelectron spectroscopy, HAXPES, PES, XPS, electronic structure, energy level alignment, mesoscopic solar cell, hole conductor, perovskite, dye-sensitized, semiconductor-sensitized, TiO2, ZnO, spiro-OMeTAD, P3HT, DEH, metal sulfide, Li-TFSI, Co(+III) complex
National Category
Atom and Molecular Physics and Optics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-221450 (URN)978-91-554-8921-2 (ISBN)
Public defence
2014-05-23, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
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Supervisors
Available from: 2014-04-29 Created: 2014-03-31 Last updated: 2014-06-30

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Publisher's full texthttp://scitation.aip.org/content/aip/journal/jap/115/4/10.1063/1.4854636

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Knut, RonnyLindblad, RebeckaRensmo, HåkanKaris, Olof

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