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Energy level alignment in TiO2/metal sulfide/polymer interfaces for solar cell applications
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: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 32, 17099-17107 p.Article in journal (Refereed) Published
Abstract [en]

Semiconductor sensitized solar cell interfaces have been studied with photoelectron spectroscopy to understand the interfacial electronic structures. In particular, the experimental energy level alignment has been determined for complete TiO2/metal sulfide/polymer interfaces. For the metal sulfides CdS, Sb2S3 and Bi2S3 deposited from single source metal xanthate precursors, it was shown that both driving forces for electron injection into TiO2 and hole transfer to the polymer decrease for narrower bandgaps. The energy level alignment results were used in the discussion of the function of solar cells with the same metal sulfides as light absorbers. For example Sb2S3 showed the most favourable energy level alignment with 0.3 eV driving force for electron injection and 0.4 eV driving force for hole transfer and also the most efficient solar cells due to high photocurrent generation. The energy level alignment of the TiO2/Bi2S3 interface on the other hand showed no driving force for electron injection to TiO2, and the performance of the corresponding solar cell was very low.

Place, publisher, year, edition, pages
The Royal Society of Chemistry , 2014. Vol. 16, no 32, 17099-17107 p.
Keyword [en]
Photoelectron spectroscopy, semiconductor sensitised solar cells, metal xhanthate, Sb2S3
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:uu:diva-221439DOI: 10.1039/C4CP01581AISI: 000340353000025OAI: oai:DiVA.org:uu-221439DiVA: diva2:709097
Funder
Swedish Research Council, 20124721EU, FP7, Seventh Framework Programme, 308997
Available from: 2014-03-31 Created: 2014-03-31 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)
Opponent
Supervisors
Available from: 2014-04-29 Created: 2014-03-31 Last updated: 2014-06-30

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Publisher's full texthttp://dx.doi.org/10.1039/C4CP01581A

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Lindblad, RebeckaCappel, UteSiegbahn, HansJohansson, Erik M. J.Rensmo, Håkan

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