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Electronic Structures and Energy Level Alignment in Mesoscopic Solar Cells: A Hard and Soft X-ray Photoelectron Spectroscopy Study
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
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 [en]
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: urn:nbn:se:uu:diva-221450ISBN: 978-91-554-8921-2 (print)OAI: oai:DiVA.org:uu-221450DiVA: diva2:709238
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
List of papers
1. Preventing Dye Aggregation on ZnO by Adding Water in the Dye-Sensitization Process
Open this publication in new window or tab >>Preventing Dye Aggregation on ZnO by Adding Water in the Dye-Sensitization Process
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2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 39, 19274-19279 p.Article in journal (Refereed) Published
Abstract [en]

ZnO based dye-sensitized solar cells have been studied using N719 and Z-907 as sensitizing dyes, with and without including water to the dye solution. The solar cells have been characterized with photoelectric measurements and the interface between the dye and the ZnO surface has been studied using photoelectron spectroscopy. It was shown that water in the dye solution greatly reduces surface dye aggregation and thereby enhances the solar cell performance for N719. For Z-907 where no sign of dye aggregation could be found, the presence of water had minor effect on the surface structure and solar cell performance.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-160379 (URN)10.1021/jp206052t (DOI)000295245500046 ()
Available from: 2011-10-24 Created: 2011-10-24 Last updated: 2017-12-08Bibliographically approved
2. Influence of Water on the Electronic: and Molecular Surface Structures of Ru-Dyes at Nanostructured TiO2
Open this publication in new window or tab >>Influence of Water on the Electronic: and Molecular Surface Structures of Ru-Dyes at Nanostructured TiO2
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2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 24, 11996-12004 p.Article in journal (Refereed) Published
Abstract [en]

The influence of water on the surface electronic and molecular properties of three Ru-dyes adsorbed at nanostructured TiO2 was investigated using photoelectron spectroscopy (PES). The dyes investigated were the Ru(dcbpy)(2)(NCS)(2) in its acid (N3) and in its 2-fold deprotonated form (N719) as well as a similar dye (Z-907) containing the hydrophobic ligand 4,4'-dinonyl-2,2'-bipyridine. Trends in surface structures depending on water exposure were followed for the three dyes. The results showed that the hydrophobic chains of the Z-907 dye effectively inhibit surface reorganization while large changes in surface electronic and molecular structure were observed for the N3 and N719 molecular layers. Specifically, large effects involving the thiocyanate ligands were detected, and the S2p and N Is core level spectra indicate that the changes involve mixing of only two dominating surface configurations. Moreover, the PES results also showed water-induced changes in the energy level matching between the dye and the TiO2, and water induced desorption of the TBA(+) counterion. Basic photoelectrochemical trends depending on water exposure to dye-sensitized solar cell systems were also verified.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-155932 (URN)10.1021/jp1076609 (DOI)000291709600016 ()
Available from: 2011-07-04 Created: 2011-07-04 Last updated: 2017-12-11Bibliographically approved
3. Energy level alignment in TiO2/metal sulfide/polymer interfaces for solar cell applications
Open this publication in new window or tab >>Energy level alignment in TiO2/metal sulfide/polymer interfaces for solar cell applications
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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
Keyword
Photoelectron spectroscopy, semiconductor sensitised solar cells, metal xhanthate, Sb2S3
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-221439 (URN)10.1039/C4CP01581A (DOI)000340353000025 ()
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
4. Electronic Structure of TiO2/CH3NH3PbI3 Perovskite Solar Cell Interfaces
Open this publication in new window or tab >>Electronic Structure of TiO2/CH3NH3PbI3 Perovskite Solar Cell Interfaces
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2014 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 5, no 4, 648-653 p.Article in journal (Refereed) Published
Abstract [en]

The electronic structure and chemical composition of efficient CH3NH3PbI3 perovskite solar cell materials deposited onto mesoporous TiO2 were studied using photoelectron spectroscopy with hard X-rays. With this technique, it is possible to directly measure the occupied energy levels of the perovskite as well as the TiO2 buried beneath and thereby determine the energy level matching of the interface. The measurements of the valence levels were in good agreement with simulated density of states, and the investigation gives information on the character of the valence levels. We also show that two different deposition techniques give results indicating similar electronic structures.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-220599 (URN)10.1021/jz402749f (DOI)000331860900002 ()
Available from: 2014-03-17 Created: 2014-03-17 Last updated: 2017-12-05Bibliographically approved
5. The electronic structure of CH3NH3PbX3 perovskites; the dependence on the halide moeity
Open this publication in new window or tab >>The electronic structure of CH3NH3PbX3 perovskites; the dependence on the halide moeity
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(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-221448 (URN)
Available from: 2014-03-31 Created: 2014-03-31 Last updated: 2016-04-20
6. Energy level alignment in TiO(2)/dipole-molecule/P3HT interfaces
Open this publication in new window or tab >>Energy level alignment in TiO(2)/dipole-molecule/P3HT interfaces
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2011 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 515, no 1-3, 146-150 p.Article in journal (Refereed) Published
Abstract [en]

Controlling the energy levels at the interface between an inorganic and an organic material is of importance to improve the properties in devices based on such hybrid interfaces, and can be obtained by the incorporation of dipole molecules between the materials. In this report interfaces containing TiO(2), a dipole molecule (benzoic acid or 4-nitrobenzoic acid) and a polymer, poly(3-hexylthiophene) (P3HT) were investigated using high kinetic energy photoelectron spectroscopy. We could successfully measure through all materials in the fully assembled systems, and thereby experimentally quantify the dipole induced change in the energy level alignment of the polymer and the TiO(2).

National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-162364 (URN)10.1016/j.cplett.2011.09.014 (DOI)000296551300027 ()
Available from: 2011-11-30 Created: 2011-11-30 Last updated: 2017-12-08Bibliographically approved
7. Energy Level Shifts in Spiro-OMeTAD Molecular Thin Films When Adding Li-TFSI
Open this publication in new window or tab >>Energy Level Shifts in Spiro-OMeTAD Molecular Thin Films When Adding Li-TFSI
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2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 50, 26300-26305 p.Article in journal (Refereed) Published
Abstract [en]

Hard X-ray photoelectron spectroscopy (HAXPES) has been used to study the effects of adding Li-TFSI to hole conducting molecular thin films of 2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD). The work shows that a procedure of mixing a Li-TFSI solution into a spiro-OMeTAD solution, and subsequent spin-coating this mixture into a solid thin film causes the Fermi level of the molecular film to move closer to the HOMO level. Hence, adding the Li-TFSI gives similar effects to spiro-OMeTAD as a p-dopant. Specific effects from doping on the valence levels were also characterized. Absorbance measurements also showed that the spiro-OMeTAD film was partially oxidized when Li-TFSI was added before spin-coating. By varying the photon energy in the photoelectron spectroscopy measurements, the probe depth varies between being surface sensitive (<1 nm) and bulk sensitive (inelastic mean free path >= 10 nm). This property was used to follow differences in the composition at different depth of the spiro-OMeTAD/Li-TFSI film. It could be concluded that there was a concentration gradient in the molecular film and that the concentration of Li-TFSI was dominating at the interface between the spiro-OMeTAD/Li-TFSI film and vacuum.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-220596 (URN)10.1021/jp306433g (DOI)
Available from: 2014-03-17 Created: 2014-03-17 Last updated: 2017-12-05Bibliographically approved
8. Controlling energy level positions in hole conducting molecular films by additives
Open this publication in new window or tab >>Controlling energy level positions in hole conducting molecular films by additives
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(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-221447 (URN)
Available from: 2014-03-31 Created: 2014-03-31 Last updated: 2016-04-20
9. Reactive ZnO/Ti/ZnO interfaces studied by hard x-ray photoelectron spectroscopy
Open this publication in new window or tab >>Reactive ZnO/Ti/ZnO interfaces studied by hard x-ray photoelectron spectroscopy
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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.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-220603 (URN)10.1063/1.4854636 (DOI)000331210800061 ()
Available from: 2014-03-17 Created: 2014-03-17 Last updated: 2017-12-05Bibliographically approved

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