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Sehati, Parisa
Publications (9 of 9) Show all publications
Sehati, P., Braun, S. & Fahlman, M. (2013). Energy level alignment in Au/pentacene/PTCDA trilayer stacks. Chemical Physics Letters, 583, 38-41
Open this publication in new window or tab >>Energy level alignment in Au/pentacene/PTCDA trilayer stacks
2013 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 583, p. 38-41Article in journal (Refereed) Published
Abstract [en]

Ultraviolet photoelectron spectroscopy is used to investigate the energy level alignment and molecular orientation at the interfaces in Au/pentacene/PTCDA trilayer stacks. We deduced a standing orientation for pentacene grown on Au while we conclude a flat lying geometry for PTCDA grown onto pentacene. We propose that the rough surface of polycrystalline Au induces the standing geometry in pentacene. It is further shown that in situ deposition of PTCDA on pentacene can influence the orientation of the surface pentacene layer, flipping part of the surface pentacene molecules into a flat lying geometry, maximizing the orbital interaction across the pentacene–PTCDA heterojunction.

Place, publisher, year, edition, pages
Elsevier, 2013
Keywords
Ultraviolet Photoelectron Spectroscopy (UPS), Orientation dependent Ionization Potential, Pentacene, PTCDA
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-81061 (URN)10.1016/j.cplett.2013.07.035 (DOI)000324304600008 ()
Available from: 2012-09-06 Created: 2012-09-06 Last updated: 2017-12-07Bibliographically approved
Fahlman, M., Sehati, P., Osikowicz, W., Braun, S., de Jong, M. P. & Brocks, G. (2013). Photoelectron spectroscopy and modeling of interface properties related to organic photovoltaic cells. Journal of Electron Spectroscopy and Related Phenomena, 190, 33-41
Open this publication in new window or tab >>Photoelectron spectroscopy and modeling of interface properties related to organic photovoltaic cells
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2013 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 190, p. 33-41Article in journal (Refereed) Published
Abstract [en]

In this short review, we will give examples on how photoelectron spectroscopy (PES) assisted by models on interface energetics can be used to study properties important to bulk heterojunction type organic photovoltaic devices focusing on the well-known bulk heterojunction blend of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and its model system P3HT:C-60. We also will discuss some of the limitations of PES as applied to organic semiconductors (OS) and photovoltaic devices and finish with reviewing recent theoretical advances that now enable calculation of relevant parameters at (hybrid) interfaces measured by PES.

Place, publisher, year, edition, pages
Elsevier, 2013
Keywords
Photoelectron spectroscopy; Photovoltaics; Organic semiconductors; Interfaces
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-103884 (URN)10.1016/j.elspec.2013.02.001 (DOI)000329376800005 ()
Available from: 2014-01-30 Created: 2014-01-30 Last updated: 2017-12-06
Sehati, P. (2012). Influence of intermolecular order at the interfaces. (Doctoral dissertation). Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>Influence of intermolecular order at the interfaces
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis covers a range of different surfaces and interfaces of organic molecules/polymers and metallic materials. It is of vita importance to understand how charge transfer processes and other electrical interactions existing at physisorped contacts can influence the electronic structure at an interface. Hence our mission in these studies was to understand the physics happening at the aforementioned surfaces and interfaces of relevance to electronic devices, mainly solar cells.

In order to explain the observed measurements at physisorped interfaces, a model previously has been put forward. The Integer Charge Transfer (ICT) model explains why and how charge transfer can occur at an interface consisting of π-conjugated molecules/ polymers and a metallic/semi-conducting substrate. The main output of this model is depicted in a typical curve and considers two different regimes, namely when charge transfer occurs and when it doesn’t happen. This curve is obtained through measuring work function of the organic semi-conducting material atop of different substrates covering a wide span of work functions. The measured work function of the organicsubstrate is visualized as the vertical axis of a graph with the substrate work function as the horizontal axis. The ultimate curve then resembles the famous, well-known “mark of Zorro”. The part with slope=1 is an indication of the vacuum level alignment. In plain text it means there is no charge transfer between deposited/ span material and substrate. On the other hand, part of the curve with slope=0 is a measure of Fermi-level pinning and occurrence of charge transfer. The threshold point between these two regimes is called pinning point which either implies ICT+ (positive Integer Charge Transfer state) or ICT- (negative Integer Charge Transfer state). It is crucial to realize that a π-conjugated molecule can be both acceptor or donor depending on the substrate work function.

Our efforts were aimed toward further improving this model and understanding impacting factors on charge transfer and the pinning point. Factors such as interface dipole and inter- and intra-molecular order are among the most important ones. Order and packing of the spin-coated and vacuumdeposited materials thus directly affect the position of the pinning point/Integer Charge Transfer states. Therefore any parameter which can influence order/packing can affect the interface dipole and resulting energy level alignment. Intra-molecular order, for example, can be tuned by annealing (twisting of the conjugated chains, crystal grain growth) while thickness of the deposited material also can modify intermolecular packing. All those factors were studied in the context of this thesis, mainly using materials common in socalled bulk heterojunction solar cells. Another way to tune the energy level alignment at interfaces is through Self-Assembled Molecules (SAMs). We have utilized such molecules to manipulate the surface of zinc oxide, a common ingredient of transparent solar cells, studying the effects on work function and therefore charge injection properties.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. p. 46
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1468
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-81066 (URN)978-91-7519-821-7 (ISBN)
Public defence
2012-09-27, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2012-09-06 Created: 2012-09-06 Last updated: 2019-12-10Bibliographically approved
Sehati, P. (2012). Manipulation of ZnO work function upon deposition of 4-TBP molecule.
Open this publication in new window or tab >>Manipulation of ZnO work function upon deposition of 4-TBP molecule
2012 (English)Manuscript (preprint) (Other academic)
Abstract [en]

ZnO is a multi dimensional material, which can be used as semiconductor, as pigment (zinc white) for coloring paper as sensor for hydrogen gas and has attracted much attention as a potential complement or even substitute to GaN. Furthermore, since it is photochemically stable, transparent in the visible region and it is largely available in the single crystal form it profitably can be used as an electrode and/or n-type semiconductor in diodes and photovoltaic cells. In order to optimize the performance of ZnO is such applications one need to tune zinc oxide’s work function (and surface energy), which potentially can be achieved by adsorption of thin interfacial layers. In this study, we investigated modification of work function of zinc oxide’s polar surfaces, ZnO(0001), and the interactions between those surfaces and monolayer and multilayer of an organic molecule, belonging to Self Assembled Molecules (SAM) family. We observed similar final work function of Zn-terminated and O-terminated zinc oxide after one monolayer deposition by SAM molecule despite the fact that Zn-terminated surfaces exhibits different chemical properties and work function than O-terminated surfaces.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-81062 (URN)
Note

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Available from: 2012-09-06 Created: 2012-09-06 Last updated: 2012-09-06Bibliographically approved
Sehati, P., Çakır, D., Brocks, G., Fahlman, M. & Braun, S. (2012). Tuning low work function contacts using molecular donor layers: intermolecular order effects.
Open this publication in new window or tab >>Tuning low work function contacts using molecular donor layers: intermolecular order effects
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2012 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Low work function cathodes were created by physisorbed multilayers of acridine orange base and leuco crystal violet on a variety of electrode materials. The resulting work functions have been studied experimentally using photoelectron spectroscopy and calculated using density functional theory and the Integer Charge Transfer model. The resulting cathodes yield work functions in the range between 3.95 eV to 4.3 eV for leuco crystal violet and between 3.2 eV to 4.43 eV for acridine orange base. The range of possible work functions are in excellent agreement with the values predicted for the respective molecules by the ICT model, where intermolecular order and intrinsic molecular dipole moments may strongly influence the socalled integer charge transfer energies and hence the resulting work function at an (hybrid) interface.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-81063 (URN)
Note

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Available from: 2012-09-06 Created: 2012-09-06 Last updated: 2012-09-06Bibliographically approved
Davis, R. J., Lloyd, M. T., Ferreira, S. R., Bruzek, M. J., Watkins, S. E., Lindell, L., . . . Hsu, J. W. (2011). Determination of energy level alignment at interfaces of hybrid and organic solar cells under ambient environment. JOURNAL OF MATERIALS CHEMISTRY, 21(6), 1721-1729
Open this publication in new window or tab >>Determination of energy level alignment at interfaces of hybrid and organic solar cells under ambient environment
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2011 (English)In: JOURNAL OF MATERIALS CHEMISTRY, ISSN 0959-9428, Vol. 21, no 6, p. 1721-1729Article in journal (Refereed) Published
Abstract [en]

Device function in organic electronics is critically governed by the transport of charge across interfaces of dissimilar materials. Accurate measurements of energy level positions in organic electronic devices are therefore necessary for assessing the viability of new materials and optimizing device performance. In contrast to established methods that are used in solution or vacuum environments, here we combine Kelvin probe measurements performed in ambient environments to obtain work function values with photoelectron spectroscopy in air to obtain ionization potential, so that a complete energy level diagram for organic semiconductors can be determined. We apply this new approach to study commonly used electron donor and acceptor materials in organic photovoltaics (OPV), including poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and ZnO, as well as examine new materials. Band alignments across the entire OPV devices are constructed and compared with actual device performance. The ability to determine interfacial electronic properties in the devices enables us to answer the outstanding question: why previous attempts to make OPV devices using 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene as the electron donor were not successful.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-65942 (URN)10.1039/c0jm02349c (DOI)000286614100014 ()
Available from: 2011-02-28 Created: 2011-02-28 Last updated: 2015-05-13
Aarnio, H., Sehati, P., Braun, S., Nyman, M., de Jong, M. P., Fahlman, M. & Osterbacka, R. (2011). Spontaneous Charge Transfer and Dipole Formation at the Interface Between P3HT and PCBM. ADVANCED ENERGY MATERIALS, 1(5), 792-797
Open this publication in new window or tab >>Spontaneous Charge Transfer and Dipole Formation at the Interface Between P3HT and PCBM
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2011 (English)In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 1, no 5, p. 792-797Article in journal (Refereed) Published
Abstract [en]

In the pursuit of developing new materials for more efficient bulk-heterojunction solar cells, the blend poly (3-hexylthiophene):[ 6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) serves as an important model system. The success of the P3HT: PCBM blend comes from efficient charge generation and transport with low recombination. There is not, however, a good microscopic picture of what causes these, hindering the development of new material systems. In this report UV photoelectron spectroscopy measurements on both regiorandom-(rra) and regioregular-(rr) P3HT are presented, and the results are interpreted using the Integer Charge Transfer model. The results suggest that spontaneous charge transfer from P3HT to PCBM occurs after heat treatment of P3HT: PCBM blends. The resulting formation of an interfacial dipole creates an extra barrier at the interface explaining the reduced (non-)geminate recombination with increased charge generation in heat treated rr-P3HT: PCBM blends. Extensive photoinduced absorption measurements using both above-and below-bandgap excitation light are presented, in good agreement with the suggested dipole formation.

Place, publisher, year, edition, pages
Wiley-VCH Verlag Berlin, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-71375 (URN)10.1002/aenm.201100074 (DOI)000295140100013 ()
Note

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Available from: 2011-10-14 Created: 2011-10-14 Last updated: 2013-04-24
Sehati, P., Braun, S., Lindell, L., Liu, X., Andersson, L. M. & Fahlman, M. (2010). Energy-Level Alignment at Metal-Organic and Organic-Organic Interfaces in Bulk-Heterojunction Solar Cells. IEEE Journal of Selected Topics in Quantum Electronics, 16(6), 1718-1724
Open this publication in new window or tab >>Energy-Level Alignment at Metal-Organic and Organic-Organic Interfaces in Bulk-Heterojunction Solar Cells
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2010 (English)In: IEEE Journal of Selected Topics in Quantum Electronics, ISSN 1077-260X, E-ISSN 1558-4542, Vol. 16, no 6, p. 1718-1724Article in journal (Refereed) Published
Abstract [en]

Ultraviolet photoelectron spectroscopy measurements in combination with the integer charge transfer (ICT) model is used to obtain the energy-level alignment diagrams for two common types of bulk-heterojunction solar cell devices based on poly(3-hexylthiophene) or poly(2-methoxy-5-(3,7 -dimethyloctyloxy)- 1,4-phenylene vinylene) as the donor polymer and (6,6)phenyl- C61-butric-acid as the acceptor molecule. A ground-state interface dipole at the donor/acceptor heterojunction is present for both systems, but the origin of the interface dipole differs, quadrupole-induced in the case of poly(2-methoxy-5-(3,7-dimethyl-octyloxy)-1,4-phenylene vinylene), and ICT state based for poly(3-hexylthiophene). The presence of bound electron-hole charge carriers (CT states) and/or interface dipoles are expected to enhance exciton dissociation into free charge carriers, thus reducing the probability that charges become trapped by Coulomb forces at the interface followed by recombination.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2010
Keywords
Interfaces, organic electronics, photoelectron, spectroscopy, solar cells
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-67163 (URN)10.1109/JSTQE.2010.2042684 (DOI)000288488400027 ()
Note

©2011 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. Parisa Sehati, Slawomir Braun, Linda Lindell, Xianjie Liu, Lars Mattias Andersson and Mats Fahlman, Energy-Level Alignment at Metal-Organic and Organic-Organic Interfaces in Bulk-Heterojunction Solar Cells, 2010, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, (16), 6, 1718-1724. http://dx.doi.org/10.1109/JSTQE.2010.2042684

Available from: 2011-04-01 Created: 2011-04-01 Last updated: 2017-12-11Bibliographically approved
Wadeasa, A., Tzamalis, G., Sehati, P., Nour, O., Fahlman, M., Willander, M., . . . Crispin, X. (2010). Solution processed ZnO nanowires/polyfluorene heterojunctions for large area lightening. Chemical Physics Letters, 490(4-6), 200-204
Open this publication in new window or tab >>Solution processed ZnO nanowires/polyfluorene heterojunctions for large area lightening
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2010 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 490, no 4-6, p. 200-204Article in journal (Refereed) Published
Abstract [en]

Hybrid inorganic-organic semiconductor heterojunctions are nowadays scrutinized for optoelectronic devices, such as solar cells and light emitting diodes. Here, ZnO nanowires/polyfluorene heterojunctions have been entirely fabricated from solution by wet chemistry and low temperature processes. The transparent plastic electrode PEDOT injects holes in the polyfluorene, while the electrons are injected via the ZnO-Au contact, thus avoiding the use of air sensitive low work function metals. The hybrid inorganic-organic light emitting diode emits almost white light. Because of its solution processability, stable cathode, low cost and low temperature process, the ZnO/polymer heterojunction devices are promising for large area lightening applications.

Place, publisher, year, edition, pages
Elsevier Science B.V., Amsterdam., 2010
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-55530 (URN)10.1016/j.cplett.2010.03.050 (DOI)000276822100018 ()
Available from: 2010-04-30 Created: 2010-04-30 Last updated: 2017-12-12Bibliographically approved
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