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Influence of intermolecular order at the interfaces
Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, The Institute of Technology.
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. , 46 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1468
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-81066ISBN: 978-91-7519-821-7 (print)OAI: oai:DiVA.org:liu-81066DiVA: diva2:550116
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: 2012-09-06Bibliographically approved
List of papers
1. Energy-Level Alignment at Metal-Organic and Organic-Organic Interfaces in Bulk-Heterojunction Solar Cells
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, 1718-1724 p.Article 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
Keyword
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
2. Spontaneous Charge Transfer and Dipole Formation at the Interface Between P3HT and PCBM
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, 792-797 p.Article 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

|

Available from: 2011-10-14 Created: 2011-10-14 Last updated: 2013-04-24
3. Energy level alignment in Au/pentacene/PTCDA trilayer stacks
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, 38-41 p.Article 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
Keyword
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
4. 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

|

Available from: 2012-09-06 Created: 2012-09-06 Last updated: 2012-09-06Bibliographically approved
5. 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

|

Available from: 2012-09-06 Created: 2012-09-06 Last updated: 2012-09-06Bibliographically approved

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