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Tuning low work function contacts using molecular donor layers: intermolecular order effects
Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, The Institute of Technology.
Computational Materials Science, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, The Netherlands.
Computational Materials Science, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, The Netherlands.
Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, The Institute of Technology.
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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.

Place, publisher, year, edition, pages
2012.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-81063OAI: oai:DiVA.org:liu-81063DiVA: diva2:550113
Note

|

Available from: 2012-09-06 Created: 2012-09-06 Last updated: 2012-09-06Bibliographically approved
In thesis
1. Influence of intermolecular order at the interfaces
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. 46 p.
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: 2012-09-06Bibliographically approved

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Sehati, ParisaFahlman, MatsBraun, Slawomir

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