| 1. |
- Sehati, Parisa, 1974-
(författare)
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Influence of intermolecular order at the interfaces
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 2. |
- Sehati, Parisa
(författare)
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Manipulation of ZnO work function upon deposition of 4-TBP molecule
- 2012
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 3. |
- Sehati, Parisa, et al.
(författare)
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Tuning low work function contacts using molecular donor layers : intermolecular order effects
- 2012
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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