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Spontaneous Charge Transfer and Dipole Formation at the Interface Between P3HT and PCBM
Abo Akad University.
Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry.
Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, The Institute of Technology.
Abo Akad University.
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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. Vol. 1, no 5, 792-797 p.
National Category
Engineering and Technology
URN: urn:nbn:se:liu:diva-71375DOI: 10.1002/aenm.201100074ISI: 000295140100013OAI: diva2:448071


Available from: 2011-10-14 Created: 2011-10-14 Last updated: 2013-04-24
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.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1468
National Category
Natural Sciences
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)
Available from: 2012-09-06 Created: 2012-09-06 Last updated: 2012-09-06Bibliographically approved

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