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Relationship of Ionization Potential and Oxidation Potential of Organic Semiconductor Films Used in Photovoltaics
Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering.
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2018 (English)In: Solar RRL, ISSN 2367-198X, Vol. 2, no 9Article in journal (Refereed) Published
Abstract [en]

Ultraviolet photoelectron spectroscopy (UPS) and cyclic voltammetry (CV) are employed to measure energy levels for charge transport in organic semiconductor films. A series of classical molecules/polymers used in organic bulk heterojunction solar cells are deposited on platinum substrates/electrodes to form thin films and a linear relationship of vertical ionization potential (IP) measured by UPS and relative oxidation potential (Eox) obtained by CV is found, with a slope equal to unity. The intercept varies with the different reference redox couples and repeated potential sweep numbers during experiment processes. The relationship provides for an easy conversion of values obtained by the two techniques and correlates well with device parameters. The precision in the CV-derived IP values is not sufficient, however, to enable precise design of energy level alignment at heterojunction and the approach does not improve upon the current ?best practice? for obtaining donor ionization potential?acceptor electron affinity gaps at heterojunctions.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2018. Vol. 2, no 9
Keywords [en]
cyclic voltammetry, ionization potential, linear relationship, organic photovoltaics, oxidation potential, semiconductor films, UV photoelectron spectroscopy
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-151711DOI: 10.1002/solr.201800122ISI: 000443972900011OAI: oai:DiVA.org:liu-151711DiVA, id: diva2:1252842
Funder
Swedish Foundation for Strategic Research , SE13‐0060Knut and Alice Wallenberg FoundationSwedish Research Council, 2016‐05498Linköpings universitet, 2009 00971Göran Gustafsson Foundation for Research in Natural Sciences and MedicineAvailable from: 2018-10-03 Created: 2018-10-03 Last updated: 2018-10-10Bibliographically approved
In thesis
1. Electronic Structure of π-Conjugated Materials and Their Effect on Organic Photovoltaics
Open this publication in new window or tab >>Electronic Structure of π-Conjugated Materials and Their Effect on Organic Photovoltaics
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The great tunability of structure and electronic properties of π-conjugated organic molecules/polymers combined with other advantages such as light weight and flexibility etc., have made organic-based electronics the focus of an exciting still-growing field of physics and chemistry for more than half a century. The application of organic electronics has led to the appearance of wide range of organic electronic devices mainly including organic light emitting diodes (OLED), organic field effect transistors (OFET) and organic solar cells (OSC). The application of the organic electronic devices mainly is limited by two dominant parameters, i.e., their performance and stability. Up to date, OLED has been successfully commercialized in the market while the OSC are still on the way to commercialization hindered by low efficiency and inferior stability. Understanding the energy levels of organic materials and energy level alignment of the devices is crucial to control the efficiency and stability of the OSC. In this thesis, energy levels measured by different methods are studied to explore their relationship with device properties, and the strategies on how to design efficient and stable OSC based on energy level diagrams are provided.

Cyclic Voltammetry (CV) is a traditional and widely used method to probe the energy levels of organic materials, although there is little consensus on how to relate the oxidation/reduction potential ((Eox/Ered) to the vacuum level. Ultraviolet Photoelectron Spectroscopy (UPS) can be used to directly detect vertical ionization potential (IP) of organic materials. In this thesis, a linear relationship of IP and Eox was found, with a slope equal to unity. The relationship provides for easy conversion of values obtained by the two techniques, enabling complementarily use in designing and fabricating efficient and stable OSC. A popular rule of thumb is that the offset between the LUMO levels of donor and acceptor should be 0.3 eV, according to which a binary solar cell with the minimum voltage losses around 0.49 V was designed here.

Introduction of the ternary blend as active layer is an efficient way to improve both efficiency and stability of the OSC. Based on our studied energy-level diagram within the integer charge transfer (ICT) model, we designed ternary solar cells with enhanced open circuit voltage for the first time and improved thermal stability compared to reference binary ones. The ternary solar cell with minimum voltage losses was developed by combining two donor materials with same ionization potential and positive ICT energy while featuring complementary optical absorption. Furthermore, the fullerene acceptor was chosen so that the energy of the positive ICT state of the two donor polymers is equal to the energy of negative ICT state of the fullerene, which can enhance dissociation of all polymer donor and fullerene acceptor excitons and suppress bimolecular and trap-assistant recombination.

Rapid development of non-fullerene acceptors in the last two years affords more recipes of designing both efficient and stabile OSC. We show in this thesis how non-fullerene acceptors successfully can be used to design ternary solar cells with both enhanced efficiency and thermal stability. Besides improving the efficiency of the devices, understanding of the stability and degradation mechanism is another key issue. The degradation of conjugated molecules/polymers often follow many complicated pathways and at the same time many factors for degradation are coupled with each other. Therefore, the degradation of non-fullerene acceptors was investigated in darkness by photoelectron spectroscopy in this thesis with the in-situ method of controlling exposure of O2 and water vapor separately.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. p. 84
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1893
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Condensed Matter Physics Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-143025 (URN)10.3384/diss.diva-143025 (DOI)9789176853931 (ISBN)
Public defence
2017-12-08, Schrödinger, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2019-10-12Bibliographically approved

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Wang, ChuanfeiOuyang, LiangqiXu, XiaofengBraun, SlawomirLiu, XianjieFahlman, Mats

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