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Interface Phenomena in Organic Electronics
Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Organic electronics based on organic semiconductors offer tremendous advantages compared to traditional inorganic counterparts such as low temperature processing, light weight, low manufacturing cost, high throughput and mechanical flexibility. Many key electronic processes in organic electronic devices, e.g. charge injection/extraction, charge recombination and exciton dissociation, occur at interfaces, significantly controlling performance and function. Understanding/modeling the interface energetics at organic-electrode/organic-organic heterojunctions is one of the crucial issues for organic electronic technologies to provide a route for improving device efficiency, which is the aim of the research presented in this thesis.

Integer charge transfer (ICT) states pre-existed in the dark and created as a consequence of Fermi level equilibrium at donor-acceptor interface have a profound effect on open circuit voltage in organic bulk heterojunction photovoltaics. ICT state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.

Doping and insertion of interlayer are two established methods for enhancing charge injection/extraction properties at organic-electrode interface. By studying the energy level alignment behavior at low to intermediate doping levels for molecule-doped conjugated polymer/electrode interfaces, we deduce that two combined processes govern the interface energetics: (i) equilibration of the Fermi level due to oxidation (or reduction) of polymer sites at the interface as per the ICT model and (ii) a double dipole step induced by image charge from the dopant-polymer charge transfer complex that causes a shift of the work function. Such behavior is expected to hold in general for low to intermediate level doped organic semiconductor systems. The unified model is further extended to be suitable for conjugated electrolyte/electrode  interfaces, revealing the design rules for achieving the smallest charge injection/extraction barrier for both thin tunneling and thick charge transporting conjugated electrolyte interlayers.

To probe into the energy level spatial extension at interfaces, we employ the original approach of building and characterizing multilayers composed of a well-defined number of polymer monolayers with the Langmuir-Shäfer method to control polymer film uniformity and thicknesses, avoiding the problems associated with spin-coating ultrathin films. The disordered/amorphous films feature smaller, and in fact negligible, energy level bending compared to the more well-ordered films, in contradiction with existing models. It is found that that energy level bending depends on the ICT state distribution rather than the density of states of the neutral polymer chains in relation to the Fermi energy, thus taking into account the Coulomb energy associated with charging the polymer chain and transferring a charge across the interface. Based on this work, a general model for energy level bending in absence of significant doping of conjugated polymer films is proposed.

Organic semiconductors are sensitive to ambient atmosphere that can influence the energetics. The degradation effects of common PCBM film induced by oxygen and water are found to be completely different. Upon exposure to oxygen, the work function is down-shifted by ~ 0.15 eV compared to the ICT curve of the pristine PCBM film, originating from the weak interaction between the fullerene part of PCBM and oxygen, and this can be reversed by thermal treatment in vacuum. The down-shift in energetics will cause a loss in open circuit voltage at electrode interface, but aids free charge generation at donor-acceptor interface. Upon exposure to water, there is irreversible extensive broadening and bleaching of the valence electronic structure features as well as a substantial decrease of work function and ionization potential, severely degrading the transport properties.

Overall, the research results in this thesis thus give a deeper understanding of interface phenomena in organic electronics, especially regard to organic solar cells, aimed to further improve the device operation efficiency and lifetime.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. , 56 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1658
National Category
Physical Sciences Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:liu:diva-118922DOI: 10.3384/diss.diva-118922ISBN: 978-91-7519-077-8 (print)OAI: oai:DiVA.org:liu-118922DiVA: diva2:817444
Public defence
2015-08-27, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2015-06-05 Created: 2015-06-05 Last updated: 2015-06-05Bibliographically approved
List of papers
1. Trap-Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics
Open this publication in new window or tab >>Trap-Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics
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2014 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 24, no 40, 6309-6316 p.Article in journal (Refereed) Published
Abstract [en]

Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular-poly(3-hexylthiophene): fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.

Place, publisher, year, edition, pages
Wiley-VCH Verlag, 2014
National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-112635 (URN)10.1002/adfm.201401513 (DOI)000344249900007 ()
Note

Funding Agencies|Swedish Energy Agency [34142-1]; European Commission [287594]; Academy of Finland [137093]; Swedish Research Council Linnaeus grant LiLi-NFM; Waldemar von Frenckell Foundation; Swedish Cultural Foundation in Finland; Advanced Functional Materials Center at Linkoping University

Available from: 2014-12-08 Created: 2014-12-05 Last updated: 2017-12-05
2. Oxygen- and Water-Based Degradation in [6,6]-Phenyl-C-61-Butyric Acid Methyl Ester (PCBM) Films
Open this publication in new window or tab >>Oxygen- and Water-Based Degradation in [6,6]-Phenyl-C-61-Butyric Acid Methyl Ester (PCBM) Films
2014 (English)In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 4, no 6Article in journal (Refereed) Published
Abstract [en]

Effects of in situ oxygen/water exposure on the energetics of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films are presented. For oxygen exposure, the work function is downshifted by ≈0.15 eV compared to the ideal integer charge transfer (ICT) curve for pristine PCBM, which is incompatible with significant introduction of electron trap states or p-doping. Water induces the highest occupied molecular orbital (HOMO) structure to undergo strong, irreversible modifications accompanied by a chemical interaction with PCBM.

Place, publisher, year, edition, pages
Wiley-VCH Verlag, 2014
Keyword
electronic structure; integer charge transfer; oxygen; water exposure; degradation; photoelectron spectroscopy
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-106975 (URN)10.1002/aenm.201301272 (DOI)000334790000005 ()
Available from: 2014-06-04 Created: 2014-06-02 Last updated: 2015-06-05
3. The energetics of the semiconducting polymer-electrode interface for solution-processed electronics
Open this publication in new window or tab >>The energetics of the semiconducting polymer-electrode interface for solution-processed electronics
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The semiconductor-electrode interface impacts the function and the performance of (opto-)electronic devices. For printed organic electronics the electrode surface is not atomically clean leading to weakly interacting interfaces. As a result, solution-processed organic ultra-thin films on electrodes typically form islands due to de-wetting. It has therefore been utterly difficult to achieve homogenous ultrathin conjugated polymer films. This has made the investigation of the correct energetics of the conjugated polymer-electrode interface impossible. Also, this has hampered the development of devices including ultra-thin conjugated polymer layers. Here, we report Langmuir-Shäfer-manufactured homogenous mono- and multilayers of semiconducting polymers on metal electrodes and track the energy level bending using photoelectron spectroscopy. The amorphous films display an abrupt energy level bending that does not extend beyond the first monolayer. Our findings provide new insights of the energetics of the polymer-electrode interface and opens up for new high-performing devices based on ultra-thin semiconducting polymers.

Keyword
Energy level bending, conjugated polymer, Langmuir-Shäfer, organic electronics
National Category
Physical Sciences Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-118913 (URN)
Available from: 2015-06-05 Created: 2015-06-05 Last updated: 2017-02-03
4. Energetics at Doped Conjugated Polymer/Electrode Interfaces
Open this publication in new window or tab >>Energetics at Doped Conjugated Polymer/Electrode Interfaces
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2015 (English)In: ADVANCED MATERIALS INTERFACES, ISSN 2196-7350, Vol. 2, no 2Article in journal (Refereed) Published
Abstract [en]

n/a

Place, publisher, year, edition, pages
Wiley: 12 months, 2015
National Category
Physical Sciences Biological Sciences
Identifiers
urn:nbn:se:liu:diva-114420 (URN)10.1002/admi.201400403 (DOI)000348287700008 ()
Note

Funding Agencies|EU [287594]; Swedish Research Council Linnaeus grant LiLi-NFM; Swedish Research Council [2013-4022]; Goran Gustafsson Foundation for Research in Natural Sciences and Medicine; Advanced Functional Materials Center at Linkoping University

Available from: 2015-03-02 Created: 2015-02-20 Last updated: 2015-06-05
5. Regular Energetics at Conjugated Electrolyte/Electrode Modifier for Organic Electronics and Their Implications of Design Rules
Open this publication in new window or tab >>Regular Energetics at Conjugated Electrolyte/Electrode Modifier for Organic Electronics and Their Implications of Design Rules
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2015 (English)In: Advanced Materials Interfaces, ISSN 2196-7350, Vol. 2, no 12, 1-6 p., 1500204Article in journal (Refereed) Published
Abstract [en]

Regular energetics at a conjugated electrolyte/electrode modifier are found and controlled by equilibration of the Fermi level and an additional interface double dipole step induced by ionic functionality. Based on the results, design rules for conjugated electrolyte/electrode modifiers to achieve the smallest charge injection/exaction barrier and break through the current thickness limitation are proposed.

Place, publisher, year, edition, pages
John Wiley & Sons, 2015
Keyword
Conjugated electrolyte, electrode modifier, energetics, organic electronics
National Category
Physical Sciences Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-118917 (URN)10.1002/admi.201500204 (DOI)000360057500011 ()
Note

On the day of the defence date the status of this article was Manuscript.

Available from: 2015-06-05 Created: 2015-06-05 Last updated: 2017-03-16Bibliographically approved
6. Effects of ultraviolet soaking on surface electronic structures of solution processed ZnO nanoparticle films in polymer solar cells
Open this publication in new window or tab >>Effects of ultraviolet soaking on surface electronic structures of solution processed ZnO nanoparticle films in polymer solar cells
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2014 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 2, no 41, 17676-17682 p.Article in journal (Refereed) Published
Abstract [en]

We systematically show the effect of UV-light soaking on surface electronic structures and chemical states of solution processed ZnO nanoparticle (ZnONP) films in UHV, dry air and UV-ozone. UV exposure in UHV induces a slight decrease in work function and surface-desorption of chemisorbed oxygen, whereas UV exposure in the presence of oxygen causes an increase in work function due to oxygen atom vacancy filling in the ZnO matrix. We demonstrate that UV-light soaking in combination with vacuum or oxygen can tune the work function of the ZnONP films over a range exceeding 1 eV. Based on photovoltaic performance and diode measurements, we conclude that the oxygen atom vacancy filling occurs mainly at the surface of the ZnONP films and that the films consequently retain their n-type behavior despite a significant increase in the measured work function.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2014
National Category
Physical Sciences Biological Sciences
Identifiers
urn:nbn:se:liu:diva-112033 (URN)10.1039/c4ta02695k (DOI)000342763300057 ()
Note

Funding Agencies|EU [287594]; Swedish Research Council Linnaeus grant LiLi-NFM

Available from: 2014-11-17 Created: 2014-11-13 Last updated: 2017-12-05

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