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On the operation of light-emitting electrochemical cells
Umeå University, Faculty of Science and Technology, Department of Physics. (OPEG)ORCID iD: 0000-0002-1903-9875
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

We are in the midst of a technological revolution that permeates nearly all human activities; artificial light is one of the most visible contributors in this societal change. If more efficient, green, and versatile light sources can be developed, they might improve the life of millions of people around the world while causing minimal damage to our climate and environment. The unique operational mechanism of the light-emitting electrochemical cell (LEC) makes it an ideal fit for some unconventional and emerging uses of light, in for example medicine and security.

By exploiting this operational mechanism, in which mobile ions enable electrochemical doping of a luminescent polymer, we have designed and fabricated new bilayer LEC architectures. The bilayer LEC features patterned light emission that is easily adjustable during fabrication, and that can be configured to suit new applications of light. Given the light-emitting nature of the LEC, it is somewhat surprising that the optical understanding of its operation is rather limited. To fill this knowledge gap, we investigate how the optical properties of the luminescent polymer respond to electrochemical doping. We find that the complex-refractive index spectrum in the active layer of an LEC, as a direct result of the doping, varies in both space and time. The thin-film structure of an LEC implies that computational predictions of its luminous output need to consider internal reflections and interference. Finally, we implement a doping dependent optical thin-film simulation model. It enables us to precisely replicate the experimental luminance and angle-dependent emission spectrum for a range of LECs with different thicknesses. Using the model we can also identify and quantify many of the different optical loss mechanisms in LECs, which has not previously been done. The insights that we have collected on the path towards our present model will be useful for computational determination of device parameters that are otherwise difficult to acquire.

The improved understanding of the optical operation of LECs is important for the maturation of the technology, as it facilitates formulation of relevant and accurate research questions. Hopefully, our results will accelerate the development of the field, so that useful products based on this technology can become available in the not too distant future.

Abstract [sv]

Just nu pågår en teknologisk revolution som genomsyrar nära nog alla samhällsfunktioner, och där artificiellt ljus har en påfallande viktig roll. Nya ljuskällor, som är mer miljövänliga, effektiva och mångsidiga, skulle kunna förbättra livskvaliten för miljoner människor över hela världen, utan att för den skull skapa problem för vår miljö och vårt klimat. Den ljusemitterande elektrokemiska cellen (LEC) är en teknik som fungerar på ett unikt sätt. Det gör att den är lämplig för nya och okonventionella användningsområden av ljus, exempelvis inom medicin och säkerhetsprodukter.

Vi har kunnat designa och tillverka en ny sorts dubbellagers-LEC genom att utnyttja den interna funktionen i en LEC. Den innebär att rörliga joner möjliggör elektrokemiska oxidations- och reduktionsprocesser (dopning) av en lysande polymer. En dubbellagers-LEC lyser i mönster som enkelt kan anpassas utefter önskemål, och skulle kunna användas i nya sorters ljusapplikationer. Med tanke på att en LEC är en lysande komponent är förståelsen för dess optik förvånansvärt begränsad. För att förbättra dessa kunskaper börjar vi med att undersöka hur den lysande polymerens optiska egenskaper förändras när den dopas. Vi finner att dess optiska egenskaper varierar i tid och rum i det aktiva skiktet i en LEC, som en direkt följd av dopningen. För att sedan med hjälp av de optiska egenskaperna kunna beräkna hur mängden ljus påverkas, måste vi också ta hänsyn till att ljus i tunna skikt kan reflekteras vid gränsytor och interagera med sig själv. Slutligen implementerar vi en dopningsberoende optisk beräkningsmodell för tunna skikt, och lyckas återskapa den experimentellt uppmätta luminansen och de vinkelberoende ljusspektrumen för en serie LECer med olika tjocklek. Utifrån modellen kan vi också identifiera och kvantifiera många av de olika optiska förlustkanalerna i en LEC, vilket inte gjorts tidigare. Vägen fram till den nuvarande modellen har bjudit oss på en rad insikter som gör att vi beräkningsmässigt kan uppskatta komponentegenskaper som annars skulle förbli okända, då de inte går att mäta med direkta metoder.

Den förbättrade optiska förståelsen för LEC-tekniken är viktig för forskningen inom fältet. Våra resultat kan förhoppningsvis accelerera utvecklingen mot bra och användbara produkter, så att dessa blir tillgängliga inom en inte alltför avlägsen framtid.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet , 2019. , p. 63
Keywords [en]
Artificial Light, Organic Electronics, Light-emitting Electrochemical Cells, Electrochemical Doping, Thin-film Optical Model, Optical Modes
National Category
Nano Technology Other Physics Topics Condensed Matter Physics
Identifiers
URN: urn:nbn:se:umu:diva-156093ISBN: 978-91-7855-000-5 (print)OAI: oai:DiVA.org:umu-156093DiVA, id: diva2:1285763
Public defence
2019-03-01, Lilla hörsalen, KB.E3.01, KBC-huset, Umeå, 09:15 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research Swedish Energy AgencyÅForsk (Ångpanneföreningen's Foundation for Research and Development)Knut and Alice Wallenberg FoundationThe Kempe FoundationsSwedish Research CouncilAvailable from: 2019-02-08 Created: 2019-02-05 Last updated: 2019-02-06Bibliographically approved
List of papers
1. Inkjet Printed Bilayer Light-Emitting Electrochemical Cells for Display and Lighting Applications
Open this publication in new window or tab >>Inkjet Printed Bilayer Light-Emitting Electrochemical Cells for Display and Lighting Applications
2014 (English)In: Small, ISSN 1613-6810, E-ISSN 1613-6829, Vol. 10, no 20, p. 4148-4153Article in journal (Refereed) Published
Abstract [en]

A new bilayer light-emitting electrochemical cell (LEC) device, which allows well-defined patterned light emission through an easily adjustable, mask-free, and additive fabrication process, is reported. The bilayer stack comprises an inkjet-printed lattice of micrometer-sized electrolyte droplets, in a filled or patterned lattice configuration. On top of this, a thin layer of light-emitting compound is deposited from solution. The light emission is demonstrated to originate from regions proximate to the interfaces between the inkjetted electrolyte, the light-emitting compound, and one electrode, where bipolar electron/hole injection and electrochemical doping are facilitated by ion motion. By employing KCF3SO3 in poly(ethylene glycol) as the electrolyte, Super Yellow as the light-emitting compound, and two air-stabile electrodes, it is possible to realize filled lattice devices that feature uniform yellow-green light emission to the naked eye, and patterned lattice devices that deliver well-defined and high-contrast static messages with a pixel density of 170 PPI.

Place, publisher, year, edition, pages
John Wiley & Sons, 2014
Keywords
inkjet printing, light-emitting electrochemical cells, displays, patterning, conjugated polymers
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-96951 (URN)10.1002/smll.201400840 (DOI)000344452500015 ()
Available from: 2015-02-25 Created: 2014-12-05 Last updated: 2019-02-05Bibliographically approved
2. Luminescent line art by direct-write patterning
Open this publication in new window or tab >>Luminescent line art by direct-write patterning
2016 (English)In: Light: Science & Applications, ISSN 2047-7538, Vol. 5, article id e16050Article in journal (Refereed) Published
Abstract [en]

We present a direct-write patterning method for the realization of electroluminescent (EL) line art using a surface-emissive light-emitting electrochemical cell with its electrolyte and EL material separated into a bilayer structure. The line-art emission isachieved through subtractive patterning of the electrolyte layer with a stylus, and the single-step patterning can be either manual for personalization and uniqueness or automated for high throughput and repeatability. We demonstrate that the light emission is effectuated by cation-assisted electron injection in the patterned regions and that the resulting emissive lines can be as narrow as a few micrometers. The versatility of the method is demonstrated through the attainment of a wide range of light-emission patterns and colors using a variety of different materials. We propose that this low-voltage-driven and easy-to-modify luminescent line-art technology could be of interest for emerging applications, such as active packaging and personalized gadgets.

Place, publisher, year, edition, pages
Nature Publishing Group, 2016
Keywords
direct-write patterning, light-emitting electrochemical cell, luminescent line art, organic electronics
National Category
Other Physics Topics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-114168 (URN)10.1038/lsa.2016.50 (DOI)000374463100006 ()
Funder
Knut and Alice Wallenberg FoundationThe Kempe FoundationsÅForsk (Ångpanneföreningen's Foundation for Research and Development)Swedish Energy AgencySwedish Research CouncilSwedish Foundation for Strategic Research
Available from: 2016-01-15 Created: 2016-01-15 Last updated: 2019-02-05Bibliographically approved
3. On the asymmetric evolution of the optical properties of a conjugated polymer during electrochemical p- and n-type doping
Open this publication in new window or tab >>On the asymmetric evolution of the optical properties of a conjugated polymer during electrochemical p- and n-type doping
2017 (English)In: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 5, no 19, p. 4706-4715Article in journal (Refereed) Published
Abstract [en]

We report on the in situ measured evolution of the spectral complex refractive index of a prototypical conjugated polymer, a phenyl-substituted poly (para-phenylenevinylene) copolymer (Ph-PPV, “Super Yellow”), during electrochemical p- and n-type doping. We find that the real part of the refractive index is lowered in a significant and continuous fashion over essentially the entire visible range with doping, as exemplified by a drop in the peak value at ∼480 nm from 2.1 for pristine Ph-PPV to 1.8 at a p-type doping concentration of 0.2 dopants per repeat unit and an n-type doping concentration of 0.6 dopants per repeat unit. The imaginary part features a concomitant distinct bleaching of the high-energy π–π* transition and the emergence of a low-energy polaron band. Interestingly, we observe that the optical response of Ph-PPV to p-type and n-type doping is highly asymmetric, with the former resulting in much stronger changes and a distinct blue-shift of all optical transitions. We tentatively attribute this difference in response to larger effective size of the p-type polaron compared to the n-type polaron. We anticipate that the presented results should be of value for the rational design of emerging optical devices that utilize the doping capacity of conjugated polymers.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2017
National Category
Materials Chemistry Physical Sciences
Identifiers
urn:nbn:se:umu:diva-135968 (URN)10.1039/c7tc01022b (DOI)000401712700013 ()
Available from: 2017-06-27 Created: 2017-06-27 Last updated: 2019-02-05Bibliographically approved
4. The Weak Microcavity as an Enabler for Bright and Fault-tolerant Light-emitting Electrochemical Cells
Open this publication in new window or tab >>The Weak Microcavity as an Enabler for Bright and Fault-tolerant Light-emitting Electrochemical Cells
Show others...
2018 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 6970Article in journal (Refereed) Published
Abstract [en]

The light-emitting electrochemical cell (LEC) is functional at substantial active-layer thickness, and is as such heralded for being fit for low-cost and fault-tolerant solution-based fabrication. We report here that this statement should be moderated, and that in order to obtain a strong luminous output, it is fundamentally important to fabricate LEC devices with a designed thickness of the active layer. By systematic experimentation and simulation, we demonstrate that weak optical microcavity effects are prominent in a common LEC system, and that the luminance and efficiency, as well as the emission color and the angular intensity, vary in a periodic manner with the active-layer thickness. Importantly, we demonstrate that high-performance light-emission can be attained from LEC devices with a significant active-layer thickness of 300 nm, which implies that low-cost solution-processed LECs are indeed a realistic option, provided that the device structure has been appropriately designed from an optical perspective.

Place, publisher, year, edition, pages
Nature Publishing Group, 2018
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-147802 (URN)10.1038/s41598-018-25287-x (DOI)000431291500022 ()29725061 (PubMedID)
Note

Publisher Correction: M. Lindh, P. Lundberg, T. Lanz, J. Mindemark, L. Edman. The Weak Microcavity as an Enabler for Bright and Fault-tolerant Light-emitting Electrochemical Cells. Scientific Reports. 2018;8 DOI: 10.1038/s41598-018-26760-3

Available from: 2018-05-22 Created: 2018-05-22 Last updated: 2019-02-05Bibliographically approved
5. Optical analysis of light-emitting electrochemical cells
Open this publication in new window or tab >>Optical analysis of light-emitting electrochemical cells
2019 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, article id 10433Article in journal (Refereed) Published
Abstract [en]

The light-emitting electrochemical cell (LEC) is a contender for emerging applications of light, primarily because it offers low-cost solution fabrication of easily functionalized device architectures. The attractive properties originate in the in-situ formation of electrochemically doped transport regions that enclose an emissive intrinsic region, but the understanding of how this intricate doping structure affects the optical performance of the LEC is largely lacking. We combine angle- and doping-dependent measurements and simulations, and demonstrate that the emission zone in our high-performance LEC is centered at ~30% of the active-layer thickness (dal) from the anode. We further find that the emission intensity and efficiency are undulating with dal, and establish that the first emission maximum at dal ~ 100 nm is largely limited by the lossy coupling of excitons to the doping regions, whereas the most prominent loss channel at the second maximum at dal ~ 300 nm is wave-guided modes.

Place, publisher, year, edition, pages
Nature Publishing Group, 2019
National Category
Nano Technology Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-156092 (URN)10.1038/s41598-019-46860-y (DOI)000475845400037 ()31320711 (PubMedID)2-s2.0-85069470003 (Scopus ID)
Note

Originally included in thesis in manuscript form.

Available from: 2019-02-05 Created: 2019-02-05 Last updated: 2019-08-08Bibliographically approved

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