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Yellow-green light-emitting electrochemical cells with long lifetime and high efficiency
Umeå University, Faculty of Science and Technology, Department of Physics. (The Organic Photonics and Electronics Group)
Umeå University, Faculty of Science and Technology, Department of Physics. (The Organic Photonics and Electronics Group)
Umeå University, Faculty of Science and Technology, Department of Physics. (The Organic Photonics and Electronics Group)
2010 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 5, 053303- p.Article in journal (Refereed) Published
Abstract [en]

 We show that the electrochemical stability window of the constituent components in light-emitting electrochemical cells (LECs), e.g., the electrolyte, should be considered in order to minimize undesired side reactions. By designing and operating LECs in accordance with straightforward principles, we demonstrate sandwich cells that turn on fast at room temperature (<2 s), and which emit significant yellow-green light (>100 cd/m2) during 25 days of uninterrupted operation at low voltage (<4 V) and high power conversion efficacy ~6 lm/W. We further demonstrate that it is possible to attain balanced p- and n-type doping and a centered p-n junction in such planar LECs based on the conjugated polymer “superyellow.”

Place, publisher, year, edition, pages
2010. Vol. 96, no 5, 053303- p.
Identifiers
URN: urn:nbn:se:umu:diva-35841DOI: 10.1063/1.3299018ISI: 000274319500102OAI: oai:DiVA.org:umu-35841DiVA: diva2:349636
Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Polymer light-emitting electrochemical cells: Utilizing doping for generation of light
Open this publication in new window or tab >>Polymer light-emitting electrochemical cells: Utilizing doping for generation of light
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

    The current implementation of conjugated polymers (“conducting plastics”) in a wide range of devices promises to bring the vision of a new generation of flexible, efficient and low-cost applications to reality. Plastic lightemitting devices in the form of polymer light-emitting diodes (PLEDs) are projected to be particularly close to the market in applications such as large area and conformable illumination panels and high-performance thin displays. However, two notable drawbacks of PLEDs are that they depend on vacuum deposition of a reactive metal for the negative electrode and that the active material must be extremely thin and uniform in thickness. As a consequence, PLEDs cannot be expected to allow for a low-cost continuous production using a roll-to-roll coating and/or printing process.

This thesis focuses on an alternative to the PLED: A light-emitting electrochemical cell (LEC). LECs comprise a mixture of a conjugated polymer and a solid-state electrolyte as the active material positioned between two electrodes. The existence of mobile ions in the active material allows for a number of interesting attributes, both from a fundamental science and an application perspective. Importantly, the ions and the related unique operation of LECs make these devices apt for the utilization of low-cost roll-to-roll fabrication of the entire device as the electrode materials can be air stable and solution-processible and the requirement on the thickness of the active material is much less stringent than in PLEDs.

   The herein presented “basic science” studies primarily focus on the operation of LECs. It is for instance firmly established that a light-emitting p-n junction can form in-situ in a LEC device during the application of a voltage. This dynamic p-n junction exhibits some similarities, but also distinct differences, in comparison to the static p-n junctions that are exploited in crystalline inorganic semiconductor devices. We have also systematically explored the role that the constituent materials (ions, conjugated polymer, ionic solvent, and electrode material) can have on the performance of LECs, and two of the more important findings are that the concentration of ions can influence the doping structure in a motivated fashion and that it is critically important to consider the electrochemical stability window of the constituent materials in order to attain stable device operation.

   With this knowledge at hand, we have executed a number of more “applied science” studies, where we have used the acquired information from the basic-science studies for the rational design of improved devices. We have demonstrated LEC devices with significantly improved device performance, as exemplified by an orange-red device that emitted significant light (> 100 cd/m2) for more than one month of uninterrupted operation, and a yellow-green device that emitted significant light for 25 days at a low voltage of 4 V and at relatively high efficiency (6 lm/W). Finally, we have conceptualized and realized a solely solution-processed and metal-free LEC comprising graphene as the negative electrode and the conducting polymer PEDOT-PSS as the positive electrode. This type of devices represents a paradigm shift in the field of solid-state lighting as they demonstrate that it is possible to fabricate an entire light-emitting device from solution-processible and “green” carbon-based materials in a process that is akin to printing.

Place, publisher, year, edition, pages
Umeå: Department of Physics, Umeå University, 2011. 67 p.
Identifiers
urn:nbn:se:umu:diva-38953 (URN)978-91-7459-124-8 (ISBN)
Public defence
2011-02-04, Mit-huset, MA 121, Umeå universitet, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2011-01-13 Created: 2011-01-11 Last updated: 2012-06-29Bibliographically approved
2. Design and Fabrication of Light-Emitting Electrochemical Cells
Open this publication in new window or tab >>Design and Fabrication of Light-Emitting Electrochemical Cells
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Design och tillverkning av ljusemitterande elektrokemiska celler
Abstract [sv]

Glödlampan, en gång symbolen för mänsklig uppfinningsförmåga, är idag på väg att försvinna. Lysdioder och lågenergilampor har istället tagit över då dessa har betydligt längre livstid och högre effektivitet. Den tidigare så hyllade glödlampan anses numera vara en miljöbov, och förbud och restriktioner mot den blir allt vanligare. Trots detta så är de nya alternativen bara att betrakta som provisoriska steg på vägen mot en ideal ljuskälla, som idag tyvärr inte existerar. Lågenergilampor innehåller exempelvis kvicksilver, och utgör därmed ett direkt hot mot en användares hälsa. Både lysdioder och lågenergilampor består även av höga halter av andra tungmetaller, och är väldigt komplicerade att tillverka. Återvinning är därför ett måste, och en fullödig energibesparingsanalys måste ta hänsyn till den betydande energin som går åt vid tillverkningen. Till viss del kan detta lösas genom att göra komponenterna små och ljusstarka, men för att göra en sådan belysning angenäm används istället utrymmeskrävande och ofta energislukande lampskärmar. Lysdioder och lågenergilampor är helt enkelt bra, men långt ifrån perfekta.All elektronisk utrustning är idag beroende av metaller och inorganiska halvledare, vilket gör återvinning viktig och tillverkning komplicerad. Detta är kanske på väg att ändras då även organiska material, t.ex. plast, har visat sig kunna ha elektroniska egenskaper. Idag är organisk elektronik ett hett forskningsområde där material med liknande egenskaper som plast, fast med funktionella elektroniska egenskaper, undersöks och appliceras. Något som gör organiska material extra intressanta är att många kan lösas upp i vätskor, vilket möjliggör för skapandet av bläck. Detta leder i sin tur till möjligheter för användandet av storskaliga trycktekniker, t.ex. tidningspressar och bläckstråleskrivare, vilka leder till en stor kostnadsreduktion och förenklad tillverkning av lysande komponenter. Idag har plast redan ersatt många andra material i en mängd olika tillämpningar. Plastflaskor är vanligare än glasflaskor, och ylletröjor konkurerar idag med kläder gjorda av fleece och andra syntetiska fibrer. Med ljusemitterande plast finns det helt klart en möjlighet att en liknande utveckling kan ske även för lampor.Den här avhandlingen fokuserar på den fortsatta utvecklingen av den ljusemitterande elektrokemiska cellen (LEC), som 1995 uppfanns av Pei et al. LEC-tekniken använder sig av organiska halvledare för att konvertera elektrisk ström till ljus, men även en elektrolyt som möjliggör elektrokemisk dopning. Detta förbättrar den organiska halvledarens elektroniska egenskaper signifikant, vilket leder till mindre resistans och högre effektivitet hos den färdiga lysande komponenten.Visionen för denna och besläktade tekniker har sedan länge varit förverkligandet av en lysande tapet. Den här avhandlingen har försökt närma sig denna vision genom att visa hur en LEC kan uppnå hög effektivitet och lång livslängd, och samtidigt tillverkas i luft med storskaliga produktionsmetoder. Orsaker till en tidigare begränsad livslängd har identifierats och minimerats med hjälp av nya komponentstrukturer och materialformuleringar. En inkapslingsmetod presenteras också, vilken skyddar komponenten från syre och vatten som annars lätt reagerar med det dopade organiska materialet. Detta resulterar i en signifikant förbättring av livslängden.Genom att använda slot-die bestrykning och sprayning, båda kompatibla med rulle-till-rulle tillverkning, har möjligheter för storskalig produktion demonstrerats. Slutligen har en speciell metod för spraymålning av stora lysande ytor utvecklats.

Abstract [en]

The incandescent light bulb, once the very symbol for human ingenuity, is now being replaced by the next generation of lighting technologies such as the compact fluorescent lamp (CFL) and the light emitting diode (LED). The higher efficiencies and longer operational lifetimes of these new sources of illumination have led to the demise of the classic traditional bulb. However, it should be pointed out that the light sources that are taking over are better, but not perfect. The complex high-voltage electronic circuits and health hazardous materials required for their operation make them far from a sustainable eco-friendly option. Their fabrication is also complex, making the final product expensive. A new path forward might be through the use of plastics or other organic materials. Though not traditionally seen as electronically active, some organic materials do behave like inorganic semiconductors and substantial conductivity can be achieved by doping. Since plastics can be easily molded into complex shapes, or made into an ink using a solvent, it is expected that organic materials could revolutionize how we fabricate electronic devices in the future, and possibly replace inorganic crystals in the same way as plastics have replaced glass and wool for food storage and clothes. This thesis has focused on the light-emitting electrochemical cell (LEC), which was invented by Pei et al. in 1995. It employs organic semiconductors that can convert electricity to light, but also an electrolyte that further enhances the electronic properties of the semiconductor by allowing it to be electrochemically doped. This allows light-emitting films to be driven by a low-voltage source at a high efficiency. Unfortunately, the electrolyte has been shown to facilitate rapid degradation of the device under operation, which has historically severely limited the operational lifetime. Realizing the predicted high efficiency has also proven difficult. The purpose of this thesis is to bridge the gap between the LEC and the CFL. This is done by demonstrating efficient devices and improved operational lifetimes. Possible degradation mechanisms are identified and minimized using novel device architectures and optimized active layer compositions. An encapsulation method is presented, and shown to increase the LEC stability significantly by protecting it from ambient oxygen and water. The thesis further focuses on up-scaled fabrication under ambient air conditions, proving that light-emitting devices are compatible with solution-based and cost-efficient printing. This is achieved by a roll-to-roll compatible slot-die coating and a novel spray-depositing technique that alleviates problems stemming from dust particles and phase separation. A practical ambient air fabrication and a subsequent operation of light-emitting electrochemical cells with high efficiency are thus shown possible.

 

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2013. 61 p.
Keyword
Light-emitting electrochemical cell, fabrication, organic semiconductors, organic electronics, ambient fabrication, roll-to-roll
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-79544 (URN)978-91-7459-691-5 (ISBN)
Public defence
2013-09-13, N300, Umeå universitet, Umeå, 09:47 (English)
Opponent
Supervisors
Available from: 2013-08-23 Created: 2013-08-22 Last updated: 2013-08-22Bibliographically approved

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Sandström, AndreasMatyba, PiotrEdman, Ludvig

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