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Doping-Induced Self-Absorption in Light-Emitting Electrochemical Cells
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
2014 (English)In: ACS PHOTONICS, ISSN 2330-4022, Vol. 1, no 3, 182-189 p.Article in journal (Refereed) Published
Abstract [en]

We report on the quantitative effects of doping-induced self-absorption in light-emitting electrochemical cells (LECs) as a function of active material (AM) thickness and doping concentration. For state-of-the-art polymer LECs with optimized doping concentration and comprising Super Yellow as the electroluminescent (EL) polymer and poly(ethylene oxide)-KCF3SO3 as the electrolyte, we find that the self-absorption loss at the EL peak wavelength is similar to 10% for a 100 nm thin AM and >70% for a 1 mu m thick AM. This implies that the utilization of micrometer-thick AMs fit for fault-tolerant large-scale fabrication can be concomitant with a notable penalty in device performance, and that spatial variations in AM thickness will be manifested in a corresponding spatial light-intensity variation. Moreover, we find that inclusion of a poly(ethylene oxide)-KCF3SO3 electrolyte can inhibit the out-coupling of light and suggest that the culprit is light scattering from dispersed crystalline-electrolyte domains. Finally, we demonstrate evidence for that the selected initial salt concentration in an LEC device dictates the maximum doping concentration that can be attained at steady-state operation.

Place, publisher, year, edition, pages
2014. Vol. 1, no 3, 182-189 p.
Keyword [en]
light-emitting device, organic electronics, electroluminescence, electrochemical doping, cyclic voltammetry, conjugated polymer
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:umu:diva-90793DOI: 10.1021/ph400050tISI: 000335802900006OAI: oai:DiVA.org:umu-90793DiVA: diva2:753947
Available from: 2014-10-09 Created: 2014-07-01 Last updated: 2015-04-24Bibliographically approved
In thesis
1. Functional and Flexible Light-Emitting Electrochemical Cells
Open this publication in new window or tab >>Functional and Flexible Light-Emitting Electrochemical Cells
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The introduction of artificial illumination has brought extensive benefits to mankind, and during the last years we have seen a tremendous progress in this field with the introduction of the energy-efficient light-emitting diode (LED) and the high-contrast organic LED display. These high-end technologies are, however, produced using costly and complex processes, and it is anticipated that the next big thing in the field will be the advent of a low-cost and “green” illumination technology, which can be fabricated in a cost- and material-efficient manner using non-toxic and abundant raw materials, and which features attractive form factors such as flexibility, robustness and light-weight. The light-emitting electrochemical cell (LEC) is a newly invented illumination technology, and in this thesis we present results that imply that it can turn the above vision into reality.

The thin-film LEC comprises an active material sandwiched between a cathode and an anode as its key constituent parts. With the aid of a handheld air-brush, we show that functional large-area LECs can be fabricated by simply spraying three layers of solution -- forming the anode, active material, and cathode -- on top of a substrate. We also demonstrate that such “spray-sintered” LECs can feature multicolored emission patterns, and be fabricated directly on complex-shaped surfaces, with one notable example being the realization of a light-emission fork!

Almost all LECs up-to-date have been fabricated on glass substrates, but for a flexible and light-weight emissive device, it is obviously relevant to identify more appropriate substrate materials. For this end, we show that it is possible to spray-coat the entire LEC directly on conventional copy paper, and that such paper-LECs feature uniform light-emission even under heavy bending and flexing.

We have further looked into the fundamental aspects of the LEC operation and demonstrated that the in-situ doping formation, which is a characteristic and heralded feature of LECs, can bring problems in the form of doping-induced self-absorption. By quantitatively analyzing this phenomenon, we provided straightforward guidelines on how future efficiency-optimized LEC devices should be designed.

The in-situ doping formation process brings the important advantage that LECs can be fabricated from solely air-stabile materials, but during light emission the device needs to be protected from the ambient air. We have therefore developed a functional glass/epoxy encapsulation procedure for the attainment of LEC devices that feature a record-long ambient-air operational lifetime of 5600 h. For the light-emission device of the future, it is however critical that the encapsulation is flexible, and in our last study, we show that the use of multi-layer barrier can result in high-performance flexible LECs.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2015. 57 p.
Keyword
all-ambient fabrication, ambient-air lifetime, encapsulation, flexible, light-emitting electrochemical cells, light-emitting paper
National Category
Nano Technology Other Physics Topics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-102400 (URN)978-91-7601-257-4 (ISBN)
Public defence
2015-05-22, N300, Naturvetarhuset, Umeå University, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2015-04-30 Created: 2015-04-23 Last updated: 2015-05-08Bibliographically approved

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