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A unifying model for the operation of light-emitting electrochemical cells
Eindhoven University of Technology.
Umeå University, Faculty of Science and Technology, Department of Physics.
Eindhoven University of Technology.
Eindhoven University of Technology.
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2010 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 132, no 39, 13776-13781 p.Article in journal (Refereed) Published
Abstract [en]

The application of doping in semiconductors plays a major role in the high performances achieved to date in inorganic devices. In contrast, doping has yet to make such an impact in organic electronics. One organic device that does make extensive use of doping is the light-emitting electrochemical cell (LEC), where the presence of mobile ions enables dynamic doping, which enhances carrier injection and facilitates relatively large current densities. The mechanism and effects of doping in LECs are, however, still far from being fully understood, as evidenced by the existence of two competing models that seem physically distinct: the electrochemical doping model and the electrodynamic model. Both models are supported by experimental data and numerical modeling. Here, we show that these models are essentially limits of one master model, separated by different rates of carrier injection. For ohmic nonlimited injection, a dynamic p−n junction is formed, which is absent in injection-limited devices. This unification is demonstrated by both numerical calculations and measured surface potentials as well as light emission and doping profiles in operational devices. An analytical analysis yields an upper limit for the ratio of drift and diffusion currents, having major consequences on the maximum current density through this type of device.

Place, publisher, year, edition, pages
American Chemical Society , 2010. Vol. 132, no 39, 13776-13781 p.
URN: urn:nbn:se:umu:diva-38943DOI: 10.1021/ja1045555ISI: 000282864100048OAI: diva2:385386
Available from: 2011-01-11 Created: 2011-01-11 Last updated: 2011-01-13Bibliographically 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.
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)
Available from: 2011-01-13 Created: 2011-01-11 Last updated: 2012-06-29Bibliographically approved

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