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Polymer light-emitting electrochemical cells: Utilizing doping for generation of light
Umeå University, Faculty of Science and Technology, Department of Physics.
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: urn:nbn:se:umu:diva-38953ISBN: 978-91-7459-124-8 (print)OAI: oai:DiVA.org:umu-38953DiVA: diva2:385412
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
List of papers
1. A unifying model for the operation of light-emitting electrochemical cells
Open this publication in new window or tab >>A unifying model for the operation of light-emitting electrochemical cells
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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
Identifiers
urn:nbn:se:umu:diva-38943 (URN)10.1021/ja1045555 (DOI)000282864100048 ()
Available from: 2011-01-11 Created: 2011-01-11 Last updated: 2011-01-13Bibliographically approved
2. The dynamic organic p-n junction
Open this publication in new window or tab >>The dynamic organic p-n junction
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2009 (English)In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 8, no 8, 672-676 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-30610 (URN)10.1038/nmat2478 (DOI)
Available from: 2010-01-08 Created: 2010-01-08 Last updated: 2011-01-13Bibliographically approved
3. Salt concentration effects in planar light-emitting electrochemical cells
Open this publication in new window or tab >>Salt concentration effects in planar light-emitting electrochemical cells
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(English)Article in journal (Refereed) Submitted
Identifiers
urn:nbn:se:umu:diva-38946 (URN)
Available from: 2011-01-11 Created: 2011-01-11 Last updated: 2011-01-13Bibliographically approved
4. The influence of electrodes on the performance of light-emitting electrochemical cells
Open this publication in new window or tab >>The influence of electrodes on the performance of light-emitting electrochemical cells
2007 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 0019-4686, Vol. 52, no 23, 6456–6462- p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate that the electrochemical properties of the electrode material can have a dramatic impact on the performance of light-emitting electrochemical cells (LECs). Specifically, we report results from planar wide-gap LECs containing a blend of poly(2-methoxy,5-(2’-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV), poly(ethylene oxide) and LiCF3SO3 as the active material. We find that Au electrodes are preferable over Al electrodes, since Au-electrode devices exhibit fast turn-on (i.e., p-n junction formation time) and clearly visible light emission during operation at 5 V and 360 K, while Al-electrode devices exhibit slow turn-on (due to a delayed onset of p-doping progression) and no visible light emission. These results are rationalized with a cyclic voltammetry study, which demonstrates that Al is oxidized at a lower potential than the p-doping (oxidation) potential of MEH-PPV, while Au is electrochemically inert over the entire voltage range spanned by the p- and n-doping potentials of MEH-PPV. Consequently, the oxidation charge injected into Al-electrode devices results in a combination of p-doping of MEH-PPV and formation of Al ions. The latter process is undesired since it results in a slow turn-on time and quenched light emission. Finally, we find that planar LECs in a bottom-electrode configuration exhibit a faster turn-on time than identical devices with the electrodes on top of the active material.

Place, publisher, year, edition, pages
Elsevier, 2007
Identifiers
urn:nbn:se:umu:diva-6544 (URN)10.1016/j.electacta.2007.04.068 (DOI)
Available from: 2007-12-13 Created: 2007-12-13 Last updated: 2011-01-13Bibliographically approved
5. Identifying and alleviating electrochemical side-reactions in light-emitting electrochemical cells.
Open this publication in new window or tab >>Identifying and alleviating electrochemical side-reactions in light-emitting electrochemical cells.
2008 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 130, 4562-4568 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-11279 (URN)10.1021/ja7113294 (DOI)
Available from: 2008-12-05 Created: 2008-12-05 Last updated: 2011-01-13Bibliographically approved
6. On the desired properties of a conjugated polymer-electrolyte blend in a light-emitting electrochemical cell
Open this publication in new window or tab >>On the desired properties of a conjugated polymer-electrolyte blend in a light-emitting electrochemical cell
2008 (English)In: Organic electronics, ISSN 1566-1199, Vol. 9, no 5, 699-710 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-11280 (URN)10.1016/j.orgel.2008.05.010 (DOI)
Available from: 2008-12-05 Created: 2008-12-05 Last updated: 2011-01-13Bibliographically approved
7. The design and realization of flexible light-emitting electrochemical cells with record-long lifetime
Open this publication in new window or tab >>The design and realization of flexible light-emitting electrochemical cells with record-long lifetime
2009 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 19, no 16, 2671-2676 p.Article in journal (Refereed) Published
Abstract [en]

Polymer light-emitting electrochemical cells (LECs) offer an attractive opportunity for low-cost production of functional devices in flexible and large-area configurations, but the critical drawback in comparison to competing light-emission technologies is a limited operational lifetime. Here, it is demonstrated that it is possible to improve the lifetime by straightforward and motivated means from a typical value of a few hours to more than one month of uninterrupted operation at significant brightness (>100 cd m−2) and relatively high power conversion efficiency (2 lm W−1 for orange-red emission). Specifically, by optimizing the composition of the active material and by employing an appropriate operational protocol, a desired doping structure is designed and detrimental chemical and electrochemical side reactions are identified and minimized. Moreover, the first functional flexible LEC with a similar promising device performance is demonstrated.

Place, publisher, year, edition, pages
Wiley, 2009
Keyword
flexible materials, light-emitting electrochemical cells, organic electronics, power conversion efficiencies
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-30612 (URN)10.1002/adfm.200900479 (DOI)
Available from: 2010-01-08 Created: 2010-01-08 Last updated: 2012-04-18Bibliographically approved
8. Yellow-green light-emitting electrochemical cells with long lifetime and high efficiency
Open this publication in new window or tab >>Yellow-green light-emitting electrochemical cells with long lifetime and high efficiency
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.”

Identifiers
urn:nbn:se:umu:diva-35841 (URN)10.1063/1.3299018 (DOI)000274319500102 ()
Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2013-08-22Bibliographically approved
9. Separating ion and and electron transport: the bi-layer light-emitting electrochemical cell
Open this publication in new window or tab >>Separating ion and and electron transport: the bi-layer light-emitting electrochemical cell
2010 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 132, no 19, 6646-6647 p.Article in journal (Refereed) Published
Abstract [en]

The current generation of polymer light-emitting electrochemical cells (LECs) suffers from insufficient stability during operation. One identified culprit is the active material, which comprises an intimate blend between an ion-conducting electrolyte and an electron-transporting conjugated polymer, as it tends to undergo phase separation during long-term operation and the intimate contact between the ion- and electron-transporting components provokes side reactions. To address these stability issues, we present here a bilayer LEC structure in which the electrolyte is spatially separated from the conjugated polymer. We demonstrate that employing this novel device structure, with its clearly separated ion- and electron-transport paths, leads to distinctly improved LEC performance in the form of decreased turn-on time and improved light emission. We also point out that it will allow for the utilization of combinations of active materials having mutually incompatible solubilities.

Place, publisher, year, edition, pages
ACS Publications, 2010
Identifiers
urn:nbn:se:umu:diva-35843 (URN)10.1021/ja102038e (DOI)000277721500017 ()
Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2013-08-22Bibliographically approved
10. Graphene and mobile ions: the key to all-plastic, solution-processed light-emitting devices
Open this publication in new window or tab >>Graphene and mobile ions: the key to all-plastic, solution-processed light-emitting devices
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2010 (English)In: ACS Nano, ISSN 1936-0851, Vol. 4, no 2, 637-642 p.Article in journal (Refereed) Published
Abstract [en]

The emerging field of “organic” or “plastic” electronics has brought low-voltage, ultrathin, and energy-efficient lighting and displays to market as organic light-emitting diode (OLED) televisions and displays in cameras and mobile phones. Despite using carbon-based materials as the light-emitting layer, previous efficient organic electronic light-emitting devices have required at least one metal electrode. Here, we utilize chemically derived graphene for the transparent cathode in an all-plastic sandwich-structure device, similar to an OLED, called a light-emitting electrochemical cell (LEC). Using a screen-printable conducting polymer as a partially transparent anode and a micrometer-thick active layer solution-deposited from a blend of a light-emitting polymer and a polymer electrolyte, we demonstrate a light-emitting device based solely on solution-processable carbon-based materials. Our results demonstrate that low-voltage, inexpensive, and efficient light-emitting devices can be made without using metals. In other words, electronics can truly be “organic”.

Place, publisher, year, edition, pages
American Chemical Society Publications, 2010
Keyword
graphene, light-emitting device, polymer, light-emitting, electrochemical cell, electroluminescence
Identifiers
urn:nbn:se:umu:diva-35840 (URN)10.1021/nn9018569 (DOI)000274635800009 ()
Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2011-01-13Bibliographically approved
11. Flexible and Metal-Free Light-Emitting Electrochemical Cells Based on Graphene and PEDOT-PSS as the Electrode Materials
Open this publication in new window or tab >>Flexible and Metal-Free Light-Emitting Electrochemical Cells Based on Graphene and PEDOT-PSS as the Electrode Materials
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2011 (English)In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 5, no 1, 574-580 p.Article in journal (Refereed) Published
Abstract [en]

We report flexible and metal-free light-emitting electrochemical cells (LECs) using exclusively solution-processed organic materials and illustrate interesting design opportunities offered by such conformable devices with transparent electrodes. Flexible LEC devices based on chemically derived graphene (CDG) as the cathode and poly(3,4-ethylenedioxythiophene) mixed with poly(styrenesulfonate) as the anode exhibit a low turn-on voltage for yellow light emission (V = 2.8 V) and a good efficiency 2.4 (4.0) cd/A at a brightness of 100 (50) cd/m2. We also find that CDG is electrochemically inert over a wide potential range (+1.2 to −2.8 V vsferrocene/ferrocenium) and exploit this property to demonstrate planar LEC devices with CDG as both the anode and the cathode.

Place, publisher, year, edition, pages
Washington D.C.: American Chemical Society, 2011
Keyword
light-emitting electrochemical cell, metal-free, flexible, solution processing, graphene, superyellow, PEDOT-PSS, doping, p−n junction
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-48543 (URN)10.1021/nn102704h (DOI)
Available from: 2011-10-21 Created: 2011-10-21 Last updated: 2017-12-08Bibliographically approved

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