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- Liu, Jiang
(författare)
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Light-Emitting Electrochemical Transistors
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since the discovery of conductive polymers in 1977, the implementation of organic conjugated materials in electronic applications has been of great interest in both industry and academia. The goal of organic electronics is to realize large-area, inexpensive and mechanically-flexible electronic applications.Organic light emitting diodes (OLEDs), as the first commercial product made from organic conjugated polymers, have successfully demonstrated that organic electronics can make possible a new generation of modern electronics. However, OLEDs are highly sensitive to materials selection and requires a complicated fabrication process. As a result, OLEDs are expensive to fabricate and are not suitable for low-cost printing or roll-to-roll process.This thesis studies an alternative to OLEDs: light-emitting electrochemical cells (LECs). The active materials in an LEC consist of a conjugated light-emitting polymer (LEP) and an electrolyte. Taking advantage of electrochemical doping of the LEP, an LEC features an in-situ formed emissive organic p-n junction which is easy to fabricate. We aim to control the electrochemical doping profile by employing a “gate” terminal on top of a conventional LEC, forming a lightemitting electrochemical transistor (LECT). We developed three generations of LECTs, in which the position of the light-emitting profile can be modified by the voltage applied at the gate electrode, as well as the geometry of the gate materials. Thus, one can use this structure to achieve a centered light-emitting zone to maximize the power-conversion efficiency. Alternatively, LECTs can be used for information display in a highly integrated system, as it combines the simultaneous modulation of photons and electrons.In addition, we use multiple LECs to construct reconfigurable circuits, based on the reversible electrochemical doping. We demonstrate an LEC-array where several different circuits can be created by forming diodes with different polarity at different locations. The thereby formed circuitry can be erased and turned into circuitry with other functionality. For example, the diodes of a digital AND gate can be re-programmed to form an analogue voltage limiter. These reprogrammable circuits are promising for fully-printed and large-area reconfigurable circuits with facile fabrication.
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| 2. |
- Matyba, Piotr, 1982-
(författare)
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Polymer light-emitting electrochemical cells : Utilizing doping for generation of light
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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