Large-area light-emission on the roll

The light-emitting electrochemical cell (LEC) is a paradigm-shifting technology, which offers unique opportunities for future lighting and display applications. LEC devices can be thin, flexible, lightweight, robust and promise energy-efficient operation and extremely low-cost production. The Swedish Foundation for Strategic Research (SSF) has sponsored this project to advance the technology towards consumer products.

Overview

Organic electronic (OE) lighting is slated to replace today’s state-of-the-art energy-saving lamps with devices that emit “warmer” light. Using solution-processable carbon-based (organic) materials also allows large-area devices to be created using inexpensive roll-to-roll processes similar to the way wallpaper is manufactured. Furthermore, the flexibility of the organic materials used in this project will allow lamps and displays to fit curved and bendable surfaces, opening a world of possibilities for architects and lighting designers that many of us have difficulty envisioning. The first OE products on the market are organic light-emitting diodes (OLEDs). These devices are formed by coating several layers of different organic materials on top of a transparent electrode (usually indium tin oxide, ITO) and are capped with an expensive and/or highly-reactive metal electrode. Although OLEDs represent an improvement in size and color over incumbent technologies, there is significant room for improvement in manufacturing cost and device flexibility. Specifically, OLEDs are extremely sensitive to the thickness of the layers in the device and the chemical properties of the electrode materials, pushing the limits of what low-cost coating and printing techniques are capable of. LECs employ mobile ions mixed with the same type of light-emitting material used in OLEDs. Simply stated, the mobile ions allow electrochemistry to occur within the emissive layer so that a sophisticated, multilayer p-n doping structure is created automatically when the device is turned on. The result requires only one layer of active material to be coated and is relatively insensitive to the thickness of that layer and the properties of the electrodes. We recently demonstrated a breakthrough in this area by creating a metal-free LEC employing graphene and plastic electrode materials that are all easily processed (from solution) and recyclable.

Project goals

Light-emitting electrochemical cells (LECs) promise a unique combination of advantages not attainable with competing emissive technologies, perhaps most easily summarized in the anticipated emergence of a low-cost and flexible “light-emitting wall-paper”. Our team has made significant contributions towards the realization of this vision, as exemplified by our recent demonstrations of: (i) an ambient roll-to-roll compatible fabrication of functional flexible LEC sheets and (ii) the pioneering demonstration of a metal-free light-emitting device comprising solely carbon-based materials. However, considerable additional work is needed before LEC devices and systems are ready for commercialization. Here, we present our ambitious research and development project for this end, which will result in the following major deliverables:

Demonstration of a high-performing LEC device through the development of novel and appropriate device architectures, materials combinations, and drive-voltage schemes. Our specific goal is to demonstrate LEC devices that emit with significant brightness (>100 cd/m2) for >10000 h of uninterrupted operation at high efficiency (>25 lm/W for green emission and >10 lm/W for red, blue and white emission). Note that we hold the current lifetime record for LECs, with an operational lifetime of 5600 h for a greenemitting device with a peak efficiency of 10 lm/W
The development and benchmarking of adaptable modeling tools, based on knowledge attained in dedicated LEC-related experiments, which will assist in a subsequent optimized design of devices and systems.
The realization of a commercially competitive production method within a dedicated facility, which will allow for low-cost roll-to-roll- fabrication and encapsulation of large-area devices.
The design and development of an all-plastic and flexible display system, comprising a speciallydesigned converter, that can be directly connected to a 230 V AC or a 12 V DC electrical outlet, and a low-information-content display, in which the switching transistor and the light-emission element are combined into one pixel unit.

Finally, we point out that we have established a company (LunaLEC AB) for the purpose of commercializing our LEC devices and systems. LunaLEC has ongoing collaborations with relevant commercial partners and this project will benefit from access to state-of-the-art industrial-grade materials and knowledge made available to, and developed by, the company; while LunaLEC in return will benefit from the know-how and developed IPR within this project, which will be made available to the company. All in all, we firmly believe that we have established an appropriate team and project plan, so that the long-term vision of a “light-emitting wall-paper” can be realized in Sweden!

Project and technology background

A device technology with similarities to the LEC is the organic light-emitting diode (OLED). OLEDs have recently been commercially introduced in ultrathin, bright, and low-voltage displays in cameras and mobile phones, and future applications in large-scale TVs and illumination are anticipated. LECs can also be fabricated in thin sandwich structures and be driven at low voltages (V = 2-4 V), but offer a number of important additional advantages over OLEDs. From a fabrication and commercial perspective, these advantages include the potential for an extremely straightforward and lowcost fabrication process, as all parts of an LEC device can be fabricated from air-stabile and solution-processable materials in a highly fault-tolerant process (something that OLEDs cannot offer). The main drawback with LECs in comparison to OLEDs has been an inadequate operational lifetime. We have recently improved upon the stability (and efficiency) of LECs significantly via dedicated and systematic studies of their complex operational mechanism. We are convinced that further improvements in device performance are attainable via the same path, and have therefore dedicated the remainder of the section to a description of the LEC operational mechanism and our studies within this field. This part will then serve as a background for some of the research projects outlined in the following sections.

An LEC consists of an ion- and electron-conducting active material connecting (often sandwiched between) two charge-injecting electrodes. The active material comprises an intimate blend of a semiconducting and fluorescent conjugated polymer (CP) and an electrolyte with mobile ions. When a voltage is applied between the electrodes, the mobile ions redistribute within the active material to form electric double layers at the electrode interfaces. If the applied voltages exceeds the “band-gap potential” of the CP (V ≥ Eg/e), electrons are injected at the negative cathode and holes at the positive anode. The subsequent processes in LECs have, however, been poorly understood and intensely debated for more than a decade, and it is clear that this lack of understanding has severely hampered the development of LECs into viable emissive devices.

An LEC consists of an ion- and electron-conducting active material connecting (often sandwiched between) two charge-injecting electrodes. The active material comprises an intimate blend of a semiconducting and fluorescent conjugated polymer (CP) and an electrolyte with mobile ions. When a voltage is applied between the electrodes, the mobile ions redistribute within the active material to form electric double layers at the electrode interfaces. If the applied voltages exceeds the “band-gap potential” of the CP (V ≥ Eg/e), electrons are injected at the negative cathode and holes at the positive anode. The subsequent processes in LECs have, however, been poorly understood and intensely debated for more than a decade, and it is clear that this lack of understanding has severely hampered the development of LECs into viable emissive devices.

Micrographs showing evolution of LEC turn-on.We have recently made a significant contribution to this end by comparing optical and scanning Kelvin probe microscopy experiments on open planar devices during operation.5 (Note: Open planar devices are an alternative device structure that allows the active area between the electrodes to be observed externally. This type of device is only possible with LECs, not with, e.g., OLEDs.) We proved that the CP becomes doped following the initial charge injection, that is to say that injected electrons (holes) attract electrostatically compensating cations (anions) from the electrolyte in a process termed n-type (p-type) doping at the cathodic (anodic) interface. These distinct doped regions grow with time and eventually make contact to form a p-n junction. (Note: This can be compared to a commercial OLED device structure in which electron- and hole-transporting layers are coated above and below the light-emitting CP. In the case of the LEC, these layers are formed automatically, in-situ, when a potential is applied.) Subsequently injected electrons and holes migrate though the high-conductivity doped regions before recombining to form excitons at the p-n junction. Such excitons can decay radiatively, and voilá a light-emitting p-n junction structure has formed in-situ within the device! The figure to the right presents a set of optical micrographs illustrating the doping process, as recorded on an open planar device structure.7 The appearance and growth of doping at the electrode interfaces are seen in the second and third micrographs as dark regions, and the formation of the lightemitting p-n junction at the meeting point of the two doped regions is clearly seen in the last micrograph.

We have managed to extract important and quite unexpected information on the doping process and structure in LECs, notably that the initial growth of doped regions takes place at constant doping concentration,8 that continued formation of doping can take place after the initial formation of the p-n junction, and that the final doping structure can be controlled via the choice of ion concentration in the pristine active material. The latter allows the doping structure to be tailored, e.g. we have been able to stop the doping progression in an LEC device under ion-depletion conditions (i.e., when all ions in the active material are electrostatically locked up in the doped regions). In the project proposed here, we will here utilize this information to demonstrate and optimize light-emitting transistor systems.

Another accomplishment has been pinpointing and identifying the electrochemical and chemical side reactions within the active material that limit LEC lifetime. The former process can, in “conventional” LEC devices, be attributed to the characteristic high LUMO level of most fluorescent CPs, as the thermodynamically preferred reduction reaction at the cathode is then not (the desired) n-type doping of the CP but instead (irreversible) reduction of the electrolyte. We have demonstrated that it is possible to reduce the extent of this undesired electrochemical side reaction by operating devices at a high applied potential for a brief period when first turned-on (in agreement with Marcus theory). The chemical side reactions in LEC devices were found to take place at, or close to, the light-emitting p-n junction, and involve detrimental interactions between the excitons on the CP and the electrolyte (ions and ion-solvating material). By fabricating devices with an “optimized” electrolyte content following rational design principles, and operating these devices at a high initial applied voltage, we have been able to reproducibly improve the operational lifetime from a typical value of a mere 2-5 hours to >1000 h at high brightness for both red- and green-emitting LECs. These are the longest operational lifetimes for CP-based LECs reported to date.

We have also initiated an effort to improve the energy efficiency of LEC devices, specifically the power conversion efficacy (PCE). The conventional unit for visible light power is lumen (lm), which takes into account that our eyes exhibit a strongly wavelength-dependent sensitivity, with the highest sensitivity in the green region at a wavelength of 555 nm. Accordingly, the maximum PCE value for such a monochromatic green-emitting device is 683 lm/W, while an “ideal” white-emitting device exhibits a lower value of ~240 lm/W and red- and blue-emitting devices demonstrate even lower maximum PCE values. As a further point of reference, it is of interest to note that the common incandescent light bulb (being phased-out in the US and Europe) has a PCE value of ~10 lm/W and an operational lifetime of ~1000 h, while its expected short-term replacement, the compact fluorescent lamp (commonly termed “the energy saving lamp”), typically exhibits a PCE value of ~60 lm/W and an operational lifetime of ~10000 h. In our laboratories, we have recently demonstrated red-emitting LEC devices with PCE values of >2 lm/W and green-emitting devices with 10 lm/W, which are record performances for CP-based LEC devices. These values are still modest in comparison to the state-of-the-art for energy-efficient illumination (such as light-emitting diodes), but our rapid progress during our first few years in this field is very encouraging. This project includes a number of promising approaches that we expect will result in significant improvements in device efficiency.

Publications

Peer-reviewed publications

Select popular science publications

Thesis

Qinye Bao: PhD Thesis Interface Phenomena in Organic Electronics, August 27, 2015

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