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The concept of a metal-free and all-organic electroluminescent device is appealing from both sustainability and cost perspectives. Herein, we report the design and fabrication of such a light-emitting electrochemical cell (LEC), comprising a blend of an emissive semiconducting polymer and an ionic liquid as the active material sandwiched between two poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) conducting-polymer electrodes. In the off-state, this all-organic LEC is highly transparent, and in the on-state, it delivers uniform and fast to turn-on bright surface emission. It is notable that all three device layers were fabricated by material- and cost-efficient spray-coating under ambient air. For the electrodes, we systematically investigated and developed a large number of PEDOT:PSS formulations. We call particular attention to one such p-type doped PEDOT:PSS formulation that was demonstrated to function as the negative cathode, as well as future attempts towards all-organic LECs to carefully consider the effects of electrochemical doping of the electrode in order to achieve optimum device performance.

The initial operation of a light-emitting electrochemical cell (LEC) constitutes the in-situ formation of a p-n junction doping structure in the active material by electrochemical doping. It has been firmly established that the spatial position of the emissive p-n junction in the interelectrode gap has a profound influence on the LEC performance because of exciton quenching and microcavity effects. Hence, practical strategies for a control of the position of the p-n junction in LEC devices are highly desired. Here, we introduce a "chemical pre-doping" approach for the rational shifting of the p-n junction for improved performance. Specifically, we demonstrate, by combined experiments and simulations, that the addition of a strong chemical reductant termed "reduced benzyl viologen" to a common active-material ink during LEC fabrication results in a filling of deep electron traps and an associated shifting of the emissive p-n junction from the center of the active material towards the positive anode. We finally demonstrate that this chemical pre-doping approach can improve the emission efficiency and stability of a common LEC device.

The shift from depleting petroleum compounds to regenerating biomass as the raw material for organic semiconductors is a prerequisite if organic electronics is to become truly sustainable. Here, we report on a one-pot solvothermal synthesis of a biomass-based carbon dot (bio-CD) fluorescent semiconductor, using birch leaves as the sole raw material. These bio-CDs are highly soluble in ethanol (45 g L^-1), and deliver deep-red and narrowband emission (peak wavelength = 675 nm, full width at half maximum, FWHM = 28 nm) at a high photoluminescence quantum yield of 26% in ethanol solution. Systematic structural characterization shows that molecular pheophytin a is the single fluorophore, and that this fluorophore is localized in the bulk of the bio-CD away from its polar surface. The functionality of the birch-leaf-derived bio-CDs in sustainable organic electronics is demonstrated by its employment as the printable emitter in a light-emitting electrochemical cell, which delivers narrowband deep-red luminance of 110 cd m^-2, with a FWHM of 29 nm, at an external quantum efficiency of 0.29%. This study thus reveals a promising avenue for the functional benign synthesis and the practical solution-based implementation of narrowband bio-CDs in sustainable optoelectronic technologies.

Our world is filling up with electronics. High-tech gadgets are integrated everywhere, from smart fridges able to track expiry dates and food usage, to microchip implants that let us unlock doors and pay with our hands. As innovative as they are, these new products and the many more to come impose new requirements on materials and fabrication methods. For instance, emerging electronic technologies that deliver light emission such as smart labels, authenticity features and light-based medical therapies, are often required to be flexible, see-through and low-cost, and in addition sustainable to fabricate, operate and recycle.

In response to these challenges, the light-emitting industry is turning to organic electronics for solutions, a field that promises resource-efficient fabrication using environmentally benign materials. An interesting proposal is that of the light-emitting electrochemical cell (LEC), which, thanks to its simple structure, is well suitedfor high-throughput fabrication. The LEC is in many aspects a smart device, able to reorganize itself during operation via the electrochemical action of mobile ions, which create the injection and transport layers that require additional fabrication steps in other technologies. This elegant behavior makes the LEC tolerant to a large array of materials and fabrication methods, and hence a good fit for many applications.

Yet the LEC is still today a scientific curiosity rather than an actual commercial solution and among the very few prototypes available on the market, none are able to meet the combined performance, resource efficiency and sustainability criteria. As a matter of fact, of the three layers that make an LEC, i.e., two electrodes surrounding an active material, only the later meet these requirements thanks to a strong recent research effort. In comparison, the electrodes have received little attention and are almost exclusively comprising metals or metal oxides deposited by time- and energy-expensive fabrication methods, making the LEC as a whole unfit for many applications.

In an effort to push the LEC toward the untapped commercial niche of low-cost lighting, we tackle the problem of electrode fabrication with resource-efficiency in mind. We first show that up-scalable spray coating of inks under ambient air is a viable mean of fabrication for both active materials and electrodes alike. However, in doing so, we find that we create electrode interfaces that are open to ion transfer; an up-to-now overlooked issue that needs careful consideration when designing solution-processed LECs. Building on our discovery, we demonstrate that it is possible to fabricate an LEC entirely by using spray coating metal-free and organic inks; thereby demonstrating that an all-organic, metal-free and resource-efficient LEC is possible.

I hope that our efforts will encourage others to work on solution-processed LECs, electrodes included, and develop ready-to-use products.

A light-emitting electrochemical cell (LEC) comprises mobile ions in its active material, which enable for in situ formation of a p-n junction by electrochemical doping. The position of this emissive p-n junction in the interelectrode gap is important, because it determines whether the emission is affected by constructive or destructive interference. An appealing LEC feature is that the entire device can be fabricated by low-cost solution-based printing and coating. Here, we show, somewhat unexpectedly, that the replacement of conventional vacuum-deposited indium-tin-oxide (ITO) for the positive anode with solution-processed poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) can result in an increase in the peak light-emission output by 75%. We demonstrate that this emission increase is due to that the p-n junction shifts from a position of destructive interference in the center of the interelectrode gap with ITO to a position of constructive interference closer to the anode with PEDOT:PSS. We rationalize the anodic p-n junction shift by significant anion transfer into the soft and porous PEDOT:PSS electrode during LEC operation, which is prohibited for the ITO electrode because of its compact and hard nature. Our study, thus, contributes with important design criteria for the attainment of efficient light emission from solution-processed LEC devices.

The light-emitting electrochemical cell (LEC) allows for energy- and cost-efficient printing and coating fabrication of its entire device structure, including both electrodes and the single-layer active material. This attractive fabrication opportunity is enabled by the electrochemical action of mobile ions in the active material. However, a related and up to now overlooked issue is that such solution-fabricated LECs commonly comprise electrode/active-material interfaces that are open for transfer of the mobile ions, and it is herein demonstrated that a majority of the mobile anions in a common spray-coated active material can transfer into a spray-coated poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) positive electrode during LEC operation. Since it is well established that the mobile ion concentration in the active material has a profound influence on the LEC performance, this significant ion transfer is an important factor that should be considered in the design of low-cost LEC devices that deliver high performance.

The light-emitting electrochemical cell (LEC) has demonstrated capacity for cost- and material-efficient solution-based fabrication of the active material under ambient air. In this context, it is notable that corresponding reports on a scalable solution-based fabrication of the electrodes, particularly the top electrode, are rare. We address this issue through the demonstration of a transparent LEC, which is fabricated under ambient air by sequential spray deposition of a hydrophobic conjugated-polymer:ionic-liquid blend ink as the active material and a hydrophilic poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) ink as the transparent top electrode. Such an optimized LEC delivers a luminance of 360 cd/m2 at a power efficacy of 1.6 lm/W, which is on par with the performance of a corresponding LEC device equipped with a vacuum-deposited and reflective metal top electrode. This implies that the entire LEC device indeed can be fabricated with solution-based processes and deliver a good performance, which is critical if the LEC technology is going to fulfil its low-cost potential.