Spontaneous buckling in flexible organic light-emitting devices for enhanced light extraction
Spotlight summary: Wrinkles are something that everyone wants to avoid. According to the work of Park and Jeon, however, things could be a little different when it comes to organic light-emitting diodes (OLEDs), whose unparalleled high quality has made them thrive amidst fierce competition in the portable electronics market. Despite their recent success, the key applications of OLEDs – display and lighting – still demand improvement in OLED efficiency. This is partly because improved efficiency not only offers a high value of lumens per watt but also helps to extend the lifetime of devices made of such OLEDs, eventually lowering the total cost of ownership.
The efficiency of OLEDs can be limited by many different factors, but the most notable one is the so-called “outcoupling bottleneck.” It accounts for the optical limitation inherent to the typical stratified geometry of OLEDs. A significant portion of the photons generated within the emissive layers in OLEDs cannot pass through the front face due to the total internal reflections occurring at several key interfaces; instead, many of the photons are confined in the substrate (referred to as “substrate modes”) or guided within the organic and transparent conductive oxide (TCO) layers (referred to as “waveguide modes”). The upper limit for the outcoupling efficiency, defined as the ratio of the number of outcoupled photons to that of internally generated photons, can be estimated under a suitable optical model. Calculations based on a simple assumption of isotropic emitters and geometrical optics show that this estimated number of internally generated photons is merely in the range of 15-20% in typical OLEDs. Although this number can vary depending on the model and its level of sophistication, it strongly suggests that there is indeed an ample amount of room for efficiency improvement in OLEDs, which is exactly what the authors in this Optics Express article have tried to achieve, based on “wrinkles” formed by a process called “spontaneous buckling.”
As the term suggests, spontaneous buckling is a process in which an object is deformed to spontaneously form structures like wrinkles under stain or stress. The process typically involves an elastic medium that can respond to mechanical / thermal strain or stress. In fact, Koo et al. previously demonstrated that the spontaneous buckling process and the nano-scale wrinkles formed thereby can be useful in extracting light that would otherwise be waveguided in organic and TCO layers and eventually lost by absorption. The major success (and difference from the prior art) in the work by Park and Jeon lies in their discovery that the spontaneous buckling process has a natural compatibility with flexible OLEDs. The authors found that wrinkles can be formed spontaneously on organic layers that are laid on top of the substrate and the conducting polymer layer during the process of “hot” cathode metal deposition and natural cooling. That is, the authors generate nanoscale wrinkled structures simply by letting the elastic/ thermal property of the plastic substrates and conducting polymers do the work. Their experimental results show that these wrinkles can disrupt the waveguide modes and convert them into outcoupled modes. Another potential benefit anticipated in this work is that the wrinkled interfaces may suppress the plasmonic resonant coupling, which is another critical loss mechanism typical in planar OLEDs. In addition, the wrinkled structure leads to a lower operating voltage due to the field enhancement commonly found in OLEDs with such corrugated structures. These effects being combined, the authors were able to demonstrate an impressive 3× improvement in power efficiency with respect to the planar, glass-based reference cells. Of course, judging a certain technology by its ‘relative enhancement’ can be rather tricky, especially when reference samples are in a geometry that is not well known or common, which is more or less the case here. Nevertheless, the achievement made by the authors can be regarded as significant given the fact that such a large enhancement can be obtained without any special process.
The final success of the proposed technology will eventually be determined by whether one can suppress, in a controllable manner, the potential side effects that can often come with corrugated structures: e.g., vulnerability to electrical leakage, spectral dependence, and optical scattering/ haze. Another thing that will be crucial to the proposed technology is to secure flexible transparent conductors with sufficiently high conductivity and the transmittance required by a given application. The characteristics of the conducting polymer used in this work, commonly referred to as PEDOT:PSS, are being steadily improved, yet they have not reached the level of ITO electrodes until now. Fortunately, researchers worldwide are actively searching for an ITO-alternative with comparable performance and flexibility. In the meantime, metal grids combined with PEDOT:PSS can be used at the expense of the optical shadowing effect.
It is further noteworthy that there have been lots of proposals for outcoupling-enhancing structures, yet it is still only the structure with simple planar geometry that has passed the stringent tests needed for commercialization. In the end, the adoption of any given outcoupling enhancement technique will be contingent on whether the technology can reach a good balance between performance and manufacturability.
Technical Division: Optoelectronics
ToC Category: Optoelectronics
|OCIS Codes:||(250.3680) Optoelectronics : Light-emitting polymers|
|(260.3800) Physical optics : Luminescence|
|(310.1860) Thin films : Deposition and fabrication|
|(310.6845) Thin films : Thin film devices and applications|
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