Light‐emitting diodes (LEDs) based on organic–inorganic hybrid perovskites, in particular, 3D and quasi‐2D ones, are in the fast development and their external quantum efficiencies (EQEs) have ...exceeded 10%, making them competitive candidates toward large‐area and low‐cost light‐emitting applications allowing printing techniques. Similar to other LED categories, light out‐coupling efficiency is an important parameter determining the EQE of perovskite LEDs (PeLEDs), which, however, is scarcely studied, limiting further efficiency improvement and understanding of PeLEDs. In this work, for the first time, optical energy losses in PeLEDs are investigated through systematic optical simulations, which reveal that the 3D and quasi‐2D PeLEDs can achieve theoretically maximum EQEs of ≈25% and ≈20%, respectively, in spite of their high refractive indices. These results are consistent with the reported experimental data. This work presents primary understanding of the optical energy losses in PeLEDs and will spur new developments in the aspects of device engineering and light extraction techniques to boost the EQEs of PeLEDs.
Systematic optical simulations on the optical modes in perovskite light‐emitting diodes (PeLEDs) are carried out. How the refractive index of perovskite materials affects the optical modes in PeLEDs is fully discussed. The results demonstrate that the out‐coupling efficiency of PeLEDs can reach more than 20% though perovskites have large refractive indices.
Organic light-emitting diodes (OLEDs) currently suffer mostly from only moderate outcoupling efficiencies, calling for many different photonic strategies to increase the share of extracted photons. ...The quantification of concepts relying on scattering based on periodic or random structures proves complicated and often deserves more attention. To evaluate light extraction methods of OLEDs, we define a procedure leading to a quantifiable extraction efficiency ηEE which can be calculated for top- or bottom-emitting OLEDs comprising external as well as internal outcoupling structures. Hence, the extraction efficiency allows to evaluate scattering approaches independently from the employed OLED system. A systematic study of 1D periodically corrugated red top-emitting OLEDs reveals the origin of a moderate enhancement due to an extraction efficiency of ηEE = 14 %. Furthermore, our work provides recommendations how OLEDs with periodic corrugations should be characterized.
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•A general quantification method of light outcoupling techniques is presented.•Exemplarily, the extraction efficiency is calculated for a series of top-OLEDs.•The method allows to judge outcoupling structures independently from architectures.
Reaching high current densities is an essential need to achieve electrically driven organic semiconductor lasers (OSLs). This requires highly conductive (usually metallic) electrodes which on the ...other hand cause unwanted optical losses. Here, we introduce a concept for organic light-emitting diodes (OLEDs) where the layer sequence is restructured in order to concentrate and maximize the current flow in a metal-free area. Thus, we do not only avoid parasitic absorption by the metal electrodes, but also drastically reduce the power dissipation. These devices enable higher current densities than the standard OLED design due to reduced self-heating as a consequence of an active area which automatically shrinks down to an appropriate size. They further allow for integration into an organic microcavity. Therefore, the new concept is well suited to realize an electrically driven OSL in future.
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•A new OLED architecture for high current density application is proposed.•The emission is expanded to the electrode-free area by a lateral transport layer.•Restructuring the layer stack results in a decreased Joule heating of the device.•Including an insulating layer increases the electrical breakdown voltage.•Current densities up to 1 kA/cm2 are achievable for an active area of 5 μm.
This paper demonstrates 2-stack and 3-stack white organic light-emitting diodes (WOLEDs) with fluorescent blue and phosphorescent yellow emissive units. The 2-stack and 3-stack WOLED comprises ...blue-yellow and blue-blue-yellow (blue-yellow-blue) combinations. The position of the yellow emitter and possible cavity lengths in different stack architectures are theoretically and experimentally investigated to reach Commission Internationale de L'Eclairage (CIE) coordinates of near (0.333/0.333). Here, a maximum external quantum efficiency (EQE) of 23.6% and current efficiency of 62.2 cd/A at 1000 cd/m
as well as suitable CIE color coordinates of (0.335/0.313) for the blue-blue-yellow 3-stack hybrid WOLED structure is reported. In addition, the blue-yellow-blue 3-stack architecture exhibits an improved angular dependence compared to the blue-blue-yellow structure at a decreased EQE of 19.1%.
In our previous work, we investigated the effects of the formation of ultrastable glasses within monochrome phosphorescent organic light‐emitting diodes.1 The ultrastable glasses are fabricated by ...heating the glass substrate to about 85% of the glass transition temperature while evaporation of the material. We observed significant enhancements of the external quantum efficiency (in the range of 20%) and device lifetime (up to 4x). These improvements are attributed to a denser packaging of the organic molecules resulting in reduced non‐radiative rates of the emitters. The change in the nano‐morphology can also have impact on the charge transport, the radiative rates of the emitter, and emitter orientation. For those properties, we have not identified a significant impact on the device performance. In this paper, we apply our concept to a green thermally activated delayed fluorescent emitter in a proof of principle device series. We find that insufficient energy transfer to the emitter holds back our devices from achieving state‐of‐the‐art efficiencies.
Providing artificial light and enhancing the quality of the respective light sources is of continued interest in the fields of solid state, condensed matter, and semiconductor physics. Much research ...has been carried out to increase the luminous efficiency, lifetime and colour stability of such devices. However, the emission characteristics of a given light source do not necessarily comply with today's often sophisticated applications. Here, beam shaping addresses the transformation of a given light distribution into a customized form. This is typically achieved by secondary optical elements often sporting elaborate designs, where the actual light source takes up only a small fraction of the system's volume. Such designs limit the final light source to a single permanent operation mode, which can only be overcome by employing mechanically adjustable optical elements. Here we show that organic light-emitting diodes (OLEDs) can enable real-time regulation of a beam shape without relying on secondary optical elements and without using any mechanical adjustment. For a red light-emitting two-unit OLED architecture, we demonstrate the ability to continuously tune between strongly forward and strongly sideward emission, where the device efficiency is maintained at an application-relevant level ranging between 6 and 8% of external quantum efficiency for any chosen setting. In combination with additional optical elements, customizable and tuneable systems are possible, whereby the tuning stems from the light source itself rather than from the use of secondary optics.
This paper reports on the latest development on ultra low‐power microdisplays. Initially this architecture has been presented in a monochrome version in 4. Due to the demand of extended applications ...beyond pure information displays towards situation aware notifications a new colored version has been developed with the main achievement of cutting down the pixel size by more than a factor of two to enable color sub‐pixels. Furthermore special attention was paid on very high brightness to also enable implementation in situation‐aware augmented reality optics.
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