Organic light‐emitting diodes (OLEDs) are established as a mainstream light source for display applications and can now be found in a plethora of consumer electronic devices used daily. This success can be attributed to the rich luminescent properties of organic materials, but efficiency enhancement made over the last few decades has also played a significant role in making OLEDs a practically viable technology. This report summarizes the efforts made so far to improve the external quantum efficiency (EQE) of OLEDs and discusses what should further be done to push toward the ultimate efficiency that can be offered by OLEDs. The study indicates that EQE close to 58% and 80% can be within reach without and with additional light extraction structures, respectively, with an optimal combination of cavity engineering, low‐index transport layers, and horizontal dipole orientation. In addition, recent endeavors to identify possible applications of OLEDs beyond displays are presented with emphasis on their potential in wearable healthcare, such as OLED‐based pulse oximetry as well as phototherapeutic applications based on body‐attachable flexible OLED patches. OLEDs with fabric‐like form factors and washable encapsulation strategies are also introduced as technologies essential to the success of OLED‐based wearable electronics.

Optoelectronic devices that are stretchable can revolutionize various fields by enabling the functionalities that are not accessible from conventional rigid platforms. In this respect, many efforts are made to realize stretchable organic light‐emitting diodes (OLEDs) for their compatibility with various deformable substrates. Two representative examples of such efforts include a method based on intrinsically stretchable materials and a method to attach ultrathin OLEDs on a prestrained elastomer substrate. However, both of those methods have limitations either in material availability for high performance or in optical clarity. Here a hybrid platform is proposed for stretchable OLEDs free from such limitations. The proposed platform is composed of an array of rigid islands connected by serpentine‐shaped interconnectors formed on a bilayer elastomer substrate in which the top layer has an ultralow Young's modulus of ≈0.9 kPa, greatly relieving the rigid islands and interconnectors of the mechanical stress applied to them when stretched. With the proposed platform, stretchable OLEDs are demonstrated that can be repeatedly stretched by 140% and are based on thermally evaporable materials well established for fabrication of high‐performance OLEDs.

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Recent advances in highly efficient organic light‐emitting diodes (OLEDs) and their emerging applications, including phototherapeutic patches, biometric sensors, and textile displays are reviewed by Kyung Cheol Choi, Seunghyup Yoo, and co‐workers in article number 1907539. For efficiency improvement, the importance of outcoupling structures coupled with horizontally oriented dipole emitters is highlighted.

The pulse oximeter (PO) is an essential healthcare sensor that monitors the heart rate and blood oxygen level. With the emergence of wearable form factors, its use is rapidly expanding from applications in clinical environments to fitness, daily activities, and point-of-care applications. However, the relatively high power consumption of commercial POs has been an obstacle to applying them to wearables, which generally have a limited on-board power source. In this work, we propose a hybrid reflectiontype (R-type) PO that adopts inorganic light-emitting diodes (LEDs) and a wrap-around organic photodiode (OPD), which are conveniently integrated via lamination. The overall structure is carefully optimized to minimize direct coupling of light from the LEDs to OPDs and to maximize the meaningful signal through an optical simulation. In particular, we provide a method for optical simulation resolving the deepest layer visited by a photon that returns back to a specific point on the skin so that one can better estimate the relative portion of photons that penetrate deeply enough to contribute to the signal in R-type configuration. The resultant hybrid POs, in which red and near-infrared LEDs are alternately turned on, are shown to be operable with the average LED driving power as low as ca. 35 μW (at 25% duty), demonstrating their immense potential for wearable POs with practical viability.