Degradation Mechanisms in Small‐Molecule and Polymer Organic Light‐Emitting Diodes

Degradation in organic light‐emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degradation mechanism can be very different. In this Progress Report, using examples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small‐molecule and polymer OLEDs is presented. For small‐molecule OLEDs, the operational degradation of exemplary fluorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance‐efficiency loss and operating‐voltage rise. For polymer OLEDs, it is shown how the charge‐transport and injection properties affect the device lifetime. Further, it is shown how the charge balance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.     

OLEDs: Degradation Mechanisms in Small‐Molecule and Polymer Organic Light‐Emitting Diodes (Adv. Mater. 34/2010)

Degradation in organic light‐emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degradation mechanism can be very different. In this Progress Report, using examples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small‐molecule and polymer OLEDs is presented. For small‐molecule OLEDs, the operational degradation of exemplary fluorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance‐efficiency loss and operating‐voltage rise. For polymer OLEDs, it is shown how the charge‐transport and injection properties affect the device lifetime. Further, it is shown how the charge balance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.     

Recent Developments in Top‐Emitting Organic Light‐Emitting Diodes

Organic light‐emitting diodes (OLEDs) have rapidly progressed in recent years due to their unique characteristics and potential applications in flat panel displays. Significant advancements in top‐emitting OLEDs have driven the development of large‐size screens and microdisplays with high resolution and large aperture ratio. After a brief introduction to the architecture and types of top‐emitting OLEDs, the microcavity theory typically used in top‐emitting OLEDs is described in detail here. Then, methods for producing and understanding monochromatic (red, green, and blue) and white top‐emitting OLEDs are summarized and discussed. Finally, the status of display development based on top‐emitting OLEDs is briefly addressed.     

Semiconductor‐Nanocrystals‐Based White Light‐Emitting Diodes

In response to the demands for energy and the concerns of global warming and climate change, energy efficient and environmentally friendly solid‐state lighting, such as white light‐emitting diodes (WLEDs), is considered to be the most promising and suitable light source. Because of their small size, high efficiency, and long lifetime, WLEDs based on colloidal semiconductor nanocrystals (or quantum dots) are emerging as a completely new technology platform for the development of flat‐panel displays and solid‐state lighting, exhibiting the potential to replace the conventionally used incandescent and fluorescent lamps. This replacement can cut the ever‐increasing level of energy consumption, solve the problem of rapidly depleting fossil fuel reserves, and improve the quality of the global environment. In this review, the recent progress in semiconductor‐nanocrystals‐based WLEDs is highlighted, the different approaches for generating white light are compared, and the benefits and challenges of the solid‐state lighting technology are discussed.     

Recent Progress in GaN‐Based Light‐Emitting Diodes

In the last few years the GaN‐based white light‐emitting diode (LED) has been remarkable as a commercially available solid‐state light source. To increase the luminescence power, we studied GaN LED epitaxial materials. First, a special maskless V‐grooved c‐plane sapphire was fabricated, a GaN lateral epitaxial overgrowth method on this substrate was developed, and consequently GaN films are obtained with low dislocation densities and an increased light‐emitting efficiency (because of the enhanced reflection from the V‐grooved plane). Furthermore, anomalous tunneling‐assisted carrier transfer in an asymmetrically coupled InGaN/GaN quantum well structure was studied. A new quantum well structure using this effect is designed to enhance the luminescent efficiency of the LED to ～72%. Finally, a single‐chip phosphor‐free white LED is fabricated, a stable white light is emitted for currents from 20 to 60 mA, which makes the LED chip suitable for lighting applications.     

Surface‐Plasmon‐Enhanced Perovskite Light‐Emitting Diodes

Perovskite light‐emitting diodes (PeLEDs) have attracted considerable attention because of their potential in display and lighting applications. To promote commercialization of PeLEDs, it is important to improve the external quantum efficiency of the devices, which depends on their internal quantum efficiency (IQE) and light extraction efficiency. Optical simulations have revealed that 20–50% of the light generated in the device will be lost to surface plasmon (SP) modes formed in the metal/dielectric interfaces. Therefore, extracting the optical energy in SP modes to the air will greatly increase the light extraction efficiency of PeLEDs. In addition, the SPs can accelerate radiative recombination of the emitter via near‐field effects. Thus, the IQE of a PeLED can also be enhanced by SP manipulation. In this review, first, general concepts of the SPs and how they can enhance the efficiency of LEDs are introduced. Then recent progresses in SP‐enhanced emission of perovskite films and LEDs are systematically reviewed. After that, the challenges and opportunities of the SP‐enhanced PeLEDs are shown, followed by an outlook of further development of the SPs in perovskite optoelectronic devices.     

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

The design of thermally activated delayed fluorescence (TADF) materials both as emitters and as hosts is an exploding area of research. The replacement of phosphorescent metal complexes with inexpensive organic compounds in electroluminescent (EL) devices that demonstrate comparable performance metrics is paradigm shifting, as these new materials offer the possibility of developing low‐cost lighting and displays. Here, a comprehensive review of TADF materials is presented, with a focus on linking their optoelectronic behavior with the performance of the organic light‐emitting diode (OLED) and related EL devices. TADF emitters are cross‐compared within specific color ranges, with a focus on blue, green–yellow, orange–red, and white OLEDs. Organic small‐molecule, dendrimer, polymer, and exciplex emitters are all discussed within this review, as is their use as host materials. Correlations are provided between the structure of the TADF materials and their optoelectronic properties. The success of TADF materials has ushered in the next generation of OLEDs.     

Monolithic Flexible Vertical GaN Light‐Emitting Diodes for a Transparent Wireless Brain Optical Stimulator

Flexible inorganic‐based micro light‐emitting diodes (µLEDs) are emerging as a significant technology for flexible displays, which is an important area for bilateral visual communication in the upcoming Internet of Things era. Conventional flexible lateral µLEDs have been investigated by several researchers, but still have significant issues of power consumption, thermal stability, lifetime, and light‐extraction efficiency on plastics. Here, high‐performance flexible vertical GaN light‐emitting diodes (LEDs) are demonstrated by silver nanowire networks and monolithic fabrication. Transparent, ultrathin GaN LED arrays adhere to a human fingernail and stably glow without any mechanical deformation. Experimental studies provide outstanding characteristics of the flexible vertical μLEDs (f‐VLEDs) with high optical power (30 mW mm^?2), long lifetime (≈12 years), and good thermal/mechanical stability (100 000 bending/unbending cycles). The wireless light‐emitting system on the human skin is successfully realized by transferring the electrical power f‐VLED. Finally, the high‐density GaN f‐VLED arrays are inserted onto a living mouse cortex and operated without significant histological damage of brain.     

Exploiting Singlet Fission in Organic Light‐Emitting Diodes

Harvesting of both triplets and singlets yields electroluminescence quantum efficiencies of nearly 100% in organic light‐emitting diodes (OLEDs), but the production efficiency of excitons that can undergo radiative decay is theoretically limited to 100% of the electron–hole pairs. Here, breaking of this limit by exploiting singlet fission in an OLED is reported. Based on the dependence of electroluminescence intensity on an applied magnetic field, it is confirmed that triplets produced by singlet fission in a rubrene host matrix are emitted as near‐infrared (NIR) electroluminescence by erbium(III) tris(8‐hydroxyquinoline) (ErQ₃) after excitonic energy transfer from the “dark” triplet state of rubrene to an “emissive” state of ErQ₃, leading to NIR electroluminescence with an overall exciton production efficiency of 100.8%. This demonstration clearly indicates that the harvesting of triplets produced by singlet fission as electroluminescence is possible even under electrical excitation, leading to an enhancement of the quantum efficiency of the OLEDs. Electroluminescence employing singlet fission provides a route toward developing high‐intensity NIR light sources, which are of particular interest for sensing, optical communications, and medical applications.