High‐Brightness Blue Light‐Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer

Single‐crystalline GaN‐based light‐emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid‐state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high‐quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high‐brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal–organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and In_xGa_1?xN/GaN multiple quantum well structures. The as‐fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one‐step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high‐brightness LEDs.     

Optimization of Low‐Dimensional Components of Quasi‐2D Perovskite Films for Deep‐Blue Light‐Emitting Diodes

Compared to efficient green and near‐infrared light‐emitting diodes (LEDs), less progress has been made on deep‐blue perovskite LEDs. They suffer from inefficient domain [various number of PbX₆^? layers (n)] control, resulting in a series of unfavorable issues such as unstable color, multipeak profile, and poor fluorescence yield. Here, a strategy involving a delicate spacer modulation for quasi‐2D perovskite films via an introduction of aromatic polyamine molecules into the perovskite precursor is reported. With low‐dimensional component engineering, the n₁ domain, which shows nonradiative recombination and retarded exciton transfer, is significantly suppressed. Also, the n₃ domain, which represents the population of emission species, is remarkably increased. The optimized quasi‐2D perovskite film presents blue emission from the n₃ domain (peak at 465 nm) with a photoluminescence quantum yield (PLQY) as high as 77%. It enables the corresponding perovskite LEDs to deliver stable deep‐blue emission (CIE (0.145, 0.05)) with an external quantum efficiency (EQE) of 2.6%. The findings in this work provide further understanding on the structural and emission properties of quasi‐2D perovskites, which pave a new route to design deep‐blue‐emissive perovskite materials.     

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.     

Highly Efficient Spectrally Stable Red Perovskite Light‐Emitting Diodes

Perovskite light‐emitting diodes (LEDs) have recently attracted great research interest for their narrow emissions and solution processability. Remarkable progress has been achieved in green perovskite LEDs in recent years, but not blue or red ones. Here, highly efficient and spectrally stable red perovskite LEDs with quasi‐2D perovskite/poly(ethylene oxide) (PEO) composite thin films as the light‐emitting layer are reported. By controlling the molar ratios of organic salt (benzylammonium iodide) to inorganic salts (cesium iodide and lead iodide), luminescent quasi‐2D perovskite thin films are obtained with tunable emission colors from red to deep red. The perovskite/polymer composite approach enables quasi‐2D perovskite/PEO composite thin films to possess much higher photoluminescence quantum efficiencies and smoothness than their neat quasi‐2D perovskite counterparts. Electrically driven LEDs with emissions peaked at 638, 664, 680, and 690 nm have been fabricated to exhibit high brightness and external quantum efficiencies (EQEs). For instance, the perovskite LED with an emission peaked at 680 nm exhibits a brightness of 1392 cd m^?2 and an EQE of 6.23%. Moreover, exceptional electroluminescence spectral stability under continuous device operation has been achieved for these red perovskite LEDs.     

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.     

Bis‐Tridentate Ir(III) Metal Phosphors for Efficient Deep‐Blue Organic Light‐Emitting Diodes

Emissive Ir(III) metal complexes possessing two tridentate chelates (bis‐tridentate) are known to be more robust compared to those with three bidentate chelates (tris‐bidentate). Here, the deep‐blue‐emitting, bis‐tridentate Ir(III) metal phosphors bearing both the dicarbene pincer ancillary such as 2,6‐diimidazolylidene benzene and the 6‐pyrazolyl‐2‐phenoxylpyridine chromophoric chelate are synthesized. A deep‐blue organic light‐emitting diode from one phosphor exhibits Commission Internationale de l'Eclairage (CIE_{(x ,y )}) coordinates of (0.15, 0.17) with maximum external quantum efficiency (max. EQE) of 20.7% and EQE = 14.6% at the practical brightness of 100 cd m^?2.     

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.     

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.     

Organic Materials for Deep Blue Phosphorescent Organic Light‐Emitting Diodes

Recently, great progress has been made in the device performance of deep blue phosphorescent organic light‐emitting diodes (PHOLEDs) by developing high triplet energy charge‐transport materials, high triplet energy host and deep blue emitting phosphorescent dopant materials. A high quantum efficiency of over 25% and a high power efficiency of over 15 lm/W have already been achieved at 1000 cd m^?2 in the deep blue PHOLEDs with a y color coordinate less than 0.20. In this work, recent developments in organic materials for high efficiency deep blue PHOLEDs are reviewed and a future strategy for the development of high efficiency deep blue PHOLEDs is proposed.     

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.