Small Molecule Host Materials for Solution Processed Phosphorescent Organic Light‐Emitting Diodes

Solution processed phosphorescent organic light‐emitting diodes (OLEDs) have been actively developed due to merits of high quantum efficiency of phosphorescent materials and simple fabrication processes of solution processed OLEDs. The device performances of the solution processed phosphorescent OLEDs have been greatly improved in the last 10 years and the progress of the device performances was made by the development of small molecule host materials for solution processes. A hybrid host of polymer and small molecules, a single small molecule host and a mixed host of small molecule hosts have effectively enhanced the quantum efficiency of the solution processed phosphorescent OLEDs. Therefore, this paper reviews recent developments in small molecule host materials for solution processed phosphorescent OLEDs and provides future directions for the development of the small molecule host materials.     

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 Advances in White Organic Light‐Emitting Materials and Devices (WOLEDs)

WOLEDs offer new design opportunities in practical solid‐state lighting and could play a significant role in reducing global energy consumption. Obtaining white light from organic LEDs is a considerable challenge. Alongside the development of new materials with improved color stability and balanced charge transport properties, major issues involve the fabrication of large‐area devices and the development of low‐cost manufacturing technology. This Review will describe the types of materials (small molecules and polymers) that have been used to fabricate WOLEDs. A range of device architectures are presented and appraised.     

Long‐Lived Flexible Displays Employing Efficient and Stable Inverted Organic Light‐Emitting Diodes

Although organic light‐emitting diodes (OLEDs) are promising for use in applications such as in flexible displays, reports of long‐lived flexible OLED‐based devices are limited due to the poor environmental stability of OLEDs. Flexible substrates such as plastic allow ambient oxygen and moisture to permeate into devices, which degrades the alkali metals used for the electron‐injection layer in conventional OLEDs (cOLEDs). Here, the fabrication of a long‐lived flexible display is reported using efficient and stable inverted OLEDs (iOLEDs), in which electrons can be effectively injected without the use of alkali metals. The flexible display employing iOLEDs can emit light for over 1 year with simplified encapsulation, whereas a flexible display employing cOLEDs exhibits almost no luminescence after only 21 d with the same encapsulation. These results demonstrate the great potential of iOLEDs to replace cOLEDs employing alkali metals for use in a wide variety of flexible organic optoelectronic devices.     

All‐Solution‐Processed Pure Formamidinium‐Based Perovskite Light‐Emitting Diodes

All‐solution‐processed pure formamidinium‐based perovskite light‐emitting diodes (PeLEDs) with record performance are successfully realized. It is found that the FAPbBr₃ device is hole dominant. To achieve charge carrier balance, on the anode side, PEDOT:PSS 8000 is employed as the hole injection layer, replacing PEDOT:PSS 4083 to suppress the hole current. On the cathode side, the solution‐processed ZnO nanoparticle (NP) is used as the electron injection layer in regular PeLEDs to improve the electron current. With the smallest ZnO NPs (2.9 nm) as electron injection layer (EIL), the solution‐processed PeLED exhibits a highest forward viewing power efficiency of 22.3 lm W^?1, a peak current efficiency of 21.3 cd A^?1, and an external quantum efficiency of 4.66%. The maximum brightness reaches a record 1.09 × 10⁵ cd m^?2. A record lifetime T₅₀ of 436 s is achieved at the initial brightness of 10 000 cd m^?2. Not only do PEDOT:PSS 8000 HIL and ZnO NPs EIL modulate the injected charge carriers to reach charge balance, but also they prevent the exciton quenching at the interface between the charge injection layer and the light emission layer. The subbandgap turn‐on voltage is attributed to Auger‐assisted energy up‐conversion process.