High Current Injection into Dynamic p–n Homojunction in Polymer Light‐Emitting Electrochemical Cells

Ions and electrons in blends of polymer–electrolyte can work in ensemble to operate light‐emitting electrochemical cells (LECs), in which the unique features of in situ formed p–n homojunctions offer efficient charge injection and transport. However, electrochemical features give rise to significant stability and speed issues due to limited electrochemical stability and low ion mobility, resulting in low brightness and a slow response of LECs. Here, these issues are overcome by the separate control of ionic and electronic charges, using a simple driving pulse superimposed on a small base voltage; ions with slow response are rearranged by a constant base voltage, while a high‐voltage pulse, superimposed upon the base, injects electrons/holes which have fast response, with minimal effect on the ions. This scheme successfully injects an extremely high current density of > 2 kA cm^?2 with a balanced electron/hole ratio, at a high‐speed response time of ≈ 50 ns; both properties demonstrate advantages of LECs in making polymers brighter. An in situ electron spin resonance measurement on the LECs further revealed that this impressive performance is due to the highly doped polymers, whose spin density reached 7 × 10¹⁹ spins cm^?3, and an ordered polymer structure in the active layer blend.     

Solar‐Energy Production and Energy‐Efficient Lighting: Photovoltaic Devices and White‐Light‐Emitting Diodes Using Poly(2,7‐fluorene), Poly(2,7‐carbazole), and Poly(2,7‐dibenzosilole) Derivatives

World energy needs grow each year. To address global warming and climate changes the search for renewable energy sources with limited greenhouse gas emissions and the development of energy‐efficient lighting devices are underway. This Review reports recent progress made in the synthesis and characterization of conjugated polymers based on bridged phenylenes, namely, poly(2,7‐fluorene)s, poly(2,7‐carbazole)s, and poly(2,7‐dibenzosilole)s, for applications in solar cells and white‐light‐emitting diodes. The main strategies and remaining challenges in the development of reliable and low‐cost renewable sources of energy and energy‐saving lighting devices 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.     

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.     

White Organic Light‐Emitting Devices for Solid‐State Lighting

White organic light‐emitting devices (WOLEDs) have advanced over the last twelve years to the extent that these devices are now being considered as efficient solid‐state lighting sources. Initially, WOLEDs were targeted towards display applications for use primarily as liquid‐crystal display backlights. Now, their power efficiencies have surpassed those of incandescent sources due to improvements in device architectures, synthesis of novel materials, and the incorporation of electrophosphorescent emitters. This review discusses the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality. Hindrances to their widespread acceptance as solid‐state lighting sources are also noted.     

Inkjet Printing—Process and Its Applications

In this Progress Report we provide an update on recent developments in inkjet printing technology and its applications, which include organic thin‐film transistors, light‐emitting diodes, solar cells, conductive structures, memory devices, sensors, and biological/pharmaceutical tasks. Various classes of materials and device types are in turn examined and an opinion is offered about the nature of the progress that has been achieved.     

Chemically Addressable Perovskite Nanocrystals for Light‐Emitting Applications

Whereas organic–inorganic hybrid perovskite nanocrystals (PNCs) have remarkable potential in the development of optoelectronic materials, their relatively poor chemical and colloidal stability undermines their performance in optoelectronic devices. Herein, this issue is addressed by passivating PNCs with a class of chemically addressable ligands. The robust ligands effectively protect the PNC surfaces, enhance PNC solution processability, and can be chemically addressed by thermally induced crosslinking or radical‐induced polymerization. This thin polymer shield further enhances the photoluminescence quantum yields by removing surface trap states. Crosslinked methylammonium lead bromide (MAPbBr₃) PNCs are applied as active materials to build light‐emitting diodes that have low turn‐on voltages and achieve a record luminance of over 7000 cd m^?2, around threefold better than previous reported MA‐based PNC devices. These results indicate the great potential of this ligand passivation approach for long lifespan, highly efficient PNC light emitters.