Inkjet Printed Bilayer Light‐Emitting Electrochemical Cells for Display and Lighting Applications

A new bilayer light‐emitting electrochemical cell (LEC) device, which allows well‐defined patterned light emission through an easily adjustable, mask‐free, and additive fabrication process, is reported. The bilayer stack comprises an inkjet‐printed lattice of micrometer‐sized electrolyte droplets, in a “filled” or “patterned” lattice configuration. On top of this, a thin layer of light‐emitting compound is deposited from solution. The light emission is demonstrated to originate from regions proximate to the interfaces between the inkjetted electrolyte, the light‐emitting compound, and one electrode, where bipolar electron/hole injection and electrochemical doping are facilitated by ion motion. By employing KCF₃SO₃ in poly(ethylene glycol) as the electrolyte, Super Yellow as the light‐emitting compound, and two air‐stabile electrodes, it is possible to realize filled lattice devices that feature uniform yellow–green light emission to the naked eye, and patterned lattice devices that deliver well‐defined and high‐contrast static messages with a pixel density of 170 PPI.     

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.     

Ink‐Jet Printed CMOS Electronics from Oxide Semiconductors

Complementary metal oxide semiconductor (CMOS) technology with high transconductance and signal gain is mandatory for practicable digital/analog logic electronics. However, high performance all‐oxide CMOS logics are scarcely reported in the literature; specifically, not at all for solution‐processed/printed transistors. As a major step toward solution‐processed all‐oxide electronics, here it is shown that using a highly efficient electrolyte‐gating approach one can obtain printed and low‐voltage operated oxide CMOS logics with high signal gain (≈21 at a supply voltage of only 1.5 V) and low static power dissipation.     

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.     

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.     

Organic Light‐Emitting Transistors: Materials,Device Configurations,and Operations

Organic light‐emitting transistors (OLETs) represent an emerging class of organic optoelectronic devices, wherein the electrical switching capability of organic field‐effect transistors (OFETs) and the light‐generation capability of organic light‐emitting diodes (OLEDs) are inherently incorporated in a single device. In contrast to conventional OFETs and OLEDs, the planar device geometry and the versatile multifunctional nature of OLETs not only endow them with numerous technological opportunities in the frontier fields of highly integrated organic electronics, but also render them ideal scientific scaffolds to address the fundamental physical events of organic semiconductors and devices. This review article summarizes the recent advancements on OLETs in light of materials, device configurations, operation conditions, etc. Diverse state‐of‐the‐art protocols, including bulk heterojunction, layered heterojunction and laterally arranged heterojunction structures, as well as asymmetric source‐drain electrodes, and innovative dielectric layers, which have been developed for the construction of qualified OLETs and for shedding new and deep light on the working principles of OLETs, are highlighted by addressing representative paradigms. This review intends to provide readers with a deeper understanding of the design of future OLETs.