A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Organic electronics are broadly anticipated to impact the development of flexible thin-film device technologies. Among these, solution-processable π-conjugated polymers and small molecules are proving particularly promising in field-effect transistors and bulk heterojunction solar cells. This Perspective analyzes some of the most exciting strategies recently suggested in the design and structural organization of π-functional materials for transistor and solar cell applications. Emphasis is placed on the interplay between molecular structure, self-assembling properties, nanoscale and mesoscale ordering, and device efficiency parameters. A critical look at the various approaches used to optimize both materials and device performance is provided to assist in the identification of new directions and further advances.

Molecular design and ordering effects in π-functional materials for transistor and solar cell applications.

Although organic light-emitting devices have been commercialized as flat panel displays since 1997, only singlet excitons were emitted. Full use of singlet and triplet excitons, electrophosphorescence, has attracted increasing attentions after the premier work made by Forrest, Thompson, and co-workers. In fact, red electrophosphorescent dye has already been used in sub-display of commercial mobile phones since 2003. Highly efficient green phosphorescent dye is now undergoing of commercialization. Very recently, blue phosphorescence approaching the theoretical efficiency has also been achieved, which may overcome the final obstacle against the commercialization of full color display and white light sources from phosphorescent materials. Combining light out-coupling structures with highly efficient phosphors (shown in the table-of-contents image), white emission with an efficiency matching that of fluorescent tubes (90 lm/W) has now been realized. It is possible to tune the color to the true white region by changing to a deep blue emitter and corresponding wide gap host and transporting material for the blue phosphor. In this article, recent progresses in red, green, blue, and white electrophosphorescent materials for OLEDs are reviewed, with special emphasis on blue electrophosphorescent materials.

Recent Progresses on Materials for Electrophosphorescent Organic Light-Emitting Devices

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.

Recent advances in white organic light-emitting materials and devices (WOLEDs).

Semiconducting quantum dots, whose particle sizes are in the nanometer range, have very unusual properties. The quantum dots have band gaps that depend in a complicated fashion upon a number of factors, described in the article. Processing-structure-properties-performance relationships are reviewed for compound semiconducting quantum dots. Various methods for synthesizing these quantum dots are discussed, as well as their resulting properties. Quantum states and confinement of their excitons may shift their optical absorption and emission energies. Such effects are important for tuning their luminescence stimulated by photons (photoluminescence) or electric field (electroluminescence). In this article, decoupling of quantum effects on excitation and emission are described, along with the use of quantum dots as sensitizers in phosphors. In addition, we reviewed the multimodal applications of quantum dots, including in electroluminescence device, solar cell and biological imaging.

https://www.mdpi.com/1996-1944/3/4/2260/pdf

Quantum Dots and Their Multimodal Applications: A Review

Organic light-emitting diodes (OLEDs) show promise for applications as high-quality self-emissive displays for portable devices such as cellular phones and personal organizers. Although monochrome operation is sufficient for some applications, the extension to multi-colour devices--such as RGB (red, green, blue) matrix displays--could greatly enhance their technological impact. Multi-colour OLEDs have been successfully fabricated by vacuum deposition of small electroluminescent molecules, but solution processing of larger molecules (electroluminescent polymers) would result in a cheaper and simpler manufacturing process. However, it has proved difficult to combine the solution processing approach with the high-resolution patterning techniques required to produce a pixelated display. Recent attempts have focused on the modification of standard printing techniques, such as screen printing and ink jetting, but those still have technical drawbacks. Here we report a class of electroluminescent polymers that can be patterned in a way similar to standard photoresist materials--soluble polymers with oxetane sidegroups that can be crosslinked photochemically to produce insoluble polymer networks in desired areas. The resolution of the process is sufficient to fabricate pixelated matrix displays. Consecutive deposition of polymers that are luminescent in each of the three RGB colours yielded a device with efficiencies comparable to state-of-the-art OLEDs and even slightly reduced onset voltages.

Multi-colour organic light-emitting displays by solution processing.

New crosslinkable hole conductors for blue-phosphorescent organic light-emitting diodes.

Summary: Modern multilayer organic light-emitting devices (OLED) are fabricated easily and at low cost by spin-coating with subsequent crosslinking of the layers. For this purpose, a low-molecular-weight hole-transport material based on triphenyl amines bearing crosslinkable oxetane groups was synthesized. Crosslinking of the spin-coated layer was initiated with UV irradiation using an iodonium-salt photoinitiator and was observed using realtime FT-IR spectroscopy. Standard photolithography techniques can be used for structuring the material on the micrometre scale.

An AFM image of the photopatterned bis-oxetane-functionalized low-molecular-weight hole-transport material based on triphenyl amines synthesized here.

Photopatterning of Crosslinkable Hole-Conducting Materials for Application in Organic Light-Emitting Devices

Organic light emitting devices (OLEDs) are one of the most promising alternatives to liquid crystal displays (LCDs) concerning flat panel display applications. The OLED technique is based on organic semiconductors used either as hole- or electron transporting materials or as emitter. Working on common problems in OLED preparation, performance and life time, novel and improved charge transport molecules and polymers of triarylamine- and poly(para-phenylene)type are introduced. This includes: (1) cyclic triarylamine-derivatives with enhanced glass transition temperature; (2) triarylamine based low molecular mass hole transport molecules and hole transport polymers with pendant oxetane groups for processing out of solution and subsequent crosslinking; and (3) fluorenyl-segmented poly(para -phenylene)s with defined electrochemical properties.

https://www.tandfonline.com/doi/pdf/10.1163/156855502760157926

Markus Rojahn

Papers

Multi-colour organic light-emitting displays by solution processing.

New crosslinkable hole conductors for blue-phosphorescent organic light-emitting diodes.

Photopatterning of Crosslinkable Hole-Conducting Materials for Application in Organic Light-Emitting Devices

Crosslinkable hole- and electron-transport materials for application in organic light emitting devices (OLEDs)

Vernetzbare Lochleiter für blau phosphoreszierende organische Leuchtdioden