Query: Efficient radical-based light-emitting diodes

Candidate papers:

abstract: Organic light emitting diodes (OLEDs)1 5, quantum dot based LEDs6 10, perovskite based LEDs11 13 and micro LEDs14,15 have been championed to fabricate lightweight and flexible units for next generation displays and active lighting. Although there are already some high end commercial products based on OLEDs, costs must decrease whilst maintaining high operational efficiencies for the technology to realise wider impact. Here we demonstrate efficient action of radical based OLEDs16, whose emission originates from a spin doublet, rather than a singlet or triplet exciton. While the emission process is still spin allowed in these OLEDs, the efficiency limitations imposed by triplet excitons are circumvented for doublets. Using a luminescent radical emitter, we demonstrate an OLED with maximum external quantum efficiency of 27 per cent at a wavelength of 710 nanometres the highest reported value for deep red and infrared LEDs. For a standard closed shell organic semiconductor, holes and electrons occupy the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs) respectively, and recombine to form singlet or triplet excitons. Radical emitters have a singly occupied molecular orbital (SOMO) in the ground state, giving an overall spin 1/2 doublet. If as expected on energetic grounds both electrons and holes occupy this SOMO level, recombination returns the system to the ground state, giving no light emission. However, in our very efficient OLEDs, we achieve selective hole injection into the HOMO and electron injection to the SOMO to form the fluorescent doublet excited state with near unity internal quantum efficiency.Organic light emitting devices containing radical emitters can achieve an efficiency of 27 per cent at deep red and infrared wavelengths based on the excitation of spin doublets, rather than singlet or triplet states.
author_names: Xin Ai

E W Evans

Shengzhi Dong

Alexander J Gillett

Haoqing Guo

Yingxin Chen

Timothy J H Hele

Richard H Friend

Feng Li
title: Efficient radical based light emitting diodes with doublet emission
venue: Nature
year: 2018

abstract: CONJUGATED polymers are organic semiconductors, the semiconducting behaviour being associated with the p molecular orbitals delocalized along the polymer chain. Their main advantage over non polymeric organic semiconductors is the possibility of processing the polymer to form useful and robust structures. The response of the system to electronic excitation is nonlinear the injection of an electron and a hole on the conjugated chain can lead to a self localized excited state which can then decay radiatively, suggesting the possibility of using these materials in electroluminescent devices. We demonstrate here that poly(p phenylene vinylene) prepared by way of a solution processable precursor, can be used as the active element in a large area light emitting diode. The combination of good structural properties of this polymer, its ease of fabrication, and light emission in the green yellow part of the spectrum with reasonably high efficiency, suggest that the polymer can be used for the development of large area light emitting displays.
author_names: Jeremy H Burroughes

Donal D C Bradley

A Robinson Brown

R N Marks

K Mackay

Richard H Friend

P L Burns

Andrew B Holmes
title: Light emitting diodes based on conjugated polymers
venue: Nature
year: 1990

abstract: A comprehensive review of the literature on electron transport materials (ETMs) used to enhance the performance of organic light emitting diodes (OLEDs) is presented. The structure property performance relationships of many classes of ETMs, both small molecule and polymer based, that have been widely used to improve OLED performance through control of charge injection, transport, and recombination are highlighted. The molecular architecture, electronic structure (electron affinity and ionization potential) thin film processing, thermal stability, morphology, and electron mobility of diverse organic ETMs are discussed and related to their effectiveness in improving OLED performance (efficiency, brightness, and drive voltage) Some issues relating to the experimental procedures for the estimation of relevant material properties such as electron affinity and electron mobility are discussed. The design of multifunctional electroluminescent polymers whereby light emission and electron and hole transport pro.
author_names: Abhishek P Kulkarni

Christopher J Tonzola

Andreas Babel

Samson A Jenekhe
title: Electron Transport Materials for Organic Light Emitting Diodes
year: 2004

abstract: Phosphorescent organic light emitting diodes (PhOLEDs) unfurl a bright future for the next generation of flat panel displays and lighting sources due to their merit of high quantum efficiency compared with fluorescent OLEDs. This critical review focuses on small molecular organic host materials as triplet guest emitters in PhOLEDs. At first, some typical hole and electron transport materials used in OLEDs are briefly introduced. Then the hole transport type, electron transport type, bipolar transport host materials and the pure hydrocarbon compounds are comprehensively presented. The molecular design concept, molecular structures and physical properties such as triplet energy, HOMO/LUMO energy levels, thermal and morphological stabilities, and the applications of host materials in PhOLEDs are reviewed (152 references)
author_names: Youtian Tao

Chuluo Yang

Jingui Qin
title: Organic host materials for phosphorescent organic light emitting diodes.
venue: Chemical Society reviews
year: 2011

abstract: The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light emitting diodes (OLEDs) from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal organic complexes exploits the normally non radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up conversion from non radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 106 decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high efficiency phosphorescence based OLEDs.
author_names: Hiroki Uoyama

Kenichi Goushi

Katsuyuki Shizu

Hiroko Nomura

Chihaya Adachi
title: Highly efficient organic light emitting diodes from delayed fluorescence
venue: Nature
year: 2012

abstract: Compact high efficiency ultraviolet solid state light sources such as light emitting diodes (LEDs) and laser diodes are of considerable technological interest as alternatives to large, toxic, low efficiency gas lasers and mercury lamps. Microelectronic fabrication technologies and the environmental sciences both require light sources with shorter emission wavelengths: the former for improved resolution in photolithography and the latter for sensors that can detect minute hazardous particles. In addition, ultraviolet solid state light sources are also attracting attention for potential applications in high density optical data storage, biomedical research, water and air purification, and sterilization. Wide bandgap materials, such as diamond and III V nitride semiconductors (GaN, AlGaN and AlN; refs 3 10) are potential materials for ultraviolet LEDs and laser diodes, but suffer from difficulties in controlling electrical conduction. Here we report the successful control of both n type and p type doping in aluminium nitride (AlN) which has a very wide direct bandgap of 6 eV. This doping strategy allows us to develop an AlN PIN (p type/intrinsic/n type) homojunction LED with an emission wavelength of 210 nm, which is the shortest reported to date for any kind of LED. The emission is attributed to an exciton transition, and represents an important step towards achieving exciton related light emitting devices as well as replacing gas light sources with solid state light sources.
author_names: Yoshitaka Taniyasu

Makoto Kasu

Toshiki Makimoto
title: An aluminium nitride light emitting diode with a wavelength of 210 nanometres
venue: Nature
year: 2006

abstract: Organic electroluminescent devices are light emitting diodes in which the active materials consist entirely of organic materials. Here, the fabrication of a white light emitting organic electroluminescent device made from vacuum deposited organic thin films is reported. In this device, three emitter layers with different carrier transport properties, each emitting blue, green, or red light, are used to generate white light. Bright white light, over 2000 candelas per square meter, nearly as bright as a fluorescent lamp, was successfully obtained at low drive voltages such as 15 to 16 volts. The applications of such a device include paper thin light sources, which are particularly useful for places that require lightweight illumination devices, such as in aircraft and space shuttles. Other uses are a backlight for liquid crystal display as well as full color displays, achieved by combining the emitters with micropatterned color filters.
author_names: Junji Kido
title: Multilayer White Light Emitting Organic Electroluminescent Device
venue: Science
year: 1995

abstract: Since the successful demonstration of a blue light emitting diode (LED)1, potential materials for making short wavelength LEDs and diode lasers have been attracting increasing interest as the demands for display, illumination and information storage grow2,3,4. Zinc oxide has substantial advantages including large exciton binding energy, as demonstrated by efficient excitonic lasing on optical excitation5,6. Several groups have postulated the use of p type ZnO doped with nitrogen, arsenic or phosphorus7,8,9,10, and even p n junctions11,12,13. However, the choice of dopant and growth technique remains controversial and the reliability of p type ZnO is still under debate14. If ZnO is ever to produce long lasting and robust devices, the quality of epitaxial layers has to be improved as has been the protocol in other compound semiconductors15. Here we report high quality undoped films with electron mobility exceeding that in the bulk. We have used a new technique to fabricate p type ZnO reproducibly. Violet electroluminescence from homostructural p i n junctions is demonstrated at room temperature.
author_names: Atsushi Tsukazaki

Akira Ohtomo

Takeyoshi Onuma

M Ohtani

Takayuki Makino

Masatomo Sumiya

Keita Ohtani

Shigefusa F Chichibu

Shunro Fuke

Yusaburo Segawa

Hideo Ohno

Hideomi Koinuma

Masashi Kawasaki
title: Repeated temperature modulation epitaxy for p type doping and light emitting diode based on ZnO
year: 2004

abstract: Since the breakthrough by Kodak in 1987, organic light emitting diodes (OLEDs) have been seen as one of the most promising technologies for future displays. A number of materials have been developed and improved in order to fulfil the requirements of this application. The materials differ from one another by their structure but also by the mechanism involved in the electroluminescence produced (fluorescence versus phosphorescence) When properly stacked, these materials result in a device that can achieve the required high efficiency and long lifetime. Such red, green and blue devices can then be combined in matrices to become the core of a display. Building up these structures onto a display backplane is one of the challenges facing the industry. The circuitry for driving the pixels can be adapted to the OLED, sometimes at the expense of the simplicity of the display, but bearing in mind that the fabrication process must remain industrially viable. 2006 Society of Chemical Industry
author_names: Bernard Geffroy

Philippe Le Roy

Christophe Prat
title: Organic light emitting diode (OLED) technology: materials, devices and display technologies
year: 2006

abstract: Organic light emitting diodes (OLEDs) based on vacuum deposited small molecules have undergone significant progress since the first efficient double layered OLEDs were reported in 1987 by Tang and Van Slyke. Recently, solution processed small molecular OLEDs are also drawing more and more research attention, as such a technology combines advantages of the facile synthesis of small molecules and the low cost solution process like polymers. The performance of OLEDs made by solution process is gradually catching up with their vacuum deposited counterparts. This feature article will review the device structures adopted to achieve high performance solution processed OLEDs, the development of solution processable small molecules, and the comparisons of the different nature of the films and devices fabricated by solution process or by vacuum deposition. Finally, the prospects and remaining problems will be discussed.
author_names: Lian Duan

Liudong Hou

Tae-Woo Lee

Juan Qiao

De-qiang Zhang

Guifang Dong

Liduo Wang

Yong Qiu
title: Solution processable small molecules for organic light emitting diodes
year: 2010