Abstract：Organic light-emitting diodes have been widely used in display and lighting fields after more than 30 years of development. Red and green emitting OLEDs have basically been able to meet the commercial requirements， but blue-emitting OLEDs still have the defect of low brightness and short operation lifetime at high brightness. Therefore， the commercial is dying to blue-emitting materials with high exciton utilization efficiency（EUE） and high device stability. Design strategies for material such as heavy-metal phosphorescent complexes， triplet-triplet annihilation （TTA）， thermally activated delayed fluorescent （TADF） and “hot exciton” have been proposed so as to meet the demands of industrial standards. It is expected to obtain blue-emitting OLEDs device with high stability and long lifetime while achieving high photoluminescence quantum yield （PLQY） and the exciton utilization efficiency as well as reduce the efficiency roll-off of devices whenever possible. This paper summarizes the recent progress of different material structure design schemes and prospects the development trends of blue-emitting OLEDs materials.
Abstract：Efficient harvesting of triplet excitons is crucial to the development of high-performance organic light-emitting diodes（OLEDs）. Other than phosphorescence， the thermally activated delayed fluorescence（TADF） mechanism enables the upconversion of triplet excitons to singlet ones via reverse intersystem crossing（RISC）， followed by fluorescent radiation from the spin-allowed singlet excited states， providing another pathway to transition metal complexes for utilizing triplet excitons. However， the past decade has witnessed a blooming research interest in the purely organic TADF materials while the TADF metal complexes has only received much less attentions. Whereas， it has been shown that the presence of metal atom can boost the RISC rate through its heavy atom effect， leading to improved TADF emissions with high efficiencies and short lifetimes. Therefore， TADF metal complexes hold advantageous for the fabrication of high-performance OLEDs in terms of high-efficiency and small efficiency roll-off. This article summarizes the advances in TADF metal complexes by focusing on the excited state nature related to different electronic configurations， the photophysical and electroluminescence performance of representative TADF metal complexes. The structure-property relationship has been discussed which is conceived to provide useful guidelines for further design of TADF metal complexes， particularly those based on inexpensive metal elements.
Keywords：thermally activated delayed fluorescence of metal complexes;organic light-emitting diodes;luminescent materials;non-precious metal
Abstract：Organic light emitting diodes （OLED） technology is considered to be the next generation of display technology， and has gradually occupied the mainstream of the market， but the performance of OLED is still largely limited by electron transport materials， especially in high-performance deep blue light devices. Because it is difficult for organic molecules to obtain high electron mobility， and the recombination zone in the device is usually close to the interface of the electron transport layer， which requires the electron transport material to have a high triplet energy level to confine the exciton， especially the blue light exciton with high energy. However， high triplet states （poor conjugation） and high mobility （strong conjugation） have always been difficult to reconcile in the design of organic molecules， and a wider band gap will also lead to poorer thermal stability. These problems have been affecting the development of OLED electronic transport materials. In this paper， several elements of high performance electronic transport materials are introduced， including thermal stability， photochemical stability， electron mobility， the frontier orbital and the triplet energy level and so on. And the important research progresses of the micromolecular electron transport materials in 21st century are summarized， in order to provide the reference for the development of the ideal electronic transfer materials in the future.
Keywords：organic light-emitting diodes;Electronic transport materials;stability;Electron Mobility;Triplet Energy
Abstract：After more than 30 years of development， organic electroluminescent materials and organic light-emitting diodes （OLEDs） have almost completed the transformation process from basic scientific research to commercialization products. However， due to the inherent contradiction between the wide band-gap （exceed 3.0 eV） of ultraviolet and deep-blue luminescent materials and the carrier-injection ability and transport balance during device operation， with the decreasing sensitivity of human eyes in blue light area， the overall performance of ultraviolet and deep-blue light-emitting devices lag far behind other colors light-emitting devices. Therefore， the ultraviolet and deep-blue materials with excellent comprehensive performance and related molecular design theories have become the key breakthrough points for OLED to take the only place to lead the “New generation of display and lighting technology”. In this review， the ultraviolet and deep-blue luminescent materials （λEL below 430 nm） which were applied for organic light-emitting diodes in recent ten years are introduced briefly and summarized from the points of material structure and luminescence mechanism， hoping to promote the development of more and more excellent electroluminescent materials with wide band-gap and the improvement of the relevant theories.
Keywords：organic light-emitting diodes;ultraviolet;deep-bule;molecular design theory
Abstract：Exciplex， characterized by intermolecular charge-transfer excited state， possesses tiny energy splitting between the first singlet state and triplet state and thus thermally activated delayed fluorescence （TADF） property， which is caused by the separated distribution of frontier molecular orbitals. The highest occupied molecular orbital （HOMO） mainly locates at the donor molecule and the lowest unoccupied molecular orbital （LUMO） mainly locates at the acceptor molecule. As a result， exciplex can achieve 100% internal quantum efficiency theoretically like intramolecular charge-transfer TADF materials. Moreover， the donor molecule usually is hole-transporting material and the acceptor molecule is electron-transporting material， which contributes to balanced carrier injection. And the electroluminescent devices based on exciplex systems as emitting layer or mixed hosts exhibit balanced carrier injection， enlarged exciton recombination region， improved efficiency and suppressed efficiency roll-off. In this mini review， ultimate principle， design strategy and recent research progress of electroluminescent materials and devices based on exciplex excited state will be discussed and summarized.
Keywords：exciplex;thermally activated delayed fluorescence;electroluminescent materials and devices
Abstract：As a new generation of display technology， organic light emitting diodes （OLEDs） have been successfully commercialized， but efforts are still needed to develop efficient and stable blue OLED devices. In recent years， a new mechanism combining thermally activated delayed fluorescence （TADF） sensitizers and narrow spectrum final emitters， namely TADF sensitized fluorescence （TSF） has attracted more and more attention. With the continuous innovation of materials and device structures， performances of blue OLEDs based on this mechanism have been significantly improved. Here， focusing on the development of stable and efficient blue sensitizers， the progress in efficiency and lifetime of blue TSF devices in recent years is reviewed， and the future development goals and challenges are further discussed.
Keywords：organic light emitting diode;thermally activated delayed fluorescence;TADF sensitized fluorescence;efficient and stable blue OLED devices
Abstract：Purely organic room-temperature phosphorescence （RTP） materials have received extensive attention in the field of organic electroluminescence in recent years for the direct utilization of the 75% triplet excitons generated by electro-excitation. However， due to the theoretical spin-forbidden properties of purely organic materials， triplet excitons generally possess slow radiation rates and long exciton lifetimes， making it prone to non-radiative dissipation. Therefore， achieving enhanced spin-orbit coupling through efficient molecular design strategies is essential to promote fast intersystem crossing and phosphorescence radiation processes， thereby achieving high phosphorescence quantum efficiency and suppressing non-radiative deactivation of long-lived triplet excitons. In this article， based on the structural design principles of RTP materials， we reviewed recent progresses of purely organic RTP electroluminescence materials and devices， and summarized the application of RTP materials containing different non-metallic heavy atoms in electroluminescent devices. Also， we pointed out the key problems that need to be solved in the current research， and prospected the potential application of purely organic RTP materials in the field of electroluminescence.
Abstract：With the continuous development of organic light emitting diodes （OLED）， the application of OLED in markets such as smartphones， smart watches， and TVs continues to expand. Especially in the small-size smartphone market， OLED has become the mainstream display technology and is expected to gradually replace LCD technology. However， in the application fields of medium and large size display products such as tablets， notebooks， monitors， TVs， etc.， limited by the high cost and reliability of the current vacuum evaporation process， the development is relatively slow， but the market demand for them is still very high. Compared with vacuum evaporation technology， the preparation of display devices by inkjet printing technology can greatly improve the utilization rate of materials and reduce equipment costs. And because it is additive manufacturing， it can reduce resource consumption and environmental pollution， which is more conducive to the realization of large-size， light， thin and flexible display manufacturing. This paper introduces the research progress of various functional layer materials for inkjet printing OLEDs， and discusses the device structure， preparation process and device performance of current printed OLEDs. Finally， the development trends of printed OLEDs are prospected.
Keywords：organic electroluminescence;additive manufacturing;inkjet printing technology;printed OLED materials and devices
Abstract：Thanks to the discovery of new mechanisms of organic electroluminescence and the innovation of organic light-emitting materials， several breakthroughs have been made in the efficiency of organic light-emitting diode （OLED） device. Theoretical exploration of the formation and decay processes of excited states of organic molecules can deepen the understanding of the mechanism of electroluminescence and boost the development of molecular materials. This review briefly introduces the rate theory of thermal vibration correlation function （TVCF） and its applications in fluorescence， phosphorescence and thermally activated delayed fluorescence small molecular materials. For these three kinds of materials， their light-emitting mechanisms have been revealed， the relationships between molecular structure and properties have been established， the descriptors of efficiencies have been proposed， and then the excellent OLED light-emitting molecules have been designed theoretically from principles.
Abstract：Organic crystalline materials possess ordered molecular arrangement， forming structures with long-range order and low density of defect states. These properties result in good thermal stability， chemical stability and high carrier mobility compared to organic amorphous materials， making organic crystalline materials have great potential in developing high-performance OLEDs. This paper reviews the recent progress on crystalline thin-film OLEDs （C-OLEDs） based on weak-epitaxy-growth（WEG） technique. From the initial single crystalline layer green-emission device to multi-layer doped deep-blue-emission device， C-OLEDs have proved the crystalline organic semiconductor route is capable of realizing efficient light emitting， and the devices have achieved superior characteristics of low turn-on voltage， low operating voltage， high photon output， high power efficiency and low Joule heat loss.
Abstract：Ultraviolet/near ultraviolet （UV/NUV） organic light-emitting diodes （OLEDs） have attracted extensive attention due to its great application potential in biological and chemical sensing， laser， high-density information storage and optoelectronic circuits， etc. This paper reviews the recent development of ultraviolet organic light-emitting diodes （UV-OLEDs）， including organic chromophores， device structures and practical applications. Firstly， the materials with high efficiency shortwave emission are introduced， focusing on the characteristics of the materials themselves and their structural characteristics. Then， we give an overview of device structures to improve the performance of devices. Finally， we discuss the progress and challenges of using UV-OLEDs as excitation sources in emerging applications， and expect that this will promote the development of ultraviolet light sources in more fields in the future.
Abstract：Researches in advanced and efficient materials have greatly promoted the fast development of organic light-emitting diodes （OLEDs）. However， the generated photons inside the device can not completely escape into the external field due to the inherent optical losses， which greatly reduces the electroluminescence efficiency of OLEDs and hinders the commercialization progress of OLEDs for lighting applications. By manipulating the light propagation in the vicinity of optical losses， light extraction schemes could efficiently suppress the substrate mode， the waveguide mode， and the surface plasmon polariton mode， leading to a maximum enhancement factor of 4 in theory. A large number of reported light extraction strategies could improve the OLED efficiency by 1.05-2 times， but still lag behind the theorical limit. With this regard， we briefly introduce the optical loss mechanism in OLEDs and summarize the recent progresses of highly effective light extraction technologies particularly with the promising enhancement factor of above 2.
Abstract：Organic light emitting diodes（OLEDs） are light-emitting devices based on organic semiconductors. They are considered as a new generation of display and lighting technology because of their advantages such as self-illumination， fast response， adjustable light-emitting color， lightness， large area flexibility and foldability， etc. The emission of OLEDs is a process of the radiative decay of excitons formed by recombination of injected electrons and holes. Therefore， how to effectively use excitons， especially the use of triplet excitons， has become an important topic in the research on materials and devices of OLEDs. Among them， how to convert triplet exciton energies into singlet excitons and finally realize the fluorescence emission of 100% excitons has more application value. In recent years， significant progresses have been made in this field. This paper introduces the effective methods for the fabrication of high efficiency fluorescence OLEDs in detail based on working mechanism and luminescence processes of OLEDs and their latest progresses， and prospects the future development directions， which provide an important reference for the research on materials and devices of OLEDs.
Keywords：Organic light-emitting;excitons;Efficient utilization;Singlet and triplet states
Abstract：Organic light-emitting diodes （OLEDs） have attracted much attention due to their huge potential applications in the field of lighting and display. In the past three decades， the efficiency and lifetime of OLED devices have been greatly improved. However， improving device lifetime is still one of the urgent problems to be solved for commercial applications. In order to further improve the stability of the device， it is necessary to study the intrinsic degradation mechanism intensively. In this paper， using examples in both small molecule and polymer OLEDs， the degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed， and recent works on degradation studies of both small-molecule and polymer OLEDs are presented. For small-molecule OLEDs， some studies show that the degradation of devices is consistent with defect formation due primarily to exciton-polaron annihilation reactions， others show that degradation is closely linked to interactions between excitons and positive polarons， which lead to its aggregation near the interface and thus destroy the interface. For polymer OLEDs， the luminance loss and voltage rise dependence on time and current density are consistent with hole trap formation due to exciton-free hole interactions. Meanwhile， this paper summarizes some current effective solutions to increase the lifetime， which will play a positive role in the subsequent development of OLED devices with higher efficiency and longer lifetime.
Abstract：With more than 30 years of development， organic light-emitting diodes （OLEDs） have made a breakthrough in overall device performances and successfully realized commercial applications due to the development of high-efficiency organic semiconductor materials， new device structures， in-depth understanding of device working mechanisms， and unremitting engineering exploration from the industry. To date， OLED display technologies have become a leading pillar industry of the new-generation of information technology. In this review， the research progress of OLED devices and display driving will be introduced from the perspective of OLED devices. Firstly， the evolution process of OLED basic device structure is introduced in combination with the performance improvement of optoelectronic devices， and then the device structures that are widely used in the industry at this stage and have great application prospects are systematically highlighted， including p-i-n OLED device structures， tandem device structures， and doping-free device structures. Finally， the OLED display driving technology will be briefly described， which is expected to provide some useful guidelines for relevant scientific researchers.
Keywords：organic light-emitting diodes;p-i-n structures， tandem devices;doping-free devices;display driving