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...

Electron Transport Materials for Organic Light-Emitting Diodes

Tetrathiafulvalenes, oligoacenenes, and their buckminsterfullerene derivatives: the brick and mortar of organic electronics.

The design and characterization of thermally activated delayed fluorescence (TADF) materials for optoelectronic applications represents an active area of recent research in organoelectronics. Noble metal-free TADF molecules offer unique optical and electronic properties arising from the efficient transition and interconversion between the lowest singlet (S1) and triplet (T1) excited states. Their ability to harvest triplet excitons for fluorescence through facilitated reverse intersystem crossing (T1→S1) could directly impact their properties and performances, which is attractive for a wide variety of low-cost optoelectronic devices. TADF-based organic light-emitting diodes, oxygen, and temperature sensors show significantly upgraded device performances that are comparable to the ones of traditional rare-metal complexes. Here we present an overview of the quick development in TADF mechanisms, materials, and applications. Fundamental principles on design strategies of TADF materials and the common relationship between the molecular structures and optoelectronic properties for diverse research topics and a survey of recent progress in the development of TADF materials, with a particular emphasis on their different types of metal-organic complexes, D-A molecules, and fullerenes, are highlighted. The success in the breakthrough of the theoretical and technical challenges that arise in developing high-performance TADF materials may pave the way to shape the future of organoelectronics.

Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics

Pyrene-based materials for organic electronics.

Coordination complexes exhibiting room-temperature phosphorescence: Evaluation of their suitability as triplet emitters in organic light emitting diodes

Bipolar compounds (referred to in general as btza) containing a benzo[1,2,5]thiadiazole core and peripheral diarylamines and/or 4-tert-butylphenyl moieties have been synthesized via palladium-catalyzed cross-coupling reactions of 4,7-dibromobenzo[1,2,5]thiadiazole with appropriate stannyl compounds. These compounds are fluorescent and the emission color ranges from green to red. The fluorescence of the compounds originates from a charge-transfer process and exhibits solvatochromism. These red-light-emitting materials are amorphous and devices of different configurations were fabricated: I) ITO/btza/TPBI/Mg:Ag; II) ITO/btza/Alq3/Mg:Ag; III) ITO/btza/Mg:Ag (where ITO = indium tin oxide, TPBI = 1,3,5-tris(N-phenylbezimidazol-2-yl)benzene, and Alq3 = tris(8-hydroxyquinoline)aluminum). The performance of some of the red-light-emitting devices appears to be very promising.

Color Tuning in Benzo[1,2,5]thiadiazole‐Based Small Molecules by Amino Conjugation/Deconjugation: Bright Red‐Light‐Emitting Diodes

A new series of 2-tert-butyl-9,10-bis(bromoaryl)anthracenes have been synthesized from 2-tert-butyl-9,10-anthraquinone. Palladium-catalyzed C−N bond formation between these bromo compounds and diarylamines provides stable 2-tert-butyl-9,10-diarylanthracenes containing two peripheral diarylamines (anth). They possess high thermal decomposition temperature (Td > 450 °C) and form a stable glass (Tg > 130 °C). Furthermore, they are fluorescent in the blue region with moderate to good quantum efficiencies. Two types of light-emitting diodes (LED) were constructed from anth, (I) ITO/anth/TPBI/Mg:Ag and (II) ITO/anth/Alq3/Mg:Ag, where TPBI and Alq3 are 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene and tris(8-hydroxyquinolinato)aluminum, respectively. In type I devices, the anth function as the hole-transporting and emitting material. In type II devices, emission from Alq3 is observed. Several blue-light-emitting type I devices exhibit good maximum brightness and physical performance. The relation between the ener...

Blue-Emitting Anthracenes with End-Capping Diarylamines

Dipolar compounds featuring quinoxaline acceptors and various triarylamine donors were prepared in good yields and successfully employed in the fabrication of organic light-emitting diodes (OLEDs). It is also demonstrated that the emission color of these compounds can be easily tuned from bluish green to orange by suitably modifying the diarylamine and quinoxaline units independently. Increasing the donor and acceptor strengths bathochromically shifts the absorption and emission bands. These molecules possess moderate glass transition temperatures (114-152 °C) and exhibit high decomposition temperatures (441-554 °C). The two-layer OLEDs fabricated using these materials as hole-transporting and emitting layers and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene or tris(8-hydroxyquino-linato)aluminum as the electron-transport layer display promising characteristics, i.e., emission color, luminance, and efficiency. Incorporation of the hole-blocking quinoxaline segments in the hole-transporting triarylamine molecules leads to the confinement of the recombination zone in it, and thus emission is realized mainly from these materials for both types of devices. The factors leading to the funneling of light through the hole-transporting layer in these layers are critically analyzed.

Quinoxalines incorporating triarylamines: Potential electroluminescent materials with tunable emission characteristics

New multiply substituted carbazole derivatives containing fluorene or phenylene conjugated oxadiazole segments and quinoxaline units were obtained by palladium-catalyzed C−N coupling reactions. They are amorphous with the glass transition temperature (Tg) in the range 104−176 °C. The emission color of the materials varies from blue to yellow and is dependent on the nature of the electron-withdrawing segments and solvents. Two reversible one-electron oxidations were observed for these molecules in cyclic voltammograms, which originate from the peripheral 3,6-diarylamino units in the 3,6,9-trisubstituted derivatives and diarylamine and carbazole segments in the 3,9-disubstituted compounds. Reductions originating from quinoxaline segments were also located for the molecules incorporating quinoxaline moieties. The double-layer organic light-emitting diodes fabricated using these compounds as hole-transporting/emitting layers and TPBI or Alq3 as an electron-transporting layer emit bluish green to yellow colors...

Green and Yellow Electroluminescent Dipolar Carbazole Derivatives: Features and Benefits of Electron-Withdrawing Segments

3-Cyano-9-diarylamino carbazoles have been synthesized. These new compounds emit in the blue to green region. Double-layer electroluminescent devices using these compounds as the hole-transport/emitting materials are highly efficient. Two of the compounds can be fabricated into single-layer devices with good performance. Green- and blue-emitting devices with good performance were also fabricated using one of the compounds as the hole-injection layer.

Chang-Hao Chuen

Papers

Color Tuning in Benzo[1,2,5]thiadiazole‐Based Small Molecules by Amino Conjugation/Deconjugation: Bright Red‐Light‐Emitting Diodes

Blue-Emitting Anthracenes with End-Capping Diarylamines

Quinoxalines incorporating triarylamines: Potential electroluminescent materials with tunable emission characteristics

Green and Yellow Electroluminescent Dipolar Carbazole Derivatives: Features and Benefits of Electron-Withdrawing Segments

Cyanocarbazole Derivatives for High-Performance Electroluminescent Devices †