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 smallmolecule-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 properties are combined in one material to achieve efficient single-layer OLEDs is also discussed. The review concludes with a brief perspective on the challenges that future research should address.

The synthesis, photophysics, cyclic voltammetry, and highly efficient blue electroluminescence of a series of four new n‐type conjugated oligomers, 6,6′‐bis(2,4‐diphenylquinoline) (B1PPQ), 6,6′‐bis(2‐(4‐tert‐butylphenyl)‐4‐phenylquinoline) (BtBPQ), 6,6′‐bis(2‐p‐biphenyl)‐4‐phenylquinoline) (B2PPQ), and 6,6′‐bis((3,5‐diphenylbenzene)‐4‐phenylquinoline) (BDBPQ) is reported. The oligoquinolines have high glass‐transition temperatures (Tg ≥ 133 °C), reversible electrochemical reduction, and high electron affinities (2.68–2.81 eV). They emit blue photoluminescence with 0.73–0.94 quantum yields and 1.06–1.42 ns lifetimes in chloroform solutions. High‐performance organic light‐emitting diodes (OLEDs) with excellent blue chromaticity coordinates are achieved from all the oligoquinolines. OLEDs based on B2PPQ as the blue emitter give the best performance with a high brightness (19 740 cd m–2 at 8.0 V), high efficiency (7.12 cd A–1 and 6.56 % external quantum efficiency at 1175 cd m–2), and excellent blue color purity as judged by the Commission Internationale de L'Eclairage (CIE) coordinates (x = 0.15,y = 0.16). These results represent the best efficiency of blue OLEDs from neat fluorescent organic emitters reported to date. These results demonstrate the potential of oligoquinolines as emitters and electron‐transport materials for developing high‐performance blue OLEDs.

The synthesis, properties, and electroluminescent device applications of a series of five new diphenylanthrazoline molecules 1a-1e are reported. Compounds 1b, 1c, and 1d crystallized in the monoclinic system with the space groups P2(1)/c, C2/c, and P2(1)/c, respectively, revealing highly planar molecules. Diphenylanthrazolines 1a-1e have a formal reduction potential in the range -1.39 to -1.58 V (versus SCE) and estimated electron affinities (LUMO levels) of 2.90-3.10 eV. Compounds 1a-1e emit blue light with fluorescence quantum yields of 58-76% in dilute solution, whereas they emit yellow-green light as thin films. The diphenylanthrazoline molecules as the emissive layers in light-emitting diodes gave yellow light with a maximum brightness of 133 cd/m(2) and an external quantum efficiency of up to 0.07% in ambient air. Bilayer light-emitting diodes using compounds 1a-1e as the electron-transport layer and poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) as the emissive layer had a maximum external efficiency of 3.1% and 2.0 lm/W and a brightness of up to 965 cd/m(2) in ambient air. These results represent enhancements of up to 50 times in external quantum efficiency and 17 times in brightness when using 1a-1e as the electron-transport materials in polymer light-emitting diodes. These results demonstrate that the new diphenylanthrazolines are promising n-type semiconductors for organic electronics.

No abstract

Binary blends of the acceptor conjugated polymer poly(2,2‘-(3,3‘-dioctyl-2,2‘-bithienylene)-6,6‘-bis(4-phenylenequinoline)) (POBTPQ) with donor conjugated polymer poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (MEH−PPV) or poly(3-octylthiophene) (POT) were nanophase-separated and observed to exhibit efficient electroluminescence in light-emitting-diodes. The 90−120 nm phase-separated morphology of MEH−PPV-containing blends was characteristic of dimixing by spinodal decomposition whereas that of POT-containing blends consisted of nucleation and growth type spherical domains (50−400 nm) dispersed in a matrix. Efficient Förster energy transfer was observed in both blend systems. Voltage-tunable orange-red ↔ yellow ↔ green electroluminescence was observed in the POBTPQ:MEH−PPV blend diodes at a composition of 10−80 wt % MEH−PPV. Only red emission characteristic of POT was observed from POBTPQ:POT blend devices due to efficient energy transfer from POBTPQ. Large enhancements in performance of the bipolar blend light-emitting diodes were observed compared to that of the homopolymer diodes. The compositional dependence of luminance and external quantum efficiency of the blend devices was very different for the MEH−PPV and POT blends, reflecting the difference in morphology. Electric-field-induced photoluminescence quenching confirmed the bipolar charge transport in the blends and associated improved electron−hole recombination and device efficiencies. These results demonstrate that efficient electroluminescence can be achieved in bipolar blends of conjugated polymers and that nanophase-separated morphology is essential to voltage-tunable multicolor light emission in blend devices.