EL devices

The simplest polymer-based EL device consists of a single layer of semiconducting fluorescent polymer, c.g., PPV, sandwiched between two electrodes, one of which has to be transparent (Fig. 1-1). When a voltage or bias is applied to the material, charged carriers (electrons and holes) are injected into the emissive layer and these earners arc mobile under the influence of the high (> 105 V enr1) elec-... 

The other way to produce EL devices is based on a pin junction [47]. A pin junction with conjugated polymers was realized by electrochemical doping of the... 

The enormous progress in the field of electroluminescent conjugated polymers has led to performances of oiganic light-emitting devices (LEDs) that are comparable and in some aspects superior to their inorganic counterparts 11). Quantum efficiencies in excess of 5% have been demonstrated [2] and show that a high fraction of the injected carriers in a polymeric electroluminescence (EL) device form electronic excitations which recombine radiatively. 

Triarylamines have been employed in arylene vinylene AB copolymers 38 by Horhold et al. using a Homer polycondensation route of aldehydes and ketones 36 with fois-phosphonate 37 (Scheme 1-12) 164]. Phenylamines have remarkably low redox potentials and their charge transport properties have been investigated extensively [65]. EL devices comprising triarylamines have demonstrated low driving voltages. 

EL Devices from Conjugated Polymers with a High Defect Concentration... 

The luminescent centers require a range of properties that include a large cross-section for the collision excitation to occur, an ionic radius and valency to fit the lattice and be stable under the applied high electronic fields, and the capability to display high luminous efficiency when excited.11 Metal ions suitable for EL devices include Mn, Tb, Sm3+, Tm3+, Pr3+, Eu2+, and Ce3+.12-17 ZnS lattices doped with Mn2+ (yellow-orange emission at ca. 585 nm) have proved to be one of the best phosphors for EL devices. 

The light-emission characteristics of a white-light-emitting EL device with a doubly doped ZnS Pr,Ce,F phosphor layer have been described. It was observed that optimization of the co-doping of Ce enhances the emission characteristics compared to an EL device with a singly doped ZnS Pr,F layer.22 An electrical characterization of Ce-doped ZnS TbOF EL thin films has been reported Ce doping was seen to improve the radiative emission efficiency leading to improved performance of Ce co-doped film.23... 

Films of SrS HoF3, useful as white EL devices, were deposited by electron beam evaporation of SrS pellets and HoF3 powder.164... 

Porphyrine chromophore units have also been introduced to the PPV backbone but the PLQY of such materials decreased rapidly with increasing ratio of the porphyrine units and no EL devices have been reported [177,178]. 

Electron acceptor dicyanovinyl and oxadiazole substituents have been recently introduced into phenylene units of the PPV block copolymers (179,180) [211]. Blue and blue-greenish PL emission was observed for 179 and 180, respectively, but the PLQY was relatively low even in solution (13 and 24%) and no EL device has yet been reported. 

Eg = 3.6 eV). However, PL efficiencies of compounds 185-187 also drop down very significantly (Scheme 2.20), and no EL devices have been reported for this series. 

Adachi et al. showed that the ionization potential (IP) of HTLs was found to be the dominant factor for obtaining high durability in organic EL devices [70]. The formation of the small energy barrier at the interface of a HTL and anode was required for high durability. However, their results showed that there are no straightforward relationships between melting point, Ts of the HTMs, and durability of the EL devices. 

The concept of using HBMs in OLEDs started with the pioneering work of Kijima et al. when they were trying to get pure blue emission from an EL device with Alq3 as ETM and NPD as an EML [346], An undesired green emission color from Alq3 was suppressed when a thin layer of BCP was added between the NPD and Alq3 layers. 

A leading material reported as a red emitter is the fluorescent dye material typified by 4-(dicyanomethylene)-2-tert-butyl-6-(l,l,7,7-tetramethyljulolidyl-9-enyl)-4f/-pyran (DCJTB) [359]. This material is typically doped into an electron transporting host matrix such as Alq3 and delivers good chromaticity with CIE (0.646, 0.351) and a reasonable EL efficiency up to 4.4 cd/A and a power efficiency of 2.09 lm/W at 20 mA/cm2 and 6.8 V. The operational stability of the DCJTB-doped EL device has a projected half-life of over 33,800 h driven at an initial brightness of 100 cd/m2 (Scheme 3.95) [360]. 

Y. Sato, S. Ichinosawa, T. Ogata, M. Fugono, and Y. Murata, Blue-emitting organic EL devices with a hole blocking layer, Synth. Met., 111-112 25-29 (2000). 

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