Hole-transporting layer multilayer

A typical multilayer thin film OLED is made up of several active layers sandwiched between a cathode (often Mg/Ag) and an indium-doped tin oxide (ITO) glass anode. The cathode is covered by the electron transport layer which may be A1Q3. An emitting layer, doped with a fluorescent dye (which can be A1Q3 itself or some other coordination compound), is added, followed by the hole transport layer which is typically a-napthylphenylbiphenyl amine. An additional layer, copper phthalocyanine is often inserted between the hole transport layer and the ITO electrode to facilitate hole injection. 

Fig. 32 Typical electrophosphorescent multilayer device architecture composed of a hole injection layer (HIL), a hole transport layer (HTL), a emission layer (EL), a hole blocking layer (HBL), an electron blocking layer (EBL), and finally an electron transporting layer (ETL). Shown are the HOMO and LUMO energy levels of the corresponding molecules [77]. The vacuum level is assumed to be constant...

In a multilayer structure applying electron-transport and hole-transport layers, the advantages, which are contributed by each layer, can be combined.122 In the case of the PHP devices, the best EL quantum efficiencies (> 2%), the lowest electric threshold field ( 0.25 MV/cm) (see Fig. 3.16), and the best stabilities up to now are obtained in a multilayer structure ITO/OAM/PHP/diaminoctofluorobiphenyl/Al (see Fig. 3.16).105,106 123... 

Uchiyama e al. [67,315] used o(,a -dimethyl-anT with n = 4, 5, and 6 on ITO substrates with Al electrodes. The emission for all devices is orange/red. The observed quantum efficiency was 2.6 x 10 for a6T which is very low if compared with the values reported above. Also multilayer devices in which a4T and a5T act as an electron blocking layer and a6T acts as an emitting layer and a hole-transporting layer, were produced, which yielded 115 times and 1230 times higher quantum efficiencies, respectively. 

Katsuma and Shirota have synthesized several dendrimers that contain multiple phenylamine moieties and investigated their potential use as components of organic light-emitting diode (LEDs). They found that these materials were well suited for use in the hole-transport layer of multilayer LEDs, exhibiting comparable performance to previously utilized materials [lOlj. 

Organic electroluminescent devices have been the subject of intense research for almost a decade. These organic molecule-based devices use a multilayer cell structure composed of emitted layers and carrier transport layers as shown by Tang et al. [264-5]. The next advance in these devices was the construction of a three-layer cell by Adachi et al. [266] in which the emitter layer was sandwiched between a hole transport layer and an electron transport layer in the belief that these layers would increase electroluminescent efficiency. The device has the type of constmction shown in Figure 12.25. Adachi et al. reported that with a 500 A° emitter thickness, the luminescence was 1000 Cd cm. ... 

KOE 07] Koehnen A., Gather M.C., Riegel N. et al, Enhanced efficiency of multilayer organic light-emitting diodes with a low-refractive index hole-transport layer an effect of improved outcoupling . Applied Physics Letters, vol. 91, no. 11, no. 113501, 2007. 

MA 07] Ma B., Lauterwasser F., Deng L. et al, New thermally cross-linkable polymer and its application as a hole-transporting layer for solution processed multilayer organic hght emitting diodes . Chemistry of Materials, vol. 19, no. 19, pp. 4827 832, 2007. 

Mason et al. [401] studied multilayer electroluminescent ITO-(CuPc)-NPB-Alq-MgAg devices, where the optional copper phthalocyanine (CuPc) served as a buffer layer, A/, iV -bis(l-naph-thyl)-AT, AT -diphenyl-1,l -biphenyl-4,4 -diamine (NPB) served as a hole-transport layer, and tris(8-hydror quinoline)alumimun (Alq) served as an emissive as well as electron transport layer prepared... 

Adachi A, Manhart SA, Okita K, Kido J, OhsMta J, Kunai A (1997) Multilayer electroluminescent device using organosUicon polymer as hole transport layer. Synth Met 91 333-334... 

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