Design of Multilayer Structures

FIGURE 2.5. Three basic OLED stmctures SH-H, SH-E, and DH. HTL hole-transport layer, ETL electron-transport layer and EML, and EML emitter-layer. SH-H and SH-E holds both carrier-transport and emitter characters. Advanced OLED structures MH-1, MH-2, and MH-3. Double HTLs and ETLs significantly increase device performance. 

Another significant merit of organic multilayer structures is keeping the energy of molecular excitons from dissipating into the metal electrodes — in other words, the retardation effect between an excited molecule and a metal mirror.53 It is well known that if excited molecules located near a metal electrode are less than 20 nm, the nonradiative energy transfer to the metal electrode dominantly occurs. Thus, the 50-70-nm-thick HTL and ETL effectively protect molecular excitons from the dissipation process. 

Based on the three basic OLED structures, the new multilayer structures (MH1-3) are proposed as summarized in Fig. 2.5b. MH-1 and MH-2 structures are composed of double HTLs54-56 and ETLs,57,58 as shown in Figs. 2.5a and 2.5b. The meaning of the double layers is a separation of carrier injection from an electrode and carrier transport. The following are five major requirements that the HTL should have  

The HTL should have a small barrier for hole injection from an anode electrode. For the low injection barrier between them, the Ip of HTL has to match with that of an anode. As we usually use an indium-tin-oxide (ITO) with Ip of 5.0 eV, the HTL should have similar value. 

The HTL transports holes with a low driving voltage. High hole mobility over 1(T3 cm2/Vs is recommended. 

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