Internal Field Distribution in Organic Light Emitting Diodes with Double Layer Structure
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ABSTRACT Double layer organic light emitting devices (OLED) are constructed by evaporating tris(8-hydroxy) quinoline aluminum (Alq 3) on a spin cast thin film of a methyl substituted ladder type poly-para-phenylene (m-LPPP). A thick layer of Mg:Ag is used as the cathode material. These organic materials are very suitable for application in OLEDs both, as transporting materials as well as active layers. Alq3 predominantly transports electrons while mLPPP is a conjugated polymer having higher hole mobilities. Due to these transport properties the formation and radiative recombination of the excitons in ITO/m-LPPP/Alq 3/Mg:Ag devices occur close to the m-LPPP/Alq 3 interface. We compare the device performance of OLEDs with varying Alq3 layer thickness (0, 50, 150, 300, 500A) and constant m-LPPP layer thickness (900A). A difference in the device parameters and performance as a function of the Alq3 layer thickness is observed. We analyze these results with respect to the internal electric field distribution of the double layer devices derived from electroabsorption measurements. INTRODUCTION The first reports on efficient electroluminescence emission (EL) from an organic multilayer device using tris(8-hydroxy) quinoline aluminum (Alq3) as the emissive material [1][2] have stimulated intensive research in the field of fluorescent molecules with the focus on future applications in display technology [3][4]. Also, the successful use of fluorescent polymers has enlarged the selection of materials which can be used in organic light emitting devices (OLEDs) [5]. Among the conjugated polymers, poly (para-phenylene) (PPP) [6] and its derivatives are very suitable materials for EL applications. One of these derivatives is methyl substituted ladder-type poly (para-phenylene) (m-LPPP), which shows bright EL emission in the blue-green region of the visible spectrum [7]. Neither Alq3 nor m-LPPP forms an efficient bipolar charge transport layer. Alq3 is known as a better electron transport material [1] whereas m-LPPP is a predominantly hole transporting material [7]. Highly efficient OLEDs with these materials typically incorporate separate transport and emission layers. We present Alq3 layer thickness dependent EL and electroabsorption (EA) measurements on these bi-layer OLEDs. This gives us insight into the field distribution in each layer. EXPERIMENT Methylated poly-para-phenylene (m-LPPP) and a metal chelate complex, tris(8-hydroxy)quinoline aluminum (Alq3) were used as the organic compounds for these double layer devices. The chemical structures of these materials are shown in Figure 1. The 121 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
synthesis routes for m-LPPP and Alq3 can be found in Ref. [8] and Ref. [9], respectively. Figure 1 shows the structure of the OLEDs investigated in this work. The substrate is an indium tin oxide (ITO) coated glass plate where ITO is the transparent anode material with a sheet resistance of about 300Ohm/sq. Prior to the organic deposition, the substrates were thoroughl
