Light-Emitting and Hole-Transporting Polymers for LEDs
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compared with that of O-LED. Improving the performance, especially the efficiency, we have prepared highly luminous copolymers of various arylene vinylenes and a bilayer device, using the copolymers and tris(8-quinolinolate) aluminum, AMq 3 , as an electron-transporting material,ETM, [4]. From the studies on the relation between the emitting properties of the polymers and the devices, the device efficiency depends not only on the fluorescence intensity of the polymers but also on the charge-injecting efficiency[5]. The hole- and electron- injections should be taken into consideration to elucidate the injection process at the polymer-ETM interface, because both carriers recombine to generate an exciton. It is, therefore, important to estimate barrier heights of hole- and electron- injections at the interface. The poly(arylene vinylene) derivatives can be used for this purpose, because their different energy levels related to the composition. Although it is a good ETM, Alq3 cannot be used for this purpose, because it shows an emission in the same wavelength region as the copolymers. Bis(2-hydroxyphenylbenzoxazolate) zinc(ZnBox)[6], having blue emission and electron transporting properties, is useful to estimate the barrier heights. When ZnBox is used in a bilayer device with the copolymers, it is easy to obtain the precise emission spectra from copolymers separetely from that of ZnBox. Polysilane, such as poly(methylphenylsilane), is known as a good hole-transporting material, 4 2 /Vs, and has been used in LEDs as a showing a hole mobility as high as 10"cm hole-transporting layer[7]. Some authors have successfully prepared a new polysilane, showing a hole mobility higher than l0 3cm 2/Vs[8]. The polysilane consists of a silicone main chain and a 21 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
triphenyl amine side group. In this paper we will discuss the charge injection to poly(arylene vinylenes) and the hole transporting of polysilane in LEDs. EXPERIMENT Random copolymers having arylene vinylene units A and B, or A and C, were prepared by the Wittig reaction, as reported previously[5]. The number-average molecular weights, measured by GPC using polystyrene as a standard, were 5x10 3 - lxl04 for all copolymers.
(-*OocH
OR
RO
R:C 8 J47
OR
(B)
RO
Fig. I The chemical structure of arylene vinylene units. Alq 3 was commercially obtained and purified by vacuum sublimation. A bis(2-hydroxyphenyl benzoxazolate) zinc(ZnBox) was synthesized using zinc acetate and 2-(2-hydroxyphenyl) benzoxazol in the same manner reported[6] and purified by sublimation. A polysilane having triphenylamine group(TPA-PS) as a side chain was obtained by the Kipping reaction of methyl(4-(N,N-diphenyl)aminophenyl)dichlorosilane and Na[8]. The hole mobility of the polysilane was obtained with a conventional time-of-flight method using Se as a carrier generating material. Me C1-
i-
Me Cl
_A_ n
Na -NaCI
Fig.2 Kipping reaction of methyl(4-(N,N-diphenyl)aminophenyl)dichlorosilane The P-LED device, consisting of a tra
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