Multi-Layer Devices

The utility and importance of multi-layer device structures was demonstrated in the first report of oiganic molecular LEDs [7]. Since then, their use has been widespread in both organic molecular and polymer LEDs [45, 46], The details of the operating principles of many multi-layer structures continue to be investigated [47—49], The relative importance of charge carrier blocking versus improved carrier transport of the additional, non-luminescent layers is often unclear. The dramatic improvements in diode performance and, in many cases, device lifetime make a detailed understanding of multi-layer device physics essential. 

It is also conceivable that one may protect the plastic with a thin ultra-violet absorbing film. If this was a glass containing, say, Ce3+, Tl+ or Bi3+23) it might provide its own luminescence, utilizing the ultra-violet energy. However, it would seem more practical to use uranyl glass as the first layer in a multi-layer device as discussed in the next section. 

The polymers have been used in different multi-layered devices using PPV as emissive layer. Typical devices were prepared on glass substrates precoated with patterned indium-tin oxide (ITO) electrodes (resistance < 20 Q/square). In a two layer LED, the oxadiazole polymethacrylates (10a or 10b) were spin-coated on top of poly(p-phenylene vinylene) (PPV), prepared on an ITO glass substrate by the sulfonium precursor route (22). Cdcium was used as the top metal contact. A comparable device, but without the polymethacrylate, was fabricated as a reference. Both devices emitted green yellow light under forward bias potential (15 V). 

Multi-layer device. High performance of the first O-LED(l) was achieved by employing a multi-layer structure of and a diamine compound. In the double-layer OLED, the electron-accumi tion takes place and hole and electron recombines to generate excitons more easily than a single-layer device. In the P-LED, Brown et al. reported that the double-layer structure of an oxadazole compound and... 

Device efficiency. The device efficiency depends not only on PL efficiency of the emitting polymer but also on the efficiency of hole-electron recombination which is related to Ae charge transporting properties of materials used in the device and to the device structure as described above. Employing a multi-layer device(23) and a Mg-Ag or Li-Al alloy electrode(9), we have already improved the device efficiency but have to further improve the efficiency for commercialization. We, thus, have studied the relationship between composition of the random copolymer and the device efficiency. 

In particular the transition to multi-layer devices should bring additional possibilities for improvement in these figures, especially regarding the applied voltage and the long-term stability. Moreover, the first promising experiments have been conducted using soluble poly(2,5-dialkyl-l,4-phenylene-l,3,4-oxadiazole)s 12 as electron transport layer [33]. 

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