Color-variable light-emitting devices

FIGURE 9.11. Schematic structure of the color-variable light-emitting devices. 

FIGURE 9.12. Current-voltage and brightness-voltage characteristics of a color-variable light-emitting device under forward and reverse bias conditions. The inset shows the colors of the device under forward and reverse biases in the CIE chromaticity diagram. 

It has been shown that insertion of phenylene group in the PT backbone improves the PL efficiency of the polymer. A green OLED based on 191 gave an external EL quantum efficiency of 0.1% [336]. Thienylene-phenylene copolymers such as 192 and 193 with ethylene oxide side chains also exhibit electroluminescent properties [337,338]. In the case of the paracyclophane derivative 193, a color variable light-emitting device generating two independent colors was demonstrated red under forward bias and green under reverse bias [338]. 

We also discuss an approach to color variable light-emitting devices which generates two independent colors of light at room temperatures. The devices consist of a layer of active electroluminescent polymers sandwiched between two different redox polymer layers. The redox polymer layers modify the charge injection and... 

The single-layer devices, the bilayer devices (with and without the PAN-CSA network electrode), and the inverted devices discussed are unipolar devices operating under a single-bias condition. We now discuss two novel device configurations that can be operated in both forward and reverse dc biases as well as in ac modes the SCALE devices and color-variable bipolar/ac light-emitting devices. 

For the devices presented here, the wrapped copolymer PPyVPV and a wrapped copolymer of poly thiophene and polyphenylene derivative, PTP, were used as the emitting materials SPAN and EB were used as the redox materials ITO and Al were used as electrodes. Figure 9.11 shows the schematic diagram of the device structure of the color-variable bipolar/ac light-emitting devices. 

Wang, Y.Z., D.D. Gebler, D.K. Fu, T.M. Swager, and A.J. Epstein. 1997. Color variable bipolar/ac light-emitting devices based on conjugated polymers. Appl Phys Lett 70 3212-3217. 

The EL spectrum of the color-variable device under reverse bias are similar to those of the single-layer PPyVPV and PTP devices, implying that the light is generated either in the bulk of the emitting polymer or at the EB interface. The EB layer functions as a charge-injection enhancement layer, playing a similar role to that in SCALE devices reported earlier.8 Under reverse bias, the SPAN layer on the cathode side plays a similar role. 

It is noted that when the blend in the color-variable devices is replaced by pure PPyVPV polymers, the devices emit red light in forward bias and red-orange light in reverse bias. This indicates that under reverse biases, the light is still generated near PPy VPV/SPAN interfaces. This can be understood in terms of different... 

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