Light-Emitting Electrochemical Cells Based on PPPs

5 Light-Emitting Electrochemical Cells Based on PPPs 

The formation of the p-i-n junction is not an instantaneous process but depends on the mobility of the ions in the three-component blend consisting of an electron conductive and light-emitting polymer, an ionic conductive polymer, and an ionic salt. Therefore, the response times of conventional LECs typically are longer than those of polymer LEDs. However, if the motion of the ions can be suppressed after the junction has been formed, the response of the LECs can be drastically increased and it, therefore, should be possible to obtain fast response times similar to LEDs. 

m-LPPP is very suitable to act as an active layer in LECs 150 by using a blend of m-LPPP, PEO, and LiCF3SC 3 in a weight ratio of 20 10 3, luminance values of around 250 Cd/m2 were obtained at an operating voltage of 10 V.150 The reported values for the external EL quantum efficiencies (0.3%)150 of these LECs are below that obtained for ITO/m-LPPP/Al LEDs (jjel 1% see Sec. 8.1), but the threshold voltages of the LECs are distinct below that for LEDs. 

The I-V and L-V characteristics of LECs based on m-LPPP do not show the characteristic symmetric shape usually observed in LECs. In contrast to polymer LEDs, the current-voltage (I-V) and the luminance-voltage (L-V) characteristics for LECs, which are not operated in a frozen junction regime (see below) should be fairly symmetric. This holds even for different electrode materials, because 

FIGURE 8.22. Current-bias and brightness-bias characteristics of an ITO/m-LPPP PEO LiC104/Al LEC ( , brightness , current). The concentration of m-LPPP PEO LiC104 is 20 10 3. The thickness of the active layer is 200 nm. The characteristics were recorded in a pulsed mode with stepwise increase in the applied voltage. 

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