The Turn-on Voltages

It is well accepted that Vj-oN is related to the built-in potential Vbi, which is the difference in the workfunction of the cathode and the anode35,36 in addition to a correction term primarily due to interfacial effects 37 

FIGURE 6.13. The I-V curves for a series of PLED devices, in which the MEH-PPV films were spun at different spin speeds (solvent xylenes). 

However, Vi on is found to increase as the temperature decreases. This can be easily seen from Fig. 6.15, where Vi on is approximately 1.60 V at 298 K and 

FIGURE 6.15. The temperature dependence of current injection voltage. Shown are the I-V curves of the same device taken under different temperature. 

On the other hand, the poor anode/polymer contact shown in Fig. 6.12b can be improved to some extent by using higher spin speeds (Fig. 6.17). At high spin speeds, the polymer coils are stretched open, allowing the conducting polymer backbone to settle closer to the ITO surface. This results in a better contact and thus a lower hole-injection barrier and lower Vl on- 

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In push pull polymer 77, both the absorption and emission maxima are red-shifted relative to 1. The LED performance of these materials appeared to be rather low (the EL efficiency of 0.002cd/A and the maximum luminance of 100cd/m2 was achieved at 30 V), and the turn-on voltage for the push-pull polymer 77 (4 V) was lower than that in more electron-deficient polymers 75 and 76. 

Neumann and coworkers [165] synthesized tetrafluorinated-PPV copolymer 133 and studied its light-emitting properties. However, this material was quite unsuccessful for LED applications increasing the amount of fluorinated comonomer resulted in a dramatic decrease of the PLQY and the turn-on voltage of the devices was above 30 V (which could only be realized in ac mode due to device shorting). The quenching was less pronounced for an analogous copolymer with MEH-PPV (134), which showed an EL efficiency of up to 0.08 cd/A (in ITO/PEDOT/134/Ca diode) [166] (Chart 2.26). 

A better PLED performance was observed by Jenekhe and coworkers [173] for ITO/PEDOT/polymer/Al devices with quinoxaline-phenylene vinylene copolymers 586 and 587 as emitting layers. The el and maximum brightness were measured as 0.012 and 0.01%, and 120 and 35 cd/m2, respectively. The turn-on voltages of these devices were reasonably low, 6.0 and 4.0 V, respectively. The performance of PLEDs with polymer 586 was further improved by blending with 5wt% of a hole transport material, 1, l-Mstdi-d-tolylami-ii ophenyI )cycIohexane (TAPC) that enhanced the d lto 0.06% and the maximum brightness to 450 cd/m2. 

As may be noted, water vapor speeds up device response to hydrogen. Figure lib gives V-p (where V-pn is the turn on voltage for transport down the channel created by the stored charge) versus time for a MOSFET based on the Pd/Si02/Si structure. These data are for an exposure to 180 ppm H2 in air at 150°C. In this case, both response and recovery behavior are shown. Figure 12 clearly shows that Pd/SiC>2/... 

Polymers based on 1,10-phenanthroline and chlorotricarbonylrhenium(I) were fabricated into single-layer light emitting devices. The turn-on voltage was 7 V with a 125 cd/m2 output. The electroluminescence maximum was broad and occurred at 700 nm [111]. 

Device structure ITO/PEDOT PSS/polymer(Pl—P3)/BCP/Alq3/LiF/Al, where the polymer (PI—P3) is an emitting layer. bVoa is the turn-on voltage. 

Figure 28 shows the voltage-current characteristics measured for MEH-PPV and the blend polymers. The forward current increases with increasing forward bias voltage for all devices. The turn-on voltages of the blend polymer devices increase as the content of DSiPV increases in the blends. 

FIGURE 9.9. Current-voltage characteristics of four devices with different structures as shown in the legend. Note that the turn-on voltage decreases as the total number of layers... 

Doping the alignment layer reduces the turn-on voltage and enhances the LED brightness. 

Figure 10.26 shows the efficiency of the LED, as a function of the current density. The emission efficiency reaches a peak of 14.5 Cd/A at about 34 mA/cm2 with 5000 Cd/m2 luminance. The corresponding external quantum efficiency is 3.86%. The inset of Fig. 10.26 shows the luminous efficiency as a function of the current density. The peak value is 2.26 lm/W at 17 mA/cm2 with 2400 Cd/m2 luminance. He and Kanicki later reported higher efficiencies (56.2 Cd/A and 9.0 lm/W) for LEDs using the same emitting and hole-transport layers for an LED fabricated on a flexible, plastic substrate.61 The turn-on voltage of the LEDs was measured to be 6 V. 

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