Density OLEDs

Another issue that can be clarified with the aid of numerical simulations is that of the recombination profile. Mailiaras and Scott [145] have found that recombination takes place closer to the contact that injects the less mobile carrier, regardless of the injection characteristics. In Figure 13-12, the calculated recombination profiles arc shown for an OLED with an ohmic anode and an injection-limited cathode. When the two carriers have equal mobilities, despite the fact that the hole density is substantially larger than the electron density, electrons make it all the way to the anode and the recombination profile is uniform throughout the sample. 

Figure 13-6. The vullage dependence ol the cuinenl densities in the dark (open triangles), under illumination (open circles) and their difference (filled squares) for an fTO/MEH-PPV/Mg OLED. The inset shows the same data plotted as tire logarithm of the current (difference)- Reproduced with permission front [97J.

Jiang et al. were the first to report a relatively stable blue OLED based on anthracene derivative JBEM (120) [240]. With the similar OLED structure as that used above by Kodak of ITO/CuPc/NPD/JBEM perylene/Alq/Mg Ag and using JBEM as a blue host material, the device shows a maximum luminance of 7526 cd/m2 and a luminance of 408 cd/m2 at a current density of 20mA/cm2. The maximum efficiency is 1.45 lm/W with CIE (0.14,0.21). A half-life of over 1000 h at initial luminance of 100 cd/m2 has been achieved. The authors also compared the device performance using DPVBI as a host, which gave them a less stable device. 

FIGURE 6.12 (a) Current density-voltage and (b) luminance-current density characteristics of OLEDs with a configuration of ITO/Alq3 interlayer/NPB/Alq3/Ca/Ag. The thickness of the Alq3 interlayer was varied over a range of 0-5.0 nm. 

Operating Voltage and Corresponding Luminous Efficiency of Identical OLEDs Made on ITO Anode with Different Alq3 Modification Layer Thicknesses, Measured at a Current Density of 100 mA/cm2... 

The enhancement in luminous efficiency achieved by inserting an ultrathin interlayer between the ITO and NPB is mainly due to the reduction of hole injection from ITO to NPB in OLEDs. For a simple approximation, luminous efficiency (rj) can be related directly to a ratio of the recombination current (/r) to the total current density of OLEDs (/tot). If one denotes the current contributions from holes and electrons in OLEDs as. /h and /e, respectively, then the sum of hole and electron currents, /tot. /h + /e, and tj can be expressed as... 

The J-V, L-V, and E V characteristics, measured for the OLEDs made with a commercial ITO anode and an AZO anode are plotted in Figure 6.18a and b, respectively. The current density measured for an OLED with an AZO anode is lower than that obtained for a device made with an ITO anode at the same operating voltage. A slight high turn-on voltage observed in the OLED using an AZO anode is attributed to its lower work function compared... 

FIGURE 6.18 Current density-voltage, luminance-voltage, and luminous efficiency-voltage characteristics measured for the OLEDs made with an AZO anode and an ITO anode. 

Figure 7.6 shows typcial current density-voltage-luminance (J-V-L) and emission characteristics of an OLED device. OLEDs have a similar electrical characteristic to that of a rectifying diode. In forward bias, the device starts with a small current at low voltages. In this region, charge carriers are injected into the device but little exciton formation, hence light... 

FIGURE 7.6 Typical current density (filled squaresj-voltage-luminance (open squares) (J-V-L) and emission characteristics (inset figure) of an OLED device. This J-V-L data is from the device discussed later in Figure 7.10. 

A similar ITO/TPD (see fig. 117) OLED was built, with [Nd(8-Q)3] as luminescent active layer the device has an operative voltage of 30 V, corresponding to a current density of 78 mAcm-2 the threshold voltage is 13 V ( 0.23 mAcm-2) and no visible luminescence is emitted (Khreis et al., 2000). 

The /3-diketonate [Nd(dbm)3bath] (see figs. 41 and 117) has a photoluminescence quantum efficiency of 0.33% in dmso-7r, solution at a 1 mM concentration. It has been introduced as the active 20-nm thick layer into an OLED having an ITO electrode with a sheet resistance of 40 il cm-2, TPD as hole transporting layer with a thickness of 40 nm, and bathocuproine (BCP) (40 nm) as the electron injection and transporting layer (see fig. 117). The electroluminescence spectrum is identical to the photoluminescence emission the luminescence intensity at 1.07 pm versus current density curve deviates from linearity from approximately 10 mA cm-2 on, due to triplet-triplet annihilation. Near-IR electroluminescent efficiency <2el has been determined by comparison with [Eu(dbm)3bath] for which the total photoluminescence quantum yield in dmso-tig at a concentration of 1 mM is Dpi, = 6% upon ligand excitation, while its external electroluminescence efficiency is 0.14% (3.2 cdm-2 at 1 mAcm-2) ... 

Fig. 4.11. Left (a) Optical microscope image of an OLED working at a luminance of 100 cd/m2 under water vapor atmosphere. Non-emitting dark spots can be seen clearly, (b) SEM image of the bubbles formed on the aluminum cathode in the dark spot area, (c) Correlation between dark spot growths (taken from the increase in diameter) and total current density [110]. Right (a) Shown here is the random pattern of carbonized areas on the surface of the cathode after operation, shown in wide field, (b) At higher resolution, the structure of one of these areas becomes more apparent, (c) and (d) show nanoscale views of carbonized areas with the extrusion of the polymer through the cathode and the resulting void underneath [111].

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