Fluorescent Dopants

The requirements for full-colour display applications of OLEDs include red, green and blue emitters with colour coordinates close to the following values: for green emitters, x = 0.3 and у = 0.6; for red x = 0.62 and у = 0.37; for blue x = 0.14 and у = 0.10. Fluorescent dopants emitting with approximately these colour coordinates are required.

Figure 6.23 Hole transport hosts CBP and CDBP Chemical Structure reproduced from Or­ganic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

An example of a green dopant is based on a coumarin dye molecule such as C-545TB, shown in Figure 6.24. This dopant yields saturated green emission with colour coordinates x = 0.3, y = 0.64, with a luminescent efficiency of 12.9 cdA-1, a power efficiency of 3.5 lm W-1 at 20 mA cm-2 and a brightness of 2585 cdm-2. Another type of green dopant is DMQA, which is an example of a quinacridone molecule. DMQA achieved a luminescent efficiency of 21.1 cd A-1 and a luminance of over 88 000 cd m-2. Coumarin-based C-545TB and quinacridone-based DMQA are shown in Figure 6.23.

Red fluorescent dopants have been developed that simultaneously exhibit satisfactory colour coordinates with good stability and efficiency based on the arylidene family of molecules. An example of a red fluorescent molecule is DCJPP, shown in Figure 6.25. There are large numbers of other candidate red fluorescent materials in this family; however, the less suitable ones suffer from a tendency to undergo unwanted chemical reactions. Some fluoresce with y-values of their colour coordinates that are too large and orange-red emission results. Still others exhibit deep-red emission but have low quantum efficiencies.

Red fluorescent molecules based on other molecular families exist. For example, an isophorone-based red emitter, DCDDC (see Figure 6.25), has been used as a red emitter in OLEDs when dissolved in the host Alq3. Emission from both Alq3 and DCDDC is observed for small concentrations of dopant; however, if the DCDDC doping level is increased to above 2% concentration, only the red emission is observed due to a strong

host-to-guest energy transfer. The emission peak is 630 nm from the DCDDC. At a 1% DCDDC concentration, which does somewhat compromise the red colour, a peak luminance of 5600 cdm-2 at a voltage of 15 V with maximum efficiency of 1.6 lmW-1 is achieved.

There has been a great deal of effort invested in blue fluorescent molecules and suitable hosts. The challenge is that the short wavelength of emission in the range of 450 nm required for blue colour coordinates with у-values near 0.1 calls for high bandgaps between guest LUMO and HOMO levels and even higher bandgaps near 3 eV between suitable host LUMO and HOMO levels. Molecules with these properties exist but the resulting OLEDs have proven less stable than red and green emitters.

An example of a blue emitter host that is a distyrylaraline derivative is DPVBI, having HOMO and LUMO levels of -5.9 eV and -2.8 eV respectively, and bandgap of 3.1 eV. A suitable fluorescent guest that is also a distyrylaraline derivative is BCzVBI, with HOMO and LUMO levels of -5.4 eV and -2.42 eV. Luminance centred at 468 nm at 10 000 cd m-2 at an efficiency of 0.7-0.8 lm W-1 has been achieved (see Figure 6.26).

Also shown in Figure 6.26 is a candidate blue system from the anthracene family. The host is JBEM with HOMO and LUMO levels -5.8 eV and -2.8 eV respectively, and the guest is the well-known perylene molecule with HOMO and LUMO levels -5.3 eV and -2.5eV respectively. Resulting OLED performance achieves 400cdm-2 at a current density of 20mAcm-2, a maximum efficiency of 1.45lmW-1 and colour coordinates of x = 0.24 and у = 0.21. A half-life (life to half initial luminance) starting at 100cdm-2 brightness of over 1000 hours can be obtained. Improvements to lower the y-component

of the colour coordinate and achieve pure blue emission can be realized by using other anthracene derivatives.