Phosphorescent and fluorescent combined white OLED

Combining both fluorescent blue emission and phosphorescent green and red emission was then studied. We used the same structure as the totally phosphorescent device and only exchanged the FIrpic doped CBP host layer into a highly efficient sky blue fluorescent layer with MADN

dopant BUBD-1, we called these devices as Set C. The emission of this blue layer has a much bluer color rather than FIrpic which is an advantage in the fabrication of white OLED devices. The emission spectrum and CIE 1931 positions are shown in the Figure 4-13. For the white device, the fluorescent blue-emitting unit is formed close to the ITO anode side in each of the devices because the plasmon-quenching effect from the Al cathode on the blue emission could be avoided with this arrangement in order to obtain a maximal blue emission[65]. Again we evaluated the dopant concentration of Ir(piq)3 inside the Ir(ppy)3 layer, for this layer nearly defines the total efficiencies and CIE positions. We tried 2.0% and 3.0%, and found that the one with a 3.0% shows a too strong red emission thus gives a purple device, and the current efficiency weakens to 7.3 cd/A. The best dopant concentration will be 2.0% in order to enter the white region. This is 0.5% lower than the totally phosphorescent device. This reason can be explained because the sky blue fluorescent dopant BUBD-1 has a deeper blue emission compared with FIrpic, which pulls the CIE positions with lower y coordinates, thus a lower red composition is needed. This is a good effect while the Ir(piq)3

material is not good for the total efficiency of the device.

Figure 4-13 EL spectrum and CIE positions

For its performance, we will point out that the fluorescent blue EL unit is a critical component in determining color gamut and efficiency of the white OLEDs. Our panel showed a peak current efficiency of 14.7 cd/A at a luminance of 147 nits, and under near 1000 nits still owns an efficiency of 12.5 cd/A, power efficiency of 6 lm/W, and EQE near 7%.

The CIE positions are at (0.40, 0.43).

The performance of the RGB OLED as the light source in an RGB color display was evaluated by mathematically applying the transmission curves of color filter arrays (CFA) to the device. The outcome is an RGB system shown in Figure 4-14. Corresponding color coordinates compared with the original NTSC values are also shown, the %NTSC color gamut improved from 56.7% to 62.9% by using the sky blue fluorescent emitter rather than FIrpic. We note that CIEx,y of red and green peaks and especially the red coordinates lies practically on the spectral locus, indicating nearly mono-chromatic light. But the blue peaks still largely

differ from the %NTSC, which means that our blue emitter is still not bluish enough and the color filter is not very well, leading to poor

%NTSC color gamut ratios, so further improvement with deeper blue materials need to be estimated.

Figure 4-14(top) The %NTSC color gamut for the devices (bottom)The devices after color filter transmission(left:

phosphorescent, right: fluorescent plus phosphorescent)

Table 4-3 EL performance of the devices (Set C)

Further improvement by shifting the recombination zone was then performed. Although our device already has good performance, we predicted the efficiency could further be improved by modifying the distance of the emission layers to the anode and cathode. We first modified the thickness of the NPB layer, which acts as the hole transportation layer. From a thickness of 450 nm till 650 nm was studied, the device efficiencies indeed further improved, but at a thickness of 650 nm, the red emission gradually weakens because lesser holes could penetrate so far.

Figure 4-15 The EL spectrum with different NPB thickness

Table 4-4 EL performance of the devices

Another study is to exanimate the effect of phosphorescent sensitizer fluorescent devices, because we considered that this process might further drive down the material cost and fabrication process.

It has been demonstrated that the internal efficiency of fluorescent can be as high as 100% by using a phosphorescent sensitizer to excite a fluorescent dye through resonant energy transfer between triplet excitons in the phosphor and singlets in the fluorescent dye. Provided a device combines this kind of emission red or yellow with a blue emission, it should be a high-efficiency WOLED. Recently, a WOLED with a maximum luminous efficiency of 6.0 cd/A by using phosphor sensitized fluorescence was reported [52]. In this device, DCJTB, Ir(ppy)3, and NPB were used as fluorescent dye, phosphorescent sensitizer, and blue emitter, respectively. In later reports, a WOLED with a higher maximum of 9.22 cd/A is performed by using more efficient materials DPVBi and rubrene to replace NPB and DCJTB as blue and yellow emitters [66]. Lei et al. also fabricated a WOLED with a maximum current efficiency of 9.2 cd/A, in which blue phosphorescent material FIrpic was used as the

Devices (V) (cd/A) (lm/W) EQE (%) CIE 1931 CRI

NPB 450 6.6 12.2 5.8 6.8 (0.40, 0.43) 80 NPB 550 6.2 13.5 6.8 7.4 (0.39, 0.43) 83 NPB 650 6.4 14.7 7.3 6.8 (0.36, 0.46) ---

sensitizer [49]. Performances of this kind of WOLED are further improved by means of introducing a red phosphorescent emitter as well as exchanging the positions of the co-doped and the blue emissive layers [51]. Phosphor sensitization works as follows: By doping a phosphor at high concentrations ~ 5–10 wt% into a conductive host, both singlet and triplet excitons can transfer onto the phosphor molecule. If the phosphor contains a heavy metal atom, spin orbit coupling transfers all excited states on the phosphor to the radiative triplet manifold. These radiative states can then be readily transferred via the dipole–dipole Fo¨rster process to the radiative singlet state of the fluorophore co-doped with both the host and phosphor molecules. By lightly doping ~1% the fluorophore, hopping from the host triplets to the nonradiative triplet state of the fluorescent molecule is discouraged. In principle, therefore, phosphor sensitization can lead to 100% internal quantum efficiency of OLEDs radiating from the singlet manifold of the fluorescent dopant molecules [50].

Figure 4-16 Devices (Set D)

Figure 4-17 (Set D) The EL spectrum with different DCJTB doping concentrations (left: normalized, right: un-normalized

intensity)

Indeed, this provides a good method for producing white light. A special issue that has not been seen in phosphorescent red emitters is that, a red spectral shift of the DCJTB peak from about 560 nm to 580 nm is observed with increasing DCJTB concentration. This effect is also studied in other reports [67].

From the standard intensity spectrum, we can see that by intensifying the red dopant concentration, the red emission tends to maintain its maximum intensity without intensifying any further, but to the decrease of the green emission.

This device shows the best efficiency with a doping of concentration of 1.2%, reaching 16.7 cd/A, and power efficiency of 8.3 lm/W, the CIE shifted slightly to (0.41, 0.47), but still shows a white emission color. But due to the red emitter is not saturated enough; the CRI value is low, R9 which is a strong red reflector is below 0.

Table 4-5 EL performance of the devices (Set D)

Devices (V) (cd/A) (lm/W) (nits) EQE (%) CIE 1931 CRI

0.3% 6.1 23.7 12.0 2357 7.7 (0.33, 0.52) --- 0.6% 6.2 21.8 11.1 2180 7.4 (0.36, 0.50) --- 0.9% 6.3 19.2 9.5 1924 6.5 (0.37, 0.49) --- 1.2% 6.3 16.7 8.3 1665 6.0 (0.41, 0.47) 63

Figure 4-18 shows the comparison of the five kinds of device structures in its power efficiency. The phosphor-sensitized fluorescence obviously shows the highest efficiency.

Figure 4-18 The comparison of the five kinds of device structures in its power efficiency