OLED with an external color tuning layer

A few papers introduced the idea of using a color emitting OLED in combination with a down-conversion layer to produce white OLED. We also tried to use this easy fabrication process, and improved the CIE positions by selecting an appropriate concentration dye dissolved in PMMA. According to the concept of complementary wavelengths, two colors added in order to produce white color is calculated as in Figure 4-19.

Figure 4-19 The concept of complementary wavelengths

So if our blue devices have emission intensity near 480 nm, we will then need a complementary wavelength near 580 nm in order to produce white light. We exanimate the effect of the doping concentration of the ECTL dye inside polymethylmethacrylate (PMMA) host, which is considered the most transparent thin film material. It is found that the spectrum tends to red-shift at higher doping condition, but the emission peak near 538 nm is nearly not changed. We estimate that the strongest absorption and emission for the dye is between 0.5wt% and 1.0wt%.

Figure 4-20 The PL emission of the ECTL under different concentration

So for our best choice, the doping of this ECTL into PMMA was achieved by mixing 0.5 to 1.0wt% of the dye with PMMA in a common solvent such as toluene. The well-stirred mixture (using ultrasonic mixing for 15 min) was applied on the substrate by using the doctor blade technique then was dried in air.

Blue devices with ECTL

For testing the performances of the color tuning layer, we fabricated three blue devices as shown in Figure 4-21. Device E-A is a fluorescent

blue emitting OLED having the structure of:

ITO/CFx/NPB/MADN:1%BUBD-1/Alq/BPhen:5%Cs2CO3/Al, while Devices E-B and E-C are phosphorescent blue emitting diodes, with either a ungraded or graded emission layer, respectively. Device E-B has the structure: ITO/CFx /NPB/mCP: 8%FIrpic/BPhen/BPhen:

5%Cs2CO3/Al, while Device E-C have the same structure except with a graded emissive layer: ITO/CFx/NPB/mCP: 5%FIrpic/mCP:

15%FIrpic/mCP: 35%FIrpic/BPhen/BPhen:5%Cs2CO3/Al, the average dopant concentration of the emission layer is controlled at 8% .

Figure 4-21 The blue devices (Set E)

As shown in Figure 4-22, Device E-A and Device E-B shows nearly the same current efficiency of 8.6 cd/A and 9.6 cd/A under 10 mA/cm², we can see that the two lines intersects at a luminance of 2500 nits. For higher brightness, Device E-A produces a steady efficiency throughout 10000 nits, while the efficiency of Device E-B starts to descent, probably

due to triplet-triplet annihilation. Device E-C, which has a gradient emissive layer has a higher current efficiency of 13.8 cd/A, nearly 150%

better than Device B. This effect is reported earliest in 2004 [68]. The reason is that in order to achieve a high efficiency and color stability for high-energy electro-phosphorescent devices, both electron and hole-blocking layers are needed [69]. By using a graded structure, FIrpic can be used as a combined hole-blocking and electron-transporting layer:

For the part nearer the cathode, the high concentration of 35% FIrpic in the mCP: FIrpic/Bphen interface can block holes and excitons from migrating out of the emissive layer and this enhanced the efficiency and luminance of the device. For the part closer to the anode, the NPB/mCP:

FIrpic interface with low dopant concentration of only 5% acts simultaneously as an electron-blocking layer. While the excitions are further controlled inside the emissive layer, a much higher efficiency can be reached. In 2008, another study exanimated clearer this effect and

Figure 4-22 The efficiencies and EL spectra of device Set E

Figure 4-23 The device structures of device Set E after applying external color tuning layer (ECTL)

Table 4-6 The EL performance of the devices before conversion(Set E)

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

E-A 4.9 8.6 5.5 4.1 (0.17, 0.32)

E-B 6.0 9.6 5.0 3.9 (0.20, 0.42)

E-C 5.7 13.8 7.6 5.9 (0.18, 0.40)

ECTLs were deposited on the three blue devices by using the doctor blade technology till that all of the three devices have reached the same conversion intensity at the ECTL main peak with an emission of 548 nm.

The devices after conversion are labeled as Device E-A’, E-B’ and E-C’.

The efficiency after conversion is shown as Figure 4-24, the devices have CIE values pretty close to the CIE white point. We can see that originally Device E-A and Device E-B have nearly the same efficiencies, but now Device E-B’ has a intensity higher than Device E-A’. This is majorly because that the ECTL have a major absorption peak near 491 nm and 525 nm (not shown), but the fluorescent Device E-A’ while having a deeper blue emission spectrum and also a weak emission near 491 nm leads to lesser energies can then be absorbed by the ECTL near 491 nm. The layer then turns to absorb the emission closer to 525 nm and emits the light with a longer wavelength trend, which leads to a much weaker current efficiency because of the lower eye sensitivity. But the efficiencies after conversion drops a lot, the conversion efficiencies are only about 60%. To notice, while the devices contain only one emissive layer, there is absolutely no color shifting issue of the devices from 200 nits till over 10000 nits.

We also modified the concentration of the ECTL for Device E-C’

and found that the conversion peaks were slightly different for 0.5wt.%

and 1.0wt.%, but the one with a 0.5wt.% ECTL concentration have higher

efficiency, which seems to be a effect of the higher eye sensitivities due to the raise of the greenish-yellow emission peaks, and the efficiency of applying 1.0wt.% was rather weaker. The 0.5wt.% and 1.0wt.% devices after conversion show the original emission from the blue device with peaks near 472, 504 nm, and also conversion peaks near 544 or 548 nm with also a longer wavelength shoulder. But since the device with a 0.5wt.% ECTL deviates from the white point some distance, it is majority useless.

Figure 4-24 The efficiencies and EL spectra of device Set E after conversion

Table 4-7 The EL performance of the devices after conversion(Set E) Devices (V) (cd/A) (lm/W) EQE (%) CIE 1931

E-A’ 4.9 5.2 3.3 2.1 (0.33, 0.33)

E-B’ 6.0 6.8 3.6 2.6 (0.31, 0.43)

E-C’ 5.8 9.6 5.2 3.6 (0.31, 0.43)

Figure 4-25 The EL spectra of device Set E-C after conversion with 0.5 or 1.0wt.% ECTL