**3. WOLED and color filters**

were 0.57 V and 0.55 V, respectively. **Figure 2(b)** shows the output curves of the a-IGZO TFT. The output curve was plotted by sweeping the drain voltage from 0 V to 20 V, while gate voltages

**Figure 3** is a graph plotting Rtot versus physical length of a-IGZO TFT with a width of 26 μm through the channel resistance method. ΔL, which is the difference between the physical channel length and the effective channel length of the coplanar a-IGZO TFT device,

Once OLED TV displays images, a-IGZO TFTs for OLED TV turn on and off repeatedly. In such operating environment, the device is stressed by continuous voltage, current, and temperature for a long time, which causes to degrade the electrical characteristics of a-IGZO TFT. The deteriorated a-IGZO TFT usually shows the change of Vth and drain current. This reduces the lifetime of

were applied from 5 to 20 V with 5 V steps.

**Figure 2.** Transfer characteristic of a-IGZO TFTs on Gen. 8.5 glass (2200 × 2500mm).

**Figure 3.** Illustration of the method used to extract effective channel length.

*2.2.2. Reliability properties of a-IGZO TFT*

was calculated as 1.2 μm.

36 Green Electronics

#### **3.1. Two-stack tandem WOLED with two colors**

WOLED employed to the first OLED TV launched in 2013 had a two-stack two-color tandem structure consisting of fluorescent blue and phosphorescent yellow-green (YG) stacks

**Figure 5.** (a) Device structure of two-stack two-color tandem WOLED (b) relative current efficiency of red, green and blue subpixels normalized by the values needed to display full-window white pattern of 100 nit.

serially connected by n- and p-type charge generation layers, as shown in **Figure 5(a)** [2, 3, 24]. Specifications on luminance for the first OLED TV were 100 nit at full-window white pattern and 400 nit at peak-luminance pattern where 25% area is turned on. However, for the second models of OLED TV, 150 and 450 nit were demanded for the full-window and peakluminance patterns, respectively, even at the lower power consumption.

correlated with the distance of EML from the cathode [27]. W1 whose both blue EMLs are farther from the cathode than YG EML shows that the blue intensity falls faster than the YG. W3 whose YG EML is farther from the cathode than the two blue EMLs shows that the YG intensity is reduced faster than the blue. In case of W2, as the blue intensities of two blue EMLs of which one is closer to and the other is farther from the cathode than YG EML, the blue efficiency is supposed to fall at the same trend with YG efficiency when the viewing angle is varied, resulting in little color shift. To comment on the viewing angle dependence of our two-stack WOLED depicted in **Figure 5(a)**, as blue EML is more distant from the cathode than YG EML, its color is supposed to shift toward YG at high incident angle.

**Figure 6.** Three candidates for three-stack WOLED consisting of two fluorescent blue stacks and 1 phosphorescent YG

Advanced Technologies for Large-Sized OLED Display http://dx.doi.org/10.5772/intechopen.74869 39

We fabricated three-stack WOLED device where B, YG, and B units are sequentially formed, based on the optical simulation. **Table 1** summarized its actual performance compared with two-stack WOLED device. Since another blue device unit is supplemented, the voltage applied across the three-stack WOLED is enlarged by 4.5 V at the same current density of

by 8% and emits cool white light with high CCT of 8500 K, owing to its enhanced blue intensity by two blue units. Generally, TV panel displays white color with high CCT up to 9300~10,000 K. To realize such a cool white color using two-stack WOLED emitting white light of 6300 K CCT, the blue subpixel should be turned on with a high intensity, resulting in a high power consumption. The experimental electroluminance (EL) spectra varied by viewing angle, as depicted in **Figure 7(a)**, show that the intensities at blue and YG regions drop simultaneously at the same rate, resulting in very low color shift of 0.011 at 60°. The efficiency of blue subpixel for the three-stack WOLED is found to be enhanced by 75%, through adding

As mentioned above, the three-stack WOLED employing two stacks of blue device is advantageous to lowering the power consumption as well as boosting luminance of OLED panel.

. However, the three-stack WOLED has the current efficiency (cd/A) enhanced

10 mA/cm2

stack.

one more blue stack, as shown in **Figure 7(b)**.

**Figure 5(b)** shows relative current efficiencies (cd/A) of RGB subpixels after transmitting through color layer. These values are normalized by the efficiency for each subpixel, required to get a certain luminance. Since it is shown that blue subpixel is a restriction factor to the panel brightness, we need to drastically increase the efficiency of the blue device by more than 1.5 times. Two approaches could be considered in respect of external quantum efficiency and internal quantum efficiency. With regard to the external quantum efficiency, blue efficiency could be improved through a strong cavity effect using an electrode of thin metal. However, color shift varied by view angle could be worse. Regarding the internal quantum efficiency, phosphorescent blue [25, 26] or thermally activated delayed fluorescent (TADF) blue is still not around the corner.

#### **3.2. Three-stack tandem WOLED with two colors**

Our solution for the higher blue efficiency is three-stack tandem WOLED, namely, adding one more blue stack [27]. It is true that three-stack tandem WOLED needs higher applied voltage, which might become a factor to increase the power consumption. Nonetheless, three-stack WOLED can realize lower power consumption as well as higher luminance, which will be demonstrated later. As shown in **Figure 6**, three device architectures with different sequences of the unit devices are considered: (a) YG unit adjacent to the cathode (W1), (b) in the middle of the device (W2), and (c) adjacent to anode (W3). The best device architecture for three-stack two-color (3S2C) tandem WOLED is decided in terms of efficiency and color shift by viewing angle, with an assistance of optical simulation [28].

The viewing angle dependence of blue and YG efficiencies in each WOLED device can be predicted on the basis of the fact that angular dependence of blue and YG mono-devices is

**Figure 6.** Three candidates for three-stack WOLED consisting of two fluorescent blue stacks and 1 phosphorescent YG stack.

serially connected by n- and p-type charge generation layers, as shown in **Figure 5(a)** [2, 3, 24]. Specifications on luminance for the first OLED TV were 100 nit at full-window white pattern and 400 nit at peak-luminance pattern where 25% area is turned on. However, for the second models of OLED TV, 150 and 450 nit were demanded for the full-window and peak-

**Figure 5.** (a) Device structure of two-stack two-color tandem WOLED (b) relative current efficiency of red, green and

**Figure 5(b)** shows relative current efficiencies (cd/A) of RGB subpixels after transmitting through color layer. These values are normalized by the efficiency for each subpixel, required to get a certain luminance. Since it is shown that blue subpixel is a restriction factor to the panel brightness, we need to drastically increase the efficiency of the blue device by more than 1.5 times. Two approaches could be considered in respect of external quantum efficiency and internal quantum efficiency. With regard to the external quantum efficiency, blue efficiency could be improved through a strong cavity effect using an electrode of thin metal. However, color shift varied by view angle could be worse. Regarding the internal quantum efficiency, phosphorescent blue [25, 26] or thermally activated delayed fluorescent (TADF) blue is still not around the corner.

Our solution for the higher blue efficiency is three-stack tandem WOLED, namely, adding one more blue stack [27]. It is true that three-stack tandem WOLED needs higher applied voltage, which might become a factor to increase the power consumption. Nonetheless, three-stack WOLED can realize lower power consumption as well as higher luminance, which will be demonstrated later. As shown in **Figure 6**, three device architectures with different sequences of the unit devices are considered: (a) YG unit adjacent to the cathode (W1), (b) in the middle of the device (W2), and (c) adjacent to anode (W3). The best device architecture for three-stack two-color (3S2C) tandem WOLED is decided in terms of efficiency and color shift by viewing

The viewing angle dependence of blue and YG efficiencies in each WOLED device can be predicted on the basis of the fact that angular dependence of blue and YG mono-devices is

luminance patterns, respectively, even at the lower power consumption.

blue subpixels normalized by the values needed to display full-window white pattern of 100 nit.

**3.2. Three-stack tandem WOLED with two colors**

38 Green Electronics

angle, with an assistance of optical simulation [28].

correlated with the distance of EML from the cathode [27]. W1 whose both blue EMLs are farther from the cathode than YG EML shows that the blue intensity falls faster than the YG. W3 whose YG EML is farther from the cathode than the two blue EMLs shows that the YG intensity is reduced faster than the blue. In case of W2, as the blue intensities of two blue EMLs of which one is closer to and the other is farther from the cathode than YG EML, the blue efficiency is supposed to fall at the same trend with YG efficiency when the viewing angle is varied, resulting in little color shift. To comment on the viewing angle dependence of our two-stack WOLED depicted in **Figure 5(a)**, as blue EML is more distant from the cathode than YG EML, its color is supposed to shift toward YG at high incident angle.

We fabricated three-stack WOLED device where B, YG, and B units are sequentially formed, based on the optical simulation. **Table 1** summarized its actual performance compared with two-stack WOLED device. Since another blue device unit is supplemented, the voltage applied across the three-stack WOLED is enlarged by 4.5 V at the same current density of 10 mA/cm2 . However, the three-stack WOLED has the current efficiency (cd/A) enhanced by 8% and emits cool white light with high CCT of 8500 K, owing to its enhanced blue intensity by two blue units. Generally, TV panel displays white color with high CCT up to 9300~10,000 K. To realize such a cool white color using two-stack WOLED emitting white light of 6300 K CCT, the blue subpixel should be turned on with a high intensity, resulting in a high power consumption. The experimental electroluminance (EL) spectra varied by viewing angle, as depicted in **Figure 7(a)**, show that the intensities at blue and YG regions drop simultaneously at the same rate, resulting in very low color shift of 0.011 at 60°. The efficiency of blue subpixel for the three-stack WOLED is found to be enhanced by 75%, through adding one more blue stack, as shown in **Figure 7(b)**.

As mentioned above, the three-stack WOLED employing two stacks of blue device is advantageous to lowering the power consumption as well as boosting luminance of OLED panel.


**3.3. Three-stack three-color tandem WOLED**

deep red color with high CIE.x value are necessary.

The next urgent demand by our customers was to widen color gamut up to digital cinema initiatives (DCI) standard color space, after the luminance was enhanced by three-stack twocolor (3S2C) tandem WOLED. The 3S2C WOLED has achieved 114% color gamut area compared to sRGB standard color but covered only 90% area in DCI standard color space [27]. In order to widen color gamut in DCI color space, deep green color with high CIE.y value and

Advanced Technologies for Large-Sized OLED Display http://dx.doi.org/10.5772/intechopen.74869 41

Since phosphorescent green EML did not show long enough lifetime for WOLED TV, we considered the insertion of an additional red EML in the three-stacked WOLED structure while maintaining the YG EML. By adding red EML, red color point was supposed to move toward higher CIE.x value. In case of green color point, we relied on optimizing green color layer.

As shown in **Figure 8**, considering contour map of the emittance [27, 28], three ways to add red EML in the three-stack WOLED could be taken into account. The two ways are as follows: a fluorescent red EML is added, adjacent to the fluorescent blue EML in the first- or third-stack unit. The other way is that a phosphorescent red EML is inserted, adjacent to the phosphorescent yellow-green in the second-stack layer. In the case of the three-stack three-color (3S3C) WOLED with fluorescent red EML, the blue EML in the third stack shares the exciton with the red EML so that the blue efficiency is decreased a little, but the overall efficiency (cd/A) is raised, thanks to the contribution of the red EML. As a result, the WOLED has warmer white. On the other hand, in the case of the WOLED with phosphorescent red EML, the YG EML in the second stack shares the exciton with the red EML. As the efficiency

**Figure 8.** (a) Emittance contour map for three-stack three color tandem WOLED where red dotted lines are drawn at the position having relatively high emittance value near 620 nm. (b) Three device architectures for three-stack three color tandem WOLED; red EML in W-R1 is fabricaed before YG EML in the second stack, red EML in W-R2, before the second

blue EML in the third stack, and red EML in W-R3, after the first blue EML in the first stack.

**Table 1.** The comparison of device characteristics between two-stack WOLED and three-stack WOLED at 10 mA/cm2 .

**Figure 7.** (a) Experimenal EL spectra of our real three-stack two-color tandem WOLED at various viewing angles showing that blue and YG intensities drop at the same ratio and (b) the comparison of efficiency of R, G, B subpixels between the two-stack tandem WOLED and three-stack tandem WOLED.

By enhancing the blue efficiency, the three-stack WOLED made our new OLED TV realize the luminance of 150 nit at full-window pattern which is higher by 50% compared to the first model using two-stack WOLED.

As regards to power consumption, we calculated time-average current values spent at each pixel of WRGB for OLED panels during playing a movie file ruled by the international standard CIE60287 at a fixed maximum luminance. The total current of OLED panel employing the three-stack WOLED is estimated to be at the 68% level of two-stack WOLED. Considering voltage rise of the three-stack WOLED, total voltage across WOLED and driving transistor in the backplane is increased by 10%. Namely, the final effect of the three-stack WOLED for saving the power consumption of OLED panel is calculated to be 24%.

To analyze the current of each subpixel in detail, OLED panel of the two-stack WOLED shows that a half of the total current is applied to the blue subpixel. In that case, by extending aperture ratio of the blue subpixel and lessening the current density, electrical stress on the blue subpixel could be reduced.

For the three-stack WOLED emitting cool white, red subpixel is more used to display warm colors, and the current to the red subpixel is slightly increased. However, the current decrease of blue subpixel as well as white subpixel is overwhelming. As a result, balancing of currents applied to four subpixels becomes better in the three-stack WOLED, which provides an advantage to OLED panel design. In addition, the lifetime for blue subpixel can be extended significantly.
