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

**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

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

**Two-stack WOLED Three-stack 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

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

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

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

between the two-stack tandem WOLED and three-stack tandem WOLED.

Voltage (V) 7.1 11.6 Efficiency (cd/A) 78.6 85.0

Color shift (Δ*u'v*')\* 0.020 0.011

Color (Wx, Wy) (0.317, 0.332) (0.287, 0.310)

saving the power consumption of OLED panel is calculated to be 24%.

model using two-stack WOLED.

40 Green Electronics

subpixel could be reduced.

significantly.

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

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.

(cd/A) of the red EML is lower than that of the YG EML, this WOLED has the less efficiency than the 3S2C WOLED, but it emits the cool white light which is more suitable for the display application. Moreover, it is well known that fluorescent red materials do not show good lifetime performance. Thus, we should choose the WOLED involving the phosphorescent red (Ph-R) EML.

In the second case, or inserting phosphorescent R-EML, we fabricated a unit device of the phosphorescent red and YG (Ph-R/Ph-YG) EMLs whose structure is in the following, ITO/HTL/x% red dopant/12% YG dopant/ETL/EIL/Al, to examine the influence between red dopant ratio and YG dopant ratio. By controlling the concentration of R dopant at 12% YG dopant concentration, we obtained the best result at 2% R dopant concentration. **Figure 9(c)** illustrates EL spectra at the R doping ratio of 2 and 4% with the YG dopant fixed at 12% concentration. As R doping ratio is increased, the intensity in green region is reduced, and the intensity in red region is enhanced, while the external quantum efficiency (EQE) is slightly changed. These phenomena can be caused by the increase in the exciton energy transfer from YG EML to red EML owing to the increased red dopant concentration.

intensity is reduced, whereas the red peak intensity grows higher with approximately equal EQE. The increasing ratio of the electron-type host makes the facile electron transfer from YG EML to R EML. As a result, the electron-hole recombination zone will shift from the YG EML to the R EML/YG EML interface, which makes the rise of the R peak in the EL spectrum. We found the optimal spectrum at h-type host:e-type host = 7:3 ratio in YG EML. In case of Ph-R/Ph-YG unit device, we can control the red intensity with the host

10:0 3.9 60.0 19.5 0.467 0.524 7:3 4.0 52.9 19.2 0.487 0.505 0:10 4.1 49.1 19.3 0.501 0.492

2 4.0 52.9 19.2 0.487 0.505 4 4.1 47.0 19.1 0.507 0.486

**Table 2.** Summary of device performance of Ph-devices (upper) with mixed host ratio at 10 mA/cm2

.

**Volt (V) cd/A EQE (%) CIEx CIEy**

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

(lower) with R

43

There is an alternative way to increase the red intensity in WOLED, i.e., co-deposition of phosphorescent red and YG dopants at one EML in second unit device. Since the optimal doping ratio for the red dopant is in the range from 0.2 to 0.4%, the doping ratio should be precisely controlled. As the doping concentration of the red dopant was raised a little above the range, it was found that the intensity in green region is reduced, while the intensity in red region is increased. However, when the red doping ratio was too low, the exact ratio control between the red dopant and green dopant was not easy in process. Therefore, we considered inserting

**Figure 10(a)** compares EL spectra of 3S3C and 3S2C WOLEDs. It found that 3S3C WOLED has distinct red peak at 620 nm and the higher red intensity than 3S2C WOLED, resulting in an efficiency enhancement of red subpixel by 38%. As shown in **Figure 10(b)**, EL spectrum at red subpixel after going through the red color layer (CL) is red-shifted by 10 nm. Consequently, by replacing 3S2C WOLED with 3S3C WOLED, color coordinates of the red subpixel are varied from (0.666, 0.332) to (0.678, 0.321), very close to the red chromaticity of the DCI standard, as depicted in **Figure 10(c)**. Regarding the color of the green subpixel, we obtained the high purity green color by developing a new green CL for high color gamut. Thanks to the new green CL, the color coordinates of the green subpixel were shifted from (0.300, 0.645) to (0.270, 0.666). As shown in **Figure 10(c)**, 3S3C WOLED with the new CLs covers most of the color space for the DCI standard, which corresponds to 99% color gamut

ratio as well as the dopant ratio.

dopant concentration at 10 mA/cm2

of the DCI standard in CIE1976 (u'v') color space.

a red EML separately.

Host ratio (H:E)

Red dopant Conc. (%)

In addition to the red dopant concentration, it is also possible to control the relative intensities of red and green colors by changing the ratio of hole transport host and electron transport host in YG EML [3]. We have been adopting the mixed-host structure of a hole-type host and an electron-type host as a phosphorescent EML layer for high efficiency and long lifetime of WOLED. We fabricated Ph-R/Ph-YG unit devices along with the host ratio in YG EML of 0:10, 7:3, and 10:0 ratios of hole transport host and electron transport host at 12% YG dopant and 2% red dopant concentration.

**Table 2** summarizes electrooptical performance of the Ph-R/Ph-YG unit devices, and **Figure 9(b)** shows EL spectra of the devices. When the hole-type host only (h-type host:etype host =10:0) is used, the EL spectrum shows mainly YG peak with a slight red peak, which means that excitons were formed largely in the YG EML. As the ratio of the electrontype host in the YG EML is increased (h-type host:e-type host =7:3, 0:10), the YG peak

**Figure 9.** (a) Schematic diagram of phosphorescent unit device, (b) EL spectra of Ph-devices with different mixed host ratio at fixed YG and R dopant concentration, and (c) EL spectra of Ph-devices with the different R dopant concentration at fixed YG dopant concentration.


(cd/A) of the red EML is lower than that of the YG EML, this WOLED has the less efficiency than the 3S2C WOLED, but it emits the cool white light which is more suitable for the display application. Moreover, it is well known that fluorescent red materials do not show good lifetime performance. Thus, we should choose the WOLED involving the phosphorescent

In the second case, or inserting phosphorescent R-EML, we fabricated a unit device of the phosphorescent red and YG (Ph-R/Ph-YG) EMLs whose structure is in the following, ITO/HTL/x% red dopant/12% YG dopant/ETL/EIL/Al, to examine the influence between red dopant ratio and YG dopant ratio. By controlling the concentration of R dopant at 12% YG dopant concentration, we obtained the best result at 2% R dopant concentration. **Figure 9(c)** illustrates EL spectra at the R doping ratio of 2 and 4% with the YG dopant fixed at 12% concentration. As R doping ratio is increased, the intensity in green region is reduced, and the intensity in red region is enhanced, while the external quantum efficiency (EQE) is slightly changed. These phenomena can be caused by the increase in the exciton energy transfer from YG EML to red EML owing to the increased red dopant

In addition to the red dopant concentration, it is also possible to control the relative intensities of red and green colors by changing the ratio of hole transport host and electron transport host in YG EML [3]. We have been adopting the mixed-host structure of a hole-type host and an electron-type host as a phosphorescent EML layer for high efficiency and long lifetime of WOLED. We fabricated Ph-R/Ph-YG unit devices along with the host ratio in YG EML of 0:10, 7:3, and 10:0 ratios of hole transport host and electron transport host at 12% YG dopant and

**Table 2** summarizes electrooptical performance of the Ph-R/Ph-YG unit devices, and **Figure 9(b)** shows EL spectra of the devices. When the hole-type host only (h-type host:etype host =10:0) is used, the EL spectrum shows mainly YG peak with a slight red peak, which means that excitons were formed largely in the YG EML. As the ratio of the electrontype host in the YG EML is increased (h-type host:e-type host =7:3, 0:10), the YG peak

**Figure 9.** (a) Schematic diagram of phosphorescent unit device, (b) EL spectra of Ph-devices with different mixed host ratio at fixed YG and R dopant concentration, and (c) EL spectra of Ph-devices with the different R dopant concentration

red (Ph-R) EML.

42 Green Electronics

concentration.

2% red dopant concentration.

at fixed YG dopant concentration.

**Table 2.** Summary of device performance of Ph-devices (upper) with mixed host ratio at 10 mA/cm2 (lower) with R dopant concentration at 10 mA/cm2 .

intensity is reduced, whereas the red peak intensity grows higher with approximately equal EQE. The increasing ratio of the electron-type host makes the facile electron transfer from YG EML to R EML. As a result, the electron-hole recombination zone will shift from the YG EML to the R EML/YG EML interface, which makes the rise of the R peak in the EL spectrum. We found the optimal spectrum at h-type host:e-type host = 7:3 ratio in YG EML. In case of Ph-R/Ph-YG unit device, we can control the red intensity with the host ratio as well as the dopant ratio.

There is an alternative way to increase the red intensity in WOLED, i.e., co-deposition of phosphorescent red and YG dopants at one EML in second unit device. Since the optimal doping ratio for the red dopant is in the range from 0.2 to 0.4%, the doping ratio should be precisely controlled. As the doping concentration of the red dopant was raised a little above the range, it was found that the intensity in green region is reduced, while the intensity in red region is increased. However, when the red doping ratio was too low, the exact ratio control between the red dopant and green dopant was not easy in process. Therefore, we considered inserting a red EML separately.

**Figure 10(a)** compares EL spectra of 3S3C and 3S2C WOLEDs. It found that 3S3C WOLED has distinct red peak at 620 nm and the higher red intensity than 3S2C WOLED, resulting in an efficiency enhancement of red subpixel by 38%. As shown in **Figure 10(b)**, EL spectrum at red subpixel after going through the red color layer (CL) is red-shifted by 10 nm. Consequently, by replacing 3S2C WOLED with 3S3C WOLED, color coordinates of the red subpixel are varied from (0.666, 0.332) to (0.678, 0.321), very close to the red chromaticity of the DCI standard, as depicted in **Figure 10(c)**. Regarding the color of the green subpixel, we obtained the high purity green color by developing a new green CL for high color gamut. Thanks to the new green CL, the color coordinates of the green subpixel were shifted from (0.300, 0.645) to (0.270, 0.666). As shown in **Figure 10(c)**, 3S3C WOLED with the new CLs covers most of the color space for the DCI standard, which corresponds to 99% color gamut of the DCI standard in CIE1976 (u'v') color space.

Generally, the image data is supplied as a data voltage via a data line and applied to the gate of the driving transistor (DR) through the switching transistor (SW). The data voltage is stored in the storage capacitor (Cst), which keeps the gate-to-source voltage (Vgs) of DR stable even when the source voltage (Vs) changes according to the current–voltage character-

istics of the OLED. The current flowing through DR is determined by Eq. 1:

*ds* <sup>=</sup> \_\_1

<sup>2</sup> *μ Cox* \_\_ *W*

The threshold voltage (Vth) determines the x intercept of the V–I1/2 diagram of the transistor, while the mobility (μ), the capacitance per area of the gate insulator (Cox), and the width-to-

These values vary for each pixel because of fluctuations in layer thicknesses, etching biases, etc. and because of TFT degradations such as Vth shifts. The pixel current thus varies for each pixel as shown in **Figure 11(b)**. In order for an OLED display to achieve a high uniformity, the

OLED pixels with compensation traditionally employ additional TFTs, capacitors, and lines as in **Figure 12(a)**, which lowers the aperture ratio and increases defects [6], or power line voltage swinging as in **Figure 12(b)**, which is difficult to adopt in large-sized high-resolution panels because of large line loads and a short charging time. In order to achieve mass production of large-sized high-resolution OLED TVs, a simple pixel structure is necessary to reduce defects and improve aperture ratio [11]. Minimizing the number of TFTs in a subpixel can not only reduce defects but also simplify driving signals, which allows a narrow bezel design. We use a single data line and a single gate line for each subpixel, and the four subpixels in a full RGBW pixel share a sensing line and a power line, which reduces line crossings and thus reduces

**Figure 11.** (a) OLED pixel circuit and operation voltages (b) OLED current at various TFT and OLED operation points.

*<sup>L</sup>* (*Vgs* − *Vth*)

<sup>2</sup> (1)

Advanced Technologies for Large-Sized OLED Display http://dx.doi.org/10.5772/intechopen.74869 45

*I*

length ratio (W/L) determine the slope.

current variation must be compensated in each pixel.

**4.2. Internal compensation and pixel circuit**

defects.

**Figure 10.** (a) Comparison of emission spectra of 3S2C and 3S3C WOLEDs (b) Emission spectra of 3S2C and 3S3C WOLEDs with color layers (c) Comparison of color gamut of 3S2C and 3S3C WOLEDs.

**Table 3** summarizes the brightness and color gamut for 3S2C and 3S3C WOLEDs which are applied to OLED TV made in 2015 and 2016, respectively. As a result of such innovations in WOLED and CL, OLED TV could realize peak brightness of 500 nit and full-window brightness of 150 nit as well as high color gamut, i.e., 129% in sRGB color space and 99% in DCI color space.


**Table 3.** The specification of OLED TVs based on 3S2C and 3S3C WOLEDs.
