**4. Compensation technologies**

#### **4.1. Pixel circuit**

OLED displays, having current-driven subpixels, require higher backplane uniformity than LCDs. **Figure 11(a)** shows a simplified pixel circuit diagram of an active-matrix OLED display pixel.

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

$$I\_{ds} = \frac{1}{2} \mu \,\mathrm{C}\_{\alpha r} \frac{W}{L} \left(V\_{gs} - V\_{th}\right)^2\tag{1}$$

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-tolength ratio (W/L) determine the slope.

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 current variation must be compensated in each pixel.

### **4.2. Internal compensation and pixel circuit**

**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.

**Year 2015 Year 2016**

sRGB 129% DCI 99%

WOLED structure 3-stack 2-color (3S2C) 3-stack 3-color (3S3C)

Brightness (nit) 450/150 500/150

DCI 90%

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

**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.

OLED displays, having current-driven subpixels, require higher backplane uniformity than LCDs. **Figure 11(a)** shows a simplified pixel circuit diagram of an active-matrix OLED display pixel.

**4. Compensation technologies**

Color gamut (%) sRGB 114%

**4.1. Pixel circuit**

44 Green Electronics

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 defects.

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

**Figure 12.** Internal compensation pixel circuits (a) 5T1C voltage programming (b) 2T2C voltage programming with VDD swing.

#### **4.3. External compensation and pixel circuit**

**Figure 13** shows the concept of our external compensation [29]. From 2003, we developed LGD's unique compensation technology, including simple pixel structure, TFT compensation algorithm, and precise monitoring technology of TFT variation. Our pixel circuit satisfies UHD requirements such as a large screen, high compensation characteristics, and a high productivity. Our external compensation method compensates the threshold voltage and mobility variation of TFT. To get initial good uniformity of the luminance, compensation method is required to compensate variations of threshold voltage and mobility that cause luminance differences. Through the real-time sensing and compensation, our methods compensate threshold voltage shift and mobility shift.

Using this method, we solve reliability issues caused by positive and negative biases, temperature, and current stress. The subpixel itself does not have a compensation circuit, and the external circuit compensates each subpixel correctly. That makes the external circuit more complicated than internal compensation methods, because we need analog-digital converters, a sensing data memory, a compensation algorithm unit, etc. It has its own advantages: we can optimize compensation methods and parameters, not only for Vth but also for mobility, by using refined algorithms.

*Vdata*

*k* = \_\_1

*Vdata*

line (Cline) and the sensing time (t) are considered:

**Figure 13.** The external compensation method and the simplified pixel circuit.

′ = *Vdata* + *Vth* (2)

<sup>2</sup> *Cline t* (3)

Advanced Technologies for Large-Sized OLED Display http://dx.doi.org/10.5772/intechopen.74869 47

*<sup>k</sup> Vdata* <sup>+</sup> *Vth* (4)

Then, Vs is sensed, which is in proportion to the product of the TFT characteristics of the pixel except the Vth, which we call *k* here as shown in Eq. 3, where the capacitance of the sensing

> <sup>2</sup> *μ Cox* \_\_ *W <sup>L</sup> Vdata*

**Figure 14.** Sensing results of TFT (a) Vth and (b) mobility using extenal compenstion pixel circuit.

Finally, by calculating the average of *k* of all pixels, we compensate each pixel by Eq. 4:

The pixel characteristics are sensed and compensated before shipment and in real time.

\_\_\_\_ *kavg* \_\_\_

′ <sup>=</sup> <sup>√</sup>

**Figure 14** shows the sensing results of TFT Vth and mobility using external compensation pixel circuit. In our case, we sense the Vth and the mobility of the driving transistor in the source follower method shown in **Figure 14(b)**, where the data voltage for sensing (Vg) is applied to the gate of the driving transistor and the source voltage (Vs) is sensed by an external circuit. The difference between Vg and Vs is the Vth of the driving transistor and is stored to the memory. Using the sensed Vth, a Vth-compensated data voltage shown in Eq. 2 is applied to the gate:

**Figure 13.** The external compensation method and the simplified pixel circuit.

**4.3. External compensation and pixel circuit**

swing.

46 Green Electronics

pensate threshold voltage shift and mobility shift.

using refined algorithms.

applied to the gate:

**Figure 13** shows the concept of our external compensation [29]. From 2003, we developed LGD's unique compensation technology, including simple pixel structure, TFT compensation algorithm, and precise monitoring technology of TFT variation. Our pixel circuit satisfies UHD requirements such as a large screen, high compensation characteristics, and a high productivity. Our external compensation method compensates the threshold voltage and mobility variation of TFT. To get initial good uniformity of the luminance, compensation method is required to compensate variations of threshold voltage and mobility that cause luminance differences. Through the real-time sensing and compensation, our methods com-

**Figure 12.** Internal compensation pixel circuits (a) 5T1C voltage programming (b) 2T2C voltage programming with VDD

Using this method, we solve reliability issues caused by positive and negative biases, temperature, and current stress. The subpixel itself does not have a compensation circuit, and the external circuit compensates each subpixel correctly. That makes the external circuit more complicated than internal compensation methods, because we need analog-digital converters, a sensing data memory, a compensation algorithm unit, etc. It has its own advantages: we can optimize compensation methods and parameters, not only for Vth but also for mobility, by

**Figure 14** shows the sensing results of TFT Vth and mobility using external compensation pixel circuit. In our case, we sense the Vth and the mobility of the driving transistor in the source follower method shown in **Figure 14(b)**, where the data voltage for sensing (Vg) is applied to the gate of the driving transistor and the source voltage (Vs) is sensed by an external circuit. The difference between Vg and Vs is the Vth of the driving transistor and is stored to the memory. Using the sensed Vth, a Vth-compensated data voltage shown in Eq. 2 is

**Figure 14.** Sensing results of TFT (a) Vth and (b) mobility using extenal compenstion pixel circuit.

$$V\_{data}' = V\_{data} + V\_{th} \tag{2}$$

Then, Vs is sensed, which is in proportion to the product of the TFT characteristics of the pixel except the Vth, which we call *k* here as shown in Eq. 3, where the capacitance of the sensing line (Cline) and the sensing time (t) are considered:

$$k = \frac{1}{2}\mu \,\mathrm{C}\_{ox} \,\frac{W}{L} \,\mathrm{V}\_{data} \,\,^2\mathrm{C}\_{llu} \,t\tag{3}$$

Finally, by calculating the average of *k* of all pixels, we compensate each pixel by Eq. 4:

$$\overline{V\_{data}} = \sqrt{\frac{k\_{xy}}{k}} V\_{data} + V\_{th} \tag{4}$$

The pixel characteristics are sensed and compensated before shipment and in real time.

### **4.4. OLED degradation compensation**

Other quality issues include image sticking. Firstly, being current-driven, OLED pixels generate heat when they emit light, and there may be a luminance change because of high temperature. Secondly, luminance will drop according to total driving time because of OLED degradation, like any other self-luminous device. For the former issue, we have designed a mechanical structure to release heat efficiently, and we use real-time temperature compensation. For the latter issue, we use a known correlation between current efficiency decrease and electric characteristic change of OLED [30]. OLED voltage–current characteristics change according to degradation, and we need a higher voltage to have the same current after OLED usage. We sense voltage for the same predetermined current to estimate OLED degradation at each subpixel and use a lookup table to translate OLED voltage shift to luminance compensation value. **Figure 15** compares time-dependent luminance curves for low stress and high stress, without and with repeated OLED voltage sensing and luminance compensation. Luminance difference can be minimized between high-stress and low-stress subpixels through the OLED degradation compensation. We sense OLED degradation with a predetermined interval because OLED degrades much slower than TFT. We do not compensate OLED luminance completely to maintain initial luminance, because OLED compensation rather accelerates degradation. Instead, we match OLED luminance to target degradation curve. **Figure 16(a)** shows the image sticking of the OLED panel due to OLED degradation. **Figure 16(b)** demonstrates the drastic disappear-

Advanced Technologies for Large-Sized OLED Display http://dx.doi.org/10.5772/intechopen.74869 49

**Figure 17** is a photograph of the 55-, 65-, and 77-inch UHD OLED TVs, which are world's first products. These OLED TVs are employing TFT backplane composed of coplanar a-IGZO TFTs, three-stack three-color tandem WOLED and advanced compensation technologies. Our technology platform allows a panel size scalability, a product reliability, and a high aperture ratio. **Table 4** shows specifications of our UHD OLED TVs with high color gamut, high con-

The reason why OLED TV is known to have the best quality of display is that it realizes high contrast ratio since OLED at each subpixel can be completely and individually turned off by oxide TFTs when the subpixel displays zero signal. Low-leakage current of oxide TFTs contributes to the high contrast ratio of OLED TV. Namely, OLED TV can easily achieve high dynamic range (HDR) without raising up the peak luminance, contrary to LCD TV. Accordingly, OLED

ance of the image sticking after the OLED degradation compensation.

**5. World's first UHD OLED TV products**

**Figure 17.** Photograph of LG's 55-, 65- and 77-inch UHD OLED TV products.

Display type WRGB OLED

Contrast ratio > 1000000:1

Resolution 3840 × 2160 (UHD)

**Table 4.** Specifications of the LG UHD OLED TV (\*DCI: Digital Cinema Initiatives).

**Item Content Unit**

Panel size 55, 65, 77 inch

Brightness 150 (full)/500 (peak) cd/m2 Color gamut (DCI\* coverage) 99 (DCI) %

trast ratio, and thin thickness.

**Figure 15.** Image sticking compensation method due to OLED degradation.

**Figure 16.** Image sticking test results (a) Before compensation, (b) After compensation.

of the OLED panel due to OLED degradation. **Figure 16(b)** demonstrates the drastic disappearance of the image sticking after the OLED degradation compensation.
