**2.2. Device characteristics**

achieve a lightweight and slim design that used less parts and attained greater resource

LG commercialized 15-inch OLED TVs in 2009. The technologies to realize the TVs were RGB deposition technology using a shadow mask and thin film transistor (TFT) using low-temperature polycrystalline silicon. However, it was difficult to produce larger-sized OLED TVs with the same technology applied to 15-inch OLED TVs. The shadow mask is applied to the mass production of small-sized OLED displays, forming RGB subpixels. However, this method is not suitable for larger OLED displays due to sagging in metal mask and defects and color mixing due to misalignment between mask and glass substrate. Also, the development of TFT for large substrate was another important obstacle. We need to further improve reliability and

Recently, there are several breakthroughs in realizing large OLED TVs. The first one is to use white OLED as the light source and implement the color via color layer. When WOLED receives an electric current, it mixes two or three wavelengths of light and produces a white light. For RGB color, the color layer was used to filter the white light. WOLED can provide solutions for manufacturing processed on large-sized substrate [2, 3]. This technique does not require sophisticated metal mask taking into consideration the misalignment margin of pixel designs. In addition, due to good process yield and high productivity, the eight-generation glass substrate can be deposited to produce large-sized OLED TVs. The second is an oxide TFT that was developed intensively to meet the large backplane requirements for OLED TVs [4, 5]. Oxide TFT is also capable of processing on eighth-generation glass substrates because of its similarity to amorphous silicon TFT and may be compatible with TFT lines for LCDs in terms of production lines. By combining WOLED and oxide TFT that can be made from large

substrate, larger OLED TVs can be produced with higher productivity and lower cost.

oxide TFTs, WOLEDs, and compensation circuit and UHD OLED TV.

We commercialized 55-inch full HD (FHD) OLED TVs in the early 2013 [6]. And then in 2016, we launched not only 65-inch and 55-inch ultrahigh definition (UHD) OLED TVs but also 77-inch UHD OLED TVs. In this chapter, we will describe advanced technologies including

The structure of TFT is classified as top and bottom gate depending on the relative position of gate and active layer. Additionally, when the gate is on the same side of source/drain electrodes, it is called coplanar structure. Two types of oxide TFT structures are shown in **Figure 1**. Double-gate TFTs with both top and bottom gates are illustrated by **Figure 1(a)**. Gate and SD2 electrode function as a bottom and top gate, respectively. As the advantageous side, this structure was utilized to increase current flow and gets better output characteristics [7]. However, large parasitic capacitances between gates and source/drain metals are disadvantages, which are a big hurdle developing UHD OLED TV. Coplanar TFT was designed to reduce parasitic capacitance avoiding overlap between gate terminal and source/drain

efficiency and recycle rates [1].

34 Green Electronics

production yield of TFT.

**2. Oxide TFT technology**

**2.1. Overview of TFT structure**

Driving TFTs for OLED TV operate in a current-driven mode, which control supplying current to OLED devices. Scan TFTs act as switches in active-matrix OLED (AM-OLED). It is important to obtain excellent device characteristics of these TFTs because device characteristics are directly connected to the performance and lifetime of OLED TV.

#### *2.2.1. Electrical properties of a-IGZO TFT*

Threshold voltage, subthreshold swing, field-effect mobility, and on/off current ratio can be extracted from I–V curve [9]. Typical values for each property are 0.5 V, 0.15 V/dec., 10㎠/Vs, and 107 at Vds = 10 V in the same order. Threshold voltage (Vth) is an important factor in the electrical characteristics, and it tells when the device turns on and off. Vth can be extracted from gate voltage when the drain current reaches 10 nA at the transfer curve under 10 V of drain voltage. Series resistance is also an important parameter in short-channel devices. The effective channel length can be obtained from the channel resistance method [10].

**Figure 2(a)** shows the transfer curves of 20pts TFTs located in six 55-inch panels which are fabricated on a Gen. 8.5 glass. The transfer curves were plotted by measuring the W/L = 26 μm/10 μm devices in the test element groups (TEG) through an inline probe station instrument. The inset picture shows the Vth distribution extracted from the real-time automatic Vth sensing instrument [11]. The variations of Vth extracted from transfer curve and automatic Vth sensing methods

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

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

the OLED TV or causes nonuniform luminance and image-sticking problems [12, 13, 14]. For this reason, a-IGZO TFT must be able to exhibit stable electrical characteristics for guaranteed lifetime. There are three typical methods to monitor device reliability such as NBTIS, positive bias temperature stress (PBTS), and current stress (CS) [15, 16, 17]. There are two well-known device degradation models of charge trapping in gate insulator [18] and defect creation in active layer models [19]. In addition, research has been going on the ambient effect model [20]. In order to improve the device reliability, it is necessary to optimize device fabrication process. Defect sites which are the source of charge trap should be reduced inside the oxide semiconductor thin film and at the interface between gate insulator and active layer. Studies have been conducted to improve device reliability. Oxide TFT was passivated to prevent external moisture and oxygen permeating into active layer [21, 22]. By applying light shield (LS) layer into TFT structure, it is possible to improve

Advanced Technologies for Large-Sized OLED Display http://dx.doi.org/10.5772/intechopen.74869 37

**Figure 4.** Shift of transfer curve after 1 h of negative gate bias at 60°C. Light emission from (a) top and (b) bottom.

**Figure 4** shows the shift of transfer curve of a-IGZO TFT before and after NBTIS test. The NBTIS test was evaluated by applying Vgs = -30 V and Vds = 0 V at 60°C for 1 hour. The 4500 nit of visible light is also applied in the top and bottom directions of the device. **Figure 4** shows NBITS reliability characteristics of a-IGZO TFT under the top light (a) and bottom light (b). In both illumination tests, transfer curves shifted to the negative direction compared to the initial transfer curves. The amount of shifts was −0.03 V when top light was applied and − 0.09 V when bottom light was applied. It is believed that the amount of light is more

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

light-induced reliability degradation preventing active layer from the light [23].

injected into the device when the bottom light is illuminated than the top light.

**3. WOLED and color filters**

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

**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, was calculated as 1.2 μm.

#### *2.2.2. Reliability properties of a-IGZO TFT*

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

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

**Figure 4.** Shift of transfer curve after 1 h of negative gate bias at 60°C. Light emission from (a) top and (b) bottom.

the OLED TV or causes nonuniform luminance and image-sticking problems [12, 13, 14]. For this reason, a-IGZO TFT must be able to exhibit stable electrical characteristics for guaranteed lifetime. There are three typical methods to monitor device reliability such as NBTIS, positive bias temperature stress (PBTS), and current stress (CS) [15, 16, 17]. There are two well-known device degradation models of charge trapping in gate insulator [18] and defect creation in active layer models [19]. In addition, research has been going on the ambient effect model [20]. In order to improve the device reliability, it is necessary to optimize device fabrication process. Defect sites which are the source of charge trap should be reduced inside the oxide semiconductor thin film and at the interface between gate insulator and active layer. Studies have been conducted to improve device reliability. Oxide TFT was passivated to prevent external moisture and oxygen permeating into active layer [21, 22]. By applying light shield (LS) layer into TFT structure, it is possible to improve light-induced reliability degradation preventing active layer from the light [23].

**Figure 4** shows the shift of transfer curve of a-IGZO TFT before and after NBTIS test. The NBTIS test was evaluated by applying Vgs = -30 V and Vds = 0 V at 60°C for 1 hour. The 4500 nit of visible light is also applied in the top and bottom directions of the device. **Figure 4** shows NBITS reliability characteristics of a-IGZO TFT under the top light (a) and bottom light (b). In both illumination tests, transfer curves shifted to the negative direction compared to the initial transfer curves. The amount of shifts was −0.03 V when top light was applied and − 0.09 V when bottom light was applied. It is believed that the amount of light is more injected into the device when the bottom light is illuminated than the top light.
