**1. Introduction**

Organic light-emitting diode (OLED) is a very promising display. OLED provides a major technological enhancement to displays and TVs, such as wide viewing angle, high contrast ratio, and extremely fast response times. Furthermore, OLEDs can be used to make thin-light displays, transparent displays, and rollable displays.

OLED is a type of display technology that makes it possible to achieve dark black levels from ultrathin screens while at the same time making TVs more efficient and eco-friendly. OLED technology actively eliminated parts containing hazardous substances such as Cd, Hg, InP, etc. In addition, self-luminous OLEDs removed the backlight, helping designers

achieve a lightweight and slim design that used less parts and attained greater resource efficiency and recycle rates [1].

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 production yield of TFT.

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.

terminals as shown in **Figure 1(b)**. Light shield (LS) layer acts as blocking light coming to active layer, which is the main source to cause device degradation under negative bias temperature illumination stress (NBTIS). a-IGZO TFT with coplanar structure needs active metallization process to make ohmic contact between a-IGZO semiconductor and source/drain metals. This process was optimized to generate increased oxygen vacancies inside active film [8]. After metallization process, we obtained effective channel length, which is found to be shorter than nominal channel length. Effective channel length should be managed with

**Figure 1.** The cross-section of oxide TFT structures. (a) Etch-stopper structure with double gates and (b) self-aligned

Advanced Technologies for Large-Sized OLED Display http://dx.doi.org/10.5772/intechopen.74869 35

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 characteris-

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,

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

**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

effective channel length can be obtained from the channel resistance method [10].

at Vds = 10 V in the same order. Threshold voltage (Vth) is an important factor in the

tics are directly connected to the performance and lifetime of OLED TV.

controlled process.

and 107

**2.2. Device characteristics**

coplanar structure with top gate.

*2.2.1. Electrical properties of a-IGZO TFT*

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