What is OLED?


Editor : Papilio
2017/01/10

 

What is OLED?

 

 Hello. This is Papilio to introduce technology related to measurement of OLED in 2017. OLED technology development seems to be very fast. A few years ago, we had to talk about the possibility of commercialization. Now, since launching commercial AMOLED products in Korea for the first time, we have seen a sharp increase in market share. Its excellence is acknowledged in the world market, and it is spreading to various application fields beyond mobile and TV. In this trend, invisible technology wars have been taking place all over the world to develop excellent OLED devices. I would like to introduce 12 OLED measurement techniques that I have been working in McScience for a while. Through this, we will communicate with related researchers and share for better ideas of measurement technology development and product development.

 

Month Topics??
1 What is OLED?
2 Overview of OLED device characterization
3 OLED devices I-V-L characterization and systems
4 OLED device lifetime characteristics evaluation and system
5 OLED device lifetime prediction technology
6 OLED device complex property evaluation and system
7 OLED device C-V and transient measurement technology
8 OLED Panel drive
9 OLED Module drive
10 Evaluation of OLED display characteristics (1)
11 Evaluation of OLED display characteristics (2)
12 Large area OLED display

 

 In this series, I will explain about ‘what is OLED?’. For your reference, please understand that I am writing a narrative form for the convenience of explanation. I apologize in advance for any misunderstandings in the explanation. I am preparing to correct at any time.
OLED is short for Organic Light-Emitting Diode. It is driven by the principle that electrons and holes injected from cathode and anode are recombined in organic compound and emit light by themselves.

 

(Figure 1) OLED device structure and luminescence principle

 

 In the past, it showed superior performance indicators compared to LCD displays such as wide viewing angle, fast response time, and color reproduction rate but these advantages are not considered to be due to OLED technology. However, the self-emission characteristics of OLEDs cannot be overcome by LCDs, which are essential for backlighting, and these features are likely to lead to the development of OLEDs as a next-generation display.
In detail, OLEDs are distinguished in various ways. In personal, it seems to use different ways to distinguish OLEDs and terms depending on their major fields. Table 2 summarizes the general classification methods.

 

Table 2. OLED separation method

 

 The voltage-current (omitted from the graph) or the shape of the voltage-luminance graph is similar with the voltage-current characteristic of a diode element commonly used in electronics. For this reason, it is said that “the electrical characteristics of the OLED device follow the characteristics of the diode curve”. The voltage-current-luminance characteristics are shown in the right-hand graph of Fig 2. From this graph, we can see that the light output characteristics of OLED devices have a linear relationship to the current density (or amount of current). Therefore, it is correct to control the current value rather than the voltage value to accurately control the light output of the OLED device.

 

Point of View Division Feature
Meterial Small Molecules ? – Vacuum Evaporation
? – Low molecules weight
? – Good lifetime relative to polymer
Polymer ? – High molecules weight
? – Spin coating or inkjet printing
Mechanism Fluorescence ? – External QE limitation (~5%)
? – Good lifetime relative to phosphorescence
Phosphorescence ? – High external QE (~20%)?
Device Structure Top Emission ? – Light emission toword the opposite direction of substrate
Bottom Emission ? – Light emission toward the substrate direction
Operation Passive Matrix ? – Row & Columc array
Active Matrix ? – Each pixel with TFT circuit

 

 The author, who deals with measurement and inspection techniques, frequently checks the distinction between device structure and operation. For example, the bottom emission and the top emission need to be configured in consideration of the position of the contact and the position of emission due to the difference in structure.
As described above in the definition of OLED, the emission principle of the OLED occurs through injection and recombination of electrons and holes, so that the amount of emitted light (luminance) varies depending on the amount of applied current. This voltage-current-luminance characteristic is shown graphically in Fig 2.

 

(Figure 2) Graph of voltage-current-luminance characteristics of OLED devices

 

 The voltage-current (omitted from the graph) or the shape of the voltage-luminance graph is similar with the voltage-current characteristic of a diode element commonly used in electronics. For this reason, it is said that “the electrical characteristics of the OLED device follow the characteristics of the diode curve”. The voltage-current-luminance characteristics are shown in the right-hand graph of Fig 2. From this graph, we can see that the light output characteristics of OLED devices have a linear relationship to the current density (or amount of current). Therefore, it is correct to control the current value rather than the voltage value to accurately control the light output of the OLED device.
The manufacturing process of the OLED display is briefly shown in Fig 3.

 

(Figure 3) OLED display manufacturing process

 

 The detailed process is much more complicated, and there are some parts that vary from manufacturer to manufacturer. The OLED display is manufactured through such a manufacturing process, and there is a method of calling the manufactured shape at each step. In practice, there are situations that cause confusion when using terminology and are summarized in Table 3. One thing to note is that the rules are not based on specific rules and are based on experiences that are frequently used in industry.

 

Table 3. Name and characteristics per OLED type

 

(Figure 3) OLED display manufacturing process

 

 It is called Unit Cell (or TEG Cell), which is made in small form for material and material research. The unit cell is tested and the product is made based on the proven material and device structure. A large glass substrate is processed by evaporation and several samples are made simultaneously, which is called Mother Glass. Here, it is divided into households, and the larger the size of the substrate, the more OLED products can be produced. An OLED panel (or OLED cell) is formed by cutting the OLED products formed on the mother glass individually, and it can be regarded as a completed form as a product. OLED panels are completed by attaching drive ICs to OLED panels, and they will be finalized by display manufacturers. After taking these parts, the handset maker or TV maker completes the final product.

 

 I have outlined the “what OLED is”. The OLED device can be thought of as a well-completed and organized technology, but there are many challenges in practice. Particularly, OLED display is competitive and it is necessary to improve efficiency and lifetime to be used in various application fields, especially flexible display in the future. In order to achieve this, performance improvement technologies are applied in various fields ranging from material and material stages to structure and product design. Finally, technical improvements are completed through accurate performance measurement and evaluation. Let’s look at what characteristic evaluation elements are in the next topic, “Outline of OLED Characteristic Evaluation”.


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