A basic explanation of OLED

1. Avijit Barua
Matriculation Id: 3051623
Selected Topics in Functional Materials
1
Organic Light Emitting Diode (OLED)

2. 2
Outline
 Introduction
 Brief History
 OLED
o Structure
o Working Principle
o Materials
o Fabrication
o Pixels
 Types of OLED
 Characteristics of Displays
 Comparison between LCD and OLED
 Advantage and Limitations
 Conclusion

3. 3
Introduction
 OLED is an Electroluminescent device which emits light when an external
voltage is applied
 Light is emitted from OLED pixels with the help of very thin organic film
layer
 Organic material is sandwiched between two conductors
 Bright electro-luminescent light is produced when external voltage is applied
 It can emit the light from 0% to 100%
 Using color films it produces three subpixels-RGB including white
[1]. http://www.agasonia.com/index.php?prm=40200
[2]. http://tech.ebu.ch/events/Electro-optic Displays for Television /Michael E. Backer /display- metrology.com
[1]
[2]

4. 4
Brief History of OLED
 In 1950, A. Bernanose and co-workers first produced electroluminescence in organic materials in France [3].
 In 1987, Ching W. Tang and Steven Van Slyke developed the first working OLED at Eastman Kodak [4].
[3]. https://www.oled-info.com/history
[4]. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51, 913 (1987)
[5]. S. Kunić and Z. Šego, "OLED technology and displays," Proceedings ELMAR-2012, Zadar, 2012, pp. 31-35.
[6]. https://peoplepill.com/people/andre-bernanose
 In1990, JH Burroughes reported highly efficient green light polymers using 100 nm thick
sheet.
 In 1998-Kodak, Sanyo show full-color AMOLED and High efficient Green OLED [5].
 In 2000, LG electronics developed organic EL displays for mobile gadgets.
Since then, extensive research on OLED were carried out.
 In recent days, January 2019, LGD launches the world’s first roll-able OLED TV.
 May 2019 Lenovo demonstrated a foldable windows laptop [3].
Fig: A. Bernanose (1912-2002) [6]

5. 5
OLED Structure
 OLED is a flat light emitting technology, made by placing a series of
Organic thin film between two conductors.
 These conductors stays between seal and substrate.
 When electrical current is applied, energy passes from cathode to anode
and a bright light is emitted.
 It does not requires backlight as LCD because of its emissive layer.
 Thinner and efficient than LCD
 Better Image quality, flexible, even roll-able.
Fig: The OLED structure
https://www.newhavendisplay.com/pkc_oledtechnology.html

6. 6
Working Principle
o Electrical current flows from
cathode to anode through
organic layers.
o It gives electrons to the
emissive layer and removes
electrons from the
conductive layer
o Removed electrons
leaves holes that need
to be filled with
electrons in the
emissive layer
o The holes jumps to the
emissive layer and
recombine with the
electrons.
o Electrons drop into the
holes they release their
extra energy as light.
[8]. https://www.newhavendisplay.com/pkc_oledtechnology.html
[9]. S. Kunić and Z. Šego, "OLED technology and displays," Proceedings ELMAR-2012, Zadar, 2012, pp. 31-35.
[9][8]

7. 7
Materials
Substrate and Electrodes:
 Plastic or glass materials are used for substrate.
 Should facilitate hole injection in Hole Transport Layer (HTL)
 Material: Indium tin oxide (ITO) and Graphene is a common anode material, both
has high work function.
 For cathode, low work function is required which enables electron injection in ETL.
 Material: Mg-Ag alloy, Al-Li alloy
HTL and ETL:
 HTL material: NPB (C44H32N2)
 Glass transition temperature (Tg) below 100⁰ C
Hole mobility range of 10−3
− 10−4
𝑐𝑚2
/(V s)
 ETL material: Alq3 (C27H18AlN3O3)
 It emits green light with emission peaking 530 nm
Electron mobility 10−6 𝑐𝑚2/(V s) at 4*105 V/cm
NPB Alq3
M. F. Rahman & Md. Moniruzzaman, “Fundamentals of Organic Light Emitting Diode”, (2015).

8. Small Molecule Materials Polymer Materials
 Emissive layer based on SMM
 It requires vacuum thermal process
 Mature materials with longer lifetimes
 EML based on polymer materials (inkjet printing)
 Deposited at atmospheric pressure
 Lower operating voltage
 Phosphorescent materials are in early age
8
Fabrication

9. 9
Pixels
 Pixels are created by the arrangement of the cathodes and anodes
 These are arranged perpendicular to each other
 Where anodes and cathodes intersect each other are called pixels.
 Electric current applied to the selected strips of anodes and cathodes
determine which pixels get turned On and Off
 The brightness of each pixel is proportional to the amount of applied
currents.

10. 10
Types of OLED
Bottom Emitting Top Emitting
 Cathode is reflective; Anode is transparent
 Mature technology
 Pixel electronics (TFTs, Capacitors) reduce aspect
ratio
 Cathode is transparent; Anode is reflective
 Still a developing technology
 Pixel electronics do not block light and enables
more complicated TFT structure
Passive Matrix OLED Active Matrix OLED
 Simpler electronic array structure
 High voltage and power needed for high resolution
 Suited for small area display application
 TFT array fabrication
 Lower voltage and power needed for high
resolution
 Suited for portable electronics where display is
larger than 2-3 inches
S. Kunić and Z. Šego, "OLED technology and displays," Proceedings ELMAR-2012, Zadar, 2012, pp. 31-35

11. 11
Characteristics of Displays
Contrast Ratio:
CR = Brightness on 100% white state / Brightness on
0% black state (without degradation)
Color Gamut Measurement:
OLED technology shows the largest color range
It shows better color than standard EBU
Response Time: Time required for the display to
transition from 90% white to 10% black=Rise Time
(TR), and from 10% black to 90% white=Decay Time
(TD)
Viewing Angle:
Angle where contrast ratio still greater than 10:1
For OLED its 178 degree

12. 12
Comparison Between LCD and OLED
Technology LCD OLED
Power consumption (W) 60-300 24-150
Brightness (cd/𝑚2) 350-500 1000
Contrast 350:1 – 1000:1 1,000,000:1
Response time (ms) 8-12 0.05
Lifetime (hrs) 50,000 – 60,000 10,000

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Advantage and Limitations
Advantage Limitations
 Self Luminous: does not require backlight
 Low power: 2-10 V
 Low cost and easy fabrication: inkjet printing
 Light weight, compact and thin devices
 Flexibility
 High brightness and high resolution
 Fast response
 Degradation by oxygen and water molecules hence
“short life time”
 Low glass transition temperature
 Low mobility of holes and electrons due to
amorphous nature of the organic molecules
 Low stability at high brightness
 Low device efficiency

14. 14
Conclusion
 Thin, Efficient and Bright display is created
 Charge carrier injection, charge transport and emission are the three vital parameters because Operating
Voltage and luminance efficiency depends on them
 Cathode materials and ITO has been developed for increasing charge injection
 Higher electron mobility are needed in ETL to get high brightness and low power consumption
 Flexible and transparent OLED are becoming popular
 Life span should be enhanced
 So, new design and materials is required to bring this technology viable

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Thank You For Your Time