1. Electroluminescence Displays
Jens Bömer
Fakultät für Elektrotechnik und Informationstechnik
Lehrstuhl für Kommunikationstechnik
Prof. Dr.-Ing. Rüdiger Kays

2. Electroluminescence: Basics & Technology
Introduction
Principles of Electroluminescence (EL)
general effect
high-field EL
Technological Realization
layer structure
Selected Approaches to
produce colours
make a display
Applications/ Products
Pros & Cons
Table of Contents

3. fast response for video services
Motivating Forces
tendency to miniaturizement
fast response for video services
low power consumption
Introduction

4. non-thermal conversion of electrical energy into light
Electroluminescence
non-thermal conversion of electrical energy into light
LED & OLED
High Field EL
pn-junction
electron-hole pair recombination
phosphorus material
hot-electrons
impact excitation
LED & OLED
light is produced by electron-hole pair recombination
bandgap Eg> 3 eV, to emitt rayes in viewable wavelength
high field EL
Introduction

5. Phosphorescence t  1min - ...
Luminescence E E1
Fluorescence t  1s
Phosphorescence t  1min - ... Eg E0
Requirements
band gap (Eg) higher than 3.0eV
special semiconductor
insulator
electrical energy is converted into light energy
operation at low temperatures possible
Principles of EL

6. High Field EL Electron transport Excitation Principles of EL
excited state
impurity ground state
top insulator
bottom insulator Al
ITO
...
High Field EL
insulator/ phosphor interface states
electrons tunnel into phosphor
electrons accelerated by electric field (high energy or “hot” electrons > 2 eV)
Electron transport
Excitation
collisions with lattice/ ímpurity atoms (e.g. Mn)
luminescent impurity absorbs electron‘s energy
excited impurity relaxes by emitting a photon
Principles of EL

7. Technological RealizationAC
electrode
insulator ( 250nm)
phosphor
( 600nm)
insulator/phosphor/insulator stack
insulators limit short current
electric field strength > 1.5 MV/cm
165 V alternating voltage
thin film EL (TFEL)
wide viewing angle
luminosity controlled by pulse width modulation T
Technological Realization

8. rare earth oxids and oxysulfats (as in CRT)
Red
ZnS:Mn
organic red filter
Green
ZnS:TbOF
SrS:Ce
Blue
SrS:Ce
SrS:Cu,Ag
Amber
ZnS:Mn
rare earth oxids and oxysulfats (as in CRT)
White
stacked layers
SrS:Ce and ZnS:Mn
Colourful Materials

9. Facts Concerning Colours
Luminance (max.)
red: 70 cd/m2
green 160 cd/m2
blue: 100 cd/m2
white: 470 cd/m2
Efficiency
red: 0.8 lm/W
green: 1.0 lm/W
blue: 0.2 lm/W
operating temperature -30° C to 65°C
Facts Concerning Colours

10. Comparison of EL and LCD
thin film EL
LCD:
Contrast ratio
Luminance
Viewing angle
Power consumption
150:1
150 cd/m2
> 160° 3W
18:1-150:1
200 cd/m2
100° (recently up to 170°)
20W
Comparison of EL and LCD

11. Selected Approach to Produce Colors
One approach: “Colour by White“
broad band “white“ phosphor
patterned colour filter
indium-tin oxide (ITO) top electrode
pixel size of 12 or 24 micron
 simple device fabrication
 low-cost production
Selected Approach to Produce Colors

12. Selected Approach to Make a Display
Active Matrix (AMEL) Display Organization
EL pixel
ITO
data line
Technological advantages
high integration level
low capacity
less current
reduction of control wires
 fast response
line drivers
column drivers
Selected Approach to Make a Display

13. Selected Approach to Make a Display
Silicon On Insulator (SOI):
Required isolation between high voltage AC of EL device and low voltage digital signals which address each pixel.
Technological features
Advantages of AMEL
aging characteristics of the phosphor
inverted structure
brightness controlled by pulse width modulation t b
high resolution (2000 lpi)
reduced power consumption
higher brightness
expanded grayscal capability
Selected Approach to Make a Display

14. ACTFEL: alternating current thin film electroluminescence
What is ACTFEL?
ACTFEL: alternating current thin film electroluminescence
head-up displays (HUD)
instrument panels
transparent
minimize driver‘s eye movement
visibility under sunlight
pixel luminance:
amber: 1200 cd/m2
green: 500 cd/m2
used in cars
high reliability
wide temperature range
luminance:
red: 23 cd/m2
amber: 140 cd/m2
green: 25 cd/m2
Applications

15. car control panel (ACTFEL)
EL foil
Products
EL display
camcorder
car control panel (ACTFEL)
head mounted display
Products

16. PRO
CONTRA
high resolution
fast response
low power consumption
wide viewing angle
ruggedness
low-cost production
use at low temperatures
visibility under sunlight
inadequate brightness and chromaticity
aging characteristics
high voltage
alternating current
Pros and Cons

17. HFEL bases on high energy electrons (hot electrons)
Self-emitting technology instead of light through (LCD)
Realization with an insulator/phosphor/insulator stack
Multi-colour displays by “white phosphor“ and colour filters
Various monochrome control panels by variation of dopants
Active matrix integration (AMEL)
Application in cars, laptops, head-mounted-displays, camcorder
Summary

18. Technological RealizationCI CP US
CI : insulator‘s capacity
CP : phosphor‘s capactiy
U [V]
Q [C]
Q = C • U V1
Vth V3
dissipated
energy
charge stored at interface lowers the diode current
remaining charge lowers turn-on voltage
Technological Realization