Electroluminescence Displays Jens Bömer jcbsystems@gmx.de Fakultät für Elektrotechnik und Informationstechnik Lehrstuhl für Kommunikationstechnik Prof. Dr.-Ing. Rüdiger Kays

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

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

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

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

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

Technological Realization AC 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

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

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

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

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

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

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

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

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

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

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

Technological Realization CI 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