eurodisplay 2013 amoled underwood v7 › files › ef-q5vmtsq56tk6 › ...sept2013 %...

Sept 2013 Eurodisplay London Workshop AMOLED Underwood p1

Pu#ng the Ac,ve Matrix in Ac,ve Matrix OLED Displays

Prof Ian Underwood FREng FRSE FInstP

There’s a lot more to OLED displays than the OLED !!!


Contents

Introduc,on to Electronic Informa,on Displays Principles of Ac,ve Matrix Addressing

Ac,ve Matrix Addressing in LCDs Ac,ve Matrix Addressing in OLED Displays

Some Recent Advances Summary & Conclusions

Introduc,on to Electronic Informa,on Displays


Goal of advanced displays •  Defini@on of the ideal electronic display

– An electronic display on which a viewed image is indis,nguishable from reality

•  Guess what -‐ that’s not yet possible!

•  Goal of today’s advanced displays – To come a close as possible to the ideal display

•  Given prac,cal restric,ons •  Size, weight, power consump,on, cost etc

•  And the problems??


Display Classifica,on

0

400

800

1200

1600

0 25 50 75 100 125Screen diagonal (cm)

Defini,on (lines)

PC

Handheld / personal

Laptop

SD Television

Camera, phone

Direct-‐view displays Microdisplays

Business projec,on

Digital cinema

HDTV

Projec,on displays

Underwood’s 5 P’s

Projec@on Permanent .

Portable Personal

Private





Principles of Ac,ve Matrix Addressing


Direct Drive •  Common (or front) electrode

•  Patterned (or back) electrode

•  Per display segment –  One driver –  One connecting wire

•  For high pixel count displays this quickly becomes unmanageable


Passive Matrix Addressing

•  Back electrode is ver,cal columns

•  Front electrode is horizontal rows

•  Per row / column –  One driver –  One connecting wire

•  MxN pixels requires only M+N drivers


Ac,ve Matrix Addressing •  Row and column electrodes on ac,ve (back) substrate •  Common electrode on front substrate •  Ac,ve storage element per pixel hugely reduces crosstalk

+V

Address this row Not addressed


LCD OLED

Passive Matrix

Ac@ve Matrix

TFT

MIM Thin Film Diode (TFD)

Ac,ve Matrix Technologies

LT Poly-‐Si (LTPS) TFT

α-‐Si TFT

LC on Si (LCoS)

Organic / plas@c TFT

IGZO TFT (Indium Gallium Zinc oxide)

Micro/Nano-‐x TFT

Feature size

Panel size

Carrier mobility

Integra,on capability

Cost p.u. area


AM LCD Module (including backlight)

Integra,on on glass capability of amorphous silicon

Integra,on on glass capability of LTPS

Integra,on on chip capability of CMOS for microdisplay





Ac,ve Matrix Addressing in LCDs


LC Characteris,cs

2011 EE4 / MSc Micro Devices - Displays - PM Addressing - Underwood Page 5

Revision – TNLC Config & Response

In this case “normally OFF” or “normally BLACK”

(c.f. previous lecture “Normally ON”)


Scanning limita,ons

VonVoff

!

"##

$

%&&max

=N +1N −1

Nmax Von/Voff

2 2.41 3 1.93 4 1.73 8 1.45 16 1.29 32 1.20 100 1.10

0 1 2 3 Applied Voltage

Contrast

0 50 10

0

N=32 8 3 2

The larger the number of rows to be mul,plexed The steeper the LC response curve must be to cope with the Von/Voff constraint OR

The lower the contrast ra,o of the LC display


Pixel Circuit for AMLCD

Ac@ve Matrix Addressing •  Reduces crosstalk •  Simplifies addressing waveforms •  Increases pixel drive ,me from 1/N of field ,me to ~ field ,me

•  Relaxes LC material & device constraints

•  Relaxes drive voltage constraints

•  Improves contrast •  Allows more rows of pixels





Ac,ve Matrix Addressing in OLED Displays


OLED Characteris,cs


OLED Op,cal response •  OLED response is highly linear with current •  Luminance propor,onal to current density over many decades of dynamic range

© 2005 CDT Ltd; reprinted with permission CDT-ES


Passive Matrix – Power

+Vs

I

t

V

t

L

t

1.  Driver compliance

1

1

Four Factors Dominate

3.  Resistive losses

3

3

2.  Column capacitive charging

2

2

2 4.  Diode Power

4

4

4

4


Passive Matrix – Power Scaling

1.  Driver compliance

2.  Column capacitive charging

3.  Resistive losses

4.  Diode Power

Four Factors Dominate

Power Consumption per Pixel in Passive Matrix Displays

0%10%20%30%40%50%60%70%80%90%100%

16 32 48 64 96 128

Number of Rows (Multiplex Ratio)

Con

trib

utio

n to

Tot

al

Pow

er C

onsu

mpt

ion

050100150200250300350400

Pow

er C

onsu

mpt

ion

per P

ixel

(µW

)

Diode Cap ResistiveDriver Total

% Useful power

AMOLED can help keep the yellow curve flat!


Luminance decrease with use

Constant current drive – Luminance drops – Voltage rises

50

60

70

80

90

100

110

3

3.5

4

4.5

5

0 1000 2000 3000 4000 5000

lum

inan

ce (

cd/m

2 )

voltage (V)

time (h)

© 2005 CDT Ltd; reprinted with permission


OLED Drive schemes

V I L

,me

Const I V I L

,me

Const V

V I L

,me

Const L


Effect of OLED Life,me – Burn-‐in

Image s@cking Caused by Differen,al pixel ageing Pixels used more become dimmer sooner


Example of Real Burn-‐in on PLASMA


Simple AM Pixel Circuit

RX store drive

M1 M2

LCD OLED

2012 Micro Devices -‐ Underwood

LC


Pixel Circuit for AMOLED Saturation region IDS =

12WLµC0 VGS −VTH( )2

Linear region IDS =WLµC0 VGS −VTH( )VDS −

12VDS

2

•  Subject to –  Manufacturing varia,on –  Manu and ,me/use varia,on

•  Degree of varia,on different for different technologies –  Eg –  α-‐Si: VTH shits with ,me & use –  LTPS: VTH varies across a panel


Threshold Voltage Correction •  Selected with 0V data, C1

off, C2 on •  C1 turned on, Discharging

the gate voltage to threshold

•  C2 turned off then C1, holding the threshold at the gate

•  Any voltage applied by the column driver is now offset by the threshold voltage

•  2 large TFTs •  VDS high (4V+0.5V) 2012 Micro Devices - Underwood

GND

Supply Volts

Column Driver

Row Select

Drive TFT Select TFT

Storage Cap

LEP diode

Control Lines 1 2


Amplifying Current Mirror •  In use primarily by Sony •  A similar feedback mechanism (to

the Sarnoff circuit) sets the current on a ‘mirror’ TFT

•  The mirror TFT is geometrically scaled by a known factor (k) to the drive TFT

•  The drive TFT will exactly pass k ,mes the mirror TFT current for a given gate voltage

•  Three small TFTs and low currents on the column line

•  Very linear and demonstrated on 13” diagonal display

•  VDS s,ll ~4V GND

Supply Volts Storage Cap

Drive TFT

LEP diode

Column Driver

Row Select

Erase



Charge programmed op,cal feedback •  Very simple circuit •  A charge is put on the

capacitor •  The photodiode will discharge

the capacitor un,l the gate voltage drops sufficiently to turn off the OLED

•  The light output should be propor,onal to the charge

•  Sensi,ve to ambient light. •  OLED is pulsed – less efficient •  Long tail of pulse can cause

nonlineari,es GND

Supply Volts Storage Cap

Drive TFT

LEP diode

Photodiode

Column Driver

Row Select



PCM Voltage Drive •  Address period split into sub-

frames

•  Drive TFT acts as a switch •  OLED effectively voltage

controlled

•  Not sensitive to TFT properties •  VDS low (~0.5V) efficient

operation •  Accelerated pixel aging – LEP

Vt increases with time reduce operating current

•  Very susceptible to burn-in and differential aging

•  Image artifacts possibly introduced

•  Higher data rates 2012 Micro Devices - Underwood

GND

Supply Volts

Column Driver

Row Select

Drive TFT Select TFT

Storage Cap

LEP diode


Active Matrix OLED Displays

Signal line Supply line

Scan line

Capacitor line

Anode

Cathode

LEP

Driving TFT

Switching TFT

Storage Capacitor

PLED TFT Pixel Circuit LEP

Cathode

TFT Glass

Anode (ITO)

Polyimide

SiO2

PLED TFT Cross-section


Aperture Ra,o for Ac,ve Matrix •  Bopom emi#ng OLED

–  Share area between opaque electronics and transparent aperture

–  Compromise

•  Top emi#ng OLED –  Electronics can fill pixel area hidden by electrode

– More circuit func,onality + higher Ap ra,o

Bottom emissive aperture

Black matrix

Clear aperture

Black matrix

Top Emissive aperture


The effect of low Fill Factor




Some Recent Advances Summary & Conclusions Some Recent Advances


OLED empirical model development (UEDIN)

•  SPICE models created for both AC and DC behaviour

•  Curve fitting approach –  Models not indicative of

underlying physical processes

•  Model confidences increases the more devices tested

•  Models useful for both drive circuit design and manufacturing process control

DC equivalent

circuit model of

PIN-OLED

AC equivalent circuit model of PIN-OLED


Bidirec,onal OLED Microdisplay •  Fraunhofer IPMS, JSID 2009 •  For eye-‐trackingHMD •  Eye sees image on OLED matrix •  Camera senses direc,on of pupil


hpp://www.disp

laybank.com/_en

g/research/

repo

rt_view.htm

l?id=932&cate=2

AMOLED -‐ It’s a hot topic … •  [New Report] Key Patent Report

– Compensa,on Circuit for AMOLED -‐ 2013 •  Patent Search and Related Patent Selec,on •  Research Period: Un,l 4 June 2013 •  Research Scope: Patents published or issued in the United States •  Patent Search DB: FOCUST, USPTO •  Scope and Methodology of Patent Analysis •  Scope of Analysis: Pixel driving circuit technology that

compensates the threshold voltage and IR-‐drop of AMOLED •  Patent Trend Analysis: A total of 244 patents selected to assess

patent applica,on trends by year, technology and assignee •  Key Patent Analysis: 50 key patents and 39 new technology

patents considered for an in-‐depth analysis (technology development trends and case analysis)


Progress is never easy … Display Daily, September 3, 2013 David Barnes -‐ “Silicon TFT Commercializa;on Wasn’t Easy Either – Lessons for IGZO Development” “TFT engineers have taken advantage of how silicon nitride compensates for amorphous silicon since the 1990s. Now they have to learn how to design all over again.” . “The more interes,ng factor in a-‐Si:H evolu,on was the nature of silicon nitride used as the insula,ng dielectric between the Gate (metal) electrode and the channel. Unlike the silicon dioxide used in typical IC, silicon nitride has lots of charge trap loca,ons near the channel interface. While this sounds nega,ve, it turned out to be a posi,ve. The ac,on of such charge traps has liple dependence on temperature nor on device geometry. More important, the traps tend to compensate for state changes in the channel. This is a rare case of two wrongs making a right.”

hpp://www.disp

lay-‐central.com

/free-‐ne

ws/display-‐daily/




Some Recent Advances Summary & Conclusions Summary & Conclusions


Some current AMOLED Displays


Some AMOLED Microdisplays

Top le{ Microoled Panasonic Lumix

Top right Sony Sony NEX

Le{ eMagin US Military


Closing Thoughts •  OLED is not a “perfect” material for displays •  Requirements of advanced displays can seriously compromise isolated device performance

•  Smart advanced electronics is one thing that can help – Boost performance of display cf isolated device – Compensate for inadequacies of simple device – Add func,onality to display system

•  Requires mul,disciplinary approach


Some key references •  Handbook of Visual Display Technologies. Pub. Canopus (Springer) 2012. Eds. Chen,

Cranton, Fihn. Esp Sec,ons 4 (Driving Displays) and 5 (TFT technology). •  Ac,ve matrix, organic light-‐emi#ng diodes (AM-‐OLEDs) for displays, A Lääperi, Chapter in

Organic light-‐emi#ng diodes (OLEDs): Materials, devices and applica,ons, Edited by A Buckley, University of Sheffield, UK. Pub. Woodhead Publishing Series in Electronic and Op,cal Materials No. 36, 2013.

•  Ac,ve Matrix for OLED Displays, R Ma, Chapter 6.6.2 in Handbook of Visual Display Technology Volume 2; J Chen, W Cranton & M Fihn (Eds). Pub canopus / Springer, 2012.

•  Technologies for AMOLED Displays, HJ Kim & BD Chin; IEEE Photonics Society News Vol 27 No2, April 2013

•  OLED Microdisplay Electronics, Chapter 2.8 in Introduc,on to Microdisplays; D Armitage. I Underwood, ST Wu. Pub SID / Wiley, 2006

•  Oxide Semiconductor Thin-‐Film Transistors: A Review of Recent AdvancesE. Fortunato, P. Barquinha, R. Mar,ns; Advanced Materials,Volume 24, Issue 22, pages 2945–2986, June 12, 2012

eurodisplay 2013 amoled underwood v7 › files › ef-q5vmtsq56tk6 › ...sept2013 %...

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