1

Temperature-Dependent Dynamic Behaviors of Organic Light-Emitting Diodes

Jongwoon ParkKyoto University

Japan

2

Outline

Basic principle of operation

Numerical model

Simulation & Experiment results

Overall conclusion

3

Low cost

Self-emitting: require no backlight, reducing thickness

Direct replacement for a conventional LCD

Viewing angle up to 160 degrees

Response of 1000 times faster than LCDs

Organic Light-Emitting Devices:40-inch Samsung

OLED TV

4

Comparison with TFT-LCD

Viewing angle Power consumption

AMOLED TFT-LCD AMOLED TFT-LCD

IMID 2006 display products by Samsung SDI

5

Color Representation

IMID 2006 display products by Samsung SDI

Outdoor Indoor

6

Considerations for Long Lifetime

Dark spot- impurity- defects

Heat- glass transition temperature

Electrochemical factors (stress)- device structure for balancing holes and electrons- doping- injection layers- surface quality between electrode and organic layers

Unencapsulated4min later

EL distribution

7

Anode

CuPc(20nm)

NPD(50nm) Alq3

(45nm)

Cathode

Energy Band Diagram

8

Numerical Models (1D):

< Poisson’s equation >

( )AD NNtxntxpqxtxE

−+−=∂

∂ ),(),(),(ε

),(),(),(),(1),( txptxntxrxtxJ

qttxn n −

∂∂

=∂

∂

),(),(),(),(1),( txptxntxr

xtxJ

qttxp p −

∂

∂−=

∂∂

xtxntxkTtxEtxntxqtxJ nnn ∂

∂+=

),(),(),(),(),(),( µµ

xtxptxkTtxEtxptxqtxJ ppp ∂

∂−=

),(),(),(),(),(),( µµ

( )

=

00

),(exp)(),,(EtxETTtxE µµ

qss

txSxQtxSdx

txSdDtxptxntxrttxS

ττ),()(),(),(),(),(),(

41),(

2

2

−−+=∂

∂

)(683)( 2 thvPn

tLsub

absηπ

=

,),(0 bibias

LVVdxtxE −=∫

)()(exp)/exp( 2/12 ESxqnfkTATJ B −−= φ

0≥t

< Drift-Diffusion equation >

< Singlet exciton rate equation >

< Luminance >

< Boundary conditions >

< Field- and temperature-dependent mobility >

Langevin recombination rate model

Poole-Frenkel typemobility model

9

NPD-Alq3 OLEDNPD-Alq3 OLED

under EL test Vacuum Evaporation Systemat Kyoto Univ.

10

EL Spectrum

11

Validation of Numerical Model

12

Simulation Results-Effects of ∆HOMO-

Electric Field

Hole charge density

Recombination rate

● Similar behaviors for ∆LUMO

13

Temperature-Dependent Device Performances

14

Temperature-Dependent Current Balance=Jr/J

anode cathodeorganic layer

Jh

Jh′

Je

Je′

JrJr Jr

J=Jh+Je′=Je+Jh′

Jr=Jh-Jh′=Je-Je′

15

Temperature-Dependent Dynamic Behaviors of OLED

300K (0.8µs)

275K (1.31µs)

250K (2.4µs)

Bias=9V

Td

16

Spatial Distribution of Carrier Density

Turn-on cycle (charge) Turn-off cycle (discharge)

17

Exciton Distribution

Dirac delta function (“seed exciton”) → Diffusion

emission zone

18

Response to a Train of Voltage Pulses

19

Summary

The luminance decreases and the turn-on voltage increases as the temperature decreases due to a reduction in thermally activated hopping speed.

It delays not only the startup of EL upon turn-on of OLEDs, but also the discharge upon turn-off.

The device efficiency is increased with decreasing temperature due to enhanced charge-balance factor.

The pulse-to-pulse interference by the space charge effects is more significant at lower temperatures.