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Temperature-Dependent Dynamic Behaviors of Organic Light-Emitting Diodes
Jongwoon ParkKyoto University
Japan
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Outline
Basic principle of operation
Numerical model
Simulation & Experiment results
Overall conclusion
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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
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Comparison with TFT-LCD
Viewing angle Power consumption
AMOLED TFT-LCD AMOLED TFT-LCD
IMID 2006 display products by Samsung SDI
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Color Representation
IMID 2006 display products by Samsung SDI
Outdoor Indoor
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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
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Anode
CuPc(20nm)
NPD(50nm) Alq3
(45nm)
Cathode
Energy Band Diagram
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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
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NPD-Alq3 OLEDNPD-Alq3 OLED
under EL test Vacuum Evaporation Systemat Kyoto Univ.
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EL Spectrum
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Validation of Numerical Model
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Simulation Results-Effects of ∆HOMO-
Electric Field
Hole charge density
Recombination rate
● Similar behaviors for ∆LUMO
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Temperature-Dependent Device Performances
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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′
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Temperature-Dependent Dynamic Behaviors of OLED
300K (0.8µs)
275K (1.31µs)
250K (2.4µs)
Bias=9V
Td
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Spatial Distribution of Carrier Density
Turn-on cycle (charge) Turn-off cycle (discharge)
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Exciton Distribution
Dirac delta function (“seed exciton”) → Diffusion
emission zone
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Response to a Train of Voltage Pulses
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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.