Organic Optoelectronic Devices: Organic Light-Emitting Diodes (OLED)
Minlu Zhang
Outline
– Small molecular OLEDs (OLED)
– Polymer OLEDs (PLED) – Applications – Challenges
Small molecular OLEDs — Structure
Single layer device
Cathode Organic Layer Anode Substrate
P-n junction device
Multiple layers device
Cathode Electron transport layer Hole transport layer Anode Substrate Cathode
Electron Injection layer
Electron transport layer
Emissive layer
Hole transport layer
Hole Injection layer
Anode Substrate
Small molecular OLEDs — Device operation principle
Cathode
Electron Injection layer
Electron transport layer
Electrons injected from cathode
Holes injected from anode
Emissive layer
Hole transport layer
Hole Injection layer
Anode Substrate
Transport and radiative recombination of electron hole pairs at emissive layer
LUMO — Ec
Light
Transparent substrate
e-
e-
e-
ITO HIL HTL EML ETL EIL Cathode
h+
h+
h+
HOMO — Ev
Small molecular OLEDs — Materials
EIL, ETL: n-type materials Alq3, PBD
Alq3 PBD
HIL, HTL: p-type materials
NPB, TPD
EML:
Fluorescent dye DCM2 Phosphorescent dye PtOEP, Ir(ppy)3
NPB
TPB
DCM2
Small molecular OLEDs — Electrodes
Anode:
Indium-tin-oxide (ITO): 4.5-5.1 eV Au: 5.1 eV Pt: 5.7 eV
Cathode:
Ca: 2.9 eV
Mg: 3.7 eV
Al: 4.3 eV Ag: 4.3 eV
Mg : Al alloys
Ca : Al Alloys
Small molecular OLEDs — Device preparation
Thermal vacuum evaporation
Substrate
Cathode Electron transport layer Hole transport layer Anode Substrate
Vacuum
Small molecules
Growth:
~10-5-10-7 Torr
Room temperature Heater ~20 Å- 2000 Å
Polymer OLEDs — Structure and Operation
Cathode Cathode
Emissive polymer Anode Substrate
Emissive polymer
Conducting polymer
Anode
Substrate
http://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0401a.pdf
Polymer OLEDs — Materials
Conducting polymers:
PANI:PSS
PANI
PDOT:PSS
PDOT
PSS
Emissive polymers:
R-PPV
PFO
Polymer OLEDs — Fabrication
Spin coating
Ink jet printing
Screen printing Web coating
Spin coating
Polymer film
Substrate
Ink jet printing
Substrate
Why Organic LED?
Vibrant colors High contrast Wide viewing angles from all directions Low power consumption
Low operating voltages
Wide operating temperature range
A thin and lightweight form factor
Cost-effective manufacturability , etc
Applications — Full color OLED display
A full color, 13-inch diagonal small-molecular-weight OLED display with 2mm thickness.
Flexible internet display screen
S. R. Forrest in Nature428, 911 (2004)
Applications — Full color OLED display
Samsung large OLED displays
KODAK OLED displays
http://www.kodak.com/eknec/PageQuerier.jhtml?pq-path=1473/1481/1491&pq-locale=en_US
Applications — Solid state white lighting
White OLEDs
From Universal Display Corporation
http://www.universaldisplay.com/white.htm
Challenges
Lifetime Red Green Blue
>10,000 hrs >10,000 hrs ~1,000hrs ?
High efficiency ? Pure color?