A. Fischer, S. Forget, S. Chnais, M.C. Castex,

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Highly efficient multilayer organic
pure-blue-light emitting diodes with substituted
carbazole compounds in the emitting layer.

• A. Fischer, S. Forget, S. Chénais, M.-C. Castex,
• Lab. de Physique des Lasers, Univ. Paris Nord,
  France

D. Adès, A. Siove, Lab. Biomateriaux et
Polymères de Spécialité, Univ. Paris Nord, France
LBPS
C. Denis, P. Maisse and B. Geffroy Lab. Cellules
et Composants, CEA Saclay, France
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Outline

• Introduction why BLUE oleds ?
• Two new carbazolic compounds PMC
  (Pentamethylcarbazole) and DEC (Dimer of
  N-ethylcarbazole)
• Devices using neat films of PMC and DEC in
  single layer and multilayer structures
• Devices using doped films of PMCDPVBi and
  DECDPVBi
• Conclusion

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Introduction

• Organic Light Emitting Diodes
• Ultrathin light sources, lightweight
• High brightness and viewing angle gt 160
• Low drive voltage (3-10 V) and low power
  consumption
• Extremely rich diversity of materials All
  visible colors available (? inorganic LEDs),
  including saturated colors
• Potentially flexible
• Long lifetimes (gt 20 000 h reported)
• Low cost potential for mass production
• Applications flat-panel RGB DISPLAYS,
  solid-state lighting,...

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Why BLUE ?

• Why Blue OLEDs with high efficiencies are needed
  ?
• different approaches for multi-color emission

RGB emitters
White emitters Filters
Color changing media
homogeneous aging - not efficient
(filters) needs efficient blue emitters to
achieve bright white
homogeneous aging - not efficient
(photoconversion)
power efficient, mature - different aging
and optimization needs efficient blue
emitters (efficient R,G already exist)
needs efficient blue emitters
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OLEDs materials

• Requirements for an efficient blue material
• Chemical stability and Electrochemical stability
• High Tg
• High quantum yield of photoluminescence in the
  solid state
• Chromaticity coordinates approaching
• the spectrum locus (saturated color)

Active research for new blue-emitting organic
materials (both fluorescent and phosphorescent)
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OLEDs materials

• Carbazolic derivatives

- Blue emitters Carbazole-substituted
Distyrylarylenes (DSA) - Hole Transport materials
PVK - Host material for triplet emitters CBP
Carbazole unit
Already used as
new

• Chemically and thermally stable (up to 430 C)
• Tg 75C
• Polaronic transport levels measured by cyclic
  voltammetry (eV)

Vacuum level
Lowest Unoccupied Molecular Orbital
2.5
2.8
5.6
penta-methyl carbazole
Highest Occupied Molecular Orbital
5.9
Dimer of N-Ethyl carbazole
DEC
PMC
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OLEDs structures

• 1st DEC-based diode single layer

D. Romero, A. Siove et al., Adv. Mater. 9, 1158
(1997)
V
Al

• Drawbacks
• Low ext. quantum efficiency ?ext. 7.10-2
• High operating voltage (20 V), crystallization
  during operation (short-circuit)

DEC
ITO

h?
Bad performance due to recombination and
quenching of excitons at Al/DEC interface, poor
charge injection

• This work Use of DEC (and PMC) in a multilayer
  OLED structure with both neat films and doped
  films configurations efficient deep-blue organic
  emitter

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Device a OLED with NEAT film of DEC
2.5
electrons
2.9
2.4
2.4
2.4
LUMO
Cathode
3.0
3.6
holes
4.7
HIL
HTL
Anode
ETL
HBL
5.3
5.4
HOMO
5.6
5.7
6.1
NPB 50 nm
Alq3 10nm
LiF / Al 1.2 / 100nm
BCP 10nm
DEC 50 nm
ITO 100-150nm
CuPc 10nm
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Device a OLED with neat film of DEC
Main recombination zone
2.5
electrons
2.9
2.4
2.4
2.4
LUMO
Cathode
3.0
3.6
holes
4.7
HIL
HTL
Anode
ETL
HBL
5.3
5.4
HOMO
5.6
5.7
6.1
NPB 50 nm
Alq3 10nm
LiF / Al 1.2 / 100nm
BCP 10nm
DEC 50 nm
ITO 100-150nm
CuPc 10nm
?ext 1.5
(optical design not optimized)
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Device a OLED with neat film of PMC
electrons
2.8
2.9
2.4
2.4
LUMO
Cathode
3.0
3.6
PMC OLED
holes
4.7
HIL
HTL
ETL
Anode
5.3
HBL
5.4
5.7
HOMO
5.9
6.1
NPB 50 nm
Alq3 10nm
LiF / Al 1.2 / 100nm
BCP 10nm
PMC 50 nm
ITO 100-150nm
CuPc 10nm
? attributed to bad electron transport properties
of PMC / electron barrier of BCP
?ext 0.6
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Device a OLED with neat film of PMC
Main recombination zone
electrons
2.8
2.9
2.4
2.4
LUMO
Cathode
3.0
3.6
PMC OLED
holes
4.7
HIL
HTL
ETL
Anode
5.3
HBL
5.4
5.7
HOMO
5.9
6.1
NPB 50 nm
Alq3 10nm
LiF / Al 1.2 / 100nm
BCP 10nm
PMC 50 nm
ITO 100-150nm
CuPc 10nm
? attributed to bad electron transport properties
of PMC / electron barrier of BCP
?ext 0.6
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Device a (neat films) Experimental results
Chromaticity coordinates

• Electroluminescence spectra

Aggregates, excimers ?
PMC CIE x 0.153 y 0.100 DEC CIE x
0.192 y 0.209
Ext. Quantum efficiency ?ext 0.6
(PMC) ?ext 1.5 (DEC)
? Bright saturated blue With PMC, but modest
efficiency
Brightness L 236 cd/m2 _at_ 60 mA/cm2
(PMC) Luminous efficiency ?power 0.2 lm/W
(PMC)
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Investigating emitting mixtures ( doping )
The role of emitting mixtures (or doping but
not in the electrical sense !)

• energy transfer doping diluting a low-gap
  guest material inside a wide-gap host Förster
  (and Dexter) energy transfers possible

? Very efficient mechanism but not useful for
blue emitters
guest
host

• other types of doping the dopant impurities
  can enhance exciton recombination by trapping
  charge carriers (and diffusing excitons)

Barrier for electrons trap for holes improved
recombination rate
Ex
guest
host
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Device b OLEDs with DPVBi doped with PMC (DEC)
5 wt.

or
2 wt.
DPVBi 4,4-bis(2,2-diphenylvinyl)-1,1-biphenyl
Vacuum level

• Doping by coevaporation from 2 resistively heated
  cells

Lowest Unoccupied Molecular Orbital
2.5
2.8
2.8
5.6
Highest Occupied Molecular Orbital
5.9
5.9
DEC
DPVBI
PMC
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OLEDs with DPVBi doped with DEC
2 DEC
2.5
electrons
2.9
2.4
LUMO
3.0
2.8
Cathode
3.6
holes
DPVBi
4.7
HIL
HTL
Anode
5.3
5.6
ETL
5.4
HOMO
5.7
5.9
NPB 50 nm
Alq3 10nm
LiF / Al 1.2 / 100nm
DECDPVBi 50 nm
ITO 100-150nm
CuPc 10nm
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OLEDs with DPVBi doped with DEC
Recombination zone
2 DEC
2.5
electrons
2.9
2.4
LUMO
3.0
2.8
Cathode
3.6
holes
DPVBi
4.7
HIL
HTL
Anode
5.3
5.6
ETL
5.4
HOMO
5.9
5.7
NPB 50 nm
Alq3 10nm
LiF / Al 1.2 / 100nm
DECDPVBi 50 nm
ITO 100-150nm
CuPc 10nm
?ext 3.3
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OLEDs with DPVBi doped with PMC
5 PMC
Recombination zone
electrons
2.9
2.4
LUMO
3.0
2.8
Cathode
3.6
holes
DPVBi
4.7
HIL
HTL
Anode
5.3
ETL
5.4
HOMO
5.7
5.9
NPB 50 nm
Alq3 10nm
LiF / Al 1.2 / 100nm
PMCDPVBi 50 nm
ITO 100-150nm
CuPc 10nm
?ext 2.8
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Comparison point OLEDs with DPVBi ALONE
Recombination zone
electrons
2.9
2.4
LUMO
3.0
2.8
Cathode
3.6
holes
DPVBi
4.7
HIL
HTL
Anode
5.3
ETL
5.4
HOMO
5.7
5.9
NPB 50 nm
Alq3 10nm
LiF / Al 1.2 / 100nm
PMCDPVBi 50 nm
ITO 100-150nm
CuPc 10nm
?ext 2.7
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Device b (doping) SUMMARY
? All spectra similar to DPVBi and NPB which
material is emitting light ? ?no shoulder in DEC
spectra suppression of aggregates by dilution
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Summary

• We demonstrated state-of-the-art external quantum
  efficiency of 3.3 with a deep-blue OLED (CIE x
  0.15 y 0.17) using a DECDPVBi emitting
  mixture
• Close to the max 5 25 (singlet/triplet
  ratio) x 20 (extraction efficiency)
• Efficiency of the doping approach DECDPVBi
  better than DPVBi alone (or DPVBIPMC)
  attributed to enhanced trapping of charged
  carriers
• PMC exhibits the most saturated color (x 0.15
  y 0.10) better efficiency would be achievable
  with a different design while keeping the CIE
  coordinates (in progress)

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