A. Fischer, S. Forget, S. Chénais, M.-C. Castex, Lab. de Physique des Lasers, Univ. Paris Nord,...
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Transcript of A. Fischer, S. Forget, S. Chénais, M.-C. Castex, Lab. de Physique des Lasers, Univ. Paris Nord,...
C2H5
C2H5
N
N
CH3 CH3
CH3 CH3
CH3
N
C2H5
C2H5
N
N
CH3 CH3
CH3 CH3
CH3
N
C2H5
C2H5
N
N
CH3 CH3
CH3 CH3
CH3
N C2H5
C2H5
N
N
CH3 CH3
CH3 CH3
CH3
N CH3 CH3
CH3 CH3
CH3
N
A. Fischer, S. Forget, S. Chénais, M.-C. Castex,Lab. de Physique des Lasers, Univ. Paris Nord, France
Highly efficient multilayer organic pure-Highly efficient multilayer organic pure-blue-light emitting diodes with blue-light emitting diodes with
substituted carbazole compounds in the substituted carbazole compounds in the emitting layer.emitting layer.
Highly efficient multilayer organic pure-Highly efficient multilayer organic pure-blue-light emitting diodes with blue-light emitting diodes with
substituted carbazole compounds in the substituted carbazole compounds in the emitting layer.emitting layer.
D. Adès, A. Siove, Lab. Biomateriaux et Polymères de Spécialité, Univ. Paris Nord, France
C. Denis, P. Maisse and B. GeffroyLab. Cellules et Composants, CEA Saclay, France
2CLEO ’06 – Long Beach (USA)
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 PMC:DPVBi and DEC:DPVBi Conclusion
3CLEO ’06 – Long Beach (USA)
Introduction
Organic Light Emitting Diodes :
Ultrathin light sources, lightweightHigh brightness and viewing angle > 160°Low drive voltage (3-10 V) and low power consumptionExtremely rich diversity of materials : All visible colors available (≠ inorganic LEDs), including saturated colorsPotentially flexible Long lifetimes (> 20 000 h reported)Low cost potential for mass production
Applications : flat-panel RGB DISPLAYS, solid-state lighting,...
4CLEO ’06 – Long Beach (USA)
needs efficient blue emitters
Why BLUE ?
Why Blue OLEDs with high efficiencies are needed ?
different approaches for multi-color emission :
RGB emitters
+ : power efficient, mature
- : different aging and optimization
needs efficient blue emitters
(efficient R,G already exist)
White emitters + Filters
+ : homogeneous aging
- : not efficient (filters)
needs efficient blue emitters
to achieve bright white
Color changing media
+ : homogeneous aging
- : not efficient (photoconversion)
5CLEO ’06 – Long Beach (USA)
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)
CIE 1931
6CLEO ’06 – Long Beach (USA)
OLEDs materials
Carbazolic derivatives CH3 CH3
CH3 CH3
CH3
N CH3 CH3
CH3 CH3
CH3
N
PMC
C2H5
C2H5
N
N
DEC
Carbazole unit :
penta-methyl carbazole Dimer of N-Ethyl carbazole
• Chemically and thermally stable (up to 430 °C)
• Tg = 75°C
•Polaronic transport levels measured by cyclic voltammetry (eV) :
- Blue emitters: Carbazole-substituted Distyrylarylenes (DSA)
- Hole Transport materials : PVK
- Host material for triplet emitters: CBP
Vacuum level
Lowest Unoccupied Molecular Orbital
Highest Occupied Molecular Orbital
PMC DEC
5.9
2.82.5
5.6
Already used as…
new
7CLEO ’06 – Long Beach (USA)
OLEDs structures
1st DEC-based diode : single layer
Drawbacks:• Low ext. quantum efficiency ext. = 7.10-2
%• High operating voltage (20 V), crystallization during operation (short-circuit)
DEC
ITO
Al
h
VD. Romero, A. Siove et al., Adv. Mater. 9, 1158
(1997)
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
Bad performance due to recombination and quenching of excitons at Al/DEC interface, poor charge injection
8CLEO ’06 – Long Beach (USA)
Device a : OLED with NEAT film of DEC
Anode
ITO
100-150nm
Cathode
LUMO
HOMO
CuPc
10nm
ET
L
NPB
50 nm
Alq3
10nm
LiF / Al
1.2 / 100nm
HIL
HT
L
HB
L
BCP
10nm
5.7
3.0
2.9
2.4
6.1
4.7
5.3
3.6
2.4
5.4 5.
6DEC
50 nm
N
N
N
N
N
N
NN
Cu
CuPc
N N
NPB C2H5
C2H5
N
N
holes
electrons2.4
N
O
AlO
N
O
N
CH3
N
CH3
N
2.5
9CLEO ’06 – Long Beach (USA)
Device a : OLED with neat film of DEC
Anode
Cathode
LUMO
HOMO
ET
LHIL
HT
L
HB
L
5.7
3.0
2.9
2.4
6.1
4.7
5.3
3.6
2.4
5.4 5.
6
C2H5
C2H5
N
N
holes
electrons2.4
2.5
Main recombination zone
ηext = 1.5 % (optical design not optimized)
ITO
100-150nm
CuPc
10nm
NPB
50 nm
Alq3
10nm
LiF / Al
1.2 / 100nm
BCP
10nm
DEC
50 nm
10CLEO ’06 – Long Beach (USA)
Anode
Cathode
LUMO
HOMO
ET
LHIL
HT
L
HB
L
5.7
3.0
2.9
2.4
6.1
4.7
5.3
3.6
2.4
5.4
N
O
AlO
N
O
N
CH3
N
CH3
N
N N
NPB
5.9
2.8
CH3 CH3
CH3 CH3
CH3
Nholes
electrons
Device a : OLED with neat film of PMC
PMC OLED
ηext = 0.6 % → attributed to bad electron transport properties of PMC / electron barrier of BCP
ITO
100-150nm
CuPc
10nm
NPB
50 nm
Alq3
10nm
LiF / Al
1.2 / 100nm
BCP
10nm
PMC
50 nm
11CLEO ’06 – Long Beach (USA)
Device a : OLED with neat film of PMC
Anode
Cathode
LUMO
HOMO
ET
LHIL
HT
L
HB
L
5.7
3.0
2.9
2.4
6.1
4.7
5.3
3.6
2.4
5.4 5.
9
2.8
CH3 CH3
CH3 CH3
CH3
Nholes
electrons
PMC OLED
ηext = 0.6 % → attributed to bad electron transport properties of PMC / electron barrier of BCP
Main recombination zone
ITO
100-150nm
CuPc
10nm
NPB
50 nm
Alq3
10nm
LiF / Al
1.2 / 100nm
BCP
10nm
PMC
50 nm
12CLEO ’06 – Long Beach (USA)
Device a (neat films) : Experimental results
Electroluminescence spectra
a
Chromaticity coordinates
PMC
DEC
Aggregates, excimers ?
PMC : CIE x = 0.153 ; y = 0.100
DEC : CIE x = 0.192 ; y = 0.209Ext. Quantum efficiency : ηext = 0.6 % (PMC)
ηext = 1.5 % (DEC)Brightness L = 236 cd/m2 @ 60 mA/cm2 (PMC)
Luminous efficiency ηpower = 0.2 lm/W (PMC)
→ Bright saturated blueWith PMC, but modest efficiency
13CLEO ’06 – Long Beach (USA)
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
guestguesthosthost
other types of doping : the dopant « impurities » can enhance exciton recombination by trapping charge carriers (and diffusing excitons)
guestguesthosthost
Ex : Barrier for electrons + trap for holes = improved recombination rate
14CLEO ’06 – Long Beach (USA)
Device b : OLEDs with DPVBi doped with PMC (DEC)
CuPc 10nm
NPB 50nm
DPVBi (PMC or DEC) 50nm
Alq3 10nm
LiF 1.2nm/Al 100nm
(b)
ITO glass
C2H5
C2H5
N
N
DEC
CH3 CH3
CH3 CH3
CH3
N
PMC
+ or
5% wt.
2% wt.DPVBi4,4’-bis(2,2’-diphenylvinyl)-1,1’-biphenyl
Vacuum level
Lowest Unoccupied Molecular Orbital
Highest Occupied Molecular Orbital
PMC DEC
5.9
2.82.5
5.6
DPVBI
5.9
2.8
Doping by coevaporation from 2 resistively heated cells
15CLEO ’06 – Long Beach (USA)
OLEDs with DPVBi doped with DEC
Anode
ITO
100-150nm
Cathode
LUMO
HOMO
CuPc
10nm
ET
L
NPB
50 nm
Alq3
10nm
LiF / Al
1.2 / 100nm
HIL
HT
L
5.7
3.0
2.9
4.7
5.3
3.6
2.4
5.4
DEC:DPVBi
50 nm
holes
electrons2.5
5.6
5.9
2.8
2% DEC
DPVBi
16CLEO ’06 – Long Beach (USA)
OLEDs with DPVBi doped with DEC
Anode
Cathode
LUMO
HOMO ET
L
HIL
HT
L
5.7
3.0
2.9
4.7
5.3
3.6
2.4
5.4
holes
electrons2.5
5.65.9
2.8
2% DEC
DPVBi
Recombination zone
ηext = 3.3 %
ITO
100-150nm
CuPc
10nm
NPB
50 nm
Alq3
10nm
LiF / Al
1.2 / 100nm
DEC:DPVBi
50 nm
17CLEO ’06 – Long Beach (USA)
Anode
Cathode
LUMO
HOMO ET
L
HIL
HT
L
5.7
3.0
2.9
4.7
5.3
3.6
2.4
5.4
holes
electrons
5.9
2.8
5% PMC
DPVBi
OLEDs with DPVBi doped with PMC
Recombination zone
ηext = 2.8 %
ITO
100-150nm
CuPc
10nm
NPB
50 nm
Alq3
10nm
LiF / Al
1.2 / 100nm
PMC:DPVBi
50 nm
18CLEO ’06 – Long Beach (USA)
Anode
Cathode
LUMO
HOMO ET
L
HIL
HT
L
5.7
3.0
2.9
4.7
5.3
3.6
2.4
5.4
holes
electrons
5.9
2.8
DPVBi
Recombination zone
ηext = 2.7 %
Comparison point : OLEDs with DPVBi ALONE
ITO
100-150nm
CuPc
10nm
NPB
50 nm
Alq3
10nm
LiF / Al
1.2 / 100nm
PMC:DPVBi
50 nm
19CLEO ’06 – Long Beach (USA)
Device b (doping) : SUMMARY
PMC:DPVBiDEC:DPVBi
DPVBi
Device (a)PMC
Device (a)DEC
Device (b)DPVBi PMC- doped (5%)
Device (b)DPVBi DEC- doped (2%)
Device (b)DPVBi
nondoped
ext (%) 0.6 1.5 2.8 3.3 2.7power (lm/W) 0.2 … 1.2 1.3 1.2
L (cd/m2) @ 60 mA/cm2
236 … 2279 2825 …
C.I.E. x 0.153 0.192 0.160 0.158 0.149
C.I.E. y 0.100 0.209 0.176 0.169 0.112
► All spectra similar to DPVBi and NPB : which material is emitting light ?
►no shoulder in DEC spectra : suppression of aggregates by dilution
20CLEO ’06 – Long Beach (USA)
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 DEC:DPVBi emitting mixture
Close to the max 5% = 25% (singlet/triplet ratio) x 20% (extraction
efficiency)
Efficiency of the doping approach : DEC:DPVBi better than DPVBi alone (or DPVBI:PMC) : 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)