Organic LEDs – part 4
ELECTROPHOSPHORESCENCE
1. OLED efficiency
2. Spin
3. Energy transfer
4. Organic phosphors
5. Singlet/triplet ratios
6. Phosphor sensitized fluorescence
7. Endothermic triplet energy transfer
Lecture by Prof. Marc Baldo
April 10, 2003 – Organic Optoelectronics - Lecture 17
1
The unit of perceived optical power
is defined as the lumen (lm).
Power efficiency of LED’s is therefore
measured in lumens/Watt.
The maximum photopic response is:
Φ ~ 20 lm (blue)
Φ ~ 680 lm (green)
Φ ~ 100 lm (red)
Photopic response (lm/W)
POWER EFFICIENCY
The eye’s response is wavelength dependent.
800
600
400
200
0
400
500
600
700
Wavelength (nm)
For an OLED of given color, we must maximize:
- quantum efficiency (ηQ = photons out/electrons in)
- electrical efficiency (Vλ/V = photon energy/operating
voltage)
Vλ
L
ηP =
= ΦηQ
VI
V
Power efficiency = photopic response of eye x quantum efficiency x electrical efficiency
2
Quantum efficiency
is the measure of a luminescent dye’s performance.
ηQ= the number of photons emitted per electron injected.
ηQ = χηoutφ PL
φPL is fundamental luminescence
efficiency of organic material.
ηout is output coupling fraction
reduced by absorption losses
and wave guiding within the device
and its substrate.
χ is fraction of luminescent molecular
excitations
(defined by spin selection rules)
typically ~ 25%
remaining energy is wasted
OLED
glass
Imposes a fundamental limit
to OLED efficiencies
3
EXCITON SPIN AND SYMMETRY
Molecular excited states (or excitons) are typically mobile,
with binding energy ~ 1eV, and spin S = 0,1
Molecular ground state
(spatially symmetric under exchange of electrons)
E
Lowest unoccupied (LUMO)
molecular orbital
S=0, singlet
Highest occupied (HOMO)
molecular orbital
1st excited state
(spatially anti-symmetric (triplet) or spatially symmetric (singlet))
E
LUMO
ψ
ψ
ψ
HOMO
ψ
}
S=1, triplet
S=0, singlet
4
Fluorescence and Phosphorescence
S1
Fluorescence
S0
Triplet has lower
Intersystem
energy
because it is spatially
crossing (ISC)
antisymmetric under
T1 exchange of
electrons.
(e--e- repulsion
Phosphorescence
lower).
Molecular ground state
Fluorescence:
Decay from singlet allowed by symmetry: fast (109 s-1) and often efficient.
Phosphorescence:
Decay from triplet disallowed by symmetry: slow ( > 1 s-1) and inefficient.
5
Efficient phosphorescence
Need to mix singlet and triplet states:
- make both singlet and triplet decay allowed.
Use metal-organic complexes with heavy transition metals:
Pt, Ir, etc...
Ligand molecular orbital
Spin orbit coupling mixes states: proportional to atomic number
Z4
Type I phosphor
Type II phosphor
Exciton localized on organic
ligand
exciton
+-
Metal-ligand charge transfer exciton
MLCT exciton
N
-
N
Pt
N
+
N
N
Ir
3
metal
ligand
PtOEP
less mixing ~ 100µs triplet lifetime
metal
Ir(ppy)3
ligand
most mixing ~ 1µs triplet lifetime
6
Structure and operation of OLEDs
- must transfer energy from host material to luminescent dopant.
This determines the quantum efficiency of luminescence.
A heterostructure OLED
holes
electrons
Hole transport layer
Electron
transport layer
Light
Cathode
Luminescent
dopant molecules
Transparent
Anode
Assume balanced currents:
every hole combines with an electron
Excitons form in host at interface.
Ideally energy is transferred to luminescent molecules
7
Energy transfer
(a) Förster energy transfer
Long range
(up to ~ 100Å)
Acceptor
(dye )
Donor* Acceptor
Donor Acceptor*
Donor
Exciton non-radiatively transferred by dipole-dipole coupling
if transitions are allowed (usually singlet-singlet).
(b) Dexter energy transfer
Donor
Donor* Acceptor
Donor Acceptor*
Exciton hops from donor to acceptor.
Acceptor
(dye )
Short range
~ 10Å
8
A red phosphor:
Emission spectrum
Platinum octaethylporphyrin
N
Intensity (a.u.)
1
N
Pt
N
• Triplet lifetime ~100ms
• Peak external quantum
efficiency in Alq3 ~ 4%
N
0
PtOEP
400 500 600 700 800 900
Wavelength (nm)
Luminance of 6% device (cd/m2)
2
Mg:Ag cathode
PtOEP in Alq3
a-NPD
Indium tin oxide
10
30 50
100
200 300
10
Quantum Efficiency (%)
Device
structure
4
1% PtOEP
6% PtOEP
20% PtOEP
efficiency roll-off due
to triplet-triplet annihilation
triplet
singlet
triplet
ground
state
1
0.1
Loss proportional
to square of triplet lifetime
0.1
1
10
100
Current Density (mA/cm2)
9
DOES PTOEP CAPTURE ALQ3
TRIPLETS?
Put fluorescent dye DCM2 in exciton formation zone.
1.8
1.6
PtOEP
Ag
Ag
Mg/Ag
Mg/Ag
Intensity (arb. units)
500Å Alq3
1.4
1.2
700Å Alq3
100Å PtOEP/Alq3
DCM2
100Å Alq3
100Å DCM2/Alq3
100Å DCM2/Alq3
1.0
350Å a-NPD
350Å a-NPD
0.8
60Å CuPc
60Å CuPc
ITO
ITO
0.6
Glass
Glass
0.4
Device 1
Device 2
0.2
Alq3
0.0
500
600
700
800
Wavelength (nm)
10
The singlet/triplet ratio in Alq3
Device 1
emits from
singlets only
Device 2
emits from
singlets and triplets
Ag
Ag
Mg/Ag
Mg/Ag
200Å Alq3
200Å Alq3
400Å DCM2/Alq3
400Å PtOEP/Alq3
450Å a-NPD
450Å a-NPD
60Å CuPc
60Å CuPc
ITO
ITO
Glass
Glass
Compare ratio of EL emission from devices to get singlet fraction.
After correction for PL efficiency of DCM2 and PtOEP, get:
χ=(22±3)%
11
IMPROVED PHOSPHORS
Must reduce triplet lifetime to minimize T-T annihilation.
Increase MLCT component of excited state.
30
20
105
104
10
103
102
4
10
2
1
1
0.1
0
2
4
6
8
Voltage (V)
10
Power efficiency (lm/W)
Luminance (cd/m2)
The first high efficiency green phosphor was iridium tris(2-phenylpyridine)
Ir(ppy)3
MLCT exciton
+
N
Ir
3
metal
ligand
Broad green emission at λ ~ 515 nm.
Phosphorescent lifetime, τ ~ 1 ms.
12
TRIPLET-SINGLET ENERGY
There areTRANSFER
many more fluorescent than phosphorescent materials.
Can we get high efficiency from a fluorescent dye?
host
fluorescent
dye
exciton
formation
S
?
exciton
formation
S
T
T
Need to prevent exciting triplet state in fluorescent dye.
Want mechanism for triplet-singlet energy transfer.
13
Phosphor sensitized
fluorescence
1. Triplet-singlet hopping transfer is disallowed
2. Triplet-singlet Förster transfer permitted if triplet
relaxation on donor is allowed
i.e. triplet-singlet transfer is possible from a
phosphorescent donor
Predicted by Förster in 1959 (†)
Observed by Ermolaev and Sveshnikova in 1963 (§)
e.g. for triphenylamine as the donor and chrysoidine as
the acceptor, in rigid media at 77K or 90K the interaction
length is 52Å
(†) Förster, Th. Dicussions of the Faraday Society 27, 7-17 (1959).
(§) Ermolaev, V.L. & Sveshnikova, E.B. Doklady Akademii Nauk SSSR
149, 1295-1298 (1963).
14
IMPLEMENTATION OF TRIPLET-SINGLET ENERGY
TRANSFER
host
phosphorescent
sensitizer
S
fluorescent
dye
S
S
ISC
T
T
T
N
N
N
N
DCM2
absorbs in the green
Ir
O
CH3
CBP
3
Ir(ppy)3
NC
CN
15
10
8
7
6
5
0.2% DCM2 and 8% Ir(ppy)3
in CBP
4
3
Note: factor of 4
difference
2
1
0.9
0.8
0.7
0.6
0.5
0.01
0.2% DCM2 in CBP
0.1
1
10
100
1000
Current density (mA/cm2)
Roll-off in efficiency is due to charge
trapping on DCM2 molecules
EL Intensity (arbitrary units)
External quantum efficiency (%)
DCM2 fluorescence sensitized by Ir(ppy)3
1.0
0.9
0.8
0.7
0.6
3
0.5
0.4
0.3
0.2
0.1
0.0
400 450 500 550 600 650 700 750 800
DCM2
Ir(ppy)
Wavelength (nm)
Nearly complete energy transfer
from Ir(ppy)3 to DCM2
16
OLED TRANSIENT RESPONSE
Intensity (arbitrary units)
100
DCM2
Examine transient response of
different spectral components
of OLED luminescence.
Natural lifetime of DCM2
only ~5ns.
10
Ir(ppy)3
1
0
100 200 300 400 500 600 700 800 900 1000
Time (ns)
Delayed DCM2 fluorescence confirms sensitizing action of
Ir(ppy)3
17
BLUE
Ir
F
N
O
F
O
2
FIrpic (blue phosphor)
(2.56±0.10) eV
CBP
kH
0.6
0.4
0.2
(2.62±0.10) eV
FIrpic
kR
6% FIrpic in CBP
FIrpic in CHCl3
FIrpic at 460nm
0.8
0.0
Peak wavelength 470nm
Triplet lifetime 10 µs
kF
1.0
450
500
550
Wavelength (nm)
CIE y coordinate
N
Inentsity (arb. units)
Problem: the most energetic charge transport hosts currently available
have blue-green triplets.
So to transfer energy to a blue phosphor, exciton transfer must be endothermic.
0.4
0.3
0.2
0.1
0.0
6% FIrpic
in CBP
FIrpic
in CHCl3
FIrpic
at 460nm
0.0 0.1 0.2 0.3
CIE x coordinate
Possible to get efficient endothermic
transfer
if decay of host triplet disallowed.
CBP
kG
N
N
Host CBP triplet lifetime ~ 1s.
18
Transient response of endothermic transfer
104
103
50 100 150 200 250 300
Temperature (K)
102
(b)
(c)
(a)
(d)
(a)T=300K
(b)T=200K
(c)T=100K
(d)T= 50K
0
10
20
Ir(ppy)3 in TPD
1000
0.5
0.0
PL Intensity (arb. units) EL Intensity (arb. units)
PL intensity (arbitrary units)
PL intensity (arbitrary units)
Firpic in CBP
1.0
30
40
50
Time (µs)
Efficiency of luminescence decreases below
T=200K as energy transfer to phosphor is frozen out.
Transient response is also consistent with
endothermic transfer:
At T=200K, apparent transient lifetime of FIrpic
(includes endothermic transfer) >> natural lifetime
(~10µs)
100
τ = (15 ±2) µs
10
6% Ir(ppy)3:TPD
1
0
10
20
Time (µs)
1000
T=292K
τ = (80±10)µs
100
T=200K
10
1
τ = (15 ±2) µs
6% Ir(ppy)3:TPD
0
20
40
60
80
100
Time (µs)
19
CIE y coordinate
COLOR PURITY
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
ppy2Ir(acac)
btp2Ir(acac)
PtOEP
FIrpic
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
CIE x coordinate
Organic materials have broad luminescent spectra.
Overcome in red and blue by shifting toward non-visible
wavelengths.
20
OLED SUMMARY
Peak power efficiency in green is 60 lm/W
Peak quantum efficiency is 19%
(corresponds to ~100% internal)
Largest remaining problem is operating voltage
J = 1 µA/cm2
phosphor
host
Φ (lm/W) ηP (lm/W) ηQ ext ηQ int Vλ V
J = 1 mA/cm2
ηP (lm/W) ηQ ext ηQ int Vλ V
ppy2Ir(acac) TAZ
530
60
0.19
0.87 0.60
20
0.15
0.68
0.25
btpIr(acac)
CBP
170
4
0.07
0.32 0.34
2.2
0.06
0.27
0.22
FIrpic
CBP
260
1.3
0.006 0.027 0.83
5.0
0.057 0.23
0.34
PtOEP
CBP
60
0.3
0.056 0.23 0.09
0.2
0.042 0.19
0.08
Table of phosphorescent device operating parameters
21
1.0
0.8
0.6
0.4
0.2
0.0
600
650
700
Intensity (arb. units)
Intensity (arb. units)
Intensity (arb. units)
PHOSPHORESCENT DEVICE PERFORMANCE
RED
GREEN
BLUE
1.0
0.8
0.6
0.4
0.2
0.0
Wavelength (nm)
500
550
600
1.0
0.8
0.6
0.4
0.2
0.0
Wavelength (nm)
450
500
550
Wavelength (nm)
CH3
O
CH3
Ir
O
Ir
N
O
2
O
F
1
btp2Ir(acac) in CBP
101
Current density
102
0.1
103
(mA/cm2)
FIrpic
ppy2Ir(acac)
100
100
10
10
1
1
10-3 10-2 10-1 100 101 102 103 104
Current density
(mA/cm2)
10
10
1
1
0.1
10-3
FIrpic in CBP
10-2
10-1
100
101
102
Power efficiency (lm/W)
1
Quantum efficiency (%)
10
Power efficiency (lm/W)
Quantum efficiency (%)
10
100
O
2
btp2Ir(acac)
10-1
N
CH3
CH3
2
0.1
10-2
F
Quantum efficiency (%)
N
Ir
O
Power efficiency (lm/W)
S
N
0.1
Current density (mA/cm222
)
PHOSPHORESCENT STABILITY
1.2
1.0
50 cd/m2
0.8
100 cd/m2
0.6
0.4
0.2
0.0
500 cd/m2
1
10
100
200 cd/m2
1000
10000
Time (hours)
Red: BtpIr(acac) in CBP
Normalized optical power
Normalized optical power
Courtesy Ray Kwong and UDC
1.0
200 cd/m2
0.8
500 cd/m2
1000 cd/m2
0.6
0.4
2000 cd/m2
0.2
0.0
1
10
100
1000
10000
Time (hours)
Green: (ppy)2Ir(acac) in CBP
Phosphors should improve OLED stability
by rapidly removing triplet excitons
and lowering drive current required.
23