1
E L E C T R O P H O S P H O R E S C E N C E
1. OLED efficiency
2. Spin
3. Energy transfer
4. Organic phosphors
5. Singlet/triplet ratios
6. Phosphor sensitized fluorescence
7. Endothermic triplet energy transfer
April 10, 2003 – Organic Optoelectronics - Lecture 17
Organic LEDs – part 4
Lecture by Prof. Marc Baldo
2
POWER EFFICIENCY
400 500 600 700
0
200
400
600
800
Pho
topi
c re
spon
se (l
m/W
)
Wavelength (nm)
The eye’s response is wavelength dependent.The unit of perceived optical poweris defined as the lumen (lm).
Power efficiency of LED’s is thereforemeasured in lumens/Watt.
The maximum photopic response is:Φ ~ 20 lm (blue)Φ ~ 680 lm (green)Φ ~ 100 lm (red)
Power efficiency = photopic response of eye x quantum efficiency x electrical efficiency
For an OLED of given color, we must maximize:
- quantum efficiency (ηQ = photons out/electrons in)
- electrical efficiency (Vλ/V = photon energy/operating voltage)
P QVL
VI Vλη η= = Φ
3
Q out PLη χη φ=
φPL is fundamental luminescence efficiency of organic material.
Quantum efficiency
is the measure of a luminescent dye’s performance.ηQ= the number of photons emitted per electron injected.
ηout is output coupling fractionreduced by absorption losses
and wave guiding within the device and its substrate.
glass
OLED
χ is fraction of luminescent molecularexcitations
(defined by spin selection rules)typically ~ 25%
remaining energy is wasted
Imposes a fundamental limitto OLED efficiencies
4
EXCITON SPIN AND SYMMETRY
Lowest unoccupied (LUMO)molecular orbital
Molecular ground state(spatially symmetric under exchange of electrons)
1st excited state(spatially anti-symmetric (triplet) or spatially symmetric (singlet))
LUMO
HOMO
ψ
ψψ
ψ
S=1, triplet
S=0, singlet
}
S=0, singlet
Highest occupied (HOMO)molecular orbital
E
E
Molecular excited states (or excitons) are typically mobile, with binding energy ~ 1eV, and spin S = 0,1
5
Intersystemcrossing (ISC)S1
T1
PhosphorescenceFluorescence
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.
Triplet has lower energybecause it is spatiallyantisymmetric under exchange of electrons.(e--e- repulsion lower).
Fluorescence and Phosphorescence
S0
6
Efficient phosphorescenceNeed to mix singlet and triplet states:
- make both singlet and triplet decay allowed.
Pt, Ir, etc...
Ligand molecular orbitalSpin orbit coupling mixes states: proportional to atomic number
Z4
-+ligand
exciton
metalligand
+-
MLCT exciton
metalligand
Use metal-organic complexes with heavy transition metals:
PtN N
NN
PtOEP
NIr
3
Ir(ppy)3
Type I phosphorExciton localized on organic
Type II phosphorMetal-ligand charge transfer exciton
most mixing ~ 1µs triplet lifetimeless mixing ~ 100µs triplet lifetime
7
Structure and operation of OLEDs
- must transfer energy from host material to luminescent dopant. This determines the quantum efficiency of luminescence.
A heterostructure OLED
Excitons form in host at interface.Ideally energy is transferred to luminescent molecules
electronsholes
Cathode
TransparentAnode
Assume balanced currents:every hole combines with an electron
Luminescentdopant molecules
Electrontransport layer
Hole transport layer
Light
8
Donor* Acceptor Donor Acceptor*
Acceptor(dye )
Donor
Long range(up to ~ 100Å)
Exciton non-radiatively transferred by dipole-dipole coupling if transitions are allowed (usually singlet-singlet).
(a) Förster energy transfer
Donor* Acceptor Donor Acceptor*
Acceptor(dye )Donor
Short range~ 10Å
Exciton hops from donor to acceptor.
(b) Dexter energy transfer
Energy transfer
9
A red phosphor:Platinum octaethylporphyrin
Wavelength (nm)400 500 600 700 800 900
Inte
nsity
(a.u
.)
0
1Emission spectrum
0.1 1 10 1000.1
1
1030020010050301042
Qua
ntum
Effi
cien
cy (%
)
Current Density (mA/cm2)
1% PtOEP6% PtOEP
20% PtOEP
Luminance of 6% device (cd/m2)
• Triplet lifetime ~100ms• Peak external quantumefficiency in Alq3 ~ 4%
triplet
triplet groundstate
singlet
efficiency roll-off due to triplet-triplet annihilation
Loss proportionalto square of triplet lifetime
Mg:Ag cathode
Indium tin oxide
a-NPD
PtOEP in Alq3
Devicestructure
PtN N
NN
PtOEP
10
500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8In
tens
ity (a
rb. u
nits
)
Wavelength (nm)
350Å a-NPD
700Å Alq3
Mg/Ag
Ag
Glass
ITO
60Å CuPc
100Å DCM2/Alq3
Device 1Device 1 Device 2Device 2Alq3
DCM2
PtOEP
100Å Alq3
100Å PtOEP/Alq3
350Å a-NPD
500Å Alq3
100Å DCM2/Alq3
Mg/Ag
Ag
Glass
ITO
60Å CuPc
DOES PTOEP CAPTURE ALQ3TRIPLETS?
Put fluorescent dye DCM2 in exciton formation zone.
11
The singlet/triplet ratio in Alq3
Glass
ITO60Å CuPc
450Å a-NPD
200Å Alq3
400Å PtOEP/Alq3
Mg/Ag
Ag
Device 2emits from
singlets and triplets
Device 1emits from
singlets only
Glass
ITO60Å CuPc
450Å a-NPD
200Å Alq3
400Å DCM2/Alq3
Mg/Ag
Ag
Compare ratio of EL emission from devices to get singlet fraction.After correction for PL efficiency of DCM2 and PtOEP, get:
χ=(22±3)%
12
IMPROVED PHOSPHORS
Must reduce triplet lifetime to minimize T-T annihilation.Increase MLCT component of excited state.
The first high efficiency green phosphor was iridium tris(2-phenylpyridine)
Broad green emission at λ ~ 515 nm.Phosphorescent lifetime, τ ~ 1 ms.
NIr
3
Voltage (V)0 2 4 6 80.1
1
10
102
103
104
105
Lum
inan
ce (c
d/m
2 )
1
2
4
10
2030
Pow
er e
ffici
ency
(lm
/W)
Ir(ppy)3
10
+-
MLCT exciton
metalligand
13
TRIPLET-SINGLET ENERGY TRANSFER
S
T
S
T
host fluorescentdye
There are many more fluorescent than phosphorescent materials.Can we get high efficiency from a fluorescent dye?
?
Need to prevent exciting triplet state in fluorescent dye.Want mechanism for triplet-singlet energy transfer.
excitonformation
excitonformation
14
1. Triplet-singlet hopping transfer is disallowed2. Triplet-singlet Förster transfer permitted if triplet
relaxation on donor is allowedi.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).
Phosphor sensitized fluorescence
15
S
T
S
T
ISC S
T
host phosphorescentsensitizer
fluorescentdye
IMPLEMENTATION OF TRIPLET-SINGLET ENERGY TRANSFER
N
O CH3
NC CN
DCM2absorbs in the greenN
Ir
3Ir(ppy)3
N N
CBP
16
0.01 0.1 1 10 100 10000.50.60.70.80.9
1
2
3
45678
10
Ext
erna
l qua
ntum
effi
cien
cy (%
)
Current density (mA/cm2)
0.00.10.20.30.40.50.60.70.80.91.0
EL
Inte
nsity
(arb
itrar
y un
its)
Wavelength (nm)400 450 500 550 600 650 700 750 800
0.2% DCM2 and 8% Ir(ppy)3in CBP
DCM2 fluorescence sensitized by Ir(ppy)3
DCM2
Ir(ppy)3
Roll-off in efficiency is due to chargetrapping on DCM2 molecules
Nearly complete energy transfer from Ir(ppy)3 to DCM2
0.2% DCM2 in CBP
Note: factor of 4difference
17
Delayed DCM2 fluorescence confirms sensitizing action of Ir(ppy)3
OLED TRANSIENT RESPONSE
0 100 200 300 400 500 600 700 800 900 1000
1
10
100In
tens
ity (a
rbitr
ary
units
)
Time (ns)
DCM2
Ir(ppy)3
Examine transient response ofdifferent spectral componentsof OLED luminescence.
Natural lifetime of DCM2only ~5ns.
18
B L U E
FIrpic (blue phosphor)Peak wavelength 470nmTriplet lifetime 10 µs
Problem: the most energetic charge transport hosts currently availablehave blue-green triplets.
So to transfer energy to a blue phosphor, exciton transfer must be endothermic.
NIrF
F
O
N
O
2
(2.62±0.10) eV
CBPFIrpic
kF
kR
kGkH
(2.56±0.10) eV
0.0 0.1 0.2 0.3
0.00.10.2
0.30.4
CIE
y c
oord
inat
e
CIE x coordinate
6% FIrpicin CBP
FIrpicin CHCl3
FIrpicat 460nm
Inen
tsity
(arb
. uni
ts)
450 500 5500.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
6% FIrpic in CBPFIrpic in CHCl3FIrpic at 460nm
Host CBP triplet lifetime ~ 1s.N N
CBP
Possible to get efficient endothermic transfer
if decay of host triplet disallowed.
19
Transient response of endothermic transfer
102
103
104
PL
inte
nsity
(arb
itrar
y un
its)
50 100 150 200 250 3000.0
0.5
1.0
PL
inte
nsity
(arb
itrar
y un
its)
Temperature (K)
(b)(c)(a)(d)
(a)T=300K(b)T=200K(c)T=100K(d)T= 50K
0 10 20 30 40 50Time (µs)
Efficiency of luminescence decreases belowT=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)
0 10 201
10
100
1000
EL
Inte
nsity
(arb
. uni
ts)
0 20 40 60 80 1001
10
100
1000
τ= (80±10)µs
τ= (15±2)µs
Time (µs)
PL
Inte
nsity
(arb
. uni
ts)
T=200K
T=292K
τ= (15±2)µs
6% Ir(ppy)3:TPD
6% Ir(ppy)3:TPD
Time (µs)
Firpic in CBP
Ir(ppy)3 in TPD
20
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.00.10.20.30.40.50.60.70.80.9
CIE
y c
oord
inat
e
CIE x coordinate
btp2Ir(acac)
PtOEP
ppy2Ir(acac)
FIrpic
Organic materials have broad luminescent spectra.Overcome in red and blue by shifting toward non-visible
wavelengths.
COLOR PURITY
21
J = 1 µA/cm2
phosphor host Φ (lm/W) V VληP (lm/W) ηQ ext ηQ int V VληP (lm/W) ηQ ext ηQ int
J = 1 mA/cm2
ppy2Ir(acac)
btpIr(acac)
FIrpic
PtOEP
TAZ
CBP
CBP
CBP
530
170
260
60
60
4
1.3
0.3
0.19
0.07
0.006
0.056
0.87
0.32
0.027
0.23
0.60
0.34
0.83
0.09
20
2.2
5.0
0.2
0.15
0.06
0.057
0.042
0.68
0.27
0.23
0.19
0.25
0.22
0.34
0.08
O L E D S U M M A R Y
Peak power efficiency in green is 60 lm/WPeak quantum efficiency is 19%(corresponds to ~100% internal)
Largest remaining problem is operating voltage
Table of phosphorescent device operating parameters
22
Inte
nsity
(arb
. uni
ts)
Wavelength (nm)450 500 550
0.0
0.2
0.4
0.6
0.8
1.0
PHOSPHORESCENT DEVICE PERFORMANCE
btp2Ir(acac)
S
N
Ir
2
O
OCH3
CH3
10-2 10-1 100 101 102 1030.1
1
10
0.1
1
10
Pow
er e
ffici
ency
(lm
/W)
Qua
ntum
effi
cien
cy (%
)
Current density (mA/cm2)
btp2Ir(acac) in CBP
600 650 7000.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(arb
. uni
ts)
Wavelength (nm)
R E D
NIr
F
F
O
N
O
2
FIrpic
B L U E
10-3 10-2 10-1 100 101 1020.1
1
10
0.1
1
10
Pow
er e
ffici
ency
(lm
/W)
Qua
ntum
effi
cien
cy (%
)Current density (mA/cm2)
FIrpic in CBP
ppy2Ir(acac)
500 550 6000.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(arb
. uni
ts)
Wavelength (nm)
N
IrO
O
CH3
CH32
G R E E N
Pow
er e
ffici
ency
(lm
/W)
Qua
ntum
effi
cien
cy (%
)
Current density (mA/cm2)10-3 10-2 10-1 100 101 102 103 1041
10
100
1
10
100
23
PHOSPHORESCENT STABILITY
Phosphors should improve OLED stabilityby rapidly removing triplet excitonsand lowering drive current required.
Courtesy Ray Kwong and UDC
1 10 100 1000 100000.0
0.2
0.4
0.6
0.8
1.0
1.2
Nor
mal
ized
opt
ical
pow
er
Time (hours)
50 cd/m2
100 cd/m2
200 cd/m2500 cd/m2
200 cd/m2
1 10 100 1000 100000.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
opt
ical
pow
er
Time (hours)
500 cd/m2
1000 cd/m2
2000 cd/m2
Red: BtpIr(acac) in CBP Green: (ppy)2Ir(acac) in CBP