Triplet formation and migration in conjugated polymer Triplet absorption cross-section Triplet diffusion length

Triplet formation and migration
in conjugated polymer
1. Triplet absorption cross-section of conducting polymer
2. Triplet diffusion length of conducting polymer
Dr. Chang-Lyoul Lee
OE Group
Cavendish Laboratory
Cambridge University
[email protected]
Content
¾Triplet absorption cross-section of conducting
polymer
ƒ
Why the triplet state of polymer is so important
ƒ
How the triplet absorption cross-section of polymer can
be measured
ƒ
Photoinduced absorption (PIA) spectroscopy
ƒ
Triplet absorption cross-section of F8BT
Cavendish-KAIST Collaboration Symposium (27/September/2006)
Dr. Chang-Lyoul Lee
Singlet-triplet formation ratio after electrical excitation
p-
p+
Recombination
↑↑
(
1
↑↓ − ↓↑
2
)
Singlet exciton
Triplet exciton
(
1
↑↓ + ↓↑
2
)
↓↓
Radiative
Radiative
decay
decay
Non-radiative
Non-radiative
decay
decay
ƒ Simple statistics suggests only 25% singlets
ƒ Maximum efficiency = 25%?
ƒ Does the 1:3 ratio hold in polymers?
Non-radiative
Non-radiative
decay
decay
Reference
1. M. A. Blado et al. Phys. Rev. B, 60, 14422 1999
2. A. S. Dhoot et al. Chem. Phys. Lett. 360, 195, 2002
3. M. Wohlgenannt et al. Nature, 409, 494, 2001
4. J. S. Wilson et al. Nature, 413, 828 2001
Cavendish-KAIST Collaboration Symposium (27/September/2006)
Dr. Chang-Lyoul Lee
How the triplet absorption cross-section
of polymer can be measured
ƒ Excited state absorption measurement and energy transfer
Recombination
TnP+D
S1P
Induced
absorption
Intersystem
crossing
P+D
T1
Polymer
Triplet exciton
of polymer and
phosphorescent dopant
Recombination
T1D
Phosphorescent
dopant
S0P
PIA
Dexter
energy
transfer
Energy transfer
ƒ Absorption measured in PIA
∆T
= − nσd
T
n : Triplet excited state population
σ : Triplet absorption cross-section (~ 10-15 cm2)
d : Thickness of film
Cavendish-KAIST Collaboration Symposium (27/September/2006)
Dr. Chang-Lyoul Lee
Photoinduced absorption (PIA)
ƒ Study of long-lived excited states in organic systems
ƒ Steady-state absorption spectroscopy of charge and neutral excitations
(min lifetime ~ 50µs)
ƒ Study triplet exciton formation and decay in LEDs
ƒ Frequency dependence of absorptions in PIA can be used to probe recombination
kinetics
-4
1.2x10
-4
S
N
N
[dt/t]x and [dt/t]y
1.0x10
-5
8.0x10
|∆T/T|
[dt/t]x
[dt/t]y
n*
*
-5
6.0x10
-5
4.0x10
-5
10
-6
10
-5
2.0x10
-7
10
0.0
500
600
700
800
900
1000
1100
1
10
Wavelength (nm)
Cavendish-KAIST Collaboration Symposium (27/September/2006)
2
10
3
10
4
10
5
10
Frequency (Hz)
Dr. Chang-Lyoul Lee
Characteristics of materials
S
N
N
N
N
n*
*
Ir
F
O
1.2
Abs of Firpic
PL of Firpic
1.0
Abs of F8BT
PL of F8BT
0.8
1.2
λexc= 325 nm
Norm. absorbance
Firpic
1.0
0.8
B
0.6
0.6
0.4
0.4
0.2
0.2
Norm. PL emission
F
F8BT
O
ƒ Polyfluorenes : Most promising materials for
polymer electronics. High luminous
efficiencies (19lm/W for green emission )
and low operation voltage (2.1V for
100cd/m2).1 Blends of electron- and holeaccepting derivatives F8BT is used in
polymeric PV
ƒ Firpic is well known triplet blue emitter.
High quantum and power efficiencies (10.4%
and 10.5 lm/W for blue emission)2,3
A
Reference
A.C. Morteani et al. Adv. Mater. 15, 1709, 2003
R. J. Holmes et al. App. Phys. Lett. 82, 2422, 2003
3 S. Tokito et al. App. Phys. Lett. 83, 569, 2003
1
2
0.0
300
400
500
600
700
C
0.0
800
Wavelength (nm)
Cavendish-KAIST Collaboration Symposium (27/September/2006)
Dr. Chang-Lyoul Lee
PL spectra of Firpic doped
F8BT films with different concentration
Firpic
PL intensity (a.u.)
400
F8BT
1% Firpic doped F8BT
5% Firpic doped F8BT
λexc= 351nm
300
S1
ISC
T
3
①
②
200
S1
ISC
③
100
0
F8BT
400
500
600
700
800
T3
900
Wavelength (nm)
Cavendish-KAIST Collaboration Symposium (27/September/2006)
Dr. Chang-Lyoul Lee