Transcript Document

Optically Detected Magnetic Resonance (ODMR)
and its Application to
p-Conjugated Materials and
Organic Light-Emitting Devices (OLEDs)
Joseph Shinar
March 30, 2009
[email protected]
1
ODMR in One Sentence:
Monitor mwave-induced changes in an optical quantity at the field for
resonance.
Since “optical quantity” can mean different quantities, ODMR is an
umbrella term, meaning we can measure, e.g.,
* Photoluminescence (PL)-detected magnetic resonance (PLDMR)
* Electroluminescence (EL)-detected magnetic resonance (ELDMR)
* Absorption-detected magnetic resonance (ADMR)
* Photoinduced absorption (PA)-detected magnetic resonance (PADMR)
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Similarly, Electrically Detected Magnetic
Resonance (EDMR) in One Sentence:
Monitor mwave-induced changes in an electrical quantity at the field
for resonance.
And similarly,, EDMR is an umbrella term, e.g.,
* Current or Conductivity-detected magnetic resonance (CDMR)
* Photoconductivity-detected magnetic resonance (PCDMR)
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Now consider basic electronic processes in an
organic semiconductor, i.e., a p-conjugated material.
TRIPLET
(EXCITON)
MANIFOLD
EXCITONS
SINGLET
(EXCITON)
MANIFOLD
EXCITONS
POLARON
MANIFOLD
m1Ag
Charge Transfer
m3Ag
1 3 Bu
ISC(a)
T
11Bu
p+
pP2
absorption
knr kr (PL 0-0 )
Phosphorescence0-0
P1
11Ag
GROUND STATE
(a)
Intersystem Crossing
4
A Typical PLDMR Spectrometer:
DATA AQUISITION
LOCK IN
DETECTOR (Si)
MAGNET
CONTROL
PL
MICROWAVE
MODULATION
Ar+ LASER (351 nm - 515 nm)
CRYOSTAT (He) 10 K - 300 K
MICROWAVE CAVITY
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The positive (PL-enhancing) spin 1/2 polaron pair PLDMR at g = 2
R
S
n
The positive PLDMR in poly(3-hexyl thiophene) (P3HT) and poly(3-dodecyl
thiophene) (P3DT) films and solutions.
L. S. Swanson et al., Phys. Rev. Lett. 65, 1140 (1990).
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The full-field (Dm = 1) triplet powder-pattern PLDMR.
R
S
7
n
And the half-field (Dm = 2) triplet powder-pattern PLDMR…
R
S
n
8
R'
Similar Polaron pair PLDMR at g = 2 of
m-LPPP and PHP
C6H13
R
16
n
C6H13
14
DIPL/IPL of
12
10
-4
10 D PL/PL
R
R'
• Photo-oxidized m-LPPP
8
DIPL/IPL = 1.4 x 10-3
6
4
•
2
m-LPPP
DIPL/IPL = 3.3 x 10-4
0
3300
3320
3340
3360
3380
M agnetic Field (G )
•
PHP
DIPL/IPL = 8 x 10-5
E. J. W. List et al., Appl. Phys. Lett. 76, 2083 (2000).
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PADMR of m-LPPP films [scan probe energy at constant
magnetic field; monitor microwave induced changes in the
photoinduced absorption (PA)].
12
R'
-4
10 (-DT/T)
10
8
6
C6H13
4
R
-6
10 (- T/T)
2
0
-2
-4
-6
-8
-10
n
C6H13
R
P2 0-1
P2 0-0
T1-->Tn
R'
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
Energy (eV)
10
1st Expt: nmw-dependence of Regular PLDMR of MEH-PPV
-3
1.0x10
-4
8.0x10
1.0x10
-4
-4
6.0x10
-4
4.0x10
DPL/PL
DPL/PL
8.0x10
-3
-4
2.0x10
0.0
0.01
6.0x10
-4
4.0x10
-4
2.0x10
-4
0.0
3.28 3.30 3.32 3.34 3.36 3.38
Magnetic Field(k gaus s)
0.1
1
10
Single modulation
PLDMR DPL/PL vs
the microwave
modulation frequency
fM.
The dashed line is a
single lifetime fit w/
t = 38 ms;
the solid line is a twolifetime fit w/
t1= 24 ms, t2= 244 ms.
f M (kHz)
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Note:
negative carrier  electron (e-)  negative polaron (p-)  radical anion
positive carrier  hole (h+)  positive polaron (p-)  radical cation
12
Monomolecular nonradiative quenching processes
 Quenching of excited states [singlet excitons (SEs) and triplet excitons (TEs)]
by the cathode & anode.
 Electric field-induced quenching (via dissociation) of SEs (and TEs?).
 Quenching by impurities.
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Bimolecular nonradiative quenching processes
 Quenching of SEs by TEs and by polarons
 Quenching of TEs by polarons.
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 Other results that bear on quenching mechanisms
1. Double modulation (DM) PLDMR (DM-PLDMR)
2. Joint thermally-stimulated luminescence (TSL) + PLDMR
3. PLDMR of the small molecules
tris(8-hydroxy quinoline) Al (Alq3) &
4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi)
Alq3
N
O
N
Al
O
O
N
DPVBi
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Double Modulation PLDMR (DM-PLDMR)
[M. K. Lee et al., Phys. Rev. Lett. 94, 137403 (2005)
M. Segal et al., Phys. Rev. B 71, 245201 (2005)]
 Modulate the laser power exciting the sample at nlaser.
 Monitor, via output of Lockin amplifier #2,
the PL that is faster than nLaser [PL(nLaserPR)]
 Detect the PLDMR of PL(nLaser) via Lockin #1,
referenced by the microwaves, which are modulated at nmw.
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17
Any contribution to the PL from delayed PL with lifetime t > 1/fL is
filtered out of the output by Lockin #2.
That output is connected to Lockin #1, synchronized to fM = 200 Hz.
As fL increases to 100 kHz, the spin 1/2 PLDMR due to delayed PL of
polaron pairs with t  10 ms should decrease to zero. In contrast, the
PLDMR due to quenching should remain essentially unchanged.
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Note:
DM-PLDMR vs wL = 2pfL ,
which is a measurement in the frequency domain,
is equivalent to
time-resolved PLDMR vs t,
which is a measurement in the time domain.
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2nd Expt: DM-PLDMR of MEH-PPV vs nLaser
Dashed line: Behavior predicted by the delayed PL model.
Behavior predicted by the quenching model is flat, as observed.
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2. New combined thermally-stimulated luminescence (TSL)
& PLDMR study of a PPV derivative
 Note that TSL is due to photogenerated polarons which are trapped at
low temperature, detrapped by warming up, find each other, &
recombine. Some of those which recombine to SEs yield the TSL.
 In other words, the TSL is delayed PL due to nongeminate polaron
recombination –
the very mechanism invoked by Wohlgenannt & Vardeny
as the origin of the positive spin ½ PLDMR & negative spin ½ PADMR.
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Consider
poly[ 2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]
(CzEh-PPV)
O
N
n
Absorbance &PL
(arb. unit)
CzEh-PPV PL &Absorption
1.0
Cz
Absorption
PL ex = 458 nm
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
22
TSL & PLDMR of CzEh-PPV
TSL (arb. units)
3
Eexe = 3.96 eV
Eexe = 3.42 eV
Eexe = 3.06 eV
Eexe = 2.84 eV
2
1
0
0
50
100
150
200
250
Temperature (K)
23
Absorbance & TSL (arb. units)
Note: Rise in TSL is not due to increased absorption
2.0
Absorbance
Integral TSL
1.5
1.0
0.5
0.0
2.5
3.0
3.5
4.0
Energy (eV)
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2E-3
DPL/PL
DPL/PL
1E-3
0E-3
3280
exc =
3320
458 nm
3360
Spin 1/2
3E-4
Spin = 1/2
2E-4
1E-4
0E-4
3400 3280
3320
3360
3400
351 + 363 nm
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4
4
10 (DPL/PL)
3
500 Hz
2 kHz
217 Hz
6.5 kHz
87 Hz
10 kHz
2
1
0
3300
3330
3360
3390
H (Gauss)
UV-excited spin-1/2 PDLMR at different microwave modulation frequencies.
Note the growth of the quenching resonance @
lower microwave chopping frequencies.
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3. PLDMR of Alq3 & DPVBi
N
O
N
Al
O
N
O
10
10.0
20K
100K
150K
250K
10 (DPL/PL)
6.0
4.0
1
5
5
10 (DPL/PL)
8.0
2.0
20K
0.0
3280
3300
3320
3340
3360
3380
3400
0.1
1
Magnetic Field (G)
10
100
Laser Power (mW)
Behavior similar to positive spin 1/2 PLDMR in polymers – cannot be
due to delayed PL mechanism.
G. Li et al., Phys. Rev. B 69, 165311 (2004).
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N
O
10
Spin 1/2 film
Spin 1/2 powder
N
Al
O
N
O
5
10 (DPL/PL)
8
6
Film
Powder
4
2
0
3280 3300 3320 3340 3360 3380 3400
H (Gauss)
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N
O
N
3.0
2.5
1.2
t = 2.7 ms
2.0
10 (DPL/PL)
1.5
1.0
4
4
10 (DPL/PL)
1.4
Spin 1/2 film
t = 2.7 ms
1.0
0.0
10
Modulation Frequency (Hz)
O
Spin 1/2 Powder
t = 6.1 ms
t = 6.1 ms
0.4
0.0
10
10000
N
0.6
0.2
1000
O
0.8
0.5
100
Al
10000
1000
100
Modulation Frequency (Hz)
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N
O
N
O
N
O
7E-5
2.0
FFpwd Alq 3
HFpwd 20K
6E-5
1.0
DPL/PL
5E-5
0.0
5
10 (DPL/PL)
Al
-1.0
4E-5
3E-5
2E-5
1E-5
0E-5
-2.0
2800
3200
3600
H (Gauss)
4000
1600
1620
1640 1660
H (Gauss)
1680
30
1700
N
O
N
2.5
2.0
1.5
1.0
t = 11 ms
1.4
4
4
10 | DPL/PL |
3.0
1.6
Powder FF FRODMR
t = 11 ms
10 (DPL/PL)
3.5
1.2
N
O
Powder HF FRODMR
t = 5.2 ms
t = 5.2 ms
0.8
0.6
0.0
0.4
100
1000
10000
Modulation Frequency (Hz)
O
1.0
0.5
10
Al
10
10000
1000
100
Modulation Frequency (Hz)
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N
O
N
2.0
7.0
10 (DPL/PL)
Powder
Film
5
3.0
5
10 (DPL/PL)
5.0
4.0
2.0
-1.0
-2.0
N
O
Dm s = 2 powder
6.0
0.0
O
Dm s = 2 film
FFpwd (film)
1.0
Al
1.0
2800
3200
3600
H (Gauss)
4000
0.0
1600
1620
1640 1660
H (Gauss)
1680
32
1700
ITO/ TPD/Alq3/buffer/Al
ELDMR
EDMR
AlOx buffer
CsF buffer
G. Li et al., Phys. Rev. B 69, 165311 (2004); Phys. Rev. B 71, 235211 (2005).
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Summary
ODMR is a powerful tool to study the dynamics of polarons, bipolarons,
trions, TEs, and SEs in p-conjugated materials & OLEDs.
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