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Optically Detected Magnetic Resonance (ODMR)
and its Application to
-Conjugated Materials and
Organic Light-Emitting Devices (OLEDs)
Joseph Shinar
March 30, 2009
ODMR in One Sentence:
Monitor wave-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)
2
Similarly, Electrically Detected Magnetic
Resonance (EDMR) in One Sentence:
Monitor wave-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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4
Now consider basic electronic processes in an organic semiconductor, i.e., a -conjugated material.
GROUND STATE
P2
P1
absorption knr kr (PL 0-0 )
ISC(a) 11Bu
m3Ag
13Bu
TRIPLET SINGLET POLARON(EXCITON) (EXCITON) MANIFOLDMANIFOLD MANIFOLDEXCITONS EXCITONS
p+ p-T
Charge Transfer
Phosphorescence0-0
(a)Intersystem Crossing
11Ag
m1Ag
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A Typical PLDMR Spectrometer:
MICROWAVE MODULATION
LOCK INDATA AQUISITION
DETECTOR (Si)
CRYOSTAT (He) 10 K - 300 K
MICROWAVE CAVITY
MAGNET CONTROL
Ar+ LASER (351 nm - 515 nm)
PL
MICROWAVE MODULATIONMICROWAVE MODULATION
LOCK INDATA AQUISITION
DETECTOR (Si)
CRYOSTAT (He) 10 K - 300 K
MICROWAVE CAVITY
MAGNET CONTROL
Ar+ LASER (351 nm - 515 nm)
PL
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The positive (PL-enhancing) spin 1/2 polaron pair PLDMR at g = 2
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).
S
R
n
7
The full-field (m = 1) triplet powder-pattern
PLDMR.
S
R
n
8
And the half-field (m = 2) triplet powder-pattern PLDMR…
S
R
n
9
Similar Polaron pair PLDMR at g = 2 of m-LPPP and PHP
PL/PL of • Photo-oxidized m-LPPP
IPL/IPL = 1.4 x 10-3
• m-LPPP
IPL/IPL = 3.3 x 10-4
• PHP
IPL/IPL = 8 x 10-5 E. J. W. List et al., Appl. Phys. Lett. 76, 2083 (2000).
R
R'
C6H13
C6H13R
R'
n
10
PADMR of m-LPPP films [scan probe energy at constant
magnetic field; monitor microwave induced changes in the
photoinduced absorption (PA)].
R
R'
C6H13
C6H13R
R'
n
11
1st Expt: mw-dependence of Regular PLDMR of MEH-PPV
P
L/P
L
0.01 0.1 1 100.0
2.0x10-4
4.0x10-4
6.0x10-4
8.0x10-4
1.0x10-3
3.28 3.30 3.32 3.34 3.36 3.38
0.0
2.0x10-4
4.0x10-4
6.0x10-4
8.0x10-4
1.0x10-3
PL
/PL
Magnetic Field(k gauss)
fM (kHz)
Single modulation
PLDMR PL/PL vs
the microwave
modulation frequency
fM.
The dashed line is a
single lifetime fit w/
= 38 s;
the solid line is a two-
lifetime fit w/
1= 24 s, 2= 244 s.
12
Note:
negative carrier electron (e-) negative polaron (p-) radical anion
positive carrier hole (h+) positive polaron (p-) radical cation
13
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.
14
Bimolecular nonradiative quenching processes
Quenching of SEs by TEs and by polarons
Quenching of TEs by polarons.
15
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)
N
OAl N
ON
O
Alq3
DPVBi
16
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 laser.
Monitor, via output of Lockin amplifier #2,
the PL that is faster than Laser [PL(LaserPR)]
Detect the PLDMR of PL(Laser) via Lockin #1,
referenced by the microwaves, which are modulated at w.
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Any contribution to the PL from delayed PL with lifetime > 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 10 s should decrease to zero. In contrast, the
PLDMR due to quenching should remain essentially unchanged.
19
Note:
DM-PLDMR vs L = 2fL ,
which is a measurement in the frequency domain,
is equivalent to
time-resolved PLDMR vs t,
which is a measurement in the time domain.
20
2nd Expt: DM-PLDMR of MEH-PPV vs Laser
Dashed line: Behavior predicted by the delayed PL model.
Behavior predicted by the quenching model is flat, as observed.
21
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.
22
Consider
poly[ 2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-
phenylenevinylene]
(CzEh-PPV)
O
N n
300 400 500 600 700 800
0.0
0.5
1.0Cz
CzEh-PPV PL & Absorption
Absorption PL
ex = 458 nm
Abs
orba
nce
& P
L (a
rb. u
nit)
Wavelength (nm)
23
TSL & PLDMR of CzEh-PPV
0 50 100 150 200 2500
1
2
3
TSL
(arb
. uni
ts)
Temperature (K)
Eexe = 3.96 eV Eexe = 3.42 eV Eexe = 3.06 eV Eexe = 2.84 eV
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2.5 3.0 3.5 4.00.0
0.5
1.0
1.5
2.0
Ab
sorb
ance
& T
SL
(ar
b. u
nit
s)
Energy (eV)
Absorbance
Integral TSL
Note: Rise in TSL is not due to increased absorption
25
3280 3320 3360 34000E-3
1E-3
2E-3
Spin = 1/2
PL/
PL
3280 3320 3360 3400
0E-4
1E-4
2E-4
3E-4
Spin 1/2
PL/
PLexc = 458 nm 351 + 363 nm
26
3300 3330 3360 33900
1
2
3
4
6.5 kHz
10 kHz
2 kHz500 Hz
217 Hz
87 Hz
104 (
PL
/PL
)
H (Gauss)
UV-excited spin-1/2 PDLMR at different microwave modulation frequencies. Note the growth of the quenching resonance @ lower microwave chopping frequencies.
27
3280 3300 3320 3340 3360 3380 3400
0.0
2.0
4.0
6.0
8.0
10.0
105 (
PL
/PL
)
Magnetic Field (G)
20K 100K 150K 250K
1 10 1000.1
1
10
105 (
PL
/PL
)
Laser Power (mW)
20K
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).
3. PLDMR of Alq3 & DPVBi N
OAl N
ON
O
28
3280 3300 3320 3340 3360 3380 3400
0
2
4
6
8
10 Spin 1/2 film Spin 1/2 powder
Powder
Film
10
5 (P
L/P
L)
H (Gauss)
N
OAl N
ON
O
29
10 100 1000 100000.0
0.5
1.0
1.5
2.0
2.5
3.0
= 2.7 ms
104 (
PL
/PL)
Modulation Frequency (Hz)
Spin 1/2 film
= 2.7 ms
N
OAl N
ON
O
10 100 1000 100000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
= 6.1 ms
Spin 1/2 Powder = 6.1 ms
104 (
PL/
PL
)
Modulation Frequency (Hz)
30
2800 3200 3600 4000-2.0
-1.0
0.0
1.0
2.0 FFpwd Alq3
105 (
PL/
PL)
H (Gauss)
N
OAl N
ON
O
1600 1620 1640 1660 1680 1700
0E-5
1E-5
2E-5
3E-5
4E-5
5E-5
6E-5
7E-5
HFpwd 20K
PL
/PL
H (Gauss)
31
10 100 1000 10000
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
= 11 ms
Powder FF FRODMR
= 11 ms
104|
PL
/PL
|
Modulation Frequency (Hz)
N
OAl N
ON
O
10 100 1000 10000
0.4
0.6
0.8
1.0
1.2
1.4
1.6
= 5.2 ms
Powder HF FRODMR
= 5.2 ms
104 (
PL/
PL
)
Modulation Frequency (Hz)
32
2800 3200 3600 4000-2.0
-1.0
0.0
1.0
2.0 FFpwd (film)
105 (
PL/
PL)
H (Gauss)
N
OAl N
ON
O
1600 1620 1640 1660 1680 17000.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0 ms = 2 film
ms = 2 powder
Powder
Film
105 (
PL
/PL
)
H (Gauss)
33
ELDMR EDMR
ITO/ TPD/Alq3/buffer/Al
AlOx buffer
CsF buffer
G. Li et al., Phys. Rev. B 69, 165311 (2004); Phys. Rev. B 71, 235211 (2005).
34
Summary
ODMR is a powerful tool to study the dynamics of polarons, bipolarons,
trions, TEs, and SEs in -conjugated materials & OLEDs.
