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Optically Detected Magnetic Resonance (ODMR) and its Application to p -Conjugated Materials and Organic Light-Emittin PowerPoint Presentation
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Optically Detected Magnetic Resonance (ODMR) and its Application to p -Conjugated Materials and Organic Light-Emitting Devices (OLEDs) Joseph Shinar March 30, 2009 shinar@ameslab.gov.

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slide1

Optically Detected Magnetic Resonance (ODMR)

and its Application to

p-Conjugated Materials and

Organic Light-Emitting Devices (OLEDs)

Joseph Shinar

March 30, 2009

shinar@ameslab.gov

slide2

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)

slide3

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)

now consider basic electronic processes in an organic semiconductor i e a p conjugated material
Now consider basic electronic processes in an organic semiconductor, i.e., a p-conjugated material.

TRIPLET SINGLET POLARON

(EXCITON) (EXCITON) MANIFOLD

MANIFOLD MANIFOLD

EXCITONS EXCITONS

m1Ag

m3Ag

13Bu

Charge Transfer

p+p-

T

ISC(a)

11Bu

P2

P1

absorptionknrkr (PL 0-0 )

Phosphorescence0-0

11Ag

GROUND STATE

(a)Intersystem Crossing

the positive pl enhancing spin 1 2 polaron pair pldmr at g 2
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).

similar polaron pair pldmr at g 2 of m lppp and php
Similar Polaron pair PLDMR at g = 2 of m-LPPP and PHP

DIPL/IPL of

  • Photo-oxidized m-LPPP

DIPL/IPL = 1.4 x 10-3

  • m-LPPP

DIPL/IPL = 3.3 x 10-4

  • PHP

DIPL/IPL = 8 x 10-5

E. J. W. List et al., Appl. Phys. Lett. 76, 2083 (2000).

slide10

PADMR of m-LPPP films [scan probe energy at constant magnetic field; monitor microwave induced changes in the photoinduced absorption (PA)].

1 st expt n mw dependence of regular pldmr of meh ppv
1st Expt: nmw-dependence of Regular PLDMR of MEH-PPV

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 two-lifetime fit w/

t1= 24 ms, t2= 244 ms.

slide12

Note: negative carrier  electron (e-)  negative polaron (p-) radical anionpositive carrier  hole (h+)  positive polaron (p-)  radical cation

slide13

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.
slide14

Bimolecular nonradiative quenching processes

  • Quenching of SEs by TEs and by polarons
  • Quenching of TEs by polarons.
slide15
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

DPVBi

slide16

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.

slide18
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.

slide19
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.

2 nd expt dm pldmr of meh ppv vs n laser
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.

2 new combined thermally stimulated luminescence tsl pldmr study of a ppv derivative
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.

consider poly 2 n carbazolyl 5 2 ethylhexyloxy 1 4 phenylenevinylene czeh ppv
Consider poly[ 2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylenevinylene](CzEh-PPV)
slide26

UV-excited spin-1/2 PDLMR at different microwave modulation frequencies.

Note the growth of the quenching resonance @

lower microwave chopping frequencies.

slide27

3. PLDMR of Alq3 & DPVBi

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).

slide33

ITO/ TPD/Alq3/buffer/Al

EDMR

ELDMR

AlOx buffer

CsF buffer

G. Li et al., Phys. Rev. B 69, 165311(2004); Phys. Rev. B 71, 235211 (2005).

summary
Summary

ODMR is a powerful tool to study the dynamics of polarons, bipolarons, trions, TEs, and SEs in p-conjugated materials & OLEDs.