1 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) 3

4 Now consider basic electronic processes in an organic semiconductor, i.e., a  -conjugated material. GROUND STATE P2 P1 absorption k nr k r (PL 0-0 ) ISC (a) 11Bu11Bu m3Ag13Bum3Ag13Bu TRIPLETSINGLET POLARON (EXCITON)(EXCITON) MANIFOLDMANIFOLDEXCITONS p+p-p+p- T Charge Transfer Phosphorescence 0-0 ( a) Intersystem Crossing 11Ag11Ag m1Agm1Ag

5 A Typical PLDMR Spectrometer:

6 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).

7 The full-field (  m =  1) triplet powder-pattern PLDMR.

8 And the half-field (  m =  2) triplet powder-pattern PLDMR…

9 Similar Polaron pair PLDMR at g = 2 of m-LPPP and PHP  PL /  PL of Photo-oxidized m-LPPP  I PL /I PL = 1.4 x m-LPPP  I PL /I PL = 3.3 x PHP  I PL /I PL = 8 x E. J. W. List et al., Appl. Phys. Lett. 76, 2083 (2000).

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

11 1 st Expt: mw -dependence of Regular PLDMR of MEH-PPV Single modulation PLDMR  PL/PL vs the microwave modulation frequency f M. 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 (Alq 3 ) & 4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi) Alq 3 DPVBi

16 Double Modulation PLDMR (DM-PLDMR) [M. K. Lee et al., Phys. Rev. Lett. 94, (2005) M. Segal et al., Phys. Rev. B 71, (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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18 Any contribution to the PL from delayed PL with lifetime  > 1/f L is filtered out of the output by Lockin #2. That output is connected to Lockin #1, synchronized to f M = 200 Hz. As f L 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 = 2  f L, 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 2 nd 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)

23 TSL & PLDMR of CzEh-PPV

24 Note: Rise in TSL is not due to increased absorption

25 exc = 458 nm nm

26 UV-excited spin-1/2 PDLMR at different microwave modulation frequencies. Note the growth of the quenching lower microwave chopping frequencies.

27 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, (2004). 3. PLDMR of Alq 3 & DPVBi

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33 ELDMREDMR ITO/ TPD/Alq 3 /buffer/Al AlOx buffer CsF buffer G. Li et al., Phys. Rev. B 69, (2004); Phys. Rev. B 71, (2005).

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