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Optically Detected Magnetic Resonance (ODMR) and its Application to p -Conjugated Materials and Organic Light-Emittin

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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Optically Detected Magnetic Resonance (ODMR) and its Application to p -Conjugated Materials and Organic Light-Emittin

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  1. 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

  2. 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)

  3. 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)

  4. 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

  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 (Dm = 1) triplet powder-pattern PLDMR.

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

  9. 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).

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

  11. 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.

  12. Note: negative carrier  electron (e-)  negative polaron (p-) radical anionpositive 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) 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 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.

  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.

  18. 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.

  19. 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.

  20. 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.

  21. Consider poly[ 2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylenevinylene](CzEh-PPV)

  22. TSL & PLDMR of CzEh-PPV

  23. Note: Rise in TSL is not due to increased absorption

  24. lexc = 458 nm 351 + 363 nm

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

  26. 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).

  27. 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).

  28. 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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