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Nature of Non-emissive Black Spots in Polymer LEDs

Nature of Non-emissive Black Spots in Polymer LEDs. Ji-Seon Kim, Peter K. H. Ho, Craig E. Murphy, Nicholas Baynes, and Richard H. Friend Reviewed by Joung-Mo Kang for 6.977, Spring 2002. The Phenomenon Observed The Great Organics Plague. S. H. Kim et al Synthetic Metals 111-112 (2000) 254.

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Nature of Non-emissive Black Spots in Polymer LEDs

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  1. Nature of Non-emissive Black Spots in Polymer LEDs Ji-Seon Kim, Peter K. H. Ho, Craig E. Murphy, Nicholas Baynes, and Richard H. Friend Reviewed by Joung-Mo Kang for 6.977, Spring 2002

  2. The Phenomenon ObservedThe Great Organics Plague S. H. Kim et al Synthetic Metals 111-112 (2000) 254 McElvain et al. J. Appl. Phys., Vol. 80, No. 10, 15 Nov 1996 6004

  3. ExperimentTest PLED materials • poly(4-styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) = PEDOT:PSS • poly(2,7-(9,9-di-n-octylfluorene-alt-benzothiadiazole)) = F8BT • poly(2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene)) = TFB

  4. ExperimentDevice Structure Al – 400nm Ca – 5nm 50:50 F8BT:TFB – 80nm 7% PEDOT in PSSH – 50nm ITO – substrate • Eight 16mm² LEDs fabricated on patterned ITO substrate • Encapsulated with a cover glass and epoxy resin • Emit yellow-green • Low drive voltage, high current density (>100mA/cm², 3V) • High power efficiency (>20lm/W) • Lifetime exceeds 5000h at 100 cd/m²

  5. ExperimentDevice Characteristics and Experimental Conditions Devices were driven in ambient atmosphere at room temp for 120h with J = 100 mA/cm² and initial brightness L = ~104 cd/m² Top left figure is an optical picture taken in reflected light. Two ~2 mm wide pinholes + disks are visible in each of the glass and ITO areas of substrate. Bottom shows same device turned on. The term “black spots” describes this dark patch in the yellow-green EL emission.

  6. AnalysisIntroduction to Raman Scattering (extremely abridged) Raleigh ScatterRaman Scatter Raleigh wavelength same as incident, Raman wavelength is different • For a given monochromatic incident beam, there will be many frequencies of Raman-scattered light • The difference in energy of the incident and scattered light is the Raman shift, and is associated with some coupled molecular vibrational mode • A Raman spectrum depends on the molecule and its environment, however: • The Raman shifts are independent of the frequency of the exciting light

  7. AnalysisAdvantages of Raman Spectroscopy • Non-destructive • Can detect beyond glass/ITO layers at appropriate frequencies • Can tune excitation frequency for greater response to molecules or structures of interest • 10x greater spatial resolution than FTIR (~0.5 mm at l = 633 nm vs ~5 mm at l = 4-10 mm) • Shifts can indicate conjugation length changes

  8. DataRaman Spectra

  9. DataInterpretation • Away from defect, spectra indicate a combination of polymer blend and doped PEDOT as expected • Within defect, PEDOT becomes “dedoped” (reduced) • Emissive polymers appear not to migrate or to suffer damage • Metal oxide formation within disc, outside of pinhole • Dedoping method is passive: defects formed over glass where no current was injected

  10. DiscussionProposed Mechanism

  11. DiscussionSo What Does It All Mean? • Non-emissive discs of reduced PEDOT and metal oxide form around pinhole defects in the cathode • Each half of this redox reaction produces a non-conducting material, cutting off local current density • Thus black spots reduce device active area and total luminescence output, but not EL efficiency • The drop in efficiency that is observed is due to other mechanisms such as interfacial degredation

  12. ComparisonWhere This Paper Fits Into the Current Canon • It is widely agreed that pinhole defects source a disc-shaped black spot in many organic devices, and that these defects are only formed during manufacture • Many papers found oxidation of the metal at organic interfaces causing loss of EL, or that spots are caused by a lack of carrier injection rather than quenching • One other paper agrees that loss of luminescence is intrinsic to device and independent of black spots • Several theories were specifically refuted as well, such as the dependence of black spot formation on carrier injection or conjugation length changes

  13. ComparisonSome Other (Possible) Degradation Defects • Gas evolution, metal bubbles • Bright-ringed, non-circular black spots • “Self-healing” point defects • Crystallization of organics

  14. CriticismInquiring Minds Want to Know • What happens without a low work function, positively charged dopant like PEDOT? • What about the many findings of water and oxygen oxidizing metal interfaces on their own? • Time-varying data?

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