Nature of Nonemissive Black Spots in Polymer LEDs

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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

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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
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ExperimentTest PLED materials

• poly(4-styrenesulfonate)-doped poly(3,4-ethylenedi
  oxythiophene) PEDOTPSS
• poly(2,7-(9,9-di-n-octylfluorene-alt-benzothiadiaz
  ole)) F8BT
• poly(2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenyle
  ne-((4-sec-butylphenyl)imino)-1,4-phenylene))
  TFB

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ExperimentDevice Structure
Al 400nm
Ca 5nm
5050 F8BTTFB 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
  (gt100mA/cm², 3V)
• High power efficiency (gt20lm/W)
• Lifetime exceeds 5000h at 100 cd/m²

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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.

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AnalysisIntroduction to Raman Scattering
(extremely abridged)

•     
  Raleigh Scatter
      
  Raman 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

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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

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DataRaman Spectra
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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

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DiscussionProposed Mechanism
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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

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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

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ComparisonSome Other (Possible) Degradation
Defects

• Gas evolution, metal bubbles
• Bright-ringed, non-circular black spots
• Self-healing point defects
• Crystallization of organics

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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?