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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. 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

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
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


Experiment test pled materials
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


Experiment device structure
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


Experiment device characteristics and experimental conditions
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.


Analysis introduction to raman scattering extremely abridged
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


Analysis advantages of raman spectroscopy
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


Data raman spectra
DataRaman Spectra


Data interpretation
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


Discussion proposed mechanism
DiscussionProposed Mechanism


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


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


ComparisonSome Other (Possible) Degradation Defects

  • Gas evolution, metal bubbles

  • Bright-ringed, non-circular black spots

  • “Self-healing” point defects

  • Crystallization of organics


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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