Download 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
McElvain et al. J. Appl. Phys.,
Vol. 80, No. 10, 15 Nov 1996
6004
Experiment
Test 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-secbutylphenyl)imino)-1,4-phenylene)) = TFB
Experiment
Device 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
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)
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
Analysis
Advantages 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
Data
Interpretation
1. Away from defect, spectra indicate a combination of
polymer blend and doped PEDOT as expected
2. Within defect, PEDOT becomes “dedoped” (reduced)
3. Emissive polymers appear not to migrate or to suffer
damage
4. Metal oxide formation within disc, outside of pinhole
5. Dedoping method is passive: defects formed over
glass where no current was injected
Discussion
Proposed Mechanism
Discussion
So 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 nonconducting 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
Comparison
Where This Paper Fits Into the Current Canon
•
It is widely agreed that pinhole defects source a discshaped 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
Comparison
Some Other (Possible) Degradation Defects
•
Gas evolution, metal bubbles
•
Bright-ringed, non-circular black spots
•
“Self-healing” point defects
•
Crystallization of organics
Criticism
Inquiring 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?