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Printed Electronics World
Posted on April 30, 2008 by  & 

How to print OLEDs

OLEDs have huge market potential including 10% of the replacement market for lighting meaning 70 square kilometers per year. This is because they will be more energy efficient and have more freedom of form. That was the message of the Holst Research Center of the High Tech Campus of Eindhoven in the Netherlands at the world's largest conference and exhibition on printed electronics in Dresden Germany in April. This was the Printed Electronics Europe event of IDTechEx. It will now be leapfrogged by the sister event Printed Electronics USA in San Jose California being even bigger.
 
The Holst Center is jointly owned by IMEC of Belgium and TNO of the Netherlands - both formidable research centers in their own right. It has used piezo inkjet and gravure printing and will also examine roll to roll slot die coating for the manufacture of OLEDs.
 
 
The HTC already hosts over 50 collaborating companies and it is growing from 5000 to 8000 researchers sharing laboratory facilities (www.miplaza.com). It is one of three large centers of excellence in Printed Electronics in Europe, the other two being Dresden and Cambridge.
 
 
Its roadmaps for lighting and signage are:
 
The inkjet issues are seen as:
 
Generic issues
  • Flexible: customized product / patterns
  • Small series, ideal lab set up, demo's
  • Small ink quantities required for testing
  • Low throughput, not a real R2R process
  • Yield, robustness (fail, clogging)
  • Small process window for material formulation
Holst Centre research issues
  • Go from glass-display-pixel to large-area-foil
  • Wetting/coalescence droplets/lines on substrates
  • Inhomogeneous drying of droplets/lines, coffee stain
  • Print/drying strategies, equipment, process settings
  • Material formulations: Functionality vs processing
  • Functionality material: work function, life time, efficiency
  • compared to spin coated devices
Unfortunately, surfactants are a double edged sword:
  • Better priming of the print-head
  • More chance on nozzle failure (air bubble generation and pumping)
  • Better spreading on the substrate
  • Worse leveling
  • Less 'coffee stain'
  • Less spreading of the LEP on the dry PEDOT
  • Unknown influence on device performance (but generally not positive)
Gravure printing offers different compromises:
  • + Fast, R2R
  • + Patterning
  • Contact, contamination
  • Leveling of cells
Here, the areas being investigated are homogeneity, leveling, modeling of cell output wetting, drying, formulations, process settings and upscaling R2R. So far, the following gravure demonstrators have been prepared using ac electroluminescence rather than OLEDs.
 
The outlook is:
 
 
IDTechEx notes that the IP for PEDOT PSS is held by H.C.Starck and licenced to Agfa for OLEDs. H.C.Starck has a complete range of inket inks using these materials.
Inkjet printing
The Holst Centre has the following research items on ink jet printing of OLEDs: homogeneity, multiple materials, increase speed, increase yield, optimize printing/drying strategies, free standing separate pixels
  • Towards flexible signage (low volumes, production)
  • White light
  • Multiple colours
  • High throughput: feasibility of large area wide format inkjet
  • Sensor materials
Gravure printing
  • Transition from Sheet to Sheet to Roll to Roll
  • Multiple layers, layer homogeneity within 2%
  • Printing over device support structures
Slot die coating
  • Taking part in Fast 2 Light
  • Multiple layers, layer homogeneity within 2%
  • Patterned coating
  • Coating over device support structures
The conclusions were:
  • Inkjet printed devices yield similar performance as spin coated devices and devices printed on glass substrates
Layer (light) homogeneity depends on:
  • PEDOT application (print strategy) and drying
  • LEP application and drying
  • Interaction (wetting behaviour) between PEDOT (dry) and LEP (wet)
First tests show promising results with gravure printing
  • Optimization of layers needs further research
  • Functional devices with printed PEDOT and LEP
Roll to roll line with slot die coater is installed
  • PEDOT layers have been coated
  • Official opening June 19th
 
 
If you are interested in Photovoltaics attend Photovoltaics Beyond Conventional Silicon.

Authored By:

Chairman

Posted on: April 30, 2008

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