Light-emitting-diode (LED) displays are typically made by the batch using vacuum deposition chambers, but researchers at Pacific Northwest Laboratory (Richland, WA) recently demonstrated a fast and continuous method for depositing a film of the active layer of small-molecule organic LEDs onto a flexible plastic substrate.
This proof-of-principle experiment used roll-coating, a method developed for plastics, paper, and other roll goods. The light-emitting small-molecule organic dopant was added to a tris-8-(hydroxyquinoline)-aluminum host and then sprayed onto the substrate as a liquid. The process allows the manufacturer to switch liquid sources without stopping the coater.
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The demonstration showed that doped films can be produced by premixing the host and dopant instead of carefully controlling co-evaporation rates. The resulting film showed quantum efficiencies similar to vapor-deposited films. Researcher Paul Burrows reports that spectroscopy shows no residual emission from the host, which is "important for color purity and as a demonstration that we got enough of the dopant in."
If the encapsulate and electrical contact layers can also be deposited by this method, then entire large-area displays could be made very quickly and relatively inexpensively.
Yvonne Carts-Powell is a freelance writer living in Belmont, MA; e-mail: yvonne@nasw.org.
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