Patterning without a mask
Optical/electronic materials
March 20, 2008
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| OVJP of an array of segmented OLEDs for white light illumination. (Credit: Michael S. Arnold, University of Michigan.) |
Researchers from the University of Michigan and Princeton University have fabricated white organic light-emitting devices (WOLEDs) using a new technique that allows printing of spatially separate red, green, and blue components [Arnold et al., Appl. Phys. Lett. (2008) 92, 053301].
The low cost, high volume manufacture, color quality, high luminous efficiency, and compatibility with lightweight, large-area, flexible substrates of WOLEDs makes them an attractive proposition for solid-state lighting.
WOLEDs can comprise multiple, vertically stacked, emissive layers, single emissive layers with multiple dopants, or spatially separated red, blue, and green emissive components. The final approach can extend the useful lifetime of devices by allowing for separate optimization, color tuning, and the differential compensation of each element as they age. However, this approach requires patterning of the device structure, which can be costly.
Various techniques have been suggested to reduce the cost of WOLED patterning. One of these, organic vapor jet printing (OVJP), provides a simple and efficient means of patterning parallel stripes of red, blue, and green electrophosphorescent OLEDs down to a resolution of 10 µm.
“Our novel approach to printing organic semiconductors is to eliminate the solvent completely,” explains Michael S. Arnold of the University of Michigan. In OVJP, the organic source is heated to produce a vapor, which is then transported to a small heated nozzle, where it is collimated to allow controlled deposition on a cooled substrate. “Spatial patterning is controlled by translating the substrate underneath the nozzle and the resolution is controlled by varying the diameter of the nozzle opening,” notes Arnold.
By vaporizing the organic source, the problems of solvent compatibility, solubility, surface tension, and drop size are avoided. “Any pattern can be generated without the use of a mask,” says Arnold.
The researchers used OVJP to co-deposit multiple organic materials for segmented OLEDs. The devices fabricated in this way show properties commensurate with devices grown using the current ‘gold-standard’ technique, vacuum thermal evaporation.
“[This] is a really nice method for printing small organic molecules,” says Zhenan Bao of Stanford University. “It allows patterning and deposition in one step.”
In the future, the researchers plan to scale up the technique using arrays of multiple nozzles. “We estimate that we could print a 10 cm x 10 cm WOLED in as little as 1.5 s,” Arnold told Materials Today.
Cordelia Sealy