Researchers at General Electric (GE; Niskayuna, NY) have been developing blue-emitting organic light-emitting diodes (OLEDs) coated with white-emitting phosphors as a candidate general-illumination source for the potentially large solid-state-lighting market (see Laser Focus World, August 2002, p. 22). But, as with every effort to transform a laboratory device into a practical consumer product, technical hurdles must be overcome. One obstacle to OLED light sources—poor scalability to large sizes—has just been surmounted by the same group of GE researchers.
As OLED devices are made larger in area, the number of local defects—which can cause short circuits—increases. Defects can be caused by contamination during fabrication, electrode flaws, or variations in organic-layer thickness. Sometimes, a short circuit in an OLED will simply disappear, leaving a small but harmless nonemissive area. Other times, however, the short circuit will remain, causing the whole device to fail. In addition, large-area OLEDs must contend with high electrical resistivity.
To solve these problems, the GE researchers divided a large-area OLED into many small emitting elements and electrically connected them monolithically in series. In this geometry, a catastrophic short circuit affects only one element rather than the entire device; also, the resistivity ceases to be a problem for small elements. A blue-emitting series-connected OLED array can be coated with white phosphor to create a large-area flat-panel illumination source, as seen here.
To test tolerance to flaws, the researchers fabricated 12-element devices with individual element areas of 1.2 cm2 on 15.2 × 15.2-cm substrates and were not overly careful in their manufacturing process, so as to produce defects. Many of the substrates had at least one nonemissive element, indicating the presence of short circuits (but no completely nonemissive arrays, indicating that open-circuit defects are not a problem). The intact elements grew slightly brighter because of an increased voltage drop stemming from the short circuits, with the result that the overall light output remained approximately the same.
REFERENCE
- A. R. Duggal et al., Appl. Phys. Lett. (April 21, 2003).