Screen Printing Creates LED Display

Daniel C. McCarthy

Organic LED displays offer better contrast, wider viewing angles and lower power consumption than liquid crystal screens. Their drawback, however -- particularly for those based on conjugated polymers -- is that they are manufactured by the spin-coating method, in which an organic polymer solution is applied to a substrate that is spun rapidly, discarding more than 99 percent of the applied material. Laser ablation removes the remaining film from the displays' electrical contact pads, further increasing the complexity, time and expense of producing them.

However, Siemens AG has proved the feasibility of manufacturing passive matrix devices -- a class of organic LED displays -- using screen printing, a manufacturing method associated more with novelty T-shirts.

"The goal is to print organic [LED] displays with the same characteristics [as] spin-coated devices," said Juliane Braun, a spokeswoman for Osram Opto Semiconductors GmbH & Co. OHG, the company that contracted for the research. Reported in the June 11 issue of Applied Physics Letters, the results could enable a faster manufacturing process with fewer steps and less material consumption.

The researchers used a commercially available screen printer to produce conventional passive matrix display structures: They printed two organic polymer layers between an indium tin oxide anode and a metallic cathode.

The equipment produced printed layers with sufficient homogeneity and finished displays measuring 43 x 26 mm. Each 640 x 640-µm pixel was separated by 30 µm. Screen printing theoretically can be applied to both glass and flexible substrates that measure more than a square meter.

The test diodes produced for the experiment demonstrated peak efficiencies of more than 10 cd/A, onset voltage of less than 3 V and luminance of 10,000 cd/m² at 8 V. All values were comparable to those demonstrated by a set of reference diodes produced by spin coating.

The screen-printed displays' rectification ratio, a value that influences intrapixel crosstalk, was higher than 10³ at ±5 V. This, explained Braun, demonstrates that printed organic LEDs may emerge as a viable alternative to conventional spin-coated devices.

One potential area of improvement is the lateral resolution that helps determine how efficiently pads of different polymers can be laid adjacently. If resolution increases, so can the color separation in multicolor displays. The researchers at Siemens reported that the lateral resolution for its screen-printed diodes ranged between 0.5 and 1 mm, depending on the polymer.