“The [new] method lends itself to low-cost manufacturing, even for large-area devices,” says Shawn Williams, vice president of technology at Plextronics, a company that makes conductive polymer inks for solar cells and LEDs.
The Siemens researchers have “looked at a very low-cost deposition technique, and not only can they make it work, but it actually works better than when made other ways,” says Jackson. The spray-coated photodiodes are more efficient than organic photodiodes made using the other techniques. This is because thicker layers can be made without disrupting the nanoscale structure of the polymer interface, which is crucial. The Siemens system has a quantum efficiency of about 75 percent; in other words, for every 100 photons that hit the diode, 75 will be registered. “That’s pretty good,” says Jackson.
Replacing silicon with polymers might have other advantages. The Siemens group has so far been using heavy, brittle glass coated with indium tin oxide as the substrate, but the photodiodes could be printed onto flexible plastic backings, making possible new imagers that are shaped to fit a particular part of the body. “The spray-coating can be performed on pretty much any substrate,” says Tedde.
Lightweight, large-area, flexible x-ray imagers “would be a really nice gadget,” says Richard Lanza, a senior research scientist in nuclear science and engineering at MIT, who develops high-resolution x-ray systems. In the case of mammograms, breasts must be compressed to conform to the flat, rigid imaging panels. Conformable organic photodiodes might make such procedures far more comfortable.