Researchers at Siemens have discovered a way to print polymer x-ray-sensing panels that work just as well as expensive silicon ones. Using a new printing method, which is similar to the way that cheap plastic solar cells are made, the researchers believe that the approach could bring down the cost of medical imaging systems and be used to make lightweight, flexible imaging panels for procedures such as more comfortable mammograms.
Electrically active polymers hold potential as a cheap alternative to silicon for devices including light sensors, solar cells, and transistors. Polymers can be processed in less stringent conditions–at room temperature and in the open air. However, their performance for all these applications is as yet unproven, says Thomas Jackson, a professor of electrical engineering at Penn State, who has no ties to Siemens. While polymer-based photodiodes have been shown to work well for solar cells, the value of using polymer materials for imaging hasn’t been clear. The new high-performance Siemens light detectors should change that, however.
The photodiodes, developed by Siemens researchers led by Sandro Tedde and Oliver Hayden, work as well as those made of silicon. The researchers describe the manufacturing technique used to make them in the March issue of the journal Nano Letters, and presented organic photodiodes designed for x-ray detection at a meeting of the Materials Research Society. Tedde says that the detectors are stable for at least six years.
The Siemens researchers make their photodiodes by spraying water-based solutions containing two kinds of polymers through a metal mask onto a glass substrate. They put down, first, several layers of a polymer with low conductivity, then several layers of a polymer with high conductivity.
The use of two different polymers is crucial. When a photon hits the polymer photodiode, it excites an electron, leaving a positive “hole” behind; to read the resulting electrical signal, the diode has to carry the electron away from the hole. The interface between the two layers of polymers helps this separation to occur: the low-conductivity polymer carries the positive holes, while the other carries the electron to an electrical contact where it can be read.
The spray-coating method works well over large areas. Normally, these polymers are spread out across the substrate by spinning or using a small scraping blade. But these techniques don’t work well over large areas, and x-ray imaging requires large panels because x-rays can’t be focused using conventional lenses. “You need the imager to be the same size as the body part you’re trying to image,” says Karim Karim, an assistant professor of electrical engineering at the University of Waterloo, who was not involved in the Siemens work. Indeed, a significant portion of the cost of today’s systems comes from the large silicon panels used to convert photons into the electrical signals: the larger the silicon panel, the more expensive it is.
“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.
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