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512-Pixel Detector Goes Organic

Photonics Spectra
Oct 2001
Kevin Robinson

Soon man-made detectors may take their cue from nature and do their work organically. Researchers have demonstrated an image-capture array based on an organic light sensor, promising manufacturers the ability to tailor the sensitivity of a sensor to a particular application simply by selecting the proper organic material.

Robert Street, senior research fellow at Xerox's Palo Alto Research Center and the lead author of a report of the work that appeared in the June 25 issue of Applied Physics Letters, explained that the researchers at the center and at the company's Research Center of Canada in Mississauga, Ontario, are investigating organic sensors for a broad range of applications, including large-area sensor arrays, displays, smart cards and transistors.

Because the team members had experience making thin-film transistors from amorphous silicon, they decided to use that technology as the base of the sensor. "We wanted to see if the integration with organic materials would not be too difficult," Street said. "In fact, we have developed a design ... that makes the integration rather easy."

Organic materials cannot be patterned as easily as silicon, so the researchers designed a 512 x 512-pixel array in which the sensor region -- benzimidazole perylene (BZP) for charge generation and tetraphenyldiamine (TPD) for hole transport -- lies over the structure. They first fabricated a silicon thin-film array, which features 100 x 100-µm depressions into which the organics can be deposited and act as pixels, and then used evaporation, spin or dip coating to deposit the two layers of organics. A semitransparent layer of gold/platinum coated the organic films and provided the bias for the device, which displayed an external quantum efficiency of 10 percent at 550 nm.

Boosts CMOS sensitivity

The technique offers the designers of a new sensor the ability to tune the detection range by selecting appropriate organic materials. In fact, Street said that the researchers have tested three sensors since reporting the BZP/TPD device: One responds to the visible, one to the visible through the near-infrared and one to the red through the IR. By using a phosphor converter, they also have imaged in the x-ray region.

He added that the organic sensors might coat CMOS devices so that the organic layer rather than the silicon substrate acts as the sensor. This could offer higher sensitivities because the other parts of the CMOS circuit would not have to obscure the organic sensor.

The group plans to continue to optimize the sensors' performance, which is a matter of choosing the appropriate organic coatings. "All the materials we have tried so far are organic small molecules," Street said. "We next plan to try some polymers to find out how they respond." It is easier to deposit polymers than small molecules, he said, and they may offer other advantages.

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