A Wet Way to Grow Flexible Electronics
UV radiation oxidizes polymer sheets in preparation for CdS bath.
Although modern electronics often make people move to a beat, the devices themselves can’t budge because they aren’t flexible. There is considerable interest in putting electronics and optoelectronics onto thin plastic substrates, which would make such devices more flexible, lighter and, potentially, less costly.
To develop a less expensive way to assemble a semiconductor device on plastic, researchers exposed a polymer substrate to UV radiation, creating native and photo-oxidized (COOH-terminated) regions. When placed in a cadmium and sulfur precursor solution (b), the semiconductor CdS forms where the UV light was blocked (c). Indium/gold contact points are laid atop the resulting CdS films (d) to form an array of photodetector devices. Images reprinted with permission from the Journal of the American Chemical Society.
One problem has been how to get semiconductors onto polymer sheets. Now a team from the University of Wisconsin-Madison has demonstrated the direct assembly and fabrication of arrays of CdS semiconductor photodetectors on transparent and flexible plastic sheets.
Song Jin, assistant professor of chemistry, said the group’s approach avoids the expensive and wasteful processing techniques traditionally used to create semiconductor devices. “We have developed a strategy capable of depositing just the right amount of functional material into predefined arrays under low-temperature aqueous conditions.”
This image depicts a close-up of arrays of polycrystalline CdS films on polymer substrates. Top left and right (a and b) are optical images of two arrays of different sizes, while (c) is a scanning electron micrograph of an individual film patch. The arrows point to precipitates of CdS that form in the solution and that randomly deposit. The surface-nucleated film is dense and continuous, as seen in a higher magnification of the same patch (d).
Jin added that the methods typically used for semiconductor fabrication involve both high-temperature and high-vacuum processing, neither of which is compatible with flexible polymers. Both also require specialized and expensive equipment. Moreover, contemporary semiconductor device production is a top-down affair, with material deposited in one step and then removed in a photolithographic patterning and etching operation. Therefore, much of the material deposited is abraded and wasted.
In contrast, the method developed by Jin’s group exploits surface chemistry and wet processing. Jin noted that the technique was inspired by — and in some ways mimics — natural biomineralization processes.
In a demonstration of the concept, the researchers used a photomask and a 254-nm UV source from Spectronics Corp. of Westbury, N.Y., to selectively photo-oxidize the surface of a 0.2-mm-thick sheet of poly(ethylene tere-phthalate). The UV radiation created carboxylic acid groups on the surface of the exposed polymer. They used various photomasks to create unexposed areas that ranged from 4 to 150 μm on a side. These arrays repeated every 12 μm for the smallest squares and every 300 μm for the largest.
The upper left images (a and b) show the CdS photodetector array and a single photodetector, respectively, on the transparent flexible polymer substrate. The dark currents and photocurrents of the device under different illumination (c), as well as its response when tested with light pulses (schematic, d, and results, e), reveal that the devices detect pulses from a light source.
They then placed the photo-oxidized films upside down in an aqueous growth solution containing precursors of cadmium and sulfur ions. They kept the solution at temperatures between 50 and 95 °C, changing the solution as needed to minimize unwanted precipitates.
Polycrystalline CdS formed on the polymer film wherever the carboxylic acid groups were not present. The resulting semiconductor films were dense, compact and continuous and were well-adhered to the polymer. The investigators measured the films using an atomic force microscope from Veeco Instruments Inc. of Woodbury, N.Y., and found them to be approximately 150 nm thick.
In a final step, they made photodetectors from the CdS/polymer film by depositing a pattern of In/Au to create contacts. They tested the performance of the detectors using an argon-ion laser from Melles Griot of Carlsbad, Calif. (now CVI Melles Griot). They chopped the 514-nm beam at 90 Hz and showed that the photodetectors responded to the on-and-off light. The films had a detectivity roughly two orders of magnitude less than that of CdS devices manufactured using high-temperature and high-vacuum steps.
However, Jin noted that materials processed the traditional way generally exhibit better performance than ones processed at low temperatures and in aqueous solution, but that the detectivity of his group’s devices is sufficient for many applications and offers advantages in scalability, generality, simplicity and cost. “Our fabricated CdS photodetector does not require expensive machinery during synthesis, which is attractive in developing low-cost, large-area devices,” he said.
The researchers are looking into improving the performance of the devices by changing the synthesis parameters, the materials used and the operating conditions.
Journal of the American Chemical Society, Nov. 21, 2007, pp. 14296-14302.
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