{"id":7310,"date":"2012-12-14T09:47:33","date_gmt":"2012-12-14T16:47:33","guid":{"rendered":"http:\/\/cafe.foundation\/blog\/?p=7310"},"modified":"2012-12-28T23:33:55","modified_gmt":"2012-12-29T06:33:55","slug":"princeton-solar-cell-is-black-hole-for-light","status":"publish","type":"post","link":"http:\/\/cafe.foundation\/blog\/princeton-solar-cell-is-black-hole-for-light\/","title":{"rendered":"Princeton Solar Cell is \u201cBlack Hole for Light\u201d"},"content":{"rendered":"

A great deal of the light that falls on solar cell panels does little to generate electricity, with a high percentage bouncing off pointlessly.\u00a0 Princeton researchers have confronted this issue with a layered assembly, otherwise known as a subwavelength \u00a0plasmonic cavity.<\/p>\n

Developed by Princeton University researcher Stephen Chou and a team of scientists, the cavity dampens reflections and traps light.\u00a0 According to Princeton\u2019s announcement<\/a>, \u201cThe new technique allowed Chou’s team to create a solar cell that only reflects about 4 percent of light and absorbs as much as 96 percent. It demonstrates 52 percent higher efficiency in converting [direct] light to electrical energy than a conventional solar cell.\u201d<\/p>\n

Overall, the team was able to increase solar cell efficiency a total of 175 percent with their nanostructured sandwich by capturing not only direct sunlight, but\u00a0angled rays and diffuse light that occurs on cloudy days.\u00a0 MIT researchers recently reported attempts to gather varying wavelengths <\/a>of light to effect the same type of increase in efficiency.<\/p>\n

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Princeton’s PlaCSH layered solar cell sandwich<\/p><\/div>\n

Chou\u2019s solar cells resemble a sandwich, with a top layer of an ultra-fine metal mesh, 30 nanometers thick, with each hole 175 nanometers in diameter and 25 nanometers apart.\u00a0 The Princeton announcement explains that, \u201cA nanometer is a billionth of a meter and about one hundred-thousandth the width of human hair.\u201d\u00a0 Usually, solar cells have a top, or window layer of materials such as indium-tin-oxide (ITO).<\/p>\n

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175 nM holes, 25 nM spacing in PlaCSH mesh top layer<\/p><\/div>\n

Subsequent layers are more conventional, the second composed of a thin strip of semi-conducting material, and the bottom layer a metal film typically used in solar panels. \u201cIt can be any type \u2014 silicon, plastic or gallium arsenide \u2014 although Chou’s team used an 85-nanometer-thick plastic.\u201d<\/p>\n

Mesh spacing, sandwich thickness, and the size of the nano-holes are all smaller than the wavelength of the light being collected \u2013 the subwavelength part of the cell\u2019s description. Chou, the Joseph C. Elgin Professor of Engineering, describes\u00a0 these unbelievably small spaces as miniature black holes, trapping the light and apparently pulling all the energy possible from it.<\/p>\n

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Reflection from conventional solar cells, absorption of light in PlaCSH cells<\/p><\/div>\n

This manifests itself in the black surface of the cells. \u201cThe team calls the system a “plasmonic cavity with subwavelength hole array” or PlaCSH.\u00a0Photos\u00a0 of the surface of the PlaCSH solar cells demonstrate this light-absorbing effect: under sunlight, a standard solar power cell looks tinted in color due to light reflecting from its surface, but the PlaCSH looks deep black because of the extremely low light reflection.\u201d<\/p>\n

Perhaps best of all, Chou says that PlaCSH solar cells can be manufactured in cost-effective wallpaper-size sheets, using \u201cnanoimprint,\u201d an imprinting technique Chou invented 16 years ago.\u00a0 Such sheets could be printed on plastic and give a new high-efficiency mark to so-called \u201corganic\u201d solar cells.\u00a0 Since the PlaCSH does not use the ITO electrode, it dispenses with the most expensive part of most solar cells, and one of the most brittle elements of traditional cell design.\u00a0 Chou\u2019s cells can bend easily, making them good candidates for use on wings or other vehicle components.<\/p>\n

Chou\u2019s lab has also used plasmonic nanodots to improve immunoassays and medical diagnoses<\/a>, and to boost sensitivity of nano-antennas <\/a>for detection of chemical and biological agents.<\/p>\n

Chou and electrical engineering graduate student Wei Ding reported their findings in the journal\u00a0Optics Express<\/em><\/a>.\u00a0 <\/strong>Their work was supported in part by the Defense Advanced Research Projects Agency, the Office of Naval Research and the National Science Foundation.<\/p>\n

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