{"id":7358,"date":"2012-12-30T15:24:19","date_gmt":"2012-12-30T22:24:19","guid":{"rendered":"http:\/\/cafe.foundation\/blog\/?p=7358"},"modified":"2012-12-30T15:24:19","modified_gmt":"2012-12-30T22:24:19","slug":"a-layer-of-graphene-a-layer-of-nanowires","status":"publish","type":"post","link":"http:\/\/cafe.foundation\/blog\/a-layer-of-graphene-a-layer-of-nanowires\/","title":{"rendered":"A Layer of Graphene, A Layer of Nanowires\u2026"},"content":{"rendered":"

Combine nano-anything with graphene, and that seems to describe most of what\u2019s driving physics and chemistry laboratories at our major universities.\u00a0 The blog reported last week on Princeton researcher<\/a>s\u00a0who’ve\u00a0created a thin, flexible solar cell that absorbs 96-percent of received light and draws energy from off-axis and varied wavelengths of light.<\/p>\n

MIT researchers, too, have created a thin, flexible solar cell, but one based on layers of flexible graphene sheets, each coated with a layer of nanowires.\u00a0 Besides flexibility, these sheets offer transparency, enabling their use on windows as well as other surfaces.<\/p>\n

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Layered structure of the new device, starting with a flexible layer of graphene, a one-atom-thick carbon material. A layer of polymer is bonded to that, and then a layer of zinc-oxide nano wires (shown in magenta), and finally a layer of a material that can extract energy from sunlight, such as quantum dots or a polymer-based material.
Illustration courtesy of the research team<\/p><\/div>\n

David Chandler, reporting for MIT<\/a> states that the new cells may prove to be far less expensive than today\u2019s silicon equivalents, which require high-purity silicon that undergoes crystallization and extremely thin slicing.\u00a0 Alternatives use indium tin oxide (ITO), itself an expensive substitute for or adjunct to silicon.\u00a0 Nanostructured cells such as that from Princeton may allow lower-priced material, although one version uses a gold foil top layer.<\/p>\n

Silvija Grade\u010dak, an associate professor of materials science and engineering, helped write a report on MIT\u2019s nano-graphene cells for the journal Nano Letters, along with MIT post-doctoral students Hyesung Park, Sehoon Chang, and eight other MIT researchers.<\/p>\n

Grade\u010dak \u00a0says, \u201cCurrently, ITO is the material of choice for transparent electrodes\u201d used in smartphone touch screens.\u00a0 Graphene, made from common carbon, should be a budget alternative to ITO.\u00a0 It\u2019s also flexible, light weight, has good mechanical strength and chemical robustness.<\/p>\n

Using the graphene material is not easy, though, because trying to place semiconductor nanostructures on graphene disturbs its electrical and structural properties.\u00a0 MIT researchers coated the graphene with polymers, then bonded a layer of zinc oxide nanowires to that, and over that, a layer of lead-sulfide quantum dots or P3HT polymer \u2013 both of which respond to light waves.<\/p>\n

\u201c\u2019We\u2019ve demonstrated that devices based on graphene have a comparable efficiency to ITO,\u2019 she says \u2014 in the case of the quantum-dot overlay, an overall power conversion efficiency of 4.2 percent \u2014 less than the efficiency of general purpose silicon cells, but competitive for specialized applications. \u2018We\u2019re the first to demonstrate graphene-nanowire solar cells without sacrificing device performance.\u2019\u201d<\/p>\n

Processing can be done at temperatures below 175 degrees Celsius, much lower than silicon solar cell manufacturing.\u00a0 The process is scalable according to Hyesung Park, the report\u2019s fellow lead author.<\/p>\n

MIT explains the process.\u00a0 \u201cThe graphene is synthesized through a process called chemical vapor deposition and then coated with the polymer layers. \u2018The size is not a limiting factor, and graphene can be transferred onto various target substrates such as glass or plastic,\u2019 Park says.<\/p>\n

\u201cGrade\u010dak cautions that while the scalability for solar cells hasn\u2019t been demonstrated yet \u2014 she and her colleagues have only made proof-of-concept devices a half-inch in size \u2014 she doesn\u2019t foresee any obstacles to making larger sizes. \u2018I believe within a couple of years we could see [commercial] devices\u2019 based on this technology, she says.\u201d<\/p>\n

Using graphene as a transparent electrode is not a new idea, but L\u00e1szl\u00f3 Forr\u00f3, a professor at the Ecole Polytechnique F\u00e9d\u00e9rale de Lausanne thinks, \u201cThis work is a real breakthrough,\u201d and \u201cExcellent work in every respect.\u201d<\/p>\n

MIT says, \u201cThe work also involved MIT professors Moungi Bawendi, Mildred Dresselhaus, Vladimir Bulovic and Jing Kong; graduate students Joel Jean and Jayce Cheng; postdoc Paulo Araujo; and affiliate Mingsheng Wang. It was supported by the Eni-MIT Alliance Solar Frontiers Program, and used facilities provided by the MIT Center for Materials Science Engineering, which is supported by the National Science Foundation.\u201d<\/p>\n