Skip to main content

Yeah, I know: every week, somebody somewhere announces some sort of solar cell "breakthrough" that turns out to be ... well, not much. I see them all the time too.

This one is different. Simple. Elegant. Easy to manufacture. And incredibly useful for solar cells of all types, especially thin-film cells. Follow below the arabesque to find out why.

The very first semiconductor ever made was a diode. It's a nifty crystalline device that allows current to flow through in one direction but not in the other direction. Since currently only "half" flows, it's a semi-conductor. (Get it? These guys are great with names.)

The simplest (and first) solar cell ever made was based on a simple diode. Normally a diode is made into a wire-shape. But imagine that you make it flat instead, and put a metal plate on one side, and a wire mesh on the other side, and make sure to orient the diode so that current only flows FROM the plate toward the mesh.

Solar Cell diagram
That's a solar cell. Why? Because when light hits the semiconductor, some electrons are knocked off the semiconductor and land on the wire mesh. Then the semiconductor is short electrons (it's positively charged) while the mesh has extra electrons (it's negatively charged). Now those extra electrons on the mesh want to get back to the positively charged semiconductor, but they can't, because the diode is oriented to prevent the flow of electrons in that direction. Instead, we attach an exit wire to the mesh, run it around to the plate on the back, and the electrons have to take that alternate pathway to get back to the semiconductor. And they do work along the way!

Now that's great, but there are a bunch of issues that make things less than ideal. First off, semiconductor material started out being only expensive stuff like silicon and gallium. Lately we've invented plastics that work as semiconductors, but their efficiency is lower than silicon. Also, you can use less semi-conductor by making it thinner (a so-called "thin film" cell), but you reach a conventional limit to thinness when you get down to the wavelength of light itself.

Sizing the mesh is tricky, because you'll capture more electrons with more mesh, but then that blocks more of the incoming light too. Some solar cells (especially thin film cells) get around that by using a transparent conductor called Indium-Tin-Oxide (ITO) instead of the mesh, but that's expensive stuff too.

And then some light just reflects off the surface of the cell and doesn't knock off any electrons. That's particularly true when the light comes in at sharp angles, which most of the light does on cloudy days.

In fact, a really good (and really cheap) thin-film solar cell is the holy grail of photovoltaics. Right now we can make thin film cells, but their cost per Watt is greater than conventional silicon, and the ITO layer is a big part of that. Plus the efficiency is low. So what if we could eliminate the ITO layer and boost efficiency at the same time? What if we could make PV cells in a continuous roll-to-roll process in a factory, instead of the current expensive batch process? And what if the price per Watt could be brought down to less than silicon, by using less-expensive plastic semiconductors?

Now here's the brilliant idea from Prof. Stephen Chou at Princeton University that addresses all of these issues. Replace the normal ITO (or wire mesh) on the front with a nano-mesh with openings sized smaller than the wavelength of light.

Subwavelength mesh for solar cell upper electrode
Chou tried several sizes of grid, but got good results with holes 175 nm in diameter, spaced 200 nm apart. Visible light has wavelengths between 380 and 700 nm, with sunlight (and human visual sensitivity) peaking at about 550 nm.

It turns out that light behaves in odd ways when it encounters a "subwavelength" grid. About a year ago, Chou noticed something very odd: when you block a hole of a subwavelength size, it's actually more likely that light will go though the hole than if the hole is unblocked! This is what led Chou to the subwavelength grid: the metal back of the cell acts to block the light, making it more likely that the light will make it through the grid and be trapped in the semiconductor layer.

Just as important, the subwavelength grid works even when light hits it at very large angles. This is of great importantance for a solar cell, because light usually doesn't hit it dead on. In the morning and evening the angles are greater, and on cloudy days the diffuse light hits from all directions at once. The subwavelength grid traps on average 90% of the incoming light, and in some cases as high as 96%. It's a quantum-mechanics roach motel for photons: they check in, but don't check out.

Another advantage: the grid has a lower electrical resistance than ITO, making the cell more efficient.

By eliminating the expensive ITO layer, a thin-film cell made this way is cheaper than current thin-film technology. (Chou used gold for his mesh, because it's easy to manipulate in the lab, but it could just as easily be made from aluminum.) Fabrication of the grid is extremely easy. You start by creating a cell-sized mold using light interference techniques, then use the mold to run off any number of grids you want. The entire thing can be done in a continuous roll manufacturing process, making it very cheap.

All together, Chou's results show that a thin-film cell using the subwavelength mesh performs about 175% better than a standard thin-film cell with ITO as the front conductor, a nearly three-fold improvement. (Because a 100% improvement is double ... you knew that, right?)

Standard disclaimers

Chou hasn't tested this grid with a silicon-based semiconductor, so far only as a thin-film organic (i.e., plastic) semiconductor. There is some reason to suspect there won't be as great a gain with silicon (because silicon is opaque, while thin-film plastic semiconductors are translucent.) But it still should be an improvement over existing large mesh structures. And plastic is a lot cheaper than various metal-based semiconductors. This could end up being as cheap as dye-sensitized solar cells, but without the degradation problems.

Yes, this is still just a lab demonstration, and no, we don't know what the final cost will be, and no, it won't be available any time soon (if by soon you mean in Home Depot next year). But it's still pretty awesome.

Links

News story from PhysOrg

Paper in Optics Express by Stephen Y. Chou and Wei Ding

Update:
Wow, I wrote this late last night with a publish time of early this morning, got up late and I'm top of the rec list. You guys are awesome. And thanks for the commenters who pointed out my typos, hopefully they're all corrected now.

Originally posted to The Numerate Historian on Fri Dec 07, 2012 at 03:08 AM PST.

Also republished by Climate Change SOS, SciTech, and Kosowatt.

EMAIL TO A FRIEND X
Your Email has been sent.
You must add at least one tag to this diary before publishing it.

Add keywords that describe this diary. Separate multiple keywords with commas.
Tagging tips - Search For Tags - Browse For Tags

?

More Tagging tips:

A tag is a way to search for this diary. If someone is searching for "Barack Obama," is this a diary they'd be trying to find?

Use a person's full name, without any title. Senator Obama may become President Obama, and Michelle Obama might run for office.

If your diary covers an election or elected official, use election tags, which are generally the state abbreviation followed by the office. CA-01 is the first district House seat. CA-Sen covers both senate races. NY-GOV covers the New York governor's race.

Tags do not compound: that is, "education reform" is a completely different tag from "education". A tag like "reform" alone is probably not meaningful.

Consider if one or more of these tags fits your diary: Civil Rights, Community, Congress, Culture, Economy, Education, Elections, Energy, Environment, Health Care, International, Labor, Law, Media, Meta, National Security, Science, Transportation, or White House. If your diary is specific to a state, consider adding the state (California, Texas, etc). Keep in mind, though, that there are many wonderful and important diaries that don't fit in any of these tags. Don't worry if yours doesn't.

You can add a private note to this diary when hotlisting it:
Are you sure you want to remove this diary from your hotlist?
Are you sure you want to remove your recommendation? You can only recommend a diary once, so you will not be able to re-recommend it afterwards.
Rescue this diary, and add a note:
Are you sure you want to remove this diary from Rescue?
Choose where to republish this diary. The diary will be added to the queue for that group. Publish it from the queue to make it appear.

You must be a member of a group to use this feature.

Add a quick update to your diary without changing the diary itself:
Are you sure you want to remove this diary?
(The diary will be removed from the site and returned to your drafts for further editing.)
(The diary will be removed.)
Are you sure you want to save these changes to the published diary?

Comment Preferences

kos, Meteor Blades, Renee, GainesT1958, Thumb, Alumbrados, tsackton, Angie in WA State, Doug in SF, Joe Bob, DeminNewJ, Marek, From the choir, PrahaPartizan, Sean Robertson, filkertom, Odysseus, melo, eleming, copymark, DawnG, Better Days, AaronInSanDiego, daninoah, Snuffleupagus, whataboutbob, HoyaChris, mimi, PeterHug, bosdcla14, Yoshimi, Debby, Shockwave, Minnesota Deb, Wintermute, billlaurelMD, cotterperson, SanJoseLady, meg, Cvstos, hyperstation, Jay C, primeq, jdld, eeff, Mnemosyne, etatauri, willyr, polecat, devtob, Do Something, freelunch, Woody, frisco, TampaProgressive, Bob Friend, Matilda, pollyusa, wenchacha, MarkInSanFran, exNYinTX, drewshaw, Paulie200, Bruce The Moose, bara, eyeswideopen, Gustogirl, concernedamerican, Zinman, Babsnc, afox, justme, litho, cskendrick, cyberKosFan, Karen Wehrstein, Wee Mama, pucklady, because, CalvinV, boadicea, samdiener, bowtiejack, roses, Arun, taonow, ovals49, ivote2004, CodeTalker, mollyd, PeteZerria, Bill Roberts, Glinda, Cedwyn, antirove, wader, revsue, Texknight, asterlil, RLF, Nag, TexDem, Dallasdoc, mrkvica, Miss Jones, pat bunny, 2laneIA, sp0t, Lawrence, VinBacchus, ccr4nine, Ophelia, homo neurotic, Jujuree, hoplite9, Bluehawk, defluxion10, never forget 2000, johnj, razajac, Timbuk3, lcrp, riverlover, BlogDog, walkshills, econlibVA, ybruti, Sembtex, isabel, randallt, Wife of Bath, jcrit, Albanius, Emmy, rickeagle, vacantlook, eve, Josiah Bartlett, weelzup, sb, nailbender, davidincleveland, humphrey, ExStr8, jabney, maybeeso in michigan, bloomer 101, radarlady, Tinfoil Hat, ichibon, UFOH1, Unit Zero, qofdisks, subtropolis, newfie, Independent Musings, chimene, OpherGopher, irate, JohnB47, basquebob, dewtx, CinDan, stagemom, Brooke In Seattle, YucatanMan, Gary Norton, farmerchuck, marathon, Ex Con, sallyfallschurch, The Grace Kelly, where4art, GreyHawk, Burned, lotlizard, Ice Blue, CompaniaHill, kaliope, Tool, rb608, brentut5, mph2005, loggersbrat, dsteffen, Cory Bantic, daddybunny, the fan man, CentralMass, Box of Rain, Alan Arizona, xaxnar, Jim R, begone, barbybuddy, RoseRash, forbodyandmind, Mother Mags, martini, third Party please, telamonides, Showman, Shirl In Idaho, detroitmechworks, Keone Michaels, myboo, RAW, Mr Bojangles, tobendaro, dopper0189, tonyahky, rl en france, Yellow Canary, victoria2dc, koNko, Patriot4peace, KenBee, Loonesta, flygrrl, Naniboujou, PlanetTreasures, raincrow, sailmaker, Lefty Coaster, blueoasis, MJ via Chicago, global citizen, StrayCat, twigg, Rosaura, NCJan, paul2port, sceptical observer, llbear, Turbonerd, onionjim, murphthesurf, SingerInTheChoir, BlueMississippi, CA Nana, Persiflage, doingbusinessas, Clive all hat no horse Rodeo, blueoregon, shaharazade, Tom Anderson, Bernie68, bstotts, Delta Overdue, mcpeg, bartcopfan, AllanTBG, sea note, Little, OHdog, Pandoras Box, Aaa T Tudeattack, seabos84, ammasdarling, clinging to hope, One Pissed Off Liberal, old wobbly, Sapere aude, bear83, fisheye, out of left field, bluicebank, high coup haiku, Cronesense, Habitat Vic, hawaii2, Loudoun County Dem, oklacoma dem, Deadicated Marxist, weneedahero, ColoTim, FishOutofWater, Via Chicago, HeartlandLiberal, certainot, deepeco, eOz, davehouck, rantsposition, artisan, jedennis, TheSpectator, SeaTurtle, jhop7, gchaucer2, rovertheoctopus, Wreck Smurfy, uciguy30, Brahman Colorado, JML9999, zorp, wblynch, Assaf, South Park Democrat, mathGuyNTulsa, TomP, fb, trivium, JDWolverton, MKinTN, seriously70, revm3up, mconvente, jack 1966, Roger Fox, flowerfarmer, greenalley, wayoutinthestix, chakadog, spacejam, ScottyUrb, mamamedusa, Brian76239, Involuntary Exile, dadadata, bythesea, elwior, filby, lineatus, Akonitum, jamess, Therapy, Calamity Jean, Lujane, TomFromNJ, tofumagoo, pamelabrown, Jake Williams, bluesheep, triplepoint, Snarky McAngus, carver, envwq, Gemina13, doppler effect, My Spin, Uncle Bob, BYw, SteveS Austin, Al Fondy, Philosophist Savant, Nica24, Quilldriver, slathe, Robobagpiper, Ellinorianne, billybam, Mike Taylor, watercarrier4diogenes, lissablack, legendmn, Fiddlegirl, Celtic Merlin, maggiejean, Rei, Bule Betawi, multilee, artmartin, WhizKid331, J M F, zmom, DemSign, ARS, divineorder, Domestic Elf, The Dead Man, ewmorr, bobatkinson, cantelow, bsmechanic, Carol in San Antonio, Mislead, Alex Budarin, LeftOfYou, Nebraskablue, shopkeeper, JesseCW, geebeebee, winkk, IreGyre, Thutmose V, Question Authority, stevenwag, sfarkash, haremoor, jfromga, astral66, boatwright, deviant24x, JEnviro, Larsstephens, Nalepoc, ruscle, Just Bob, ribeye, mamamorgaine, flitedocnm, Kayess, The Jester, serendipityisabitch, gramofsam1, politik, fidellio, eb23, Crabby Abbey, angelajean, mookins, 2questions, ATFILLINOIS, RJP9999, Eddie L, cordgrass, samanthab, ItsSimpleSimon, Benintn, anonevent, DiegoUK, NYWheeler, cai, Yasuragi, sharonsz, kalika, addisnana, rja, nirbama, Floande, petesmom, Oh Mary Oh, FrankSpoke, soaglow, slice, Mac in Maine, Quantumlogic, Onomastic, Maximilien Robespierre, spindr27, kerflooey, MidwestTreeHugger, I love OCD, ozsea1, coquiero, Liberal Capitalist, bgblcklab1, slowbutsure, afisher, sostos, hooktool, cooper888, Seitanist, FarWestGirl, MrSpock, Sand Hill Crane, kiwiheart, Arizona Mike, cleduc2, lillyspad, Alice Olson, Ebby, Late Spring, molunkusmol, mrsgoo, marleycat, zukesgirl64, Cinnamon Rollover, thomask, Muskegon Critic, slooterdam, muddy boots, createpeace, Reinvented Daddy, MarketFarces, RMForbes, enhydra lutris, antooo, JayFromPA, Tyto Alba, n8rboy, bluedust, MinistryOfTruth, sound of progress, poliwrangler, VTCC73, disconnect the dots, diffrntdrummr, Marihilda, Vatexia, Hayate Yagami, jolux, Miggles, Anthony Page aka SecondComing, too many people, dawnspantry, MikeBoyScout, RLMiller, DRo, Mentatmark, SouthernLiberalinMD, Kurt from CMH, Apost8, jobobo, PrometheusUnbound, No one gets out alive, Porterhouse, DawnN, Rejoinder, BlueDragon, jellyyork, roosterpoot, KiB, anodnhajo, Williston Barrett, Mindful Nature, Siri, Citizenpower, IndieGuy, wordfiddler, dance you monster, S F Hippie, orpurple, barkingcat, Deep Texan, AreDeutz, FloridaSNMOM, Mr Robert, DrCoyle65, Syoho, Karelin, rivercard, radical simplicity, MartyM, aznavy, oldpotsmuggler, Noddy, Victim of Circumstance, FrY10cK, AverageJoe42, T C Gibian, George3, Kayjay, hungeski, databob, mumtaznepal, Windowpane, Robynhood too, tripodisblack, CalBearMom, davethefave, sfn61, nthelurch, Raven Song, kiga, Blue Dream, Lily O Lady, Chaddiwicker, John Crapper, DamselleFly, Dallas L, countwebb, The grouch, Late Again, Cruzankenny, ZenManProject, 84thProblem, PHScott, jbeck, mtnlvr1946, averblue, The Hamlet, D minor, Unsung81, akze29, Icicle68, ET3117, Gary of Austin, Another Grizzle, MBishop1, Portia Elm, Friend in Miami, UncleYoder, Catkin, jerakami, CA wildwoman, AnotherProgressive

Subscribe or Donate to support Daily Kos.

Click here for the mobile view of the site