The quest for a bet­ter so­lar cell

UT re­search team seeks to turn more sun­light into elec­tric­ity

Austin American-Statesman - - FRONT PAGE - By Ralph K.M. Hau­r­witz

Sun­light is free. But it’s not cheap or easy to wring abun­dant amounts of power from it.

En­ter some tiny crys­tals known as quan­tum dots and a chem­istry pro­fes­sor at the Uni­ver­sity of Texas named Xiaoyang Zhu, or, as his stu­dents call him, XYZ.

A re­search team led by Zhu, who refers to his Cen­ter for Ma­te­ri­als Chem­istry as the XYZ Lab, has shown that it’s pos­si­ble to con­vert much more of the sun’s en­ergy to elec­tric­ity than con­ven­tional so­lar cells are able to gen- er­ate. The con­ven­tional cells, which are made of sil­i­con, turn no more than about 20 per­cent of the en­ergy into juice, and their max­i­mum the­o­ret­i­cal ef­fi­ciency is only 31 per­cent be­cause of tech­ni­cal lim­i­ta­tions.

By us­ing a com­pound called lead se­lenide in the form of quan­tum dots, also called semi­con­duc­tor nanocrys­tals, and by rout­ing elec­trons stirred up by the sun­light from the lead se­lenide to an­other com­pound called ti­ta­nium diox­ide, the re­searchers showed that it’s the­o­ret­i­cally pos­si­ble to har­vest 66 per­cent of the en­ergy.

To put it an­other way, the XYZ group has fig­ured out some of the ABCs of a bet­ter so­lar cell. Any com­mer­cial ap­pli­ca­tion is years in the fu­ture be­cause con­sid­er­ably more sci­en­tific and en­gi­neer­ing work needs to be done.

Con­tin­ued from A

The team’s find­ings, pub­lished re­cently in the schol­arly jour­nal Sci­ence, are part of a grow­ing body of re­search aimed at im­prov­ing the ef­fi­ciency — and re­duc­ing the cost — of so­lar cells. The goal is to make so­lar en­ergy a vi­able al­ter­na­tive to fos­sil fu­els that con­trib­ute to global warm­ing and to de­pen­dence on sup­plies in po­lit­i­cally volatile parts of the world.

“I’m hop­ing by the time I re­tire, we have so­lar cells like this on the roof,” said Zhu, 46. “That would be my dream.”

Zhu, who be­gan this re­search at the Uni­ver­sity of Min­nesota, was re­cruited to UT in July 2009. His so­lar cell lab in UT’s Nanoscience and Technology Build­ing doesn’t look like a con­ven­tional chem­istry lab. There’s not a test tube in sight; rather, the place is jammed with lasers, op­ti­cal equip­ment and a vac­uum cham­ber that looks vaguely like a per­son wrapped in foil.

Re­searchers use the gear to study the en­ergy, move­ment and speed of elec­trons, the neg­a­tively charged sub­atomic par­ti­cles whose flow con­sti­tutes elec­tric­ity. Liq­uid helium is used to cool sam­ples to mi­nus 450 de­grees Fahren­heit, and ex­per­i­ments take place atop spe­cial ta­bles that rest on a thin cush­ion of air. The tem­per­a­ture and ta­bles min­i­mize un­wanted vi­bra­tions and move­ments.

In con­ven­tional so­lar cells made of sil­i­con, much of the in­com­ing sun­light con­tains en­ergy that is too high for the cells to cap­ture and use. That en­ergy, in the form of high-en­ergy elec­trons, or hot elec­trons, is lost as heat. If those hot elec­trons could be cap­tured and put to work, the ef­fi­ciency of the so­lar cell would in­crease dra­mat­i­cally.

“There are a few steps needed to cre­ate what I call this ul­ti­mate so­lar cell,” Zhu said. “First, the cool­ing rate of hot elec­trons needs to be slowed down. Sec­ond, we need to be able to grab those hot elec- trons and use them quickly be­fore they lose all of their en­ergy.”

Pre­vi­ous re­search by other sci­en­tists has shown that quan­tum dots can slow the cool­ing of hot elec­trons, thanks in part to the dots’ ul­tra-small size: They mea­sure 3 to 10 nanome­ters across. By com­par­i­son, a sheet of paper is about 100,000 nanome­ters thick.

Zhu’s team took the re­search a step fur­ther, show­ing that the hot elec­trons can be trans­ferred from the quan­tum dots to an elec­tri­cal con­duc­tor made of ti­ta­nium diox­ide, a min­eral used in sun­screen. In fact, treat­ing the sur­face of the quan­tum dots with cer­tain chem­i­cals caused the trans­fer to oc­cur more rapidly than the re­searchers ex­pected.

Lead se­lenide quan­tum dots have other ad­van­tages over sil­i­con. They can be grown in the lab from the el­e­ments lead and se­le­nium in a process that is eas­ier and cheaper than pre­par­ing sil­i­con for so­lar cells, said Ken­rick Wil­liams, a grad­u­ate stu­dent at UT and a mem- ber of the re­search team.

The other mem­bers of the team were Wil­liam Tis­dale, now at the Mas­sachusetts In­sti­tute of Technology, and Brooke Timp, David Nor­ris and Eray Ay­dil, all of the Uni­ver­sity of Min­nesota.

Tian­quan Lian, a chem­istry pro­fes­sor at Emory Uni­ver­sity who was not in­volved in this re­search but stud­ies the use of nanoscale ma­te­ri­als for so­lar en­ergy, told Pop­u­lar Me­chan­ics mag­a­zine that the find­ings, if ver­i­fied, make highly ef­fi­cient quan­tum dot so­lar cells more re­al­is­tic.

Zhu, who re­ceived $490,000 in fund­ing from the U.S. Depart­ment of En­ergy for the project, re­cently re­ceived an ad­di­tional $630,000 to con­tinue the re­search.

The next chal­lenge is to trans­fer the hot elec­trons to a con­duct­ing wire with­out los­ing too much en­ergy as heat.

“We want to cap­ture most of the en­ergy of sun­light,” Zhu said. “That’s the ul­ti­mate so­lar cell.”

Jay Jan­ner

Ken­rick Wil­liams and other re­searchers at the Uni­ver­sity of Texas are break­ing ground in try­ing to cre­ate so­lar cells that are more ef­fi­cient, cheaper and eas­ier to make than con­ven­tional so­lar col­lec­tors.

Mar­sha Miller

UT’s Xiaoyang Zhu leads a team that aims to make so­lar en­ergy a vi­able al­ter­na­tive to fos­sil fu­els.

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