Media Releases

U of T‑led research improves performance of next-generation solar cell technology

September 19, 2011

TORONTO, ON – Researchers from the Uni­ver­si­ty of Toron­to (U of T), King Abdul­lah Uni­ver­si­ty of Sci­ence & Tech­nol­o­gy (KAUST) and Penn­syl­va­nia State Uni­ver­si­ty (Penn State) have cre­at­ed the most effi­cient col­loidal quan­tum dot (CQD) solar cell ever.

The dis­cov­ery is report­ed in the lat­est issue of Nature Mate­ri­als.

Quan­tum dots are nanoscale semi­con­duc­tors that cap­ture light and con­vert it into elec­tri­cal ener­gy. Because of their small scale, the dots can be sprayed onto flex­i­ble sur­faces, includ­ing plas­tics. This enables the pro­duc­tion of solar cells that are less expen­sive than the exist­ing sil­i­con-based ver­sion.

“We fig­ured out how to shrink the wrap­pers that encap­su­late quan­tum dots down to the small­est imag­in­able size – a mere lay­er of atoms,” states Pro­fes­sor Ted Sar­gent, cor­re­spond­ing author on the work and hold­er of the Cana­da Research Chair in Nan­otech­nol­o­gy at U of T.

A cru­cial chal­lenge for the field has been strik­ing a bal­ance between con­ve­nience and per­for­mance. The ide­al design is one that tight­ly packs the quan­tum dots togeth­er. The greater the dis­tance between quan­tum dots, the low­er the effi­cien­cy.

Until now, quan­tum dots have been capped with organ­ic mol­e­cules that sep­a­rate the nanopar­ti­cles by a nanome­ter. On the nanoscale, that is a long dis­tance for elec­trons to trav­el.

To solve this prob­lem, the researchers uti­lized inor­gan­ic lig­ands, sub-nanome­ter-sized atoms that bind to the sur­faces of the quan­tum dots and take up less space. The com­bi­na­tion of close pack­ing and charge trap elim­i­na­tion enabled elec­trons to move rapid­ly and smooth­ly through the solar cells, thus pro­vid­ing record effi­cien­cy.

“We wrapped a sin­gle lay­er of atoms around each par­ti­cle. This allowed us to pack well-pas­si­vat­ed quan­tum dots into a dense sol­id,” explains Dr. Jiang Tang, the first author of the paper who con­duct­ed the research while a post-doc­tor­al fel­low in The Edward S. Rogers Depart­ment of Elec­tri­cal & Com­put­er Engi­neer­ing at U of T.

“Our team at Penn State proved that we could remove charge traps — loca­tions where elec­trons get stuck — while still pack­ing the quan­tum dots close­ly togeth­er,” says Pro­fes­sor John Asbury of Penn State, a co-author of the work.

“At KAUST, we used visu­al­iza­tion meth­ods with sub-nanome­ter res­o­lu­tion and accu­ra­cy to inves­ti­gate the struc­ture and com­po­si­tion of the pas­si­vat­ed quan­tum dots,” states co-author Pro­fes­sor Aram Amass­ian of KAUST in Sau­di Ara­bia. “We proved that the inor­gan­ic pas­si­vants were tight­ly cor­re­lat­ed with the loca­tion of the quan­tum dots and that it was the chem­i­cal pas­si­va­tion, rather than nanocrys­tal order­ing, that led to the remark­able col­loidal quan­tum dot solar cell per­for­mance,” he adds.

“It is very impres­sive that the team was able to make solar cells with pow­er con­ver­sion effi­cien­cy up to 6% from quan­tum dots,” states Pro­fes­sor Michael McGe­hee of Stan­ford Uni­ver­si­ty, a world-renowned expert in solu­tion-processed organ­ic solar cells. “There is a lot of sur­face area in these films that could have dan­gling bonds which would hin­der the per­for­mance of solar cells by cre­at­ing traps states.

The team’s quan­tum dots had the high­est elec­tri­cal cur­rents and the high­est over­all pow­er con­ver­sion effi­cien­cy ever seen in CQD solar cells. The per­for­mance results were cer­ti­fied by an exter­nal lab­o­ra­to­ry, New­port, which is accred­it­ed by the US Nation­al Renew­able Ener­gy Lab­o­ra­to­ry.

“This work proves the pow­er of inor­gan­ic lig­ands in build­ing prac­ti­cal devices,” states Pro­fes­sor Dmitri Talapin of The Uni­ver­si­ty of Chica­go, a pio­neer in inor­gan­ic lig­ands and mate­ri­als chem­istry. “This new sur­face chem­istry pro­vides the path toward both effi­cient and sta­ble quan­tum dot solar cells. It should also impact oth­er elec­tron­ic and opto­elec­tron­ic devices that uti­lize col­loidal nanocrys­tals. Advan­tages of the all-inor­gan­ic approach include vast­ly improved elec­tron­ic trans­port and a path to long-term sta­bil­i­ty.”

As a result of the poten­tial of this research dis­cov­ery, a tech­nol­o­gy licens­ing agree­ment has been signed by U of T and KAUST, bro­kered by MaRS Inno­va­tions (MI), which will enable the glob­al com­mer­cial­iza­tion of this new tech­nol­o­gy.

“The world — and the mar­ket­place — need solar inno­va­tions that break the exist­ing com­pro­mise between per­for­mance and cost. Through the part­ner­ship between U of T, MI and KAUST, we are poised to trans­late excit­ing research into tan­gi­ble inno­va­tions that can be com­mer­cial­ized,” said Sar­gent.

To read the pub­lished paper in its entire­ty, please con­tact Liam Mitchell, Com­mu­ni­ca­tions & Media Rela­tions Strate­gist for the Fac­ul­ty of Applied Sci­ence & Engi­neer­ing, Uni­ver­si­ty of Toron­to.


For more infor­ma­tion, please con­tact:

Pro­fes­sor Edward Sar­gent
The Edward S. Rogers Sr. Depart­ment of Elec­tri­cal & Com­put­er Engi­neer­ing
Fac­ul­ty of Applied Sci­ence & Engi­neer­ing, Uni­ver­si­ty of Toron­to
416–946-5051 |

Liam Mitchell
Com­mu­ni­ca­tions & Media Rela­tions Strate­gist
Fac­ul­ty of Applied Sci­ence & Engi­neer­ing, Uni­ver­si­ty of Toron­to
647–228-4358 |

* Images and back­grounder avail­able on request.