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CNM Users Meeting Focus SessionNanostructured Materials for Solar Energy Utilization

October 6, 2009Organizers: SethDarling,CenterforNanoscaleMaterials,ArgonneNationalLaboratory

CharlesBlack,CenterforFunctionalNanomaterials,BrookhavenNationalLaboratory

Sponsors: Plextronics,Inc.

ArgonneNorthwesternSolarEnergyResearch(ANSER)Center

Manyessentialphysicalprocessesinvolvedinsolarenergyconversionoperateonnanometer

length scales. Nanostructured materials provide promising pathways to improving the

performance of nextgeneration photovoltaic, thermoelectric, and photocatalytic devices.

Technologicaladvancesintheseimportantfieldsrequirebothathoroughunderstandingofthephotophysicsandelectricalpropertiesinsolarenergysystems,aswellasrationalcontrolover

the assembly of optoelectronically active nanomaterials. This fullday workshop provided a

forum forpresentationanddiscussionof current researchadvances in thisimportant topical

area.Thecollectionofspeakersincludedsomeoftheleadingresearchersandpioneersinthe

fields of organic optoelectronics, semiconductor photophysics, and nanomaterials for

photovoltaics.Allmajorsectorsofresearch(academic,nationallaboratory,andindustrial)were

represented,providingadiverseperspective.

Prof.Rakesh Agrawal (Purdue)kicked off theworkshop presenting his

groupseffortstodevelopsolutionprocessibleinorganicsolarcells.They

have developed an innovativemethod ofusing copper indium gallium

disulfide (CIGS) nanocrystals as building blocks for fabrication of bulk

CIGSSe thin films. The nanocrystal ink solution is applied directly on

varioussubstratestoformathinfilmcoatingandthenconsolidatedinto

largecrystallinechalcopyritedomainsbyabriefthermaltreatmentunder

Sevapor.TheyarealsoworkingtoadapttheCIGSmethodtoCu2ZnSnS4

(CZTS) nanocrystals and the associated PV devices. In the second

presentationofthemorning,Prof.AngusRockett(UIUC)providedaninformativeandthought

provoking summary of the issues and prospects for photovoltaic devices based on

nanomaterials. He pointed out where there were fundamental limitations with various

technologies and where opportunities for advancement exist. This talk served as perfectgroundworktowhichtherestofthespeakerscouldrespondwiththeirvariousapproachesto

understandingandworkingaroundtheinherentpropertylimitations.

Thesecondmorningsessionshiftedtoorganicandhybridorganicinorganicnanomaterialsfor

photovoltaics. Dr. Garry Rumbles (NREL) focused on the process of exciton dissociation in

conjugatedpolymers.Hediscussedthepresentlowdeviceperformanceefficiency,howitmight

beincreased,andhowperformancecompareswithmoreconventionalphotovoltaicdevices.He

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then shifted to describe the use of timeresolved microwave

conductivityusingpulsedlaserexcitationasatoolforprobingboth

the production and loss of free carriers resulting from exciton

dissociation.Inadditiontotraditionalorganicblends,hepresented

data on new donor polymers and dendrimers, as well as new

acceptors such as singlewall carbon nanotubes (SWNTs) andcolloidal quantum dots. Dr. Michael Pellin (Argonne) presented

work using atomic layer deposition (ALD) to fabricate

interdigitated morphologies for advanced dyesensitized solar

cells. ALD is a synthetic method capable of functionalizing

nanoporous substrates with conformal, nanometerscalelayered, transparent conducting

oxides and semiconducting materials. They designed and fabricated a new photoelectrode

architectureconsistingofconcentricconductingandsemiconductingnanotubesforuseindye

sensitizedandothertypesofsolarcells.Thecellsexhibitbothhighphotonabsorptiondueto

thehighsurfaceareaofthenanoporoussubstratesandhighcurrentdensitiesduetotheradial

collection ofelectrons.The industrial perspectiveonorganicphotovoltaicswas presented byDr. Christine McGuiness (Plextronics). Plextronics is among the select group of institutions

exploringthescalabilityoforganic technologyfromlaboratorycellstomodules.Shereported

on performance of their laboratoryscale organic photovoltaic device aswell as the largest

organicphotovoltaicmodule tobecertifiedbyNREL. In labscaledevices, their Plexcore inks

havedemonstratedNRELcertifiedpowerconversion efficiencyof5.98%.Thetechnologyhas

beenscaleduptolargerareamodules,whichwerecertifiedbyNRELashaving2.38%totalarea

efficiency(4.95%activeareaefficiency).Dr.McGuinessalsopresentedaformalizedapproach

toascertainingthelifetimeperformanceinorganicsolarcells.

Following interestingdiscussions over the lunch break,Prof.

TobinMarks(Northwestern)kickedofftheafternoonsessionswitha summary ofhis groupswork investigating interfacial

layers in organic photovoltaic devices. He presented their

abilitytofabricatetailoredinterfacestoselectivelymodulate

chargetransport,facilitatingtransportof thedesiredcharges

while blocking that of the complementary charges. He also

showed how these approaches can stabilize and control

interfacial defect densities. This work aims not only to

understand the interfacial processes, but also to replace

materialsused indeviceswith those havinggreater sustainabilityand superiorperformance.

Working with collaborators at Solarmer, Prof. Luping Yu (Univ. of Chicago) has recentlyreportedanewworldrecordinorganicsolarcellefficiencybasedonnovelpolymericmaterials

designedand synthesized inhis laboratory. These polymers are designedtohaveanenergy

bandgapwelltailoredtoharvestsolarenergy.Thequinoidalstructuresprovidethesepolymers

withhighrigidityandrelativelyhighchargecarriermobility.Adetailedstructuralvariationled

themtodiscoverseveralpolymersexhibitingapowerconversionefficiencyof7.5%.

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The final session of the workshop included two

groundbreaking researchers in energy conversion research:

Dr. Art Nozik (NREL) and Prof. Michael Wasielewski

(Northwestern). Dr. Noziks presentation was on the

controversial topic of multiple exciton generation in

semiconductor quantum dots. Using timeresolved ultrafastspectroscopy they observed efficient multiple exciton

generationinPbSe,PbS,PbTe,andSicolloidalnanocrystalsat

threshold photon energies of 23 times the HOMOLUMO

transition. Recently, controversy has arisen regarding the reproducibility of reported MEG

efficiencies in InAs and CdSe quantum dots. They addressed this controversy for lead

chalcogenides with detailed studies, and find reproducible multiple exciton generation

dependinguponsurfacechemistryanddynamicalconditionsofthecolloid.Theyalsostudied

multipleexcitongenerationinclosepackedarrayswherethecrystalsareelectronicallycoupled

and thus exhibit good carriermobility.Prof.Wasielewski finishedoff the daywith beautiful

examples of using advanced characterization techniques to probe photodriven chargeseparationandtransportinselforganizeddonoracceptormolecularassemblies.Heprepareda

series of donoracceptor triad building blocks consisting ofa PDIacceptor, anaminopyrene

primarydonor,andadiaminobenzenesecondarydonor.SmallanglescatteringattheAPSwas

used todetermine theseselfassemble intohelical hexamers in solution. Timeresolved EPR

spectroscopywasusedtomeasuretheaveragedistancebetweenthephotogeneratedradical

ionswithinthehexamer.Thisresultshowedthatnonbondedsecondarychargetransportwithin

ordered stacked donors and/or acceptors can be kinetically competitive with charge

recombination within the covalent building block, making these materials of interest as

photoactivenanostructuredassembliesforartificialphotosynthesisandorganicphotovoltaics.

Inordertofullyrealizethepotentialofsolarenergyonaglobalscale,basicresearchintotheprocessesunderlyingphotovoltaicenergyconversionisneeded.Thespeakersatthisworkshop

capture a crosssection of the research in this area,and specifically how nanomaterials are

centraltomanyofthemostexcitingfuturetechnologies.ArgonneNationalLaboratoryandthe

Center forNanoscaleMaterials aimto havemajor impact indevelopingnovelmaterialsand

processesfornextgenerationtechnologiesandwerethrilledtohostsuchastimulatingforum

onsolarenergy.Weanticipatemanynewdiscussionsandcollaborationstoemergefromthis

event.