WIXLIB

Colloidal Particles Engineer reactions to precipitate quantum dots fromsolutions or a host material (e.g. polymer) In some cases, need to “cap” the surface so the dotremains chemically stable (i.e. bond other molecules onthe surface)f) Can form “core-shell” structures Typically group II-VI materials (e.g. CdS, CdSe) Size variations ( “size dispersion”)CdSe core with ZnSshell QDsRed: bigger dots!Blue: smaller dots!

Synthesis approach:12 nm (CdO : Se 0.5:1)CdO TOPO/OA TDPATOP-Se/Ar gas7 nm (CdO : Se 1:1) 300o CSize regulating factor » Cdto Se precursor ratio9 nm (CdO : Se 3:1)TOPO/OA- capped CdSe5 nm (CdO : Se 2:1)optimized conditionPreparation of Polymer:CdSe NanocompositesToluene solvent:- CappedCd CdSCdSe particlesti lPyridine solvent:pp CdSe pparticles- UncappedFigure shows the Capped and Uncapped CdSe nanoparticles dispersed inPolymer matrix09020240

Synthesis of CdSe Quantum DotsConclusions9 We have successfully synthesized high quality CdSequantum dotsooNearly-monodispersedHighly Crystalline9 Cd:Se 2:1 is the optimized condition as for both the capping casessmallest particles were achievedCdS (OA) 2:1CdSe(OA)21 7 nmCdSe(TOPO) 2:1 5 nm9 CdSe(OA) 2:1 particles show better properties compared to CdSe(TOPO) 2:1- Steric stability- Photoluminescence- Photostability09020241

Effect of CdSe quantum dots on hole transporti poly(3-hexylthiophene)inl (3 h l hi h ) thinhi filfilmsAuP3HTPEDOT:PSSITOGlass substrateDevice 120 nmTEM image of CdSe quantumdots (size 5 nm) dispersedin P3HT matrix in 1:1 weightratioAuP3HT:CdSePEDOT PSSPEDOT:PSSITOGlass substrateDevice 2The incorporation of CdSe quantum dots in P3HT results in enhancement inhole current and switches the transport from dual conduction mechanismmechanism, vizviz.,trap and mobility models to only trap model090202Kusum et al, Appl. Phys. Lett., 92, 263504 (2008)42

Demonstration of Solar Cell.P3HT: PCBMITO/ PEDOT:PSS/ P3HT:PCBM/ LiF/ AlJ (A/cm 2)P3HT: CdSe: PCBMITO/ PEDOT:PSS/ P3HT:CdSe:PCBM/ LiF/ Al-35.0x10-0.250.00.00Jsc 6.32 x 10-3 A/cm2 Jsc 8.88 x 10-3 A/cm20.250.50V (Volts)-3-5.0x10-2-1.0x10Voc 0.44 VVoc 0.48 VFF 0.435FF 0.36η 1.23 %η 1.91 %0.75P3HT: PCBMP3HT: CdSe: PCBM-2-1.5x109090202Reduction of barrier at active layer- acceptor interface43

Demonstration of Solar Cell.MEH-PPV:PCBMITO/ PEDOT:PSS/ MEHPPV:PCBM/ LiF/ AlMEHMEHPPV:CdSe:PCBMITO/ PEDOT:PSS/ MEHPPV:CdSe:PCBM/ LiF/ AlJ (AA/cm2 000.00-32.50x10-2.50x10MEH-PPV: PCBMMEH-PPV: CdSe: PCBM0.25A0.50V (Volts)Jsc 2.88 x 10-3 A/cm2 Jsc 7.37 x 10-3 A/cm2Voc 0.37 VVoc 0.41 VFF 0.46FF 0.40η 0.620 62 %η 1.471 47 %-5.00x10-3-7.50x10-3-1.00x10-2B CdSe QDs have a range of electron affinities reported from 3.5-4.5 eV helpin matching energy levels PCBM090202 provides additional conducting path allowing significant44enhancement of electron transport at even low doping levels

PolymerNanoparticles Voc JscEQE PCE(V) (mA/cm2)(%)OC1C10-PPVCdSe tetrapods0.759.10.522.8B. Sun et al., J Appl Phys97 (2005) 014914P3HTCdSe nanorods0.628.790.702.6B. Sun et al., Phys Chem Chem Phys 8(2006) 3557APFO-3CdSe nanorods0.957.230.442.4P. Wang et al., Nano Lett 6 (2006) 1789P3HTCdSe hbranch0 600.607 107.10222.2II. Gur et al.,al Nano Lett7 (2007) 409–14P3HTCdSe nanorods0.706.070.561.7W. U. Huynh et al., Science 295 (2002)2425–7MDMO-PPVZnO0.812.400.391.6WJE Beek et al., Adv Mater 16 (2004)1009–13MEH-PPVCdSe tetrapods0.692.860.461.13Zhou Y, Nanotechnology17 ((2006)) 4041–7MDMO-PPVZnO1.142.300.261.1WJE Beek et al., Adv Funct Mater 15(2005) 1703–7MEH-PPVCdSexTe1 x(CdSe0.78Te0.22)0.691.570.49Yi Zhou et. al., Nanotechnology 17(2006) 4041–40474041 4047MEH-PPVCdTenanocrystals0.770.190.42T. Shiga et al., Solar Energy Materials& Solar Cells 90 (2006) 1849–1858MEH-PPVPbS1 001.000 130.130 700.70AAR Watt etet. alal., J PhysD: Appl Phys 38 (2005) 2006–12P3HT090202PbSe0.351.080.14D Cui et al., Appl Phys Lett88 (2006) 1831110 210.21References45

The Potential ofOSCwww.konarka.comKonarka’s solar bags es/ Appotential cottageg industryy Production is distributedwww.scienceknowledge.org

Summaryy and Conclusions We have reviewed the status ofnovell solarl cellsll Nanoparticles are used inseveral different approaches toimprove the solar energyconversion efficiency New materials are the key toprogresspgto improvepabsorptionpfor longer wavelenghts There are several groupsworkingkion ththese aspectst iniIndia.

I thank numerous persons who havepcontributed to this presentation

First ggeneration PV

Second Gen PV

3rd Gen PV

4th Gen

CuPC(organic)Narrow BandEnergy (eV)Energy (eV)Organic absorbs in a narrow bands Can use multiple layers((tandem solar cells))Abssorptionn Coefficcient (cmm-1)Abbsorptionn Coefficcient (cmm-1)Absorption BandGaAs(Inorganic)Narrow BandEnergy (eV)Inorganics absorb in wider bandsAbsorbs all photons in solar spectrumwith energy above bandgap energyThe absorption coefficient values are usually higher in organic solar

Exciton FormationhνLUMO levelElectronEltexcitedit dto higher energy levelHOMO level Absorbed photon creates an exciton Excitons do NOT always form free electrons andholeso es– This is especially true in organic semiconductors– Here, strong field are needed to dissociate excitons

SunlightPhotovoltaic Process In OrganicgSolarCellsCouplingof sunlightintosolar cellAbsorptionofincidentphotonsLightLihtPh iciency of this step is 100%for inorganic solar cellsSeparationof chargesby built-inE field&Collectionof chargesatelectrodesChargesChargesRecombine Recombine''

Built in FieldBuilt-inφbiEfEbuilt-inb ilt iOOrganicMateriali M t i lφbip-typeEbuilt-inn-typeInorganic semiconductordNo charge of its ownBuilt-in potential depends on electrode work function difference

In Typical OSCOSC is typically different from inorganic solar cells in the following ways: Absorption in a narrower spectral band Usuallyy highg absorptionpcoefficient Exciton bindingg energygy higherg Poor chargeg mobilityy Built-in potential dependent on electrodes

At IIT Kanpur

Product Oriented Research220 m2 ISO 6 Cleanroomhttp://www.iitk.ac.in/scdt/

Members of SCDTFacultyScientists/ ResearchEngineers Dr. Deepak Gupta Dr. Y.N.Mohapatra Dr. Dr. B.Mazhari Dr.Dr Monica Katiyar Dr. Satyendra Kumar Dr. S.S.K.Iyer Dr. Ashish Garg Dr.Dr Sidhartha Panda Dr. J. Narain Dr. Vandana Singh Dr. Ashish GuptaVisiting ResearchEngineersSupport StaffMr. Dharmendra SwainMr. Arvind KumarMr. Ramnath YadavMr. Dinesh KumarMr. Ajay NaikMs. Mamata RaiMs. Shewta MauryaStudentsPh.D: 19M.Tech: 22B Tech/ MB.Tech/M.Sc.:Sc : 15 Dr. Unni Narain Dr. Asha Awasthi Mr. I. V. Kameshwar Rao Mr.Mr Ranbir Singh Mr. Boby C. Villari Mr. Pankaj Uttwani Dr. Ganesan Palaniswami

OLED Displays from IIT KanpurDisplayMetal LinesCOFDriverImages Captures on the displaymade at SCDTDisplay Module,Full Colour 96 (3) x 6464, 1”1,Passive Matrix, consisting of theOLED Display, COF and Driver.

SCDT Projects DisplaysLightingPrintable ElectronicsSolar CellsSensorsPushing the Envelope ofUnderstandingg of OrganicgDevicesOOLEDThe new age off Macro-electronicsPrintable, Flexible, Large, Cost Effective !CoreExpertise &Facilities

Technology StatusO-SensorsOSensorsWOLEDO-TFT (printable)O-Solar CellsOLED ithIndustrial Partner

On-going work at IIT Kanpur

From Materials to ApplicationsOrganic MoleculesDiscrete DevicesModuleApplications

Material SystemsWet ProcessingVacuum Thermal DepositionR Hexyl Imidazolin 5 oneImidazolin-5-oneModified Chromophor of GreenFluorescent Protein Prof. R. GurunathMg-chlorophyllModified Porphyrin moleculesProf. S.P. Rath

Four Pronged Approach Characterising the devices– Spectral response characterisation– Spectral ellipsometry Increasing efficiency of device– Physics and circuit model of organic solar cells– Choice of material– StructureSt t– blend,bl d bilbilayer, ttandemd – Process optimisationReliability and stability– Choice of material– Mechanism of degradation– EncapsulationptechniquesqNew & emerging technology issues– Novel methods of fabrication– System level issues

Suman BanerjeeSpectral Response AnalysisSuman Banerjee

P3HT:PCBM Solar CellCalcium-AluminiumCalciumAluminium CathodeP3HT:PCBM BlendGlassPEDOT:PSSITOPEDOT:PSS - 30 nm;P3HT:PCBM (1:1) - 90 nm;Ca - 6 nm; Al - 70 nm.Characterstics of a typical Organic (Polymer) Solar CellArun Tej Mallajosyula

Anirban BaguiEffect of Post Process AnnealP3HT: PCBM BlendH tHeterostructurettVinod Pagare 2007Aluminium CathodePolymer BlendPEDOT:PSSGlass ITO

Solvent Drying in EE-FieldFieldElectric Field Annealing during solvent drying step of active layerimproves device efficiencyAnirban BaguiIndian Patent being filed

Modifying Interface byAnnealingAs deposited CuPcSpecially Annealed CuPcSmoother with pillarsAnnealed CuPcSmootherAlActiveAreaC60Modification of interfacebetween CuPc and C60CuPcITOGlassAnukul Prasad ParhiIndian Patent being filed

Nanotubes in Polymer OSC241.20.9P3HT:PCBM:SWNT (0.75 %)P3HT:PCBM5.0JL (mmA cm-2)060.60.30.00.00.1Voltage (V)0.20.30.4(mA cm ) (%)18η 2.01 %η 2.99 %-0.6-1.2565Voc(mV)560214.5Jsc-2 FF4.0-0.3-0.9090.5570FFVocJscAM 1.5 GIntensity 6 mW cm-25553.5153.00.00.20.40.60.81.0SWNT wt% Incorporation of single walled nanotubes can improve solar cell performance Main role of nanotube is in charge transport within the solar cell NanotubeNanotube does not have much effect in exciton generation or dissociationAurn Tej Mallajosyula550

Degradation ModelsDegradation under Electrical & Optical Stress Statistically arrive at parameters that matter most Identify the physics of degradation Use learning to increase device lifetimeMunish Jassi 2006

PlasmonicsBulk MetalNanoscale metalDecreasingthe size Unoccupied states5 nmoccupiedpstatesClose lyingy gbandsSeparation betweenthe valence andconduction bandsUnbound electrons havemotion that is not confinedElectron motion becomesconfined, and quantization sets inParticle size mean free pathpof electrons

Unusual Properties on the nm Scale are realizedin centuries backIf you take gold and makeparticles about 10 nm indiameter, it looks wine-redor blue-grayblue gray, depending onhow close the particles aretogetherruby-red stainedglass from goldnanoparticles

Surface Plasmon Absorption of AuNanoparticles 520 nmA b s o r p t io n0.30.20.1Au colloids Size 7-8 nm0300400500600700800Wavelength (nm)Optical response of a nanoparticle depends on its size, shape,collectivell ti behaviourb h iandd llocall didielectricl t i environmentitSurface plasmon absorption is due to coherent motion of conduction band electronsafter incident EM radiation Au NPs- - --- - --

Metal Nanoparticles -Localized surface plasmonspCollective motion of electrons is called “plasmaoscillation” Plasma frequency is in UV region – Au,AuAg, Al7-8 nmBulk GoldRough surface Grating structureNanoparticles - - -- - -Metal nanoparticles Conduction electrons acts like oscillatorsystem

Plasmonics for photovoltaics¾ Conventional Si solar cellscells, light trapping istypically achieved using a pyramidal surfacetexture that causes scattering of light into thesolar cell over a large angular rangerange, therebyincreasing the effective path length in the cell.¾ Such large-scale geometries are not suitable for thin-film cells, forgeometrical reasons (as the surface roughness would exceed the filmthickness) and because the greater surface area increases minority carrierrecombination in the surface and junction regions.¾ A method for achieving light trapping in thinthin-filmfilm solar cells is the useof metallic nanostructures that support surface plasmons:o excitations of the conduction electrons at the interface between ametal and a dielectricdielectric.o By proper engineering of these metallodielectric structures, lightcan be concentrated and ‘folded’ into a thin semiconductor layer,thereby increasing the absorptionabsorption.o Both localized surface plasmons excited in metal nanoparticlesand surface plasmon polaritons (SPPs) propagating at themetal/semiconductor interface are of interest.

Plasmonic light-trapping geometries for thin-filmsolar cells(a) Light trapping by scattering from metal nanoparticlesat the surface of the solar cell. Light is preferentiallyscattered and trapped into the semiconductor thin filmby multiple and high-angle scatteringscattering, causing anincrease in the effective optical path length in the cell.(b) Light trapping by the excitation of localized surfaceplasmons in metal nanoparticles embedded in thesemiconductor. The excited particles’ near-fieldcauses the creation of electron–hole pairs in thesemiconductor.(c) Light trapping by the excitation of surfaceplasmon polaritons at the metal/semiconductorinterface. A corrugated metal back surface couplesg to surface pplasmon ppolariton or pphotonic modeslightthat propagate in the plane of the semiconductorlayer.Nature materials VOL 9, 2010,205

Surface plasmon enhanced Sisolar cella. Enhancement from a doublesidedid d polishedli h d Si waferfcharacterized optically using aspectrophotometer with particlesizes formed from differentmass thickness of silver byythermal evaporation followed byannealing.b Totalb.T t l andd diffusediffreflectancefl tplots from a double-sidedpolished Si wafer with 30 nm topoxidePillai et al JAP,101, 093105 2007

Phosphorsp Photon accounting of a Si solar cells shows about29% photons with energy 11.11 eV are not effective incontributing to the output energy due to thethermalization of the excited electrons/holes to therespective band edges. The photons with energy below the band gap are notabsorbed in the base region and are mostly lost.These two optical lossescombine to result in 50% ofthe photon energy notcontributing to thephotovoltaic conversionp

Downconversion The second loss mechanism is imperfect collection dueto recombination close to or at the surface.surface Since high energy photons are absorbed in this regiontheyy are more likelyy to be affected and the result is areduced spectral response at shorter wavelengths. This loss can be reduced by using phosphors thatabsorb near UV to 500 nm,nm whereby emitting energy inthe higher spectral response region (500 -1000 nm) ofthe Solar cell.

Upconversion ThThe thirdthi d losslmechanismh iisi transmission,ti iwhichhi hoccurs because photons with energy less than theband ggapp of silicon are not absorbed. Transmission losses can be reduced by employingup-conversion (UC) processes, whereby two or morellowenergy photonsh tcombinebi tot createt one higherhi henergy photon. Spectrum modification is the most promisingsolution for enhancing the cell efficiency butunfortunately, no breakthrough has been reportedyett ini thisthi area. ThisThi makesk theth issueimore targetttoriented and challenging to pursue research forsuitable phosphor/nanophosphor coatings forimprovements in the efficiency of solar cell.

NPLINDIALOSSESSOLUTIONThermalization.Wavelengthg 2Egg of SiDown Conversion/ PhotoluminescencePhosphorsTransmission.Wavelength Eg of SiUp Conversion Phosphors

Up-Conversion (UC) Phosphor: Absorbs IR and emits in VIS-NIRDown Conversion (DC) Phosphor: Absorbs UV-VIS and emits in VIS-NIRThe Phosphor Layer should be transparent to wavelengths other than its absorption.NPLINDIAIIT, Delhi

Solar Spectrum Modification Using Novel Nano and Bulk Phosphors for EnergyEfficient Solar CellsA ppromisingo s g coconceptcept foro eefficientc e t harvestinga est g oof sosolara eenergye gy ususingg sosolara cecellss issSpectrum Modification using phosphorsPhotographshh off YVOO4:Eu3 colloidal solution and redemission from spin coatedllayerunderd 254 nm.PL spectra of YVO4:Eu nanophosphor layerannealed at (a) 300 K, (b) 573 K, and (c) 773 K.TTablebl showingh i threehtimes increment in Isc at λ 300 nm.λ ceCell withYVO4:Eu3 47395005749.380019.217.0APL 93 (2008) 073103

Thin-film structures can reduce the costof solar power by using inexpensivesubstratesb t t andd a lowerlquantitytit anddquality of semiconductor material.However, the resulting short optical pathg and minorityy carrier diffusionlengthlength necessitates either a highabsorption coefficient or excellent lighttrapping.

Silicon NanowiresLight absorptionLongitudinal directionC i collectionCarrierll tiTTransversedirectiondi tiExcellent Light trapping structuresImproved material properties due to miniaturizationOffers new ggeometries for solar cells - not ppossible with bulk or thin filmsA SiNWs based solar cell (with radial p-n junctions):Theoretically(JAP 97, 114302(2005))mg-Si can be usedginstead of sg-SiSiNWs based solar cells are much less sensitive to the impurities ascompared with planar p-n junction solar cells half of the total energy required to fabricate a solarcell can be savedDirect impact on: Processing cost & Energy payback time

Fabrication of Nanowires¾ Gas-Phase Synthesis (Bottom up Approach)– Vapor-Liquid-Solid (VLS) Method(both by CVD and PVD methods)- PECVD, MBE etc.– Oxide Assisted Growth (OAG)¾ Etching Methods (Top down Approach) Metal catalyzed wet Chemical etching/Template-Based Synthesis Dry Etching such

- Physics and circuit model of organic solar cells - Choice of material - St tStructure - bl d bil t dblend, bilayer, tandem - Process optimisation Reliability and stability - Choice of material - Mechanism of degradation - Encappqsulation techniques New & emerging technology issues - Novel methods of fabrication