2012 tus lecture 5

• 1. Lecture 5Experiment: Types of Solar cells

2. Lecture 5.Experiment: Types of Solar CellsGeneration I solar cells:Single Crystal Si, Polycrystalline SiGrowth, impurity diffusion, contacts, anti-reflection coatingsGeneration II Solar cells:Polycrystalline thin films, crystal structure, deposition techniquesCdS/CdTe (II-VI) cellsCdS/Cu(InGa)Se2 cellsAmorphous Si:H cellsGeneration III Solar Cells:High-Efficiency Multijunction Concentrator Solar cells based onIII-Vs and III-V ternary analoguesDye-sensitized solar cellOrganic (excitonic) cellsPolymeric cellsNanostructured Solar Cells including Multicarrier per photon cells,quantum dot and quantum-confined cells 3. Background and Cost Photovoltaics convertsunlight directly toelectric power Carbon-neutral Highly abundantthe earth receives 120 quadrillion watts of power from the sun, humans currently use about 13 trillion watts Lewis, et al. Basic Research Needs for Solar CostsEnergy Utilization. Module cost Balance of system cost Power conditioning cost Currently about $0.30/kWh, a factor of 5-10 behind total cost of fossil fuel generation 4. Figure 3. The three generations of solar cells. First-generation cells are based on expensive silicon wafers and make up 85% of the current commercial market. Second-generation cells are based on thin films ofmaterials such as amorphous silicon, nanocrystalline silicon, cadmium telluride, or copper indium selenide. The materials are less expensive, but research is needed to raise the cells efficiency to thelevels shown if the cost of delivered power is to be reduced. Third-generation cells are the research goal:a dramatic increase in efficiency that maintains the cost advantage of second-generation materials. Theirdesign may make use of carrier multiplication, hot electron extraction, multiple junctions, sunlightconcentration, or new materials. The horizontal axis represents the cost of the solar module only; it mustbe approximately doubled to include the costs of packaging and mounting. Dotted lines indicate the cost per watt of peak power (Wp). (Adapted from ref. 2,) Green.) 5. Generation I. 6. Single Crystal Ingot-based PVs Single crystal wafers made byCzochralski process, as in siliconelectronics Comprise 31% of market Efficiency as high as 24.7% Expensivebatch process involvinghigh temperatures, long times, andmechanical slicing Wafers are notthe ideal geometry Benefits from improvementsdeveloped for electronics industry http://hydre.auteuil.cnrs-dir.fr/dae/competences/cnrs/images/icmcb03.jpg 7. Production-Processmono- or multi- crystalline Siliconcrystal growth processClemson Summer School6.6.06 - 8.6.06Dr. Karl Molter / FH Trier / molter@fh- 7 trier.de 8. Production process 1. Silicon Wafer-technology (mono- or multi-crystalline) Most purely silicon 99.999999999% melting / crystallizationOccurence: Siliconoxide (SiO2)Tile-production = sand Mechanical cutting: Plate-production Thickness about 300m typical Wafer-size:Minimum Thickness:cleaning10 x 10 cm2about 100mQuality-controlWaferLink toSiO2 + 2C = Si + 2CO Producers of Silicon WafersClemson Summer School6.6.06 - 8.6.06Dr. Karl Molter / FH Trier / molter@fh- 8 trier.de 9. Energa FotovoltaicaCeldas Solares De Silicio monocristalinoMaterial: Silicio monocristalinoTemperatura de Celda: 25C Intensidad luminosa: 100%rea de la celda: 100 cm2 Voltaje a circuito abierto: Vca = 0.59 volts Corriente a corto circuito: Icc = 3.2 A Voltaje para mxima potencia: Vm = 0.49 volts Corriente para mxima potencia: Im = 2.94 A Potencia mxima: Pm = 1.44 Watts 10. Polycrystalline Ingot-based PVs Fastest-growing technology involves casting Siin disposable crucibles Grains mm or cm scale, forming columns insolidification direction Efficiencies as high as 20% in research Production efficiencies 13-15% Faster, better geometry, but still requiresmechanical slicing 11. Polycrystalline Si Ribbon PVs String method Two strings drawn through melt stabilize ribbon edge Ribbon width: 8 cm Carbon foil method (edge-defined film-fed growth,EFG) Si grows on surface of a carbon foil die Die is currently an octagonal prism, with side length 12.5 cm Pros and Cons Method can be continuous Requires no mechanical slicing Efficiencies similar to other polycrystalline PVs Balancing growth rate, ribbon thickness and width 12. Generation II. 13. Flat-Plate Thin-Films Potential for cost advantages over crystalline silicon Lower material use Fewer processing steps Simpler manufacturing technology Three Major Systems Amorphous Silicon Cadmium Telluride Copper Indium Diselenide (CIS) 14. Production ProcessThin-Film-Process (CIS, CdTe, a:Si, ... )semiconductor materials are evaporated onlarge areasThickness: about 1mFlexible devices possibleless energy-consumptive than c-Silicon-processonly few raw material neededTypical production sizes:1 x 1 m2 CIS ModuleClemson Summer School6.6.06 - 8.6.06Dr. Karl Molter / FH Trier / molter@fh-17 trier.de 15. Photon Energy 16. Amorphous Silicon a-Si:H Discovered in1970s Made by CVD from SiH4 http://www.solarnavigator.net/images/uni_solar_triple_junction_flexible_cell.jpg 17. MaterialLevel ofLevel of efficiency in %efficiencyin % Lab ProductionMonocrystalline SiliconApprox. 24 14 to 17Polycrystalline SiliconApprox. 18 13 to 15 Amorphous SiliconApprox. 135 to 7 18. Amorphous SiliconGrowth by Thermal CVD 19. Basic Cell Structure p-i-n structure Intrinsic a-Si:Hbetween very thin p-njunction Lower cells can be a-Si:H, a-SiGe:H, ormicrocrystalline Si Produces electricfield throughout thecellhttp://www.sandia.gov/pv/images/PVFSC36.jpg 20. CdTe 21. Cadmium Telluride One of the mostpromising approaches Made by a variety of http://www.nrel.gov/cdte/images/cdte_cell.gifprocesses CSS HPVD http://www.sandia.gov/pv/images/PVFSC29.jpg 22. Cadmium Telluride Solar CellsD.E.Carlson, BP Solar CdS/CdTe heterojunction: typicallychemical bath CdS deposition, andCdTe sublimation. Cd Toxicity is an issue. Best lab efficiency = 16.5% First Solar plans 570 MWpproduction capacity by end of2009.John A. Woollam, PV talk UNL 2007 23. CdTe and CIGS Review: 2006 World PV Conference Noufi and Zweibel, NREL/CP -520-39894, 2006John A. Woollam, PV talk UNL 2007 35 24. Nano-Structured CdS/CdTe Solar CellsGraphiteCdTe Nanocrystalline CdSITOGlassNano CdS/ CdTe device Structure. Band gap of CdS can be tuned in the range 2.4 - 4.0 eV. Nano-structured CdS can be a better window material and mayresult in high performance, especially in short circuit currents. 25. Pros and Cons Pros A material of choice for thin-flim PV modules Nearly perfect band-gap for solar energy conversion Made by a variety of low-cost methods Future efficiencies of 19% "CdTe PV has the proper mix of excellent efficiency and manufacturing cost to make it a potential leader in economical solar electricity." Ken Zweibel, National Renewable Energy Laboratory Pros Health Risks Environmental Risks Safety Risks Disposal Fees 26. Modulos Solares de CdTe Costo 60% de Si 20 aos garantia Modulos de peliculasdelgadas Potencia 50 60 W Eficiencia 9% 27. Modulos Solares de CdTe Costo 60% de Si 20 aos garantia Modulos de peliculasdelgadas Potencia 50 60 W Eficiencia 9% 100 kW 1 MW 28. Copper Indium Diselenide Also seen as CIGS Several methods ofproduction http://www.sandia.gov/pv/images/PVFSC25.jpg http://www.sandia.gov/pv/images/PVFSC27.jpg http://www.sandia.gov/pv/images/PVFSC26.jpg 29. A New Contender? Cu2ZnSnS4 30. Tandem-cellPattern of a multi-spectral cell on thebasis of theChalkopyriteCu(In,Ga)(S,Se)2Clemson Summer School 6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 51 trier.de 31. Generation III. 32. High Efficiency ConcentratorSolar Cells 33. Multijunction Concentrators Similar in technique Exotic Materials More expensive processing (MBE) http://www.nrel.gov/highperformancepv/entech.html 34. Spectrolabs Triple-Junction Solar CellD.E.Carlson, BP Solar Spectrolab: 40.7% conversion efficiency at ~ 250 suns.John A. Woollam, PV talk UNL 2007 35. [edit] Gallium arsenide substrateTwin junction cells with Indium gallium phosphideand gallium arsenide can be made on galliumarsenide wafers. Alloys of In.5Ga.5P throughIn.53Ga.47P may be used as the high band gapalloy. This alloy range provides for the ability tohave band gaps in the range of 1.92eV to 1.87eV.The lower GaAs junction has a band gap of1.42eV.The considerable quantity of photons in the solarspectrum with energies below the band gap ofGaAs results in a considerable limitation on theachievable efficiency of GaAs substrate cells. 36. Dye-Sensitized solar cells 37. Dye-sensitized Solar Cells ORegan and Grtzel 1991 Organic dye molecules + nanocrystallinetitanium dioxide (TiO2) 11% have been demonstrated Benefits: low cost and simplicity ofmanufacturing Problems: Stability of the devices 38. OperationSunlight enters the cell through the transparent SnO2:F topcontact, striking the dye on the surface of the TiO2. Photonsstriking the dye with enough energy to be absorbed will create anexcited state of the dye, from which an electron can be "injected"directly into the conduction band of the TiO2, and from there itmoves by diffusion (as a result of an electron concentrationgradient) to the clear anode on top.Meanwhile, the dye molecule has lost an electron and themolecule will decompose if another electron is not provided. Thedye strips one from iodide in electrolyte below the TiO2, oxidizingit into triiodide. This reaction occurs quite quickly compared to thetime that it takes for the injected electron to recombine with theoxidized dye molecule, preventing this recombination reactionthat would effectively short-circuit the solar cell.The triiodide then recovers its missing electron by mechanicallydiffusing to the bottom of the cell, where the counter electrode re-introduces the electrons after flowing through the external circuit. 39. Organic and Nanotech Solar CellsBenefits: 10 times thinner than thin-film solar cells Optical tuning Low cost for constituent elements High volume productionProblems: Current efficiencies < 3-5% Long term stability 40. Organic Solar Cells 41. Fig. 1. The scheme of plastic solar cells. PET -Polyethylene terephthalate, ITO - Indium TinOxide, PEDOT:PSS - [[Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate), Active Layer (usually apolymer:fullerene blend), Al - Aluminium. 42. Nanostructured Solar cells 43. Nanostructured Solar Cells Nanomaterials as lightharvesters leading todirect conversion orchemical productionalone or imbedded ina matrix.Questions: [email protected] 44. Fig.2 (a) Nanostructure of anodically formed Al2O3 template. (b) its cross-section,(c) catalyst deposited at the bottom of the pores, (e) vertically aligned nanotubes, and (f) TEM image of a nanotube. 45. Cu2S/CdS bulk and nano heterojunction solar cells Bulk heterojunctionNano heterojunction Cr contacts Cu/Cr top contactThin layer of Cu ~10 nm Copper SulfideCu2S Inter-pore spacingCdSNano-porous AluminaTemplateITOCadmium SulfideGlass ITO 46. ITO n-CdSAlumina z p-CIS zMo/Glass 47. PTCBI Porous Al2O3 CuPc ITOAl or Ag PTCBICuPcITO 48. Quantum Dots 49. Figure 3. Photoexcitation at 3Eg creates a 2Pe-2Ph exciton state. This state is coupled to multiparticle states with matrix element V and forms a coherent superposition of single and multiparticle exciton states within 250 fs. The coherent superposition dephasesdue to interactions with phonons; asymmetric states (such as a 2Pe-1Sh) couple strongly to LO phonons and dephase at a rate of -1. 50. To study MEG processes in QDs, we detectmultiexcitons created via exciton multiplication(EM) bymonitoring the signature of multiexciton decay inthetransient absorption (TA) dynamics, whilemaintaining apump photon fluence lower than that needed tocreatemultiexcitions directly. The Auger recombinationrate isproportional to the number of excitons per QDwith thedecay of a biexciton being faster than that of thesingleexciton. By monitoring the fast-decay componentof theTA dynamics at low pump intensities we canmeasure thepopulation of excitons created by MEG. 51. The work reported here provides a confirmation of theprevious report of efficient MEG in PbSe. We observed apreviously unattained 300% QY exciting at 4Eg in PbSe QDs,indicating that we generate an average of three excitons perphoton absorbed. In addition, we present the first knownreport of multiple exciton generation in PbS QDs, at anefficiency comparable to that in PbSe QDs. We have shownthat a single photon with energy larger than 2Eg cangeneratemultiple excitons in PbSe nanocrystals, and we introduce anew model for MEG based on the coherent superposition ofmultiple excitonic states. Multiple exciton generation incolloidal QDs represents a new and important mechanismthat may greatly increase the conversion efficiency of solarcell devices. 52. For the 3.9 nm QD (Eg = 0.91 eV), the QY reaches asurprising value of 3.0 at Ehn/Eg = 4. This means that onaverage every QD in the sample produces threeexcitons/photon. 53. Fig. 2. Calculated efficiencies for different QYIImodels. 54. Modules 55. PV Module Conversion Efficiencies D.E.Carlson, BP Solar ModulesLab Dye-sensitized solar cells 3 5% 11% Amorphous silicon (multijunction)6 - 8% 13.2% Cadmium Telluride (CdTe) thin film 8 - 10%16.5% Copper-Indium-Gallium-Selenium (CIGS)9 - 11%19.5% Multicrystalline or polycrystalline silicon12 - 15% 20.3% Monocrystalline silicon14 - 16% 23% High performance monocrystalline silicon 16 - 19% 24.7% Triple-junction (GaInP/GaAs/Ge) cell (~ 250 suns)-40.7% John A. Woollam, PV talk UNL 2007 56. The Future?