Lecture 5. Experiment: Types of Solar Cells•Generation I solar cells:Single Crystal Si, Polycrystalline SiGrowth, impurity diffusion, contacts, anti-reflection coatings•Generation II Solar cells:Polycrystalline thin films, crystal structure, deposition techniquesCdS/CdTe (II-VI) cellsCdS/Cu(InGa)Se2 cellsAmorphous Si:H cells•Generation III Solar Cells:•High-Efficiency Multijunction Concentrator Solar cells based onIII-V’s and III-V ternary analogues•Dye-sensitized solar cell•Organic (excitonic) cells•Polymeric cells•Nanostructured Solar Cells including Multicarrier per photon cells,quantum dot and quantum-confined cells
Background and Cost• Photovoltaics convertsunlight directly toelectric power – Carbon-neutral – Highly abundant—the 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• Costs Energy 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
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 of materials 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. Their design 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.)
Single Crystal Ingot-based PVs• Single crystal wafers made by Czochralski process, as in silicon electronics• Comprise 31% of market• Efficiency as high as 24.7%• Expensive—batch process involving high temperatures, long times, and mechanical slicing Wafers are not the ideal geometry• Benefits from improvements developed for electronics industry http://hydre.auteuil.cnrs-dir.fr/dae/competences/cnrs/images/icmcb03.jpg
Production- Process mono- or multi- crystalline Silicon crystal growth process Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 7 trier.de
Production process 1. Silicon Wafer-technology (mono- or multi-crystalline) Most purely silicon 99.999999999% melting / crystallization Occurence: Siliconoxide (SiO2) Tile-production = sand Mechanical cutting: Plate-production Thickness about 300µm typical Wafer-size: Minimum Thickness: cleaning 10 x 10 cm2 about 100µm Quality-control Wafer Link to SiO2 + 2C = Si + 2CO Producers of Silicon Wafers Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 8 trier.de
Energía Fotovoltaica Celdas Solares De Silicio monocristalinoMaterial: Silicio monocristalinoTemperatura de Celda: 25ºC Intensidad luminosa: 100%Área de la celda: 100 cm2 Voltaje a circuito abierto: Vca = 0.59 volts Corriente a corto circuito: Icc = 3.2 A Voltaje para máxima potencia: Vm = 0.49 volts Corriente para máxima potencia: Im = 2.94 A Potencia máxima: Pm = 1.44 Watts
Polycrystalline Ingot-based PVs• Fastest-growing technology involves casting Si in disposable crucibles• Grains mm or cm scale, forming columns in solidification direction• Efficiencies as high as 20% in research• Production efficiencies 13-15%• Faster, better geometry, but still requires mechanical slicing
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
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)
Production Process Thin-Film-Process (CIS, CdTe, a:Si, ... )semiconductor materials are evaporated onlarge areasThickness: about 1µmFlexible devices possibleless energy-consumptive than c-Silicon-processonly few raw material neededTypical production sizes:1 x 1 m2 CIS Module Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 17 trier.de
Basic Cell Structure• p-i-n structure – Intrinsic a-Si:H between very thin p-n junction – Lower cells can be a- Si:H, a-SiGe:H, or microcrystalline Si• Produces electric field throughout the cell http://www.sandia.gov/pv/images/PVFSC36.jpg
Cadmium Telluride• One of the most promising approaches• Made by a variety of http://www.nrel.gov/cdte/images/cdte_cell.gif processes – CSS – HPVD http://www.sandia.gov/pv/images/PVFSC29.jpg
Cadmium Telluride Solar CellsD.E.Carlson, BP Solar CdS/CdTe heterojunction: typically chemical bath CdS deposition, and CdTe sublimation. Cd Toxicity is an issue. Best lab efficiency = 16.5% First Solar plans 570 MWp production capacity by end of 2009. John A. Woollam, PV talk UNL 2007
CdTe and CIGS Review: 2006 World PV Conference Noufi and Zweibel, NREL/CP -520-39894, 2006 John A. Woollam, PV talk UNL 2007 35
Nano-Structured CdS/CdTe Solar Cells Graphite CdTe Nanocrystalline CdS ITO Glass Nano 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 may result in high performance, especially in short circuit currents.
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
Modulos Solares de CdTe• Costo 60% de Si• 20 años garantia• Modulos de peliculas delgadas• Potencia 50 – 60 W• Eficiencia 9%
Modulos Solares de CdTe• Costo 60% de Si• 20 años garantia• Modulos de peliculas delgadas• Potencia 50 – 60 W• Eficiencia 9% 100 kW – 1 MW
Copper Indium Diselenide• Also seen as CIGS• Several methods of production http://www.sandia.gov/pv/images/PVFSC25.jpg http://www.sandia.gov/pv/images/PVFSC27.jpg http://www.sandia.gov/pv/images/PVFSC26.jpg
Multijunction Concentrators• Similar in technique• Exotic Materials• More expensive processing (MBE) http://www.nrel.gov/highperformancepv/entech.html
Spectrolab’s Triple-Junction Solar CellD.E.Carlson, BP Solar Spectrolab: 40.7% conversion efficiency at ~ 250 suns. John A. Woollam, PV talk UNL 2007
 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.
Dye-sensitized Solar Cells• O’Regan and Grätzel 1991• Organic dye molecules + nanocrystalline titanium dioxide (TiO2)• 11% have been demonstrated• Benefits: low cost and simplicity of manufacturing• Problems: Stability of the devices
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.
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
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.
Nanostructured Solar Cells• Nanomaterials as light harvesters leading to direct conversion or chemical production alone or imbedded in a matrix. Questions: email@example.com
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.
Cu2S/CdS bulk and nano heterojunction solar cells Bulk heterojunction Nano heterojunction Cr contacts Cu/Cr top contact Thin layer of Cu ~ 10 nm Copper Sulfide Cu2S Inter-pore spacing CdS Nano-porous Alumina Template ITO Cadmium Sulfide Glass ITO
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 ô-1.
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.
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.
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.
Fig. 2. Calculated efficiencies for different QYIImodels.