Solar Cells:Energy for the Future

Basic Solar Cell DesignDOE - Solar Energy Technologies ProgramNational Renewable Energy Laboratory

Measures of EfficiencyShort Circuit Current40~50mA/cm2Illumination currentOpen Circuit Voltage500~700mVFill FactorSquare area0.7-0.85EfficiencyProduction: 10-15%Laboratory: 20-25%Green, Martin A. Solar Cells Operation Principles, Technology and System Applications

Efficiency LossesLight reflectionSiliconElectrical contact coverage

Cell thicknessLower collection probability away from depletion regionRecombinationDefect statesWavelength of LightMaterial dependentMaterial resistancesBoth bulk and contactTemperatureMetal and semiconductor dependence

Silicon Various TypesSingle-crystal siliconCzochralskiFloat-zonePolycrystalline siliconRibbonAmorphous siliconDOE - Solar Energy Technologies ProgramEvergreen Solar Technology

Materials -SilconSiliconIndirect bandgap Eg = 1.142eVLow absorptivityPhoton travels farther before absorbed >100m thickPhoton + Phonon absorption processes (indirect)RecombinationDominated by defectsImpurities and surface statesGreen, Martin A. Solar Cells Operation Principles, Technology and System Applications

Materials-SiliconSilicon (continued)Doping (~1016 cm-3)P-type: BoronTrace amounts in Cz growth processN-type: PhosphorusPOCl3 + oxygen gas stream in heated furnace to oxidize SiDiffusion of P from oxide into SiContactsVacuum evaporationThree layersTi for good Si adherenceAg for high conductivity Pd barrier layer inbetweenSintering at high T (500-600C) for low resistance and high adherence

Materials- SiliconContacts (continued)Back is completely coveredMetal grid on frontAntireflective CoatingVacuum evaporationVarious oxides of Si, Al, Ti, TaEncapsulationStructural back for support and moisture resistanceAl, Steel, GlassTransparent front for light transmission Glass

Typical Silicon Cell DesignSingle and Polycrystalline Silicon Amorphous SiliconDOE - Solar Energy Technologies ProgramThe Solarserver Forum

Improving Silicon Cell Design (I)Textured top surfaceSelective etching to couple light into cellSurface passivationSiOx or SiNX Restores bonding state of dangling surface Si bondsBack Surface FieldLow recombination velocity interfaceScreen print Al and fire to alloyGreen, Martin A. Solar Cells Operation Principles, Technology and System Applications

Improving Silicon Cell Design (II)Layer thicknessThinner = lower light absorption Carrier diffusion length and surface passivation importantIf high recombination, then want thinnerContact placementBoth on back: ~25% efficiencyGreen, Martin A. Solar Cells Operation Principles, Technology and System ApplicationsHandbook of Photovoltaic Science and Engineering

Silicon Cell EfficiencyWikipedia.org

CostsHandbook of Photovoltaic Science and Engineering

Structure ComparisonHighest efficiencyMany processing techniquesPurity = Process dependentExpensiveCircular cellsHuge marketHigh waste (ingot)Excellent electrical propertiesSingle CrystallinePolycrystallineCheaper than single crystallineLess efficient More easily formed into squaresHigh waste

Advantages/Disadvantages of SiliconSecond most abundant element in the crustWell-developed processing techniquesHuge market for crystalline SiHighest efficiencyADVANTAGESDISADVANTAGESNeed thick layer (crystalline)BrittleLimited substratesExpensive single crystalsSome processing wasteful

Other Inorganic Solar CellsAmorphous Si-based Solar Cells

Cu(InGa)Se2 Solar Cells

Cadmium Telluride Solar Cells

GaAs

InN Solar Cells

Motivation for Other MaterialsGraph of Semi-conductor band gap vs. EfficiencyA band gap of ~1.4eV matches the photon energies where the suns spectral intensity is strongestGaAs is an example of a material with an optimal band gapSilicon Band Gap is 1.1 eV, not optimalThis explains why there is a maximum in efficiency for single layer devicesGreen, Martin A. Solar Cells Operation Principles, Technology and System Applications

Amorphous Si Solar CellsAmorphous Silicon SemiconductorFirst made 1974Plasma depositedDopingp-type: B2H6n-type: PH3Hydrogen helps propertieshydrogenated amorphous silicon (a-Si:H)Alloying changes the band gapGe, C, O, or NGe used for bilayer devices

a-Si:H: Photodiode DesignPhotodiode: three layers(typical example)20 nm p-type layerFew hundred nm intrinsic layer20 nm n-type layerBuilt-in E-Field~ 104 V/cmVocVaries with band gapBand gap varies with alloyingHandbook of Photovoltaic Science and Engineering:Depiction of an a-Si:H photodiode

a-Si:H: Photodiode DesignDirection of incoming lightPhotons reach p-type firstAsymmetry in the drift of holes and electronsPower drop if lighted from the n-type sideWidth of Intrinsic layerThicker cells do not absorb much more lightBest thickness around 300nm (power saturates)

Handbook of Photovoltaic Science and Engineering:Computer calculation of Power vs. Intrinsic Layer Thickness for different absorption coefficients. Solid symbols indicate illumination through the p-layer. Open Symbols indicate illumination through the n-layer

a-Si:H: Cell DesignTwo types of cell designSuperstrate (left): better for applications in which the glass substrate can be an architectural elementSubstrate (right): Substrate can be flexible Stainless SteelSubstrate affects the properties of the first photodiode layer depositedHandbook of Photovoltaic Science and Engineering:Design of the cell

Advantages of a-Si:HTechnology simple and inexpensive compared to crystalline technologyStill need to lower costsAbsorbs more light: need less material than c-SiBetter high temperature stability than c-SiBand gap: variable, 1.4-1.8 eVEfficiency ~15%Handbook of Photovoltaic Science and Engineering: IV curves for amorphous silicon solar cells at two different times

Further AdvantagesMust be hydrogenatedLow efficiencyPoor electrical propertiesHigh light absorptionVery little needed (~1/100th)Produced at lower TMany substratesLow costDisadvantages

Advantages of Other MaterialsCu(InGe)Se2 (CIGS)Thin film: easy fabrication, low costBand gap: variable, 1.0-1.2 eVHigh efficiency up to 18.8%High radiation resistanceCan take large variations in composition without appreciably affecting the optical properties

Cadmium Telluride (CdTe)Also Thin FilmBand gap in optimal range: 1.5eVEfficiencies of about 7%

Advantages of Other MaterialsGaAsBand gap in the optimal range: 1.4 eVEfficiencies of >20% shown (1982)

InNOptical band gap is also a good match to the suns spectrum: can tune the band gapThis means that multiple layers can be used to absorb different wavelengths and the crystal structures wont mismatchBand gap: 0.7 eVLarge heat capacity, resistant to radiation many defects but this does not affect light emitting diodes of the same material

Dye Sensitized Solar Cell (Grtzel Cell)Overall power conversion efficiency of 10.4% has been attained (US National Renewable Energy Laboratory)

General Structure:GlassTransparent Conductor (ITO)Semiconducting Oxide (TiO2)DyeElectrolyteCathode (Pt)Glass

M. Grtzel, Dye Sensitized Solar Cells, Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 4, 145153 (2003)

Components (I)Mesoporous oxide films:Network of tiny crystals measuring a few nanometers across. Can be TiO2, ZnO, SnO2, Nb2O5, CdSeExceptional stability against photo-corrosionLarge band gap (>3eV) = transparency for large part of spectrumM. Grtzel, Photoelectrochemical cells, Nature, 414, 338 (2001).SEM of the surface of a mesoporous anatase film prepared from a hydrothermally processed TiO2 colloid.

Components (II): The dyeM. Grtzel, Dye Sensitized Solar Cells, Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 4, 145153 (2003)Dye absorbs light and generates current in the entire visible spectrum

Components (III)Mesoscopic pores filled with a semiconducting or a conducting medium (such as a p-type semiconductor, a polymer, a hole transmitter or an electrolyte)Traditional electrolyte material consists of iodide (I-) and triiodide (I3-) as a redox couple.

M. Grtzel, Photoelectrochemical cells, Nature, 414, 338 (2001).

DSSC: OperationMesoporous dye-sensitized TiO2, receives electrons from the photo-excited dye Oxidized dye in turn oxidizes the mediator in electrolyte Mediator is regenerated by reduction at the cathode.M. Grtzel, Photoelectrochemical cells, Nature, 414, 338 (2001).

DSSC: DegradationPhoto-chemical or chemical degradation of the dye (e.g. desorption of the dye, or replacement of ligands by electrolyte species or residual water molecules)Direct band-gap excitation of TiO2 (holes in the TiO2 valence band act as strong oxidants)Photo-oxidation of the electrolyte solvent, release of protons from the solvent (change in pH)Dissolution of Pt from the counter-electrode in contact with electrolyteAdsorption of decomposition products onto the TiO2 surface.

J. Halme, Dye-sensitized nanostructured and organic photovoltaic cells: technical review and preliminary tests, Helsinki University of Technology, Masters Thesis (2002).

DSSC: BenefitsRelatively cheap to fabricatethe expensive and energy-intensive high-temperature and high-vacuum processes needed for the traditional devices can be avoidedCan be used on flexible substratesCan be shaped or tinted to suit domestic devices or architectural or decorative applications.Stable even under light soaking for more than 10,000 h (with certain conditions/materials that are less efficient).M. Grtzel, Photoelectrochemical cells, Nature, 414, 338 (2001).

DSSC: DrawbacksEfficiencies not yet commercially competitive with Si-based alternatives.Degradation still an issueEC Cell cycles important to operationEncapsulation necessaryHigh temperature stability a problemProduction only at small scale

DSSC: Costs$0.40/Wp at 5% module efficiency (Zweibel 1999)J. Halme, Dye-sensitized nanostructured and organic photovoltaic cells: technical review and preliminary tests, Helsinki University of Technology, Masters Thesis (2002).

Organic Heterojunction Solar CellsEfficiency of 3.5% has been achievedP.Peumans, S.Uchida, S.R.Forrest. Nature, 425, 158 (2003).BilayerBulk Heterojunction

Summary of PV & PEC cellsM. Grtzel, Photoelectrochemical cells, Nature, 414, 338 (2001).

Photovoltaic Efficiency ComparisonSPIE Magazine of Photonics Applications and Technologies

Environmental Impact CO2 EmissionsPV will be responsible for the displacement of millions of metric tons of CO2 per year, even under the most modest estimatesV Fthenakis, S Morris PREDICTIONS OF FUTURE PV CAPACITY AND CO2 EMISSIONS' REDUCTION IN THE US. 2003

Environmental Impact Other PollutantsAccording to economic models, PV will result in the reduction of NOx, soot, and SO2 V Fthenakis, S Morris

High light absorptionVery little neededProduced at lower TMany substratesMust be hydrogenatedLow efficiencyAmorphous Silicon as a semiconductor discovered in 1973 in Dundee Scottland by Walter Spear and Peter LeComberMade by Carlson at RCA in Princeton. Findings reported in 1976 by Carlson and Wronski. (5,6,7)Figure 12.1 Current density versus voltage under solar illumination for a very early single-junction amorphous silicon solar cell (Carlson and Wronski [5]) and from a recent triple-junction cell (Yang, Banerjee, and Guha [8]). The stabilized efficiency of the triple-junction cell is 13.0%; the active area is 0.25 cm2Alloying: Ge used in bilayer cells

Figure 12.3 In a pin photodiode, excess electrons are donated from the n-type to the p-type layers, leaving the charges and electric fields illustrated. Each photon absorbed in the undoped, intrinsic layer generates an electron and a hole photocarrier. The electric field causes these carriers to drift in the directions shown. pin diodes are incorporated into solar cells in either the superstrate or substrate designs. For amorphous siliconbased cells, photons invariably enter through the p-type window layer as shown here

Figure 12.17 Computer calculation of the power output from a pin solar cell as a function of intrinsic layer thickness. The differing curves indicate results for monochromatic illumination with absorption coefficients from 5000/cm to 100 000/cm; for typical a-Si:H, this range corresponds to a photon energy range from 1.8 to 2.5 eV (cf. Figure 12.2). Solid symbols indicate illumination through the p-layer and open symbols indicate illumination through the n-layer. Incident photon flux 2 1017/cm2s; no back reflector18.8% for .5 cm2, 13.4% for 3459 cm2A single layer of dye molecules absorb less than one percent of the incoming light.Only dye in contact with semiconductor can inject electrons.Mesoporous Semiconductor has a surface area available for dye chemisorption over a thousand times that of a flat, unstructured electrode of the same size.

mesoporous dye-sensitized TiO2, receives electrons from the photo-excited dye (which is thereby oxidized).Oxidized dye in turn oxidizes the mediator (redox species dissolved in electrolyte).Mediator is regenerated by reduction at the cathode (by electrons circulated through the external circuit).

Most have been linked to degradation of electrolyte. some can be alleviated by use of different solvents, or adding other things.

Causes are temp, light, and UV based

dye molecule must sustain at least 108 redox cycles of photo-excitation, electron injection and regeneration, to give a device service life of 20 years.Can be achieved with certain solvents for electrolyte formulation (valeronitrile, or -butyrolactone)

tin oxide coated glass is about $10/m2 (Zweibel 1999),suggesting $20/m2, or $0.40/Wp at 5% module efficiency,

Light Absorption (creation of exciton (electron hole pair)Exciton diffusion (diff length very short, therefore use bulk heterojunction)Dissociation at juntion

Made by spin coating or evaporating solution of ETL and HTL evaporating out the solvent, and annealing it to phase separate

performance of the cell declined rapidly within hours of exposure to sunlight