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Photovoltaics 101

Photovoltaics 101

Photovoltaics 101

Ryne P. Raffaelle Vice President for Research and Associate Provost Rochester Institute of Technology It would be great to talk to you about your tutorial for the FOCUS CSP folks. There are some specific topics (high T solar cells' performance vs G and T, III‐V materials choices, durability at high T, antireflection) that it would be good for you to cover, in addition to the basic PV material.

Solar

Solar Constant (1 = 1360 W/m2) Insolation Air Mass Atmospheric Attenuation Rayleigh Scattering by molecules (mostly UV) Scattering by aerosols and dust Absorption by gases and water Air Mass = 1/cos 1

S= length of shadow from vertical structure of height = h

Standard Test Conditions (STC) Air Mass 1.5 (1000W/m2, ASTM G173‐03 and IEC 60904‐3) at 25 oC

Quantum Theory of Electromagnetic

Planck's law states that

where I(ν,T) is the energy per unit time (or the power) radiated per unit area of emitting surface in the normal direction per unit Rayleigh‐Jeans angle per unit by a black body at temperature T, also known as spectral radiance; UV Catastrophe

h is the ; c is the speed of in a vacuum; k is the Boltzmann constant; ν is the frequency of the electromagnetic radiation; and T is the absolute temperature of the body.

Wien's displacement law

Quanta (Lennard, Helmholtz, Planck, Einstein) The wavelength at which the intensity per unit wavelength of the radiation produced by a black body is at a maximum is a function only of the temperature, where the constant, b, known as Wien's displacement constant, is equal to 2.8977721(26)×10−3 K m. (Einstein) Atomic and Molecular Nuclear Physics Direct vs. Diffuse

The amount of sunlight, as well as the relative amounts of direct versus diffuse sunlight will vary geographically.

Direct versus Global Geographical Distribution PV History 1839 Edmund Becquerel discovers the .

1860 ‐ 1881 Auguste Mouchout was the first man to patent a design for a motor running on . 1873 Willoughby Smith experimented with the photocoductivity properties of .

1883 Charles Fritts made a solid state by coating selenium with a thin layer of gold. 1940 Russell Ohl discovers the “p‐n junction”

1941 Russell Ohl receives a US Patent 2402662, "Light sensitive device" 1954 AT&T unveils it new “solar battery” developed by , , and Calvin Fuller which was the first modern solar cell. 1950’s Space PV

• 1839 ‐ photovoltaic effect discovered • 1883 ‐ cell created • 1941 ‐ modern pn junction solar cell demonstrated • 1954 ‐ doped silicon first used in solar cells • 1958 ‐ first spacecraft to use solar panels (Vanguard I) • 1977 ‐ SERI begin operation • 1989 ‐ first dual junction cell created • 1991 ‐ NREL is established • 1991 ‐ terrestrial PV shipments exceeds 50 MW • 1994 ‐ 30% efficiency barrier broken • 1996 ‐ National Center for chartered • 1997 ‐ terrestrial PV shipments exceed 100 MW • 2004 ‐ terrestrial PV shipments exceeds 1 GW • 2006 ‐ 40% efficiency barrier broken • 2009 ‐ terrestrial PV shipments exceeds 10 GW Silicon Photovoltaic Solar Cell

n‐type V DC Voltage p‐type holes current

Built‐in Voltage and ‐Hole Pairs Silicon Solar Cell

Metallic Grid Fingers Anti‐reflection Coatings Texturing Selective Emitter Back Surface Fields Back Surface Passivation Back Point Contacts Thin Si p‐tyoe Wafer

Bonding

Crystal Structure

Electronic Band Structure

Metals, , Insulators

Electrons, Plasmons, , , …

Semiconductor Devices Atomic Bonding

Metallic, Ionic, or Covalent Bonding

Bravais Lattices

Crystal Structures

Zinc blende

Chalcopyrite

Si GaAs

CuInSe2 Band Theory of

Zeeman Effect, Stark Effect

Metals or Insulator (depends on size of the gap) Quantum Mechanical Particles • Wave‐particle duality • Defined by their properties (charge, spin, mass, …) • Interact with each other

Photons: mediating particle of electromagnetic radiation

Electrons: charge, fundamental building blocks of matter

Hole: defined in terms of the semiconductor lattice, absence of and electron

Excitons: bound electron hole pair

Plasmons: collective oscillation of “free electrons” in matter

Phonons: quanta of lattice vibrational energy, conductors of most heat in matter

Polarons: bound electron pairs Electronic Transitions

Empty States

Filled States (available electronic energy levels are occupied by electrons)

Electrons can be promoted from filled states to empty state by absorbing energy. This energy can be provided optically, thermally, or through impact.

• Optically means they absorb a (h or hc/)

• Thermally means they absorb a phonon (hkB) • Impact means energy is transferred by a collision with another particle. Photon is Below the

E

k Photon Slightly Above the Band Gap

E

Electron

Hole k Photon is Well Above the Band Gap

E

“Thermalization”

k Direct versus Indirect Bandgaps

e.g., GaAs, InGaP, … e.g., Si Si Electronic Band Structure Optical Absorption

Si “band edge” Fermi‐Dirac Statistics

E

Ef

k

Half way between the highest filled level and the lowest empty level.

Increasing the temperature will increase the number of electrons in the conduction band. Semiconductor Electrical Conductivity versus Temperature

• Insulating at low temperatures

• KbTat room temperature 0.0256 eV • Poor electrical conductor a most temperatures Semiconductor Doping Fermi Energy with Doping

E E E

E Ef f

Fermi Level Ef k k k

Intrinsic –no doping p–type n‐type Electrical Conductivity Temperature Coefficients

Law of Wiedemann and Franz “a good electrical conductor is a good thermal conductor”

Metallic Behavior Misleading. Most of the heat is carried by phonons not electrons Semiconductor Conductivity Versus Temperature

E

Ed “donor level”

k The p‐n Junction

Homojunction: same semiconductor on both sides (Si) Heterojunction: two different semiconductors form the junction (CdS/CIGS) The Built‐in Voltage The Curve p‐n Junction Solar Cell

Depletion Region

Photo‐excited carriers created at or near the depletion region contribute to the photo‐current.

Photo‐excited carriers that are absorbed but not “collected” either radiatively recombine or non‐radiatively recombine (i.e., heat up the cell). The Diode Equation

Reverse Saturation Current

“Light” I‐V Measurement light

dark

light “Light” I‐V Measurement

 = solar cell efficiency (%) J V J = short‐circuit current density (mA/cm2)   FF sc oc sc 2 Voc = open‐circuit voltage (V) 1000W / m FF = fill factor

Solar cell efficiency is determined by inherent losses (i.e., solar photons with below Jsc, Voc, and FF are intimately and fundamentally tied. the bandgap of the host material) and those It is difficult to affect one without changing the others. primarily due to optical or electrical losses (i.e., reflectance, series resistance, etc.). Jsc, Voc, & MPP

Maximum Power Point Solar Simulation and Testing

Solar Simulators • Spectral match 'A' is the top • inhomogeneity ‐ rating an 'C' is the spatial uniformity over the lowest rating (IEC illumination area 60904‐9 Ed. 2.0.) • Temporal Instability ‐ stability over time.

“1 Sun” Simulators, LAPSS –Large Area Pulsed Solar Simulators, Outdoor Test Facility (OTF), SolarTac Theoretical Conversion Efficiency Monochromatic Efficiency Tuned to • Shockley‐Quiesser Limit the Bandgap can be >50% • Detailed Balance (in thermal equilibrium each process is equilibrated by its reverse process: generation and recombination) • Treats a solar cell just like a heat engine.

SQ Limit 33.7%

1.45 eV Optimum PV Bandgap Isc and Voc vs. Eg

Lowering the bandgap of a solar cell • Absorb more of the spectrum • Increase the current • Decrease the voltage Solar Cell Efficiency versus Temperature

Slight Increase in

Jsc with increasing T

Decrease in

Voc with increasing T

Net effect:  decreases with increasing temperature Solar Cell Efficiency versus Illumination Intensity or Concentration Concentrated Photovoltaics PV 101 Question 1:

If the intensity of the light is increased, what happens to the efficiency of a solar cell? Answer: it increases PV 101 Question 2:

But if the light that doesn’t get converted to goes to heat, what happens to the solar cell if the intensity of the light is increased? Answer: the temperature of the cell increases PV 101 Question 3:

If the temperature of the cell increases, what happens to its efficiency? Answer: it decreases PV101 Question 4:

So, who wins? Answer: increase in efficiency with concentration beats the decrease due to heating, at least for a while.  Active Cooling

“A CPV company who hasn’t had a fire is either lying or isn’t a real company”

Why not actively cool? Majority of CPV systems are passively cooled.

Cost and Reliability Efficiency Losses

• Optical and Electrical • Reflection from the Surface • Grid Obscuration • Radiative Recombination • Non‐Radiative Recombination • Series Resistance Reflective Losses Surface Recombination

Junction Bulk Recombination

Direct Shockely‐Read‐Hall Auger Radiative Defect‐assisted Recombination Recombination Recombination Quantum Efficiency Silicon Solar Cell

Metallic Grid Fingers Anti‐reflection Coatings Texturing Selective Emitter Back Surface Fields Back Surface Passivation Back Reflector Back Point Contacts Thin Si p‐type Wafer Antireflection (AR) Coating Surface Texturing Si Efficiency Improvement

SQ Limit Multi‐Junction Approach

• Split the spectrum over multiple junctions • Reduces thermalization loss (can exceed SQ limit) • Junctions connected in series need to be “current‐matched” • Voltages of individual junctions add • Can use diffractive elements • Can be mechanically stacked orr epitaxially grown Latticed‐Matched Triple Junction

InGaP

Ge Triple Junction Space Solar Cell Lattice‐Matched Multi‐Junctions Lattice‐Matched vs. Metamorphic

• Inverted MM • Wafer Bonding • GaAs Substrates • InP Substrates Tunnel Junctions Multi‐Junction Spectral Response and Current Matching Multijunction Efficiency and Concentration Spatial Spectrum Splitting Concentrated Photovoltaics (CPV)

• Reduced Thermalization • Better at Higher Temp • Uses Optical concentration to reduce necessary cell area “Thermalization Fustration” Concentrating Photovoltaics

Inverted Metamorphic III‐V Solar Cell

> 40% Efficient Tracking Systems

Horizontal single axis tracker (HSAT) Vertical single axis tracker (VSAT) Tilted single axis tracker (TSAT) Polar aligned single axis trackers (PASAT) Tip–tilt dual axis tracker (TTDAT) Azimuth‐altitude dual axis tracker (AADAT) Photoluminescent Emitters and Monochromatic Efficiency Photovoltaic Energy Systems

Residential

Photovoltaic Effect

Regulator and Inverter Utility Scale Solar Energy Potential

3 TW Solar Energy Potential

6 Boxes at 3.3 TW Each Solar Energy Potential

10% 20% 30% 40%

3.6 TW US Consumption PV Production PV Production PV Costs and Production Experience Curve

Agenda Slide (Arial Narrow, 28 pt)

= Q3 2009 Price = End‐of‐Year 2010 Price Materials Cost Comparison of Production Costs for Conventional Silicon and CdTe Thin Film Modules

Silicon $2.10/W CdTe $1.10/W

Encapsulation 27% Encapsulation 50% Coated Glass 29% Feedstock 23%

Ingot 12% Materials 3% Cell 24% Wafer 14% Equipment 13%

Operating 5% “$1/Wp” Thin Film PV Production

• First Solar (CdTe): first company to achieve >1 GW per year • Showa Shell Solar (CIGS): 1 GW within the year • (a‐Si/nc‐Si): 1 GW within the year Building Integrated PV

85kW Shell Solar CIGS in Wales

In Southern California 216 Würth CIGS modules in Tübingen, Germany Making PV More Sustainable

Economical • Raw materials usage • Abundant Materials • Manufacturability • Efficiency • Durability • Market Assessment

Environmentally Safe • Non-toxic alternatives • Aqueous based materials • Re-use, Reman, Recycle Sustainable development is • Environmental Impact Assessment development that meets the needs of the present without Societal compromising the ability of • Reliability future generations to meet • Building Integrated (BIPV) • Productization their own needs – UN Bruntland Commission Capacity Factor

National Laboratory Innovation for Our Energy Future Capacity Factor

The sun rises and sets and doesn’t shine at night. Therefore, by definition its capacity factor is quite low.

This chart indicates the range of recent capacity factor estimates for energy technologies. The dots indicate the average, and the vertical lines represent the range: Average + 1 Source: NREL (July 2010) standard deviation; and average ‐ 1 standard deviation.

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