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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 Energy Solar Constant (1 Sun = 1360 W/m2) Insolation Air Mass Attenuation 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 Radiation 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 solid Rayleigh‐Jeans angle per unit frequency by a black body at temperature T, also known as spectral radiance; UV Catastrophe h is the Planck constant; c is the speed of light 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 Quantum Mechanics only of the temperature, where the constant, b, known as Wien's displacement constant, is equal to 2.8977721(26)×10−3 K m. Photoelectric Effect (Einstein) Atomic and Molecular Physics Nuclear Physics Direct vs. Diffuse Sunlight 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 photovoltaic effect. 1860 ‐ 1881 Auguste Mouchout was the first man to patent a design for a motor running on solar energy. 1873 Willoughby Smith experimented with the photocoductivity properties of selenium. 1883 Charles Fritts made a solid state solar cell 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 Bell Labs unveils it new “solar battery” developed by Gerald Pearson, Daryl Chapin, and Calvin Fuller which was the first modern silicon solar cell. 1950’s Space PV • 1839 ‐ photovoltaic effect discovered • 1883 ‐ first solar 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 Photovoltaics 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 electrons n‐type V DC Voltage p‐type holes current Built‐in Voltage and Electron‐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 Condensed Matter Physics Bonding Crystal Structure Electronic Band Structure Metals, Semiconductors, Insulators Electrons, Plasmons, Photons, Phonons, … Semiconductor Devices Atomic Bonding Metallic, Ionic, or Covalent Bonding Bravais Lattices Crystal Structures Zinc blende Chalcopyrite Si GaAs CuInSe2 Band Theory of Solids Zeeman Effect, Stark Effect Metals Semiconductor 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 phonon 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 photon (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 Band Gap 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 Spectroscopy Si “band edge” Fermi‐Dirac Statistics E Ef Fermi Level 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 Diode 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 = solar cell efficiency (%) 2 J V Jsc = short‐circuit current density (mA/cm ) FF sc oc 2 Voc = open‐circuit voltage (V) 1000W / m FF = fill factor Solar cell efficiency is determined by inherent losses (i.e., solar photons with energies 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 • Irradiance 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 electricity 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
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