Chapter 4 solar resource+pv materials feb 2011

Green Energy CourseRenewable Energy Systems Biên sọan: Nguyễn Hữu Phúc Khoa Điện- Điện Tử- Đại Học Bách Khoa TPHCM The Solar Resource • Before we can talk about solar power, we need to talk • • • • • about the sun Need to know how much sunlight is available Can predict where the sun is at any time Insolation : incident solar radiation Want to determine the average daily insolation at a site Want to be able to chose effective locations and panel tilts of solar panels The Sun and Blackbody Radiation • The sun – – 1.4 million km in diameter 3.8 x 1020 MW of radiated electromagnetic energy • Blackbodies – – – Both a perfect emitter and a perfect absorber Perfect emitter – radiates more energy per unit of surface area than a real object of the same temperature Perfect absorber – absorbs all radiation, none is reflected The Solar Resource • Before we can talk about solar power, we need to talk • • • • • about the sun Need to know how much sunlight is available Can predict where the sun is at any time Insolation : incident solar radiation Want to determine the average daily insolation at a site Want to be able to chose effective locations and panel tilts of solar panels Plank’s Law • Plank’s law – wavelengths emitted by a blackbody depend on temperature 3.74 108 E   14400     exp    1  T    (7.1) • λ = wavelength (μm) • Eλ = emissive power per unit area of blackbody • (W/m2-μm) T = absolute temperature (K) Electromagnetic Spectrum Visible light has a wavelength of between 0.4 and 0.7 μm, with ultraviolet values immediately shorter, and infrared immediately longer Source: en.wikipedia.org/wiki/Electromagnetic_radiation 288 K Blackbody Spectrum The earth as a blackbody Figure 7.1 Area under curve is the total radiant power emitted Stefan-Boltzmann Law • Total radiant power emitted is given by the Stefan – Boltzman law of radiation E  A T • • • • (7.2) E = total blackbody emission rate (W) σ = Stefan-Boltzmann constant = 5.67x10-8 W/m2-K4 T = absolute temperature (K) A = surface area of blackbody (m2) Wien’s Displacement Rule • The wavelength at which the emissive power per unit area reaches its maximum point max • • • • 2898  T (7.3) T = absolute temperature (K) λ = wavelength (μm) λmax =0.5 μm for the sun , T = 5800 K λmax = 10.1 μm for the earth (as a blackbody), T = 288 K Extraterrestrial Solar Spectrum Figure 7.2 CdTe by CSS Sealed Volume • Closed Closed space sublimation is the primary method for deposition of CdTe Gas inlet He + O2 CdTe Source 600-800 °C Diffusion of CdTe (1-50 Torr) Substrate Pump 600 °C Deposition and re-evaporation rates from the substrate and source are controlled by relative temperatures of source and substrate O2 increases acceptor density in the CdTe (more p-type) For example of process, see C.S Ferekides et.al High efficiency CSS CdTe solar cells Thin Solid Films, vol.361-362, 21 Feb 2000, pp.520-6 2/18/2012 Part 1: Slide 137 CdTe Treatment with CdCl The CdCl2 is not incorporated into the film Some oxygen may be added to the gas phase as well to increase p-type doping 2/18/2012 CdCl2 CdTe CdCl2 acts as a flux, allowing an increase in grain size and grain quality 400°C • CdTe grain structure is significantly improved by CdCl2 treatment +O2 Part 1: Slide 138 CdS by Chemical Bath Deposition • CdTe/CdS and CuInSe2/CdS heterojunctions Sample formed by dip coating Holder Temperature • Typical dip recipe: probe 2/18/2012 Solution Water bath Solutions: • 0.015 M Cd salt [e.g.: CdSO4] • 1.5 M Thiourea [SC(NH2)2] • 30% Ammonium Hydroxide [HN4OH] • Deionized water • 60-80°C • Reaction occurs spontaneously over several minutes • Nanocrystalline Cd deposited on all surfaces Samples Magnetic Stirrer Part 1: Slide 139 CdTe Device Contacts • “Top” contact (glass side) • “Back” contact • Best devices use SnO2:F • Best devices use Cudoped graphite • Surface defects (especially O vacancies) are critical to the contacts properties • Cu reacts at the interface to form Cu2Te • These affect the Fermi energy at the interface 2/18/2012 • This forms the contact • Instabilities are a problem Part 1: Slide 140 Thin Film Devices • Cadmium Telluride • Issues – – – 2/18/2012 Cathode contact to the back of the device is unstable Cd causes cancer Single junction devices allow great improvement potential Part 1: Slide 141 • • • • • 2/18/2012 Shell Solar CIS modules Thin Film Devices Copper-Indium Diselenide Hard & little understood Works great when done right Basic methods complex Yield is difficult to obtain Part 1: Slide 142 Cu(In1-xGax)Se2 Solar Cells Non-CIGS Layers: Non• ZnO Top Contact • Sputtered or by MOCVD • One layer intrinsic • One layer n++ • Intrinsic CdS • Grown from solution n++ ZnO contact i-ZnO i-CdS Cu(In1-xGax)Se2 [p-type] Mo contact Glass • Mo Back Contact • Rf or dc magnetron sputtered • High stress typical 2/18/2012 Part 1: Slide 143 Deposition of Cu(In1-xGax)Se2 Current Processes: • Evaporation • High rate • Easy control • Difficult to scale up • High temperature process • Solid Phase Reaction • Easy to scale up • High residual stresses • Sequential room temperature process followed by a high temperature reaction 2/18/2012 Se Part 1: Slide 144 Issues in CIGS Deposition • Control of point defects in CIGS • Ordered point defects modify energy gap • Point defects control type and carrier concentration • Back contact • Selenization (solid phase reaction) causes stress that produces adhesion failures • Mo produces a 0.3 eV barrier Schottky contact to CIGS • Supply of Na is critical to device optimization 2/18/2012 Part 1: Slide 145 Point Defects in CIGS • b-phase: ordered defect compound • n-type • Egap~1.2 eV without Ga d: sphalerite a: chalcopyrite b: “P” chalcopyrite Temperature (°C) • Cu Cu deficient Cu(In,Ga)Se2 dissolves point defects • p-type • Egap and p depend upon Ga content 900 800 700 2/18/2012 ad 600 500 a b 400 300 a+ Cu2Se (HT) ab 200 100 Phase diagram from T Haalboom et.al Inst Phys Conf Ser No 152, Proc 11th Int Conf on Ternary & Multinary Compounds (ICTF-11) [IOP Publishing, Bristol, 1998], p 249 d 15 a + Cu2Se (LT) 20 25 Atomic % Cu Part 1: Slide 146 30 • • Cu(In,Ga)Se2 Issues – – – – 2/18/2012 Shell Solar CIS modules Thin Film Devices What limits the device performance is unknown Limited supply of In and Ga Hard to make by simple methods Single junction devices allow great improvement potential Part 1: Slide 147 (Ga1-xInx)N • • Inorganic alloy that covers the entire solar spectrum for multijunction devices Defects and their impact are little known • Issues: • Epitaxial devices: processing is expensive & difficult • Ga & In are rare 2/18/2012 Part 1: Slide 148 Novel Concepts: Nano • Carrier extraction: how you get carriers out of a nanoparticle? • Coulomb interaction: amplification in the dot enhances multiexcitons but enhances carrier loss and increases exciton energy • Indirect gap nano: reduces recombination but loses energy • Exciton extraction: requires two identical contacts with equal carrier transmission • Surface recombination: Almost impossible to eliminate • Exciton barriers: slow extraction Novel Concepts: Organics • Exciton binding energy: Hundreds of meV binding energies are energy losses that are intrinsic • Diffusion length: so small that it requires bulk heterojunctions Large junction areas produce large dark currents • Molecular distortions: increase HOMO/LUMO gap and create trap states • Low mobility: produces Coulomb barriers at contacts • Molecular stability: carriers are intrinsic reaction sites Novel Concepts: Photo Hydrogen • Some proposals are to make hydrogen directly from sunlight in a photoelectrochemical process • Problem: compromises both electrochemical cell and the photovoltaic device • Better: design these components separately because voltages and currents can be adapted with high efficiency in a circuit ... cos 40 cos 23 .45 cos  ? ?45   sin 40 sin 23 .45  0.7527   sin 1  0.7527   48 .8 Example 7.3 – Where is the Sun? • The sin of the azimuth angle is found from (7.9) cos 23 .45  sin  ? ?45  ... -0.9 848 S • • cos 48 .8 Two possible azimuth angles exist S = sin 1  -0.9 848   80 S = 180 -sin 1  -0.9 848   260 or  100 Apply the test (7.11) tan  tan 23 .45   cos H  cos  ? ?45 ... Court Ordered Pruning Source: NYTimes, 4/ 7/08 Solar Time vs Clock Time • Most solar work deals only in solar time (ST) • Solar time is measured relative to solar noon • Two adjustments – – – For