Photovoltaics: BASICS

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1 Laurea Magistrale in Scienza dei Materiali Materiali Inorganici Funzionali Photovoltaics: BASICS Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova

2 AIR MASS NUMBER defines the direct optical path length through the Earth s atmosphere, expressed as a ratio relative to the path length vertically upwards, i.e. at the zenith. AM number is always 1 S = length of a shadow cast by an object of height H Air Mass = [1 + (S/H) 2 ] 0.5 A widely used standard for comparing solar cell performance is the AM1.5 spectrum normalized to a total power of 1 kw/m 2 AM0 = Zero atmospheres solar cells for space power applications AM1 = (0 degrees) Sea level with the sun directrly overhead AM2-3 = A useful range for estimating the overall average performance of SC at high latitudes (Northern Europe) or in wintertime

3 The spectral content of sunlight at the Earth s surface has also a diffuse (indirect) component owing to scattering and reflection in the atmosphere and surrounding landscape and can account for up to 20%of the light incident on a solar cell. AM1.5g (global) = includes the diffuse component; AM1.5d (direct) = does not includes the diffuse component

4 SOLAR CELL I-V CHARACTERISTICS V OC V at open circuit (I = 0) Maximum power point, V MP, I MP It is the point of the curve where the power produced is at a maximum; it define a rectangle whose area (P MP = I MP V MP ) is the higher for any point Fill Factor, FF It is a measure of the squareness of the I-V characteristic and is always less than one FF = P MP V OC I SC = V MP I MP V OC I SC

5 FROM SILICA TO SILICON SINGLE CRYSTAL 1. METALLURGICAL SILICON Carbothermic reduction of silica SiO 2 + 2C Si + 2 CO Aluminothermic reduction of silica 3 SiO Al 3 Si + 2 Al 2 O 3

6 FROM SILICA TO METALLURGICAL GRADE SILICON

7 FROM SILICA TO METALLURGICAL GRADE SILICON Liquid crude silicon contains 1-3% impurities depending on the raw materials and the type of electrodes: Fe: % Al: % Ca: % Ti: % C: % Refining: 3 SiO Al 3 Si + 2 Al 2 O 3 SiO Ca Si + 2 CaO SiO Mg 3 Si + 2 MgO

8 FROM METALLURGICAL GRADE TO SEMICONDUCTOR GRADE (POLYSILICON) 1. Preparation/synthesis of the volatile chlorosilanes 2. Purification 3. Decomposition to elemental silicon 4. Recycling of by-products

9 Synthesis and purification of chlorosilane The reactor is held at 650 C and the exothermic reaction yields preominantly trichlorosilane and hydrogen Si + 3HCl = SiHCl 3 + H 2 Flow chart of the preparation and refining of trichlorosilane a) Fluidized-bad reactor, b) Dust filter, c) Condenser, d) Tanks, e) Distillation of low-boiling impuritie, f) Distillation of high boilers, g) Tanks, h) Storage tanks

10 Decomposition to elemental Si by CVD Siemens Process (Siemens & Halske 1952 Patented 1956) 4 SiHCl H 2 = 3 Si + SiCl HCl the process is strongly endothermic and requires a high reaction temperature (ca K) CVD of Si a) Electrical current b) Starting silicon slim rod c) CVD polycrystalline rod (1400 K) d) Reactor In this CVD process much energy is lost by thermal radiation from the Si rod to the cold reactor walls The upper limit for deposition rate is determined by the required silicon quality (th 2 mm/h real = 1 mm/h) Maximun ø = 25 cm (Thermal conductivity K = 1.5 W/cm K) Automatic process (corrent, T, P, gas flow, composition )

11 Decomposition to elemental Si by CVD Union Carbide (ASiMI) Process The CVD is carried out at 1000 K on a reactor similar to the Siemens one (catalized process) 6 SiHCl 3 = 3 SiCl SiH 2 Cl 2 4 SiH 2 Cl 2 = 2 SiHCl SiH 3 Cl 2 SiH 3 Cl = SiH 4 + SiH 2 Cl 2 SiH 4 = Si + 2 H 2 Ethyl Corporation Process (Texas Instrument Patent) 20 % SiH % H 2 + Si powder at 1000 K Final product = grain Si KRICT Korea: heating with microwaves CVD of granular Si a) Si powder, b) powder-feeding device, c) exhaust, d) reactor, e) heater (resistance or mw), f) removal of coarse Si granules, g) container for Si granules, h) gas injection

12 BULK CRYSTAL GROWTH AND WAFERING FOR PV Czochralski (CZ) Crystal Growth CZ Si crystal puller a) seal, b) seed shaft, c) optical system, d) silica crucible, e) graphite crucible, f) graphite heater, g) thermal insulation, h) crucible shaft, i) viewing port, j) separation valve, k) seed crystal, l) seed holder, m) front opening chamber, n) front opening door, o) valve. A silicon melt is held in a crucible and a single-crystal seed of the proper orientation is dipped into the melt and slowly withdrawn vertically. With suitable pulling speed and crystal and crucible rotation rate, a single crystal of the desired shape can be grown. : mm (150 mm) 300 mm Speed: 10 mm/min =5 mm 1 mm/min =150 mm Crucible: silica/graphite Residual melt in the crucible (quality/cost); impurities in the melt Pyrometers and optical system allow following the process very precisely

13 BULK CRYSTAL GROWTH AND WAFERING FOR PV Float-Zone (FZ) Crystal Growth FZ Si crystal puller a) Feed rod shaft, b) feed rod holder, c) feed rod polycristalline, d) single crystal, e) neck, f) seed, g) seed holder, h) seed shaft, i) seal, j) to the vacuum pump, k) camera, l) viewing port, m) RF heating coil, n) valve. Absence of a crucible: low oxygen content, lower impurities concentration Small fused volume Entire environment cold The temperature gradients are larger in growing FZ crystals because FZ rods cool faster (lower diameters) Speed: 12 mm/min = 5 mm 2 mm/min = 150 mm

14 Dopants and defects distribution Relative axial concentrtion variation C x /C 0 in silicon crystals producing from starting material (melt or feed rod) in which the impurities are distributed homogeneously A) CZ crystals B) FZ crystals k = Distribution coefficient

15 Ingot fabrication Solidification of high quality multicrystalline silicon ingots with weights of kg, 70x70 cm 2 and heights of more than 30 cm Crystallisation speed: 1cm/h (ca. 10kg/h for large ingots) Bridgman (commonly used) technology Block-casting process (Kyocera Jpn and Deutsche Solar) melting and crystallisation of the Si is performed in a Si 3 N 4 -coated quarz crucible

16 QUASI-CONTINUOUS MELTING Quasi-continuous melting process in situ and pulling downward through a so-called cold crucible which is open at the bottom Cold crucible induction casting for multicrystalline silicon ingots; a) valve, b) granule feeder, c) silicon granules d) cooling water e) auxiliary heater f) ingot puller g) ingot h) vacuum port i) induction coil j) inerg gas inlet

17 WAFERING Cost distribution for modules and silicon wafers Sawing cost is connected with high material losses (about 50%)

18 Single Crystal Wafering Technique Inner-diameter sawing machine for slicing silicon; a) crystal tilting, b) crystal support and vertical feeding c) crystal d) sawblade drive unit e) saw blade f) horizontal crystal feeding Morphologic defects elimination Slicing in inner-diameter saws; the saw blade is a 0.1 mm thick high-grade steel foil with a large circular opening in its center the edge of which is covered with small diamonds (2000 rpm, 0.3 mm cutting loss): > 150 μm thickness wafers Lapping between two large counterrotating steeldisks, whereby the wafers are held and led by a lapping carrier; abrasives : SiC, Al 2 O 3 with specific grain size and strength Chemical etch Polishing machine for silicon wafers a) polishing slurry feeding b) polishing pressure control c) wafer carrier d) wafers e) polishing plate f) drive unit

19 Multi-wire Wafering Technique Schematic diagram depicting the principle of the multi-wire sawing technique Cutting is achieved by an abrasive slurry which is supplied through nozzles over the wire web and carried by the wire into the sawing channel Abrasive = suspension of hard grinding particles (SiC, diamond) Slurry = oil based, water based, water-washable based (ethylene glycol) Wire speed, solid fraction of abrasive, viscosity of the suspension, particle size distribution and shape

20 SILICON RIBBON AND FOIL PRODUCTION Vertical methods: Edge-defined Film-fed Growth (EFG) String Ribbon (STR) Dendritic Web (WEB) Horizontal methods: With substrate Ribbon Growth on Substrate (RGS) Silicon Film (SF) Without substrate

21 PROCESSES Dendritic Web (WEB) Edge-defined Filmfed Growth (EFG) String Ribbon (STR) Ribbon Growth on Substrate (RGS) Continuous production of Si ribbon