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Home , Crystalline silicon

7/21/2010

EE580 – Solar Cells Technologies Todd J. Kaiser • Lecture 06 • A) Crystalline • Solar Cell Materials & Structures • B) Thin Film • C) Group III-IV Cells

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A) Crystalline Silicon Types of Crystalline Silicon

• Most common for commercial applications • Carefully made Silicon forms . Different levels of structure may exist ranging from to • Advantages totally non-crystalline – Well known standard processing – Single crystal silicon – Silicon is very a bun dan t – Multi-crystal silicon – Polycrystalline • Disadvantages – Ribbon silicon – Requires expensive highly pure silicon – – Competes for silicon with electronics industry • The main difference between each is the crystal grain size and their growth technique

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Different Forms of Silicon Single Crystal Silicon

Crystal Type Symbol Crystal Grain Common Growth Size Techniques Single-crystal sc-Si > 10 cm Czochralski (Cz), Float-Zone (FZ) Multicrystalline mc-Si 10cm Cast, Spheral, Sheet, ribbon Polycrystalline pc-Si 1µm – 1mm Evaporation , CVD, sputtering

All atoms arranged in pattern, one single crystal of silicon

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Single Techniques

• Czochralski Growth (Cz) • Pure Silicon is melted in a crucible under – Most single crystal silicon made this way vacuum or inert gas and a seed crystal is dipped – Lower quality silicon than FZ with Carbon and into the melt Oxygen present • The seed cryyystal is slowly withdrawn and slowl y – Cheaper pro duc tion than FZ rotated so that the molten silicon crystallizes to – Produces cylinders and circular wafers the seed (Rock Candy) • Float Zone (FZ) • The melt temperature, rotation rate and pull rate – Better Quality than Cz are controlled to create a ingot of a certain – More Expensive than Cz diameter – Produces cylinders and circular wafers

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Czocharlski Technique Czochralski Growth

• Entire ingots of silicon produced as one big crystal Spinning rod with “Seed” Crystal lowered into the molten silicon • Very high quality material with few defects Slowly pulled up to • No boundaries between crystals allow silicon to crystallize on the because it is one crystal in one seed layer orientation • Si crystal inevitably contains oxygen

Molten Silicon Once to the size impurities dissolved from the quartz desired, the crystal crucible holding the molten silicon is pulled faster to maintain the needed diameter Montana State University: Solar Cells 9 Montana State University: Solar Cells 10 Lecture 6: Solar Cells Lecture 6: Solar Cells

Float Zone Method Slicing into Wafers • Ingots are cut into thin • Produced by cylindrical polysilicon rod wafers for solar cells (300 that already has a seed crystal in its lower µm) end • Two Techniques • An encircling inductive heating coil melts – Wire sawing the silicon material – Diamond blade sawing • The coil heater starts from the bottom and is raised pulling up the molten zone • Both results in loss of silicon from “kerf losses” Æ • A solidified single crystal ingot forms silicon saw dust below • Time consuming • Impurities prefer to remain in the molten silicon so very few defects and impurities • Water Cooled, Dirty remain in the forming crystal

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Single Crystal Silicon Ribbon Silicon

• What we are using • Ribbon silicon is a technique used to grow multi- • Currently supplies a significant but declining solar cell crystalline silicon market share • Two graphite filaments are placed in a crucible of molten • Advantages silicon – PdProduce dfd for e lec tron ics idtindustry • The molten silicon is grown horizontally through capillary – Allows for higher efficiency solar cells action along the filaments • Disadvantages • Produces a ribbon-like sheet of multi-crystalline silicon which is already a long Æ no kerf losses – Requires higher quality of feed stock – More expensive and slower to produce – Circular shape to lower packing density in panels or larger waste of silicon Montana State University: Solar Cells 13 Montana State University: Solar Cells 14 Lecture 6: Solar Cells Lecture 6: Solar Cells

Ribbon Silicon (+/-) Ribbon Silicon Method

• Advantages – Thickness can be varied by the filament width & the Growing Ribbon pull speed Solid-Melt – Cheaper - less wasted silicon due to sawing wafers Interface • Disa dvan tages – Lower Solar Cell Efficiencies due to more defects – Irregular surface characteristics leading to poorer cell Silicon Feed Molten Silicon performance Crucible

Filaments

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Poly-crystal Silicon Poly& Multi-Silicon Method

• Produced by melting silicon source material in a large rectangular crucible • The material is slowly directionally cooled • Impurities drift to the edges which cool last • These edges are sawn or acid etched off • These blocks are sawn into smaller blocks and then sawn into thin wafers

Grain Grains Boundary

Regions of single crystalline silicon separated by grain boundaries with irregular bonds Montana State University: Solar Cells 17 Montana State University: Solar Cells 18 Lecture 6: Solar Cells Lecture 6: Solar Cells

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Multicrystalline Silicon Wafer Fabrication Multi-crystalline

• Significant differences in the size of crystal grains • Advantages – Cheaper – FtPFaster Processi ng • Disadvantages – Less efficient than single crystal due to grain boundaries where electrical losses occur Clearly shows different crystals formed during the casting process Montana State University: Solar Cells 19 Montana State University: Solar Cells 20 Lecture 6: Solar Cells Lecture 6: Solar Cells

Amorphous Silicon B) Thin Film • Thin film of 1-10 microns compared to Hydrogen Passivation 200-300 microns • Created by depositing a thin expensive semiconductor on a cheaper glass substrate • Advantages Dangggling Bond – Requires little semiconductor material – Cheaper to produce: • glass is cheap • semiconductor expensive

Less regular arrangement of atoms leading to • Disadvantages dangling bonds and passivation by hydrogen – Difficult to manufacture good films – Lower efficiencies Montana State University: Solar Cells 21 Montana State University: Solar Cells 22 Lecture 6: Solar Cells Lecture 6: Solar Cells

Three Main Thin Films Amorphous Silicon (a-Si)

• Amorphous Silicon (a-Si) • Made by evaporating silicon onto a glass base • Telluride (CdTe) • More random orientation than crystalline • Copper Indium Diselenide (CIGS) • More electrons not bound to SI atoms • Unbounded electrons attract impurities and degrade the electrical performance of the cell • Hydrogen is often added to the material to de- activate the dangling bonds

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a-Si Cross-Section Amorphous Silicon (+/-)

• Advantages • Disadvantages – Absorbs low and high – Lower Efficiency Glass (2-5 mm) Transparent intensity light (lower grade Si) Conducting Oxide – Less Semiconductor – Long term degradation (TCO) TCO (0 .5 µm) needddedÆ LCtLower Cost oftildf material under p+ a-Si:H (0.02 µm) sunlight i a-Si:H (0.5 µm) – High temperatures do n+ a-Si:H (0.02 µm) not significantly reduce – Production requires TCO (0.5 µm) performance hazardous gases Metal (0.5 µm)

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Cadmium Telluride (CdTe) CdTe Cross-Section

• p-type made from Cadmium and Telluride • n-type from • Advantages – High Efficiencies compared to a-Si (over 16%) • Disadvantages – Requires high processing temperatures – CdTe is unstable and will degrade – Cadmium is toxic and costly to dispose of – Sensitive to water ingress and cell degradation

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Copper Indium Gallium CIGS Cross-Section Diselenide (CIGS) • Extremely good light absorption (99% of light absorbed in the first micron) – Æ an optimal and effective PV material • The addition of gallium boosts its light absorption band gap for the solar spectrum • No performance degradation over time • Much higher efficiencies than other thin films (19%)

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CIGS (+/-) C) Group III-V

• Advantages • Disadvantages • Compounds of Group III and V on periodic table – Highest efficiency for – Gallium and Indium • Compound is a material that combines multiple thin film cells are scarce materials elements in a single structure (not just a mixture) – Clear pathways to – Requires expensive • Used extensively in the electronics and ifimprove performance vacuum processing and efficiencies optoelectronics industries as well as space satellites • Makes excellent but very expensive solar cells • Can create multi-junction cells for higher efficiency

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Single Junction III-V Cells Single Junction III-V Cells (+/-)

• Made from combination of two materials • Advantages • Disadvantages – (GaAs) – Very high efficiencies – Expensive – Low weight – Required materials are – Indium Phosphide (InP) – Resistant to damage not abundant • Best effi ci ency i s at 27 . 6% from cosmic radiation – 1000 W/m2 of sunlight produces 276 Watts of usable power

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Multi-Junction III-V Cells Solar Cell Features

• Stacked p-n junctions on top of each other • Each junction has a different band gap energy so each will respond to a different part of the solar spectrum • Very high efficiencies, but more expensive • Each junction absorbs what it can and lets the remaining light pass onto the next junction • Widely used for space applications because they are very expensive • Overall record for electrical efficiency is 35.2% Montana State University: Solar Cells 35 Montana State University: Solar Cells 36 Lecture 6: Solar Cells Lecture 6: Solar Cells

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Top Surface Features Texturing

• Light striking the surface • Reduce the reflectivity – Absorbed - Converted into electricity (GOOD) of the surface of – Reflected – Optical loss (BAD) wafers by forming – Transmitted – Optical loss if escapes microscopic stttructures • Bare silicon is highly reflective Pyramids • Works mainly for • Top of PV cell is designed to improve the light single crystal surfaces trapping (reduce reflection & confine transmitted) – Texturing – Anti-Reflection Coating (AR coating)

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Texturing Anti-reflection (AR) Coatings

• Formed by Anisotropic Reflected and etching different crystal Lost • Finished PV cell is coated with a material to planes etch at different reduce the amount of reflected light (just like eye rates Transmitted glasses) • Reduces total reflection and Absorbed by reflecting light into • Usuallyyg used on cells unsuitable for texturing another pyramid instead • Can reduce reflection to 5% of away • AR Coating Materials • Increases the chances of Less loss from absorbing the light Reflection – Silicon nitride • 30% reflection from – Silicon dioxide polished Si reduced to More Absorbed – Zinc oxide 10% for textured Si Montana State University: Solar Cells 39 Montana State University: Solar Cells 40 Lecture 6: Solar Cells Lecture 6: Solar Cells

AR mechanism AR Coating Superposition of two waves:

Constructive Interference Waves in phase add Incoming wave Silicon

Destructive Interference Waves out of phase cancel Only Reflection at air-glass λ optimized at Reflection at glass-silicon This destructive one 4 interference is created by wavelength Out of Phase Æ Cancel AR coatings When thickness is a quarter wavelength Montana State University: Solar Cells 41 Montana State University: Solar Cells 42 Lecture 6: Solar Cells Lecture 6: Solar Cells

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Front Metal Contacts Front Metal Contacts Process • Grid of metal contacts are used to collect the current from • The main process used by industry is silk-screening the p-n junction (blocks sunlight to silicon) because it is quick and cheap • Silver is mainly used: Highest conductivity but very – Silver paste is squeezed through a mesh with a expensive (we use aluminum…cheaper) pattern onto the cell’s top surface • High conductivity reduces the resistance the traveling • Other Processes eltlectrons exper ience so thlthey lose less energy as they – Eti(thd)Evaporation (our method) move – Laser Grooving and Electroplating • There will be some additional resistance provide by the • Slower and more expensive but gives good alloying of the metal and silicon (contact resistance) efficient cells • A well formed bond minimizes this resistance by allowing – Inkjet Printing electrons to flow between the materials with out any edge • Fast and cheap in theory effects, barriers or opposing voltages • Yet to achieve improved performance Montana State University: Solar Cells 43 Montana State University: Solar Cells 44 Lecture 6: Solar Cells Lecture 6: Solar Cells

Rear Metal Contacts Encapsulation

• Does not need to let sunlight through usually covers the • Solar cells are thin & brittle and need to be exposed to entire back surface outdoor conditions for 25-30 years • This reduces the back surface contact resistance • A physical casing (encapsulation) protects the PV cells • Aluminum is usually used for the rear metal contact and provide structural strength because a lot is used • Accomplishes • Aluminum is a good conductor (not as good as silver) but – Electrically isolate cells and make contacts much less expensive – Protect from water and oxygen ingress • Made thicker to compensate for lower conductivity – Withstand heavy winds, hail & installation – Maintain protection for decades – Allow modules to attach to each other

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Materials Module Structure

• Glass(low Iron): – Used for the transparent top surface. – Needs to be highly transparent, scratch-resistant & rain, wind, hail, human… proof. Glass • Tedlar EVA – Typical back layer because it is strong material – Gives structural support Solar Cells – Removes excess heat that reduces efficiency EVA • Ethylene Vinyl Acetate (EVA) Tedlar – Transparent encapsulant – Fills all the spaces between the front, rear edges and between layers

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Solar Cell Operation

n p Rear Contact Emitter Base Antireflection Absorption of photon coating creates an electron hole pair. If they are within a diffusion length of the depletion region the electric field separates them.

The electron after passing through Front Contact the load ‐ recombines with + the hole completing the External Load circuit Montana State University: Solar Cells 49 Lecture 6: Solar Cells

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