solarcell_technology

Generations of Solar Cells
• First Generation
– Single crystal silicon wafers (c-Si)
• Second Generation
– – – – Amorphous silicon (a-Si) Polycrystalline silicon (poly-Si) Cadmium telluride (CdTe) Copper indium gallium diselenide (CIGS) alloy
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First Generation: Crystalline Si-based Cells
• Larger, Si-based photovoltaic cells
– Typically made of a crystalline Si wafers sawed from Si ingots
• Dominant technology in the market
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Photovoltaic Effect
• Incident light causes an excitation of electrons from the valence band into the conduction band everywhere in the device
– Ehν < EG : the incident light transparents – Ehν ≥ EG : photons are absorbed and EHP are photogenerated – Ehν > EG : energy generated is lost as heat
– More than 86% of the commercial production of solar cells
• High-cost, high-efficiency
– Maximum theoretical efficiency of 33.7%
• Advantages
– Broad spectral absorption range (Eg=1.12eV)
• •
Low-cost, Low-efficiency cells Advantages
– – Lower manufacturing costs Much less material require • Lower cost/watt can be achieved • Lighter weight (reduced mass) Flexibility: allows fitting panels on curved surface, light or flexible materials like textiles Even can be rolled up
• Third Generation
– Nanocrystal solar cells – Photoelectrochemical (PEC) cells • Gräetzel cells – Polymer solar cells – Dye sensitized solar cell (DSSC)
SOLAR CELL TECHNOLOGY
Cho, Woo-Suhl Xufeng Wang James E Moore Tom Adams
Outlines
• What is a Solar Cell: Overview and Fundamentals • First Generation: Silicon Solar Cell • Second Generation: Thin Film Solar Cell • Beyond Solar Cell: Betavoltaic Cell Technology
• Fundamental functions of solar (photovoltaic) cell
– Photogeneration of charge carriers (electrons and holes) in a light-absorbing semiconductor material – Separation of the charge carriers to a contact to transmit electricity – An array of solar cells converts solar energy into a usable amount of DC electricity
Second Generation: Thin-Film Cells
• Developed to reduce the costs of the first generation cells
– – – – – Deposition of thin layers of materials on inexpensive substrates: Mounted on glass or ceramic substrates Reduce high temperature processing Production costs will then be dominated by material requirements Compared to crystalline Si based cells they are made from layers of semiconductor materials only a few micrometers thick Reduces mass of material required for cell design
– It supplies voltage and current to a resistive load (light, battery, motor) – It supplies DC power
• Solar Module or Solar Panel
– Solar Module: Solar cells are wired in series – Solar Panel: Solar Modules are assembled together and placed into a frame
• The photo current produces a voltage drop across the resistive load, which forward biases the pn junction 1. 2. 3. 4. 5. 6. Absorption of a photons (Ehν ≥ EG) Formation of e-h pair (EHP) EHP diffusion to Junction Charge separation Charge transport to anode (holes) and cathode (electrons) Supply a direct current for the load
History
• 1839 Alexandre-Edmond Becquerel: Photovoltaic effect
– Light dependant voltage immersing between two electrodes
• 1883 Carles Fritts: First solar cell
– Coated semiconductor selenium with an extremely thin layer of gold to form the junctions – 1% efficiency
• 1941 Russell Ohl: First Si-based solar cell
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I-V Characteristics
• I = IL – Is{exp(qV/nkT)-1} – {(V+Irs)/rsh} • ISC = IL : light induced current • VOC = kT/q{ln(IL/IOC)+1}
• Vm and Im : Operating point yielding the max power output • FF = Vm Im / VOC ISC • Power conversion efficiency: η = Pmax / Pin = Vm Im / Pin = FF VOC ISC / Pin
– Only a fraction better efficiency than selenium cells – Less expensive Si basis: a step towards greater efficiency
– Diffused Si p-n junction: Experimenting with semiconductors, accidentally found that Si doped with certain impurities was very sensitive to light – Array of thin Si strips: 6% efficiency
• Very different from the previous semiconductor devices
• Disadvantages
– High costs: Expensive manufacturing technologies • Extracting Si from sand and purifying it before growing the crystals • Growing and sawing of ingots is a highly energy intensive process – Much of the energy of higher energy photons, at the blue and violet end of the spectrum, is wasted as heat – Not more energy-cost effective than fossil fuel sources 9