Solar Cell Technology (Si)

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Presentation transcript:

Solar Cell Technology (Si) FIRST AND SECOND GENERATIONS

Outlines What is a Solar Cell Generations of Solar Cells History Basic physics of solar cells Generations of Solar Cells First Generation Second Generation

History 1839 Alexandre-Edmond Becquerel 1883 Carles Fritts Photovoltaic effect: Light dependant voltage immersing between two electrodes in an electrolyte 1883 Carles Fritts First solar cell: Coated semiconductor selenium with an extremely thin layer of gold to form the junctions (1% efficient) 1941 First silicon based solar cell demonstrated 1946 Russell Ohl Patented the modern solar cell 1954 Beginning of modern solar cell research Bell laboratories: Experimenting with semiconductors, accidentally found that Si doped with certain impurities was very sensitive to light

What Is a Solar Cell? A structure that converts solar energy directly to electricity by the photovoltaic effect It supplies voltage and current to a resistive load (light, battery, motor) It is like a battery It supplies DC power It is not like a battery The voltage supplied by the cell changes with the changes of the load resistance The solar (photovoltaic) cell fulfills two fundamental functions: Photogeneration of charge carriers (electrons and holes) in a light-absorbing material Separation of the charge carriers to a conductive contact to transmit electricity

Illumination and Generation Ehν < EG : the incident light transparents Ehν ≥ EG : photons are absorbed and EHP are photogenerated Ehν > EG : energy generated is lost as heat

Photovoltaic Effect Solar cells are: p-n junctions Minority carrier devices Voltage is not directly applied Itotal = IF - IL = Is{exp(qV/kT)-1} – IL The photo current produces a voltage drop across the resistive load, which forward biases the pn junction Absorption of a photon Formation of e-h pair (exciton) Exciton diffusion to Junction Charge separation Charge transport to anode (holes) and cathode (electrons) Supply a direct current for the load

Forward Bias vs. Photogeneration Voltage applied externally Current is dominated by diffusion Photogeneration Voltage is generated internally from EHP being swept across the junction by and E field Current is dominated by drift

Cell Structures Homojunction Device Heterojunction Device Single material altered so that one side is p-type and the other sideis n-type p-n junction is located so that the maximum amount of light is absorbed near it Heterojunction Device Junction is formed by contacting two different semiconductor Top layer - high bandgap selected for its transparency to light Bottom layer - low bandgap that readily absorbs light. p-i-n and n-i-p Devices A three-layer sandwich is created Contains a middle intrinsic layer between n-type layer and p-type layer Light generates free electrons and holes in the intrinsic region.

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 Third Generation Nanocrystal solar cells Photoelectrochemical (PEC) cells Gräetzel cells Polymer solar cells Dye sensitized solar cell (DSSC) Fourth Generation Hybrid - inorganic crystals within a polymer matrix

First Generation: Overview Dominant technology in the market More than 86% of the commercial production of solar cells High-cost, high-efficiency Maximum theoretical efficiency of 33% Generally, Si based solar cells are more efficient and longer lasting than non-Si based cells. However, they are more at risk to lose some of their efficiency at higher temperatures (hot sunny days), than thin-film solar cells

First Generation: Crystalline Si-based Cells Cells are typically made using a crystalline Si wafers Wafers about 0.3mm thick, sawn from ingot with diameter of 10-15cm Consists of a large-area, high quality and single layer p-n junction diode A single junction for extracting energy from photons Approaches Ingots can be either monocrystalline or multicrystalline Most common approach is to process discrete cells on wafers sawed from silicon ingots. More recent approach which saves energy is to process discrete cells on Si wafers cut from multicrystalline ribbons Band gap ~1.12 eV

Crystalline Si-based Cells Monocrystalline Si (c-Si) Made by Czochralski process, cut from cylindrical ingots Not completely cover a square solar cell module without a substantial waste Expensive Extremely pure refined Si Poly- or Multi-crystalline Si (poly-Si or mc-Si) Made from cast square ingots; melted Si is poured into a mold. Large square blocks of molten Si carefully cooled and solidified Less waste of space, more expensive to produce than c-Si, but less efficient Ribbon Si A type of mc-Si Formed by drawing flat thin films from molten Si Lower efficiencies than poly-Si Save on production costs due to a great reduction in Si waste Not require sawing from ingots

First Generation: Research Cells Source: National Renewable Laboratory

First Generation: Evaluation Advantages Broad spectral absorption range High carrier mobilities 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 Fairly easy for an electron generated in another molecule to hit a hole left behind in a previous photo excitation 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 With the max efficiency of 33%, it achieves cost parity with fossil fuel energy generation after a payback period of 5-7 years

Second Generation: Overview Thin-film solar cells Based on the use of thin-film deposits of semiconductors Intense development for the 90s and early 2000s Developed to reduce the costs of the first generation cells Alternative manufacturing techniques to reduce high temperature processing evolves production costs Production costs will then be dominated by material requirements Inherent defects due to lower quality processing methods reduces efficiencies compared to the first generation cells Low-cost, Low-efficiency cells

Second Generation: Thin-Film Cells Use minimal materials and cheap manufacturing processes 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 Deposition of thin layers of materials on inexpensive substrates Mounted on glass or ceramic substrates Devices initially designed to be high-efficiency, multiple junction photovoltaic cells

Second Generation: Types (a-Si) Amorphous Si cells deposited on stainless-steel ribbon Non-crystalline-Si deposited over large areas by PECVD Used to produce large-area photovoltaic solar cells Hydrogenated amorphous Si (a-Si:H) Plasma-deposited amorphous Si contains a significant percentage of H atoms Essential to the improvement of the electronic properties of the material Cells are built up in the sequence from bottom to top Metal base contact, n-layer, intrinsic layer, p-layer, transparent contact, glass substrate Instead of one layer, several thinner layers are used to prevent efficiency drop Complex production methods, but less energy intensive For a given layer thickness, absorbs much more energy than c-Si (×2.5) Not stable, less efficient than c-Si Bandgap~1.7eV

Second Generation: Types (poly-Si) Polycrystalline (Micro Crystalline) Si Consists solely of crystalline silicon grains(1mm), separated by grain boundaries Use antireflection layers to capture light waves with wavelengths several times greater than the thickness of the cell itself Using a material with a textured surface both in front and back of the cell Light change directions and be reflected, and thus travels a greater distance within the cell thickness Carrier mobilities can be orders of magnitude larger than amorphous Si Material shows greater stability under electric field and light-induced stress Low efficiency Fragile: Can be broken if hit by a falling branch or reasonably heavy object flying through a strong wind Bandgap~1.1eV

Second Generation: Types (CdTe) Cadmium telluride (CdTe) cells deposited on glass Represents the second most utilized solar cell material in the world Crystalline compound formed from Cd and Te with a zincblende (cubic) crystal structure Usually sandwiched with cadmium sulfide (CdS) to form a p-n junction photovoltaic solar cell Simplified manufaturing compared to the multi-step process of joining two different types of doped Si CaTe absorbs sunlight at close to the ideal wavelength, capturing energy at shorter wavelengths than is possible with Si panels Perfectly matched to the distribution of photons in the solar spectrum in terms of optimal conversion to electricity Cheaper than Si, especially in thin-film technology Low efficiency levels (10.6%) Toxicity of Cd Bandgap~1.58eV

Second Generation: Types (CIGS) Copper indium gallium diselenide (CIGS) alloy cells One of the best light absorber known About 99% of the light is absorbed before reaching 1μm into the material Deposited on either glass or stainless steel substrates More complex hetero-junction than CdTe The most common material for the top/window layer is CdS hard to produce in mass quantities at competitive prices Highest efficiency among the thin film material Reached efficiency levels of 20% in the laboratory Better resistance to heat than Si-based solar cells Less toxic than CdTe solar cells Uses a much lower level of Cd in CdS So far the cost cannot compete with the other solar cells Bandgap~1.38eV

Second Generation: Research Cells Source: National Renewable Laboratory

Second Generation: Evaluation Advantages Lower manufacturing costs Much less material require Lower cost/watt can be achieved Lighter weight (reduced mass) Their flexibility allows fitting panels on curved surface, light or flexible materials like textiles Less support is needed when placing panels on rooftops Even can be rolled up Disadvantages Typically, the efficiencies are lower than first generation cells

Summary Wafer Si 15 25 2 8 17 a-Si 6.5 13 1.2 4.5 21.7 c-Si 5 10 1.3 Technology Com Eff (%) Champ Eff Module ($/W) Installed ($/W) LCOE (cents/kWh) Wafer Si 15 25 2 8 17 a-Si 6.5 13 1.2 4.5 21.7 c-Si 5 10 1.3 4.8 18.3 CdTe 9 16.5 1.21 19.9 CIGS 9.5 19.5 1.8 6.3 22.2 Coal - 5 ~ 8 Nov. 2007