Solar Cells: Alternative Power

Solar Cell (Photovoltaics) Light Electricity Solar Cell (PV) 발표자 : 20075418 Ju Dae-Hyun Renewable Nonrenewable Enargy 재생에너지원 (Renewable Energy) 일회용에너지원 (Nonrenewable Energy) What is a Solar Cell? • A structure that converts solar energy directly to DC electric energy. – It supplies a voltage and a current to a resistive load (light, battery, motor). – Power = Current x Voltage=Current2 x R= Voltage2/R • It is like a battery because it supplies DC power. • It is not like a battery because the voltage supplied by the cell changes with changes in the resistance of the load. Basic Physics of Solar Cells • Silicon (Si) is from group 4 of the period table. When many Si atoms are in close proximity, the energy states form bands of forbidden energy states. • One of these bands is called the band gap(Eg) and the absorption of light in Si is a strong function of Eg. The Sun as Energy Source • The Sun daily provides about 10 000 times more energy to the Earth than we consume • Photovoltaic technology directly converts solar energy into electricity • No moving parts – no noise – no emissions – long lifetime • Large industrial potential - cost reductions needed • Feedstock for PV industry is silicon - the second most abun dant element in the crust of the Earth Solar Energy status • • • • Market is exploding The solar industry is very profitable Lack of highly purified silicon (polysilicon) Cost of solar electricity is too high, R&D focus on reducing cost and increasing efficiency Actual Growth vs. Historic Forecasts Actual market development Solar Energy status • • • • Market is exploding The solar industry is very profitable Lack of highly purified silicon (polysilicon) Cost of solar electricity is too high, R&D focus on reducing cost and increasing efficiency - Gross revenue development 1800 2454 1600 1400 (MNOK) 1705 1200 1000 800 600 400 200 0 2001 2002 2003 2004 2005 159 435 857 Solar Energy status • • • • Market is exploding The solar industry is very profitable Lack of highly purified silicon (polysilicon) Cost of solar electricity is too high, R&D focus on reducing cost and increasing efficiency Solar Grade Silicon Supply-Demand (MT/year) 25 000 20 000 15 000 10 000 5 000 0 2002 2003 2004 2005 2006 2007 2008 2009 2010 SOG Polysilicon supply SOG Polysilicon demand Solar Energy status • • • • Market is exploding The solar industry is very profitable Lack of highly purified silicon (polysilicon) Cost of solar electricity is too high, R&D fo cus on reducing cost and increasing effici ency Cost reductions – existing technologies • Thinner wafers - Wire sawing - Laser cutting and etching • Higher efficiencies - Semiconductor technologies on single crystal wafers (examples Sanyo / SunPower) • Thin film technologies (flat panel display) Public incentives are important Cost goals for third generation solar cells Efficiency and cost projections for first-, second- and third generation photovoltaic technology (wafers, thin-films, and advanced thin-films, respectively) Source: University of New South Wales Next generation technology • Silicon nanostructures Bandgap engineering of silicon. Applications could be tandem solar cells and ene rgy selective contacts for hot carrier solar cells. Fabrication of silicon nanostructures consisting o f quantum well and quantum dot super lattices to achieve band gap control Next generation technology (cont.) • Up/Down converters Luminescent materials that: EITHER absorb one high energy photon and emit m ore than one low energy photon just above the bad gap of the solar cell (down-conversion) OR that absorb more than one low energy photon b elow the band gap of the cell and emit one photon ju st above the band gap (up-conversion). Understanding cell efficiency SOLAR SPECTRUM AM 1,5 (1000 watt/m2) 18 16 14 Irradiation AM 1,5 Useful irradiation (c-Si) Irradiance, watt/m2 12 10 8 6 4 2 0 200 300 400 500 600 700 800 900 1000 1100 1200 1300 wavelength, nm Next generation technology (cont.) • Hot carrier Cells This concept tackles the major PV loss mechanism of thermalisation of carriers. The purpose is to slow down the rate of photoexcited carrier cooling caused by phonon interaction in the lattice to allow time for the carrie rs to be collected whilst they are still hot, and hence increasing the vo ltage of a cell. Next generation technology (cont.) • Thermoelectric solar cells Application of the concept of energy –selective elec tron transport used in hot carrier solar cells, to devel op thermo electrics and thermo-ionics devices. The PV Value Chain (multi-crystalline) Polysilicon Wafer Solar Cell Solar Module Systems Chemical Process (purification) Casting Cutting Surface Treatment Assembly Installation Operation Prices are actually increasing Silicon Solar cell How does solar energy work? Solar Electric or Photovoltaic Systems convert some of the energy in sunlight directly into electricity. Photovoltaic (PV) cells are made primarily of silicon, the second most abundant element in the earth's crust, and the same semiconductor material used for computers. When the silicon is combined with one or more other materials, it exhibits unique electrical properties in the presence of sunlight. Electrons are excited by the light and move through the silicon. This is known as the photovoltaic effect and results in direct current (DC) electricity. PV modules have no moving parts, are virtually maintenance-free, and have a working life of 20 - 30 years. Photovoltaics Most current solar cells are photovoltaic Typically made from silicon or amorphous silicon. Typical efficiency ~ 12%. Best efficiency ever in laboratory: ~30%. Theoretical maximum, including concentrating light: 43% Generic design: doped pn junction. Photons come in and photoionize donors. Built-in electric field at junction causes carriers to flow, building up a potential (voltage) btw the p and n sides. Clearly one can play with different band gap systems to arrive at materials with different absorption spectra. Also, good mobility of charge essential for this to work well trapping of charge or poor mobility will kill efficiency. Silicon Solar cell Principle p-n Junction Diode. Ref. Soft Condensed Matter physics group in univ. of Queenland 반사방지막 전자 앞면전극 n층 p n접합 p층 전 기 부 하 뒷면전극 전자 정공 정공 Poly-Si Solar cell Making process 기판준비 : Si ingot  330m  2cm x 2cm Surface cleaning Texturing : chemical v-groove p-n junction : POCl3 (900ºC) ITO increasing minorty carrier correction, ARC Forward surface Electrode Back Surface Field Deposition Al and Ag ohmic-contact Anti-reflection coating (ARC) TiO2 deposition H2 H2 H2 H2 H2 diffusion dangling bond H2 bonding  Decreasing recombination Measure Solar Cell, Module, Array - An individual PV cell typically produces between 1 and 2 watts Concentrator collectors • decrease the area of solar cell material being used in a system Flat-Plate Systems • Flat-plate collectors typically use large numbers or areas of cells that are mounted on a rigid, flat surface.  substrate ; metal, glass, plastic • They are simpler to design and fabricate. • They do not require special optics, specially designed cells, or mounting structures that must track the sun precisely. plus, flat-plate collectors can use all the sunlight Uses for Solar Energy Main Application Areas – Off-grid Space Water Pumping Telecom Solar Home Systems Main Application Areas Grid Connected Commercial Building Systems (50 kW) Residential Home Systems (2-8 kW) PV Power Plants ( > 100 kW) Conclusions • Solar energy will become the most important and cost-efficient en ergy source in the future. • The present lack of silicon feedstock is promoting a rapid develop ment of next generation technology. • Immediate actions are taken to cut thinner wafers and increase ce ll efficiencies for crystalline silicon. • New thin film technologies are being developed • Stronger influence from semiconductor industry will accelerate the development of better technologies • Nanosilicon and other third generation technologies may offer a lo ng-term solution for the future solar energy technology.