ppt51507

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Published on February 24, 2008

Author: yilmar

Source: authorstream.com

Eng/Phys. 160, 5/15/07:  Eng/Phys. 160, 5/15/07 Will email your project 1 grades by tomorrow, bring graded papers Thursday. In the news Will cover fusion next time (Thursday) Photovoltaics – basics  Insulators, metals, conductors  Photovoltaic effect  Need for an np junction (conventional)  Efficiency  Alternatives to silicon - organic PVs, Gratzel cells  New ideas - carrier multiplication + hot electrons Debates In the news:  In the news Bush announces `plan’ to address emissions by increased MPG, ethanol production (Critics: stalling tactic) Big gain by Iran on nuclear enrichment (to 5%, 1300 working centrifuges) Big Kudos to global ecology center at Stanford Big cooling in Europe triggered by big warming of Greenland ice (fresh water pulse) unlikely Natanz: Facts on the ground:  Natanz: Facts on the ground The centrifuge site in question in the media discussions is at Natanz, very close to the city of Esfahan, the third largest city of Iran, with a metropolitan area of 2.5 million people. ___ Stanford/Carnegie Institute Global Ecology Center (from EHDD design) :  Stanford/Carnegie Institute Global Ecology Center (from EHDD design) Top 10 green project from American Insitute of Architects Recycled products in construction (redwood siding from old wine casks!) Passive Solar Natural air conditioning Low flow plumbing No irrigation landscaping Quantum theory and metals, insulators, semiconductors:  Quantum theory and metals, insulators, semiconductors ``Diffraction’’ of electron ``waves’’ through crystals produces bands of allowed energies with gaps. Metals have partly filled bands (odd number of electrons per repeat unit)—Costs nearly infinitesimal energy to excite conduction Insulators/Semiconductors have filled bands (even number of electrons per repeat unit)—gap to excitation of conduction—only difference is scale for semiconductors vs. insulators (~ 1 eV vs. several eV) + localization Need for n-p junction:  Need for n-p junction Dope the semiconductor – As – add extra electron – n-type Ga – one less electron ~ adding hole  p type p type n type Photovoltic Efficiency – Shockley and Queisser (J. App. Phys. 1961):  Photovoltic Efficiency – Shockley and Queisser (J. App. Phys. 1961) Assume one e-h pair created at energy Egap for each photon absorbed above threshold Peaks for gap energy about 2 x blackbody temperature of sun Absorbed Incident One way to improve efficiency--multilayer cells:  One way to improve efficiency--multilayer cells Idea is to capture more spectrum as you pass through Problem: much more expensive From NREL (national renewable energy lab) From http://resosol.org/Futur/PV/optimumcells.html Polymer Photovoltaics:  Polymer Photovoltaics Alan Heeger UCSB Chem Nobel Laureate Plastic photovoltaics from Heeger’s Company Bulk heterojunction :  Bulk heterojunction Every molecule in the film is like an n-p junction Best efficiency about 2.5% Concerns about durability--but if cost gets low enough…. From http://www.quantsol.unibe.ch/pub_80.htm Gratzel dye and chlorophyll:  Gratzel dye and chlorophyll Gratzel Cell: mimic photosynthesis :  Gratzel Cell: mimic photosynthesis Beat the limit with nanostructures:  Beat the limit with nanostructures Nanostructures have intrinsically multiple bands from ``quantum confinement’’ Can increase the open circuit voltage by inhibiting carrier relaxation (emission of ``phonons’’) Can increase the current by increasing number of particle hole pairs induced per photon (Klimov, Los Alamos) Typical efficiencies of current photovoltaics :  Typical efficiencies of current photovoltaics Debate Format:  Debate Format 5 minutes presentation 4 minutes discussion between 3 minutes rebuttal (15 minutes prep before debates) Image credits:  Image credits Working Gratzel Cell: http://www.bath.ac.uk/physics/news/gratzel.jpg Gratzel schematic (liquid electrolyte): http://www.bath.ac.uk/chemistry/electrochemistry/images/mechanism_small.jpg Gratzel schematic (polymer electrolyte): http://www.rsc.org/ejga/CC/2005/b416461j-ga.gif Polymer photovoltaic photo: http://www.polymercentre.org.uk/industry/pics/03photovoltaic.jpg Alan Heeger: http://www.konarkatech.com/images/heeger.jpg Konarka Power plastic: http://energypriorities.com/graphs/konarka-power-plastic.jpg Chlorophyl: http://amanda.berkeley.edu/~bramall/work/astrobiology/images/chlorophyll.jpg PV schematic: http://www.speedace.info/speedace_images/solar_cell_schematic.jpg Met/ins/semic schematic: http://www.physics.udel.edu/~watson/scen103/semi1.gif

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