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PHOTOVOLTAICS Direct Conversion of Sunlight to Electricity David T Britton, NanoSciences Innovation Centre University of Cape Town Photovoltaic effect.

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Presentation on theme: "PHOTOVOLTAICS Direct Conversion of Sunlight to Electricity David T Britton, NanoSciences Innovation Centre University of Cape Town Photovoltaic effect."— Presentation transcript:

1 PHOTOVOLTAICS Direct Conversion of Sunlight to Electricity David T Britton, NanoSciences Innovation Centre University of Cape Town Photovoltaic effect Early (simple) solar cells: 1839 – Heterojunction solar cells: 1946 – principle of operation commercial solar cells and modules Distributed junction solar cells State of the Art?

2 PHOTOVOLTAIC EFFECT Photochemical Edmond Becquerel, Comptes Rendus 9, 561 – 567 (1839) Solid State W.G. Adams & R.E. Day, Proc Roy Soc Lond 25, 113 -117 (1876) Light generates free charges electrons and ions electrons and holes

3 Single homogeneous material between two contacts Diffusion/drift of charge to one electrode gives current Voltage is low SIMPLE SOLAR CELLS Becquerel 1839 Adams & Day 1876 illuminated electrode is cathode (photoresistors)

4 First thin film solar cell Charles Fritts 1882 Selenium with thin gold leaf front electrode, (transparent to green light) tin plated back electrode Am J. Sci 26, 465 – 482 (1883) PV discovered in Fritts cell by W. Siemens Sitzungsber. Preuss. Akad.Wiss zu Berlin 1885 (I), 147-148 Screen printed CdS, CdSe Y. T Sivhonen & S.G. Parker 1963, (Texas Instruments) US 3208022 (1965) First fully printed silicon cell 2008 SIMPLE SOLAR CELLS (photoconductors)

5 HETEROJUNCTION SOLAR CELLS (photodiodes) electrons & holes produced in the junction electrons pulled to n-type layer holes puled to p-type layer Inbuilt electric field Anode and Cathode given by diode direction dark light Short circuit current j sc is from n to p Open circuit : charge collection gives V oc

6 Standard test conditions (IEC 6125) 1kW/m 2 25 0 C AM1.5 sunlight spectrum 1 cm 2 cell Cell efficiency  : maximum electrical power out/ radiant energy in Module efficiency: useful power out/radiant energy in HETEROJUNCTION SOLAR CELLS Equivalent circuit Quantum Efficiency (QE) = number charge carriers/number of photons Internal quantum efficiency (IQE): only absorbed photons External quantum efficiency (EQE): all photons hitting the cell

7 Modern commercial solar cells Silicon pn junction cell (Bell Labs) 1941 R.S. Ohls, US Patent 2402662 (1946) HETEROJUNCTION SOLAR CELLS First organic PV (OPV) cell D. Kearns & M, Calvin, J Chem. Phys 29, 950-951 (1958) CrystallineThin film Thin film OPV, C.W. Tang, Appl Phys Lett 42, 183 – 185 (1985) GaAS solar cells 1970 Thin film a-Si:H (RCA 1976) Other geometries include tandem cells and graded junctions

8 DISTRIBUTED JUNCTION SOLAR CELLS Only one layer of mixed material More light reaches the junction Larger junction shorter distance for charge to travel Dye Sensitised Solar Cell (Photochemical) B. O’Regan & M. Grätzel, Nature 353, 737-740 (1991) Polymer – fullerine blends (OPV) G. Yu, J. Gao, J.C. Humelen, F. Wudl, and A.J. Heeger Alan Heeger & Michael Grätzel are founders of Konarka

9 2010/06/14 Sharp 44.4% STATE OF THE ART? Other factors to discuss: Industrial scalability price per Wp cost per kWh Are they green?

10 Sharp Develops Concentrator Solar Cell with World's Highest Conversion Efficiency of 44.4% Sets New Record with Concentrator Triple-Junction Compound Solar Cell http://sharp-world.com/corporate/news/130614.html

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