MIT 3 15 - Photoconductors, Photovoltaics and Photodetectors (3 pages)

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Photoconductors, Photovoltaics and Photodetectors



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Photoconductors, Photovoltaics and Photodetectors

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Lecture Notes


Pages:
3
School:
Massachusetts Institute of Technology
Course:
3 15 - Electrical, Optical & Magnetic Materials and Devices
Electrical, Optical & Magnetic Materials and Devices Documents

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3 15 Photoconductors Photovoltaics and Photodetectors C A Ross Department of Materials Science and Engineering Reference Pierret chapter 9 2 and 9 3 Photoconductors conductivity a function of light Photovoltaics generate power from light Photodetectors use a pn junction to detect light Photoconducting materials CdS ZnS for camera lightmeters amorphous As Se Te for photocopiers Photodiode and Photovoltaic PV Carriers created within Ln or Lp of junction contribute to reverse current I Io IG Photodiode operates in reverse bias A PIN diode has a wide depletion region operates much faster than a pn junction photodetector because it doesn not rely on diffusion A PV operates in the fourth quadrant positive V negative I When connected to a load e g a battery charger or a lightbulb with resistance RL V I RPV RL also I Io exp eV kT 1 IG these two relations define the voltage and current that the PV produces Power IV Solar Cells the PV must respond to the visible spectrum 400 700 nm or 2 3 eV note that l mm 1 24 E eV Ideally we would use a bandgap of about 1 2 eV but Si does not absorb light well because it has an indirect band gap Direct and indirect gap 2 On an E k plot m h 2 E k 2 1 Momentum of an electron typically p a 1010 m 1 Momentum of a photon 2p l 107 m 1 If the band gap is indirect a phonon plus a photon are eeded to make an e h pair so light absorption and emission is less efficient Amorphous Si uncertainly principle DxDp h the localization of carriers gives them an uncertain momentum so direct absorption of light can occur Use PIN design because mobility is low Handout 5 1 Scanned article removed due to copyright restrictions Please See This Month in Physics History October 22 1938 Invention of Xerography APS News 12 2003 2 Handout 5 2 ASE Americas Inc Solar Panel Model BC 16 DG Voltage per Cell Volts 0 2 0 4 0 6 4 0 04 2 0 02 Darkened l V Behavior 0 2 Fully Illuminated 0 4 5 0 12 0 5 32 9 23 0 41 6 0 19 8 28 5 37 0 43 8 Power Watts 10 15 20 0 00 0 02 Current



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