† † 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 On an E-k plot: m* = h2(∂2 E /∂k2)-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 1Handout 5 2 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 4 Darkened l-V BehaviorFully Illuminated0012.023.032.941.643.837.028.519.8Power (Watts)-5-4-22400-0.04-0.020.020.040.005 10 15 20 25Voltage -- 36 Cells in Series (Volts)0.0 0.2 0.4 0.6ASE Americas, Inc. Solar Panel, Model BC-16-DGVoltage per Cell (Volts)Current Density (Amps/cm2)Current (Amps)Figure by MIT