b1b8d350   
   XPost: sci.physics, sci.electronics.design, sci.chem   
      
   On May 2, 9:10 am, Phil Hobbs    
   wrote:   
   > On 5/2/2013 7:04 AM, Jasen Betts wrote:   
   >   
   > > On 2013-04-30, Tim Wescott  wrote:   
   > >> On Mon, 29 Apr 2013 21:36:22 -0700, RichD wrote:   
   >   
   > >> I'm not sure why quantum efficiency would suffer as recombination time   
   > >> goes down unless (a) they're referring to the overall quantum efficiency   
   > >> (in which case power efficiency and quantum efficiency are just synonyms)   
   >   
   > > I can't see power efficiency being synonymous with quantum efficiency   
   > > solars photons go in with an mean energy of around 2eV and you get   
   > > electron current out (of a silicon photocell) at less than a third of that.   
   >   
   > Solar cells are diodes working in forward bias.  If you short-circuit   
   > the cell, you lose practically nothing to recombination (i.e. forward   
   > conduction), so you get all of the photocurrent, and therefore the   
   > maximum operating quantum efficiency.  Unfortunately you get zero power,   
   > because P = VI.   
   >   
   > If you open-circuit it, you get the maximum terminal voltage, i.e. the   
   > maximum energy per electron, but you waste all of the photocurrent   
   > forward biasing the diode, i.e. the operating quantum efficiency is zero.   
   >   
   > In between, you get less than maximum voltage and less than maximum   
   > current, but since both are nonzero you also deliver power to the load.   
   >   
   > The maximum power point is where d(VI)/dV = 0, i.e.   
   >   
   > I + V dI/dV =0    so   I/V = - dI/dV   
   >   
   > If you increase the temperature, the forward voltage of the diode   
   > decreases just like any other diode, so if you keep the same operating   
   > voltage you start to lose current (i.e. the operating quantum efficiency   
   > goes down).  To maintain maximum power, you have to reduce the operating   
   > voltage, which will increase the current some, but not all the way back   
   > to its lower-temperature value.   
      
   You can get higher quantum efficiency by throwing a tarp over it,   
   reducing the temperature.  Makes 'em last longer too.   
      
   > So if you want to maintain maximum power, you have to run at lower   
   > operating QE as the temperature increases, and the power you get is reduced.   
      
   --   
   Cheers,   
   James Arthur   
      
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