061cdce9   
   XPost: sci.physics, sci.electronics.design, sci.chem   
   From: zenadsl6186@zen.co.uk   
      
   On 30/04/13 21:55, RichD wrote:   
   > On Apr 30,  benj  wrote:   
   >>>> I recently attendesd a seminar on photovoltaic research.  The   
   >>>> speaker showed a graph, how the power efficiency drops   
   >>>> as the recombination time of the minority carriers decreases.   
   >>>> Can anyone expound on this?   
   >>   
   >>> If the electron recombines before it reaches the external   
   >>> connection that the energy that it had is just thermalised as heat.   
   >>   
   >>>> And, the quantum efficiency drops, as well.  What does that   
   >>>> mean?   
   >>   
   >> So what is "quantum efficiency"? It's simply the ratio of photons   
   >> in to electrons out the wires. So if a photon comes in creates an   
   >> electron-hole pair but they recombine before being collected   
   >> for the output. Then that photon has no output. Hence QE is lower.   
   >   
   > How is that different than power conversion efficiency?   
      
      
      
   Photoelectric efficiency is the power of the light falling on the cell   
   divided by the energy output of the cell. It is seldom more than 20%.   
      
   There are several reasons for this - first, not all quanta are absorbed,   
   some are reflected.   
      
   Second, some of the quanta which are absorbed do not create   
   electron/hole pairs, and some electrons and holes recombine before they   
   reach the cell's electrodes.   
      
   The proportion of quanta absorbed which produce electrons which reach   
   the electrodes is often called the quantum efficiency, though   
   technically the term is to mean the proportion of the quanta which fall   
   on the cell which produce electrons which reach the electrodes. It is   
   measured in electrons per photon.   
      
      
   Third, light falls on the cell in quanta with energies somewhere between   
   1.6eV (for red light) and 2.8 eV (for blue light). When a quantum is   
   absorbed, some will have high initial energy and some lower initial   
   energy, but the electrons they push out will all have the energy of the   
   bandgap when they leave the electrodes, and some energy is always lost here.   
      
   Fourth, the cell has some electrical resistance.   
      
   fifth ...   
      
      
   -- Peter Fairbrother   
      
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