PV 2

PV 2

1. RECAP PV1

   • electron / hole pair generation
     • photon = absorbed - electron goes from VB to CB
       • hole left in VB - electron hole pair
   • indirect band gap
     • complex - decreases absorption efficiency

2. Electron - hole annhilation

   • three methods
     • Auger
       • Three carrier process
         • Electron/hole recombine
           • dont emit heat / photon - give enrgy to a third carrier = electron in conduction band
             • electron rasies up energy levels in conduction band, then thermalizes back down to CB edge
     • Radiative
       • band - band recombination
         • electron from CB combines with Hole from VB
           • releases photon - photon energy approx that of band gap so can exit
         • dominates in direct band gap semi conductors
     • SRH (Schokly-Read-Hall)
       • "defects recombination"
         • does not occur in pure undefected material
         • two step process
           • electron drops from CB to a energy state in the "forbidden" region
             • level introduced because of material defects
               • hole moves up to the same energy state before the electron is thermally re-emitted into conduction band = recombines

3. Current generation

   • electrons passing across band gap = electric current
     • Ip=qNA
       • N = fraction of photons in usable region of spectrum
       • A= surface area of semiconductor exposed to light
   • light induced current generates a voltage
     • electric power is generated by separating the electrons and holes at device terminals
       • Vmax=Eg/q
         • Vmax = bandgap of semicondcutor in eV

4. creating a semiconductor junction

   • 4 main types
     • PN
     • PIN
     • Schottky Barrier
     • Heterojunction
     • each has a built in potential
   • Junction generates strong electric fields
     • between two semiconductors
       • n-type electrons near interface cross to p-type side = leave a positively charged area
         • P-type holes transfer to n-type side = leaves a negatively charged area
           • creates a depletion zone - no mobile charges
             • prevents flow of electrons / holes
               • photon = absorbed
                 • electron/hole pair from depletion zone
                   • electron flows into N-type, hole into p-type, flows around circuit
   • Biasing junctions
     • no net current at equilibrium
     • forward bias = reduced barrier height occurs - increases current through diode..
       • negative termianl of battery connected to n-type, repulsion pushes electrons towards depletion zone
         • Postive termainl replusle the hoels towards the depletion zone
           • battery ahs enough power to overcome depletion zone, holes and electons can combine in depletion and create a current
     • reverse bias
       • draws electrons and holes away from depletion zone, increase depltion zone, prevetns current flow

5. how a solar cell works

   • Photon absorbed
     • energy of photon absorbed in depletion zone
       • creates a free electron and a hole - enough energy to jump out of the depletion zone
         • negative electron is attracted to +ve P type, so flows through n-type, around circuit, into back of p-type where it combines with hole

6. PN junction under external bias

   • Curve properties
     • short-circuit current = Isc
       • occurs through solar cell when voltage across teh cell is zero
     • Voc - open circuit votlage
       • is the maximum votlage available from the cell, occuring at zero current
     • Fill factor / small area/ big area
       • ff=imp*vmp/Isc&Voc
     • solar cell efficiency = Pmax(pout)/Pin = FFVocIsc/Pin
   • Temperature effects
     • increasing temperature reduces Eg
       • viewew as increasing the energy of electrons in semiconductor
         • less energy needed to break the bonduing, rduction in bond energy = reduction in band gap
           • Voc - affected most by increasing T

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