PV Flashcards
(32 cards)
What is the power density as the Earth’s surface?
1000 W/m^2
What is the power density at the atmosphere?
1360 W/m^2
What is the band gap between insulators?
Big gap (large difference in energies)
What is the band gap between semiconductors?
Small Gap
What is the band gap between conductors?
Overlapping of conduction and valence bands
What is the efficiency of a PV cell?
15 - 21 %
What is the density of a silicon atom?
10^28 m^-3
Why do some electric become free at certain temperatures?
Around 300 kelvin a few electrons gain enough energy to break one of the bonds and become free
What does 4N purity mean?
99.99 %
What does the number of electrons equal?
Number of holes
In doped silicon, what is the equation for equilibrium?
n . p = n_i^2
What is the relationship for n and p?
Inverse
What is more in a n-type material?
n goes up, vice versa for p-type
What does shining light on a semiconductor do?
Bonds are broken creating an electron-hole pair
What is the minimum energy required to achieve an electron-hole pair?
Band gap energy of the semiconductor
What happens when the minimum energy is reached?
the electron is excited from the valence band to the conduction band
What is the equation for lambda_max?
lambda_max (in micro m) = 1.24 / E_g (in eV)
Is it above or below these wavelengths that electron-hole pairs are created?
Below lamba_max
What is an advantages of having more junctions in a solar cell?
Higher efficiency - each junction is tuned to a specific part of the spectrum
What is the downside of having more junctions in a solar cell?
Complex manufacture
Mismatch losses if one layer underperforms
What happens to the intrinsic carrier concentration as temperature increases?
Carrier concentration increases exponentially - more energy to jump band gap
both n and p increase
but if heavily doped p is majority carrier (NA) and n is minority carrier (n)
How does the greenhouse gas effect work in homes with glass?
Short wave IR travels through glass and is absorbed by objects and surfaces. It is then re-emitted as longwave IR which glass is opaque to. Heat is trapped in the budling.
What are the two main intrinsic losses in solar cells?
Thermalisation = When photon energy is greater than band gap - excess energy is lost as heat before relaxing down.
Transmission = When photon energy is less than band gap - photon passes through material without being absorbed. these photons do not contribute at all.
What two properties should a good tunnel junction possess?
Band gap alignment and current matching
