Photovoltaics Flashcards
Explain how the depletion layer of a pn-junction is created.
When a p-doped and n-doped semiconductor are put together, there is an abundency of electrons on the n-side and hole on the p-side and in the region of the interface will they try to even out, the holes and electrons will pair up leaving immobile ions left, pos. on the n-side an neg. on the p-side. Dye to this there will be a drift current from the n-side to the p-side since the electrons are going in the other direction.
Explain band bending at the pn-junction.
When putting a p-doped and a n-dopen semiconductor together the bends bend so that the fermi levels are at the same level. This means that the n-doped conduction band comes lower down than the p-doped, and for the electrons to move over the depletion region there is an energy barrier due to the electric field created there by the charges.
Explain how forward and reverse bias affects the band bending.
If applying forward bias to the pn-junction, that is a voltage opposed to the field induced by the charges in the depletion region, this voltage will lower the energy barrier and help electrons to overcome the barrier and travel from the n-side to the p-side. If the voltage is high enough there will be a current going in the opposite direction as the electrons, ie. from the p-side to the n-side.
When applying reverse bias on the other hand the energy barrier is increased, because the voltage is applied in the same direction as the already existing field from the charges. No current can travel in the opposite direction but when the voltage if large enough the current breaks through, this is called break down voltage.
Explain what happens in the depletion region when light shines on it.
When light shines on the depletion region, the photons can excite excitons in the interface and if they are not tightly bound, ie. free excitons, they can separate if they have enough energy left after the excitation. Then the electron goes to the n-side due to the pos. charged ions on this side and the hole goes to the p-side.
When a load is applied to the circuit these abundant electrons travel, creating a current in the opposite direction.
The current from the solar cell is now this current from the excitons minus the drift current in the opposite direction.
Draw the IV-curve and explain it. How is the maximum power calculated?
Where the curve intersect the y-axis is the short circuit current J_sc and when the current is 0 we find the open circuit voltage V_oc. But these are not possible to realise, the maximum current J_m and voltage V_m are instead find a little bit lower.
The maximum power is then calculated as P_m = J_m*V-m.
What is special for a solar cell compared to a battery? How can the current become larger?
For a solar cell is the current always constant opposed to a battery that has constant voltage.
The short circuit current can be calc. as
J_sc = qint (b(E)QE(E)) dE,
b is the number of photons with enough energy hitting the solar cell and QE the quantum efficienfy, ie. the probability of a photon creating an exciton.
Explain the fill factor and how it can be used to calc. the efficiency.
The fill factor is the ratio between the power from J_sc and V_oc and the real maximum power.
FF = (J_mV_m)/(J_scV_oc)
The efficiency of the solar cell is the power that it delivers divided by the solar power.
eta = J_mV_m/P_s = FFJ_sc*V_oc/Ps
one has more often the J_sc and V_oc.
Define R_s and R_shunt and explain what they should be for ideal work and when they are not, how it affects FF.
R_s is the resistance in series with the solar cell and therefore need to be 0 for maximum current out, and R_shunt is in parallel with the solar cell and thus should be infinity for maximum current output.
When they are not their ideal values this makes FF decrease, ie. increasing R_s and decreasing R_shunt is bad.
Give some pros and cons of inorganic solar cells.
Pros:
+ they are mechanically easy, only one piece
+ dc current means battery storage is simple
+ no noise or exhaust
+ can give electricity in remote places
Cons:
- is expensive
- ac inverter must be used
- battery storage means more maintenance
- some materials used are toxic
Explain the basics of organic solar cells.
The pn-junction is replaced by an organic material. Need to have donor and acceptor in this material. Then on both sides of this there is an electrode.
But a limit factor is that the exciton diffusion is very short, approx. 10nm, which means that the donor and acceptor regions need to be very close. The exciton is created in the donor region and diffuses to the interface where the electron goes into the acceptor region.
An ideal structure is like a comb of donor on one side and acceptor on the other.
It is important that they are only connected to one electrode each, if to both have short circuit, or if to none don’t have a current through the electrodes.
How does the band structure look for organic photovoltaics?
For organic molecules there are not a conduction and valence band as for semicond. but instead there is the HOMO (highest occupied molecular orbit) and the LUMO (lowest unoccupied molecular orbit) which works similarly, an electron can be excited from the HOMO to LUMO.
When having more molecules is the levels below the HOMO more like continuum and the band gap in between is decreasing.
Pros and cons of organic solar cells compared to inorganic.
- organic solar cells are less efficient due to that excitons are more closely bound and thereby harder to separate and the energy barrier is higher
- organic solar cells have shorter lifetime due to photobleaching, oxidization, corrosion etc.
- amorphous structure makes charge transfer harder
+ cheaper, easier and lighter material
+ no toxic materials used
+ can have really thin layer but still absorb as much, 100nm compared to 300 micro
Explain how a dye-sensitized solar cell works.
There is a dye, a TiOx part connected to an electrode, on the other electrode happens the iodine-cycle.
- Photon comes in and excites the dye. Dye° –> Dye*.
- The excited electron escapes the dye and has enough energy to travel in the conduction band of the TiOx to the electrode, it goes through the load to the other electrode. 3. There converts it tri-iodine to iodide. I3 - + 2e –> 3I-
- The iodide reduces the oxidized dye. Dye* –> Dye°
