3. Solar PV

Question 1

Define Voc and Isc

Answer 1

Open circuit current and short circuit current.
Voc occurs when solar cell terminals not connected.
Isc occurs when terminals connected with no resistance.

Question 2

Define Quantum Efficiency

Answer 2

Likelihood that incoming photon has sufficient energy to deliver electron to external circuit.

Question 3

What does QE depend on?

Answer 3

• Absorption coefficient of cell material.
• Efficiency of charge separation.
• Efficiency of charge collection.

Question 4

Define scenarios of 0Voc

Answer 4

0Voc: Device consumes power

Question 5

Plot graph defining fill factor

Answer 5

Include Jsc, Jm, Voc, Vm

Question 6

Describe losses in PV cell.

Answer 6

• Leakage currents through cell
• Leakage currents around side of device
• Leakage currents between contacts
• Cell material resistance

Question 7

Outline Pauli-Exclusion Principle

Answer 7

No two electrons can have same 4 quantum numbers.
n, describes shell
l, describes sub shell,
m1, describes orbital
ms, describes spin

Question 8

Describe band gaps in metals

Answer 8

When atoms form a crystal, energy levels are so close together that they form a continuous energy band.

Question 9

How do PV cells work?

Answer 9

1. Light is made up of packets of energy called photons. When a material absorbs light, these photons are destroyed, which excites an electron.
2. If the energy of the photon is greater than the energy of the band gap, the electron is excited to a higher energy level.
3. The band gap must be big enough to slow down the relaxation process of the excited electrons. If the band gap is too small (metals), the electron will relax back to ground state, producing phonons, generating heat.
4. Semiconductors have a band gap large enough to slow down the relaxation process of the electrons sufficiently long enough for charge separation to occur.
5. For charge separation to occur, the cell must have a built in asymmetry, such as a p-n junction.
6. Charge separation allows the electrons to be fed into an external circuit to do useful work.
7. Increased potential energy of an electron creates a voltage which drives a current in an external circuit.

Question 10

Outline p-n junctions.

Answer 10

• Creates a built in electric field.
• One side of Silicon crystal doped with phosphorus, introducing extra electron (n-doping).
• Other side of crystal doped with boron extracts electron (p-doping).

Question 11

Define the standard test conditions.

Answer 11

-AM1.5, 1000W/m2, 25C. Sun at 41.8 angle

Question 12

Explain trade off of energy gap size.

Answer 12

• Must be great enough to slow down relaxation process effectively.
• Small band gap will absorb a large amount of photons with energy greater than Eg (large current density).
• Large band gap means energy per photo is higher (large voltage)

Question 13

What does PV efficiency depend on?

Answer 13

• Energy Gap
• Light Absorption
• Charge Separation
• Transport Losses
• Load Resistance

Question 14

How can light absorption be increased?

Answer 14

Increase material thickness; however, very high purity needed to reduce carrier recombination at defects.

Question 15

How can transport losses be reduced?

Answer 15

Use material with good transport properties (high electron and hole density)

Question 16

How is load resistance managed?

Answer 16

Load resistance should be chosen to match optimum operating point of solar cell array.

Question 17

Describe how Modules and Arrays are used.

Answer 17

• Cells connected into modules, which combine to form an array.
• Bypass and blocking diodes in place to prevent entire array shut down should one cell fail.
• Inverter required to convert from AC to DC.

Question 18

Describe Solar PV from a high level.

Answer 18

• PV modules produce a DC current from sunlight.
• Charge controller protects batteries from being over charged / discharged.
• Battery in place to store electricity.
• Battery can drive DC loads directly.
• Inverter required to convert DC to AC for grid.