Photovoltaics

Question 1

What is planks constant

Answer 1

6.626 x10^-34 m^2 kg/s

Question 2

What is the speed of light

Answer 2

2.998 x10^8 m/s

Question 3

What is the absorption coefficient

Answer 3

determines how far light of a certain wavelength can travel into a material before being absorbed

Low abso co = travels far into material
High abso co = absorbed quickly

Question 4

If photon energy (Ep) is less than the bandgap energy (Eg), what happens

Answer 4

If Ep < Eg… the photon has a low energy and thus a larger wavelength (infra, radio etc). The photon travels through the material.

Question 5

If photon energy (Ep) is greater than the bandgap energy (Eg), what happens

Answer 5

If Ep > Eg… the photon has a high energy and thus is a shorter wavelength. The photon is absorbed quickly and an electron is promoted to the conduction band (hole created in valance band).

Question 6

If photon energy (Ep) is approx. equal to the bandgap energy (Eg), what happens

Answer 6

If Ep = Eg… the photon just has enough energy to be absorbed. An electron is promoted to the conduction band and a hole is created in the valance band.

Question 7

When a photon is absorbed by a semiconductor, what happens

Answer 7

Photon absorbed -> electron promoted to conduction band and a hole is created in valance band

Question 8

Explain a multi junction solar cell

Answer 8

Has multiple bandgaps to ‘capture’ different photons with different energies…

High energy photons absorbed on the surface

Low energy photons travel through the surface and are absorbed further in material.

Question 9

What is the fermi energy

Answer 9

The max energy in an atom at 0 kelvin

Question 10

What is the relationship between resistivity and temperature in a semiconductor

Answer 10

As temperature increases, resistivity decreases

Question 11

What is the hall effect, what is it exclusive to

Answer 11

Exclusive to semi conductors… When a voltage can be measured perpendicular to the flow of current

Question 12

What is electron/hole mobility and which dominates

Answer 12

How far an electron/hole can travel before a collision and energy is lost. Electron mobility dominates

Question 13

What is the boltzman constant

Answer 13

1.38e-23

Question 14

What is the permittivity of free space

Answer 14

8.854e-12

Question 15

Avagadros Constant

Answer 15

6.02e23

Question 16

Charge of an electron

Answer 16

1.602e-19

Question 17

Intrinsic carrier boundry condition

Answer 17

p x n = ni^2

Holes x Electrons = intrinsic carriers squared

Question 18

In P and N type, what are the majority and minority carriers… and what is the minority carrier density

Answer 18

P type
Holes (p) are the majority = Na acceptor
Electrons (n) are the minority
Minority density = n = ni^2 / p

N type
Electrons (n) are the majority = Nd donor
Holes (p) are the minoirty
Minority density = p = ni^2 / n

Question 19

Draw the bandgap for a pure semiconductor

Answer 19

Top band = Conductance
Bottom band = Valance

Spaced equally between the 2 should be the fermi energy level

Question 20

Equation for voltage (Vbi)

Answer 20

V = ( kT / ec ) ln( Na Nd / ni^2 )

Question 21

Equation for intrinsic carriers (ni)

Answer 21

ni = sqrt( Nc Nv ) exp( -Eg / 2kT )