Photovoltaics

Question 1

What is radiative forcing?

Answer 1

A measure of energy in vs energy out.

Specifically, the difference between solar irradiance absorbed by earth and energy radiated back into space.

Question 2

Photovoltaics as a fuel source

Answer 2

Advantages

• Vast fuel source: The amount of solar energy is arriving on Earth is 10,000×current global consumption
• No emissions/harmful by-products
• No moving parts-high reliability (30 years manufacturers’ guarantees)
• Low operating costs (no fuel)
• Rapid installation at point-of-use
• Can be integrated into new or existing structures
• Ambient temperature operation (fewer safety issues)

Disadvantages

• The fuel source is low density
• High initial costs
• Unpredictable variations in hourly and daily output
• Requires energy storage

Question 3

Describe Simple Model of a Solar Cell

Answer 3

The solar cell has a similar band structure to a basic semiconductor.

When the solar cell is illuminated, the photon is absorbed into the cell via the generation of electron-hole pairs.

The photon excites an electron, it passes from the conduction band to external load via a selective pn junction. A second selective pn-junction returns the electron to the valence band.

Question 4

How is energy lost in a solar cell?

Answer 4

Energy is lost as excess energy from the photon-electron collision is lost. In addition, when photons do not have enough energy to excite electrons to the conduction band, the energy of the photon is lost.

Question 5

Describe bandgap diagrams

Answer 5

A band diagram is a diagram plotting various key electron energy levels (Fermi level and nearby energy band edges) as a function of x.

They only resemble a parabola around the actual gap between the conduction and valence bands.

Question 6

Direct Band Gap vs Indirect bandgap as solar cells

Answer 6

Direct Bandgap materials are more suitable than solar cells as the electron can easily move from the valence band to the conduction band.

Indirect Bandgap Materials require a phonon to do this as the electron requires extra momentum to move between bands.

Question 7

What is the absorption coefficient?

Answer 7

Measured as α dependent on wavelength. It is a measure of how well a material can absorb light.

Question 8

How does the bandgap material relate to absorption?

Answer 8

The indirect bandgap material shows slightly higher absorption at low energy levels,

However, mid energy levels the direct bandgap material has a higher absorption level. They both saturate at the same absorption level as energy increases.

Question 9

Describe the Lamber Beer Law

Answer 9

N = N₀e^-ax

Where N is photon flux at depth x,

a is the material absorption coefficient.

Units are photons/unit area/second

Question 10

The equation for energy in terms of momentum

Answer 10

Question 11

Normalised Photon Flux

Answer 11

N/N₀ = e^-ax

Used in graphs.

Question 12

Absorptance Equation

Answer 12

Question 13

What is G

Answer 13

G is the generation rate, number of e-h pairs created each second per unit volume.

G(x) = -dN/dx = a N₀ e^-ax

Question 14

Why does the generation rate decrease with high energy and increased depth into the silicon wafer

Answer 14

High energy doesn’t allow further travel into a semiconductor as high energy photons are more quickly absorbed compared to longer wavelength light.

Question 15

What is the approximate normal incidence at sea level on a normal clear day?

Answer 15

1kW/m²

Question 16

What is approx power at normal incidence on Earth?

Answer 16

1.36kW/m²

Question 17

Describe PN junction operation

Answer 17

Both p-type and n-type have no net charge.

The electrons in the n-type semiconductor diffuse to the p-type and the holes in the p-type drift to the n-type semiconductor.

Under an electric field the minority carriers i.e. holes from n-type to p-type and electrons from p-type to n-type, move. This is the drift current.

The diffusion and drift current are equivalent to each other.

Question 18

What is the light generated current?

Answer 18

I_L is the light generated current, it is defined as the current generated due to the movement of carriers in an illuminated solar cell

Question 19

What is the concept of collection probability?

Answer 19

The probability an EHP will reach the depletion region and be collected before recombination.

The probability of collection varies with distance from the depletion region.

Question 21

What is recombination due to bulk defects called?

Answer 21

Shockley Read Hall aka (SRH) recombination

Question 22

What is crystalline sillicon wafer technology?

Answer 22

Uses indirect bandgap materials, this means the device requires a large carrier diffusion length in order for photons to be successfully absorbed. Crystalline silicon devices are known as 1st gen devices.

There are two types, mono and multi-crystalline silicon

Multicrystalline silicon wafers are not as efficient as monocrystalline silicon wafer but are cheaper.

The operation of such devices are the same as the simple model, however, has defects.

Question 23

Describe the expected IV characteristics of a single crystal solar cell, when illuminated

Answer 23

Question 24

Describe expected IV characteristics of a solar cell in the dark

Answer 24

Question 25

How to measure I_sc from IV characteristics graph

Answer 25

To derive short circuit current I_sc from the IV characteristics graph, measure current decrease when voltage is 0.

Question 26

How do you derive the V_oc from an IV characteristics graph?

Answer 26

Observe the highest value of voltage where the current is 0 when the cell is illuminated.

Question 27

How is an open circuit generated? in a solar cell?

Answer 27

• More light-generated minority carriers are separated by the junction
• Creates electric field opposing the built-in field.
• Decreases the barrier height,
• Diffusion current increases to balance IL.
• When these currents are balanced, the voltage, Voc exists across the terminals.

Question 28

How do you indicate the max power current point?

Answer 28

You can utilise a max power rectangle, the I_mp point is simply the current at which the power is maxed.

Question 29

How to derive the voltage max power point?

Answer 29

Similar to I_mp, V_mp, is indicated by the max power rectangle and is simply the voltage at which power is maxed.

Question 30

The equation for fill factor

Answer 30

Question 31

The equation for power conversion efficiency

Answer 31