Photovoltaics Flashcards
What is radiative forcing?
A measure of energy in vs energy out.
Specifically, the difference between solar irradiance absorbed by earth and energy radiated back into space.
Photovoltaics as a fuel source
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
Describe Simple Model of a Solar Cell
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.
How is energy lost in a solar cell?
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.
Describe bandgap diagrams
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.
Direct Band Gap vs Indirect bandgap as solar cells
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.
What is the absorption coefficient?
Measured as α dependent on wavelength. It is a measure of how well a material can absorb light.
How does the bandgap material relate to absorption?
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.
Describe the Lamber Beer Law
N = N0e-ax
Where N is photon flux at depth x,
a is the material absorption coefficient.
Units are photons/unit area/second
The equation for energy in terms of momentum
Normalised Photon Flux
N/N0 = e-ax
Used in graphs.
Absorptance Equation
What is G
G is the generation rate, number of e-h pairs created each second per unit volume.
G(x) = -dN/dx = a N0 e-ax
Why does the generation rate decrease with high energy and increased depth into the silicon wafer
High energy doesn’t allow further travel into a semiconductor as high energy photons are more quickly absorbed compared to longer wavelength light.
What is the approximate normal incidence at sea level on a normal clear day?
1kW/m2
What is approx power at normal incidence on Earth?
1.36kW/m2
Describe PN junction operation
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.
What is the light generated current?
IL is the light generated current, it is defined as the current generated due to the movement of carriers in an illuminated solar cell
What is the concept of collection probability?
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.
What is recombination due to bulk defects called?
Shockley Read Hall aka (SRH) recombination
What is crystalline sillicon wafer technology?
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.
Describe the expected IV characteristics of a single crystal solar cell, when illuminated
Describe expected IV characteristics of a solar cell in the dark
