Semester 2 Flashcards
If the wavelength of light is 1.24 μm, what is the energy of each photon?
1 eV
What is the Pauli-exclusion principle?
No two electrons with the same set of quantum numbers can have the same energy.
What is the De Broglie relation?
λ=h/p
What is the bandgap energy range for suitable semiconductor materials to be used in photovoltaics
1-1.7eV
What is the Fermi level?
The energy for which the probability of occupancy by an electron is exactly 0.5.
What is the probability an electron has an energy E?
F(E) = 1/[1 + exp(E-Ef)/kT]
When is a semiconductor highly resistive at room temperature?
When it has an energy bandgap >1eV and it is pure and free from crystal imperfections.
What is it called when impurities are introduced into a semiconductor?
Doping
When is a semiconductor “n-type”?
When it is doped and the impurity atoms release an ELECTRON each.
When is a semiconductor “p-type”
When it is doped and each impurity atom releases a HOLE.
Why is doping carried out in semiconductors?
The introduction of foreign atoms as substitutional impurities greatly increases the electrical conductivity.
What is the intrinsic Fermi level?
Eg/2
What are the 5 types of crystal defect?
- 3D volume defects.
- 2D planar defects.
- 1D line defects.
- 0D point defects.
- Phonons
What are the 3D volume defects?
- Included grains of 2nd phase.
- Voids.
What are the 2D planar defects?
- Grain boundaries
- Stacking faults
- Twins
What are the 1D line defects?
- Edge dislocations.
- Screw dislocations.
What are the 0D point defects?
- Vacancies.
- Interstitial atoms.
- Substitutional impurity atoms.
- Interstitial impurity atoms.
What are phonons?
Quantised lattice vibrations.
What are the 3 main effects of crystal defects?
- impurity atoms and other defects may “dope” the semiconductor altering the carrier concentration.
- Impurity atoms and other defects will “scatter” charge carriers, reducing their mobility.
- Impurity atoms and other defects may act as recombination centres reducing the minority carrier lifetime diffusion length.
What is the dominant absorption process in a solar cell?
The intrinsic or band-to-band optical absorption by a single electron across the bandgap of the semiconductor.
In a solar cell, what happens when a photon with Ephoton = Eg illuminates the cell?
An electron-hole pair is generated.
In a solar cell, what happens when a photon with Ephoton > Eg illuminates the cell?
An electron-hole pair is generated and excess energy is dissipated as heat.
In a solar cell, what happens when a photon with Ephoton < Eg illuminates the cell?
The photon is only absorbed if there is an available state within the forbidden energy gap.
What causes a minority carrier injection in doped semiconductors?
When an electron-hole pair is created, with electrons and holes created at the same rate.
What is the formula for penetration depth in terms of the absorption coefficient?
W = 1/α
What are the issues with single bandgap devices?
Inefficient use of high energy photons, low energy photons are not collected, from solar spectral irradiance distribution.
What does each term in the equation I = neAVd represent?
I=current n=number of electrons e=charge of an electron A=cross-sectional area Vd=drift velocity
How do you calculate the number of charge carriers/unit volume taking part in conduction?
n=(N/V) x valence
N=number of atoms
V=volume
What is the formula for the hall constant?
Rh = 1/ne
n=carrier concentration
What is the formula for thermal velocity?
Vt = √(3kT/m)
How do you determine a materials absorption coefficient for a given energy from an E-K diagram?
Find the material at the given energy and draw the line along to absorption coefficient.
What is the formula for photon flux?
Φ = number of photons/tA
where t is time and A is area.
What are the key features of the band structure of silicon?
- Indirect bandgap.
- Maximum VB energy occurs where the wave vector k is equal to zero.
- Minimum in CB occurs along the [100] direction.
- Offset in minima and maxima positions.
What are two semiconductor materials commonly used in PV with indirect bandgaps?
- Silicon (Si)
- Germanium (Ge)
What are two semiconductor materials commonly used in PV with direct bandgaps?
- CdTe
- GaAs
