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Advanced Photovoltaics > Semester 2 > Flashcards

Semester 2 Flashcards

1
Q

If the wavelength of light is 1.24 μm, what is the energy of each photon?

A

1 eV

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2
Q

What is the Pauli-exclusion principle?

A

No two electrons with the same set of quantum numbers can have the same energy.

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3
Q

What is the De Broglie relation?

A

λ=h/p

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4
Q

What is the bandgap energy range for suitable semiconductor materials to be used in photovoltaics

A

1-1.7eV

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5
Q

What is the Fermi level?

A

The energy for which the probability of occupancy by an electron is exactly 0.5.

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6
Q

What is the probability an electron has an energy E?

A

F(E) = 1/[1 + exp(E-Ef)/kT]

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7
Q

When is a semiconductor highly resistive at room temperature?

A

When it has an energy bandgap >1eV and it is pure and free from crystal imperfections.

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8
Q

What is it called when impurities are introduced into a semiconductor?

A

Doping

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9
Q

When is a semiconductor “n-type”?

A

When it is doped and the impurity atoms release an ELECTRON each.

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10
Q

When is a semiconductor “p-type”

A

When it is doped and each impurity atom releases a HOLE.

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11
Q

Why is doping carried out in semiconductors?

A

The introduction of foreign atoms as substitutional impurities greatly increases the electrical conductivity.

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12
Q

What is the intrinsic Fermi level?

A

Eg/2

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13
Q

What are the 5 types of crystal defect?

A
  • 3D volume defects.
  • 2D planar defects.
  • 1D line defects.
  • 0D point defects.
  • Phonons
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14
Q

What are the 3D volume defects?

A
  • Included grains of 2nd phase.

- Voids.

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15
Q

What are the 2D planar defects?

A
  • Grain boundaries
  • Stacking faults
  • Twins
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16
Q

What are the 1D line defects?

A
  • Edge dislocations.

- Screw dislocations.

17
Q

What are the 0D point defects?

A
  • Vacancies.
  • Interstitial atoms.
  • Substitutional impurity atoms.
  • Interstitial impurity atoms.
18
Q

What are phonons?

A

Quantised lattice vibrations.

19
Q

What are the 3 main effects of crystal defects?

A
  • 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.
20
Q

What is the dominant absorption process in a solar cell?

A

The intrinsic or band-to-band optical absorption by a single electron across the bandgap of the semiconductor.

21
Q

In a solar cell, what happens when a photon with Ephoton = Eg illuminates the cell?

A

An electron-hole pair is generated.

22
Q

In a solar cell, what happens when a photon with Ephoton > Eg illuminates the cell?

A

An electron-hole pair is generated and excess energy is dissipated as heat.

23
Q

In a solar cell, what happens when a photon with Ephoton < Eg illuminates the cell?

A

The photon is only absorbed if there is an available state within the forbidden energy gap.

24
Q

What causes a minority carrier injection in doped semiconductors?

A

When an electron-hole pair is created, with electrons and holes created at the same rate.

25
Q

What is the formula for penetration depth in terms of the absorption coefficient?

A

W = 1/α

26
Q

What are the issues with single bandgap devices?

A

Inefficient use of high energy photons, low energy photons are not collected, from solar spectral irradiance distribution.

27
Q

What does each term in the equation I = neAVd represent?

A 
I=current
n=number of electrons
e=charge of an electron
A=cross-sectional area
Vd=drift velocity
28
Q

How do you calculate the number of charge carriers/unit volume taking part in conduction?

A

n=(N/V) x valence
N=number of atoms
V=volume

29
Q

What is the formula for the hall constant?

A

Rh = 1/ne

n=carrier concentration

30
Q

What is the formula for thermal velocity?

A

Vt = √(3kT/m)

31
Q

How do you determine a materials absorption coefficient for a given energy from an E-K diagram?

A

Find the material at the given energy and draw the line along to absorption coefficient.

32
Q

What is the formula for photon flux?

A

Φ = number of photons/tA

where t is time and A is area.

33
Q

What are the key features of the band structure of silicon?

A
  • 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.
34
Q

What are two semiconductor materials commonly used in PV with indirect bandgaps?

A
  • Silicon (Si)

- Germanium (Ge)

35
Q

What are two semiconductor materials commonly used in PV with direct bandgaps?

A
  • CdTe

- GaAs

Advanced Photovoltaics (7 decks)

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