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Climate Change - Renewables > Solar - PV Cell Mechanism > Flashcards

Solar - PV Cell Mechanism Flashcards

1
Q

Photon absorption in a semi-conducting material…

A

This excites valence band electrons to conduction bands with free movement in conduction bands.

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2
Q

What does valence band excitation create?

A

A hole in the valence band that can propogate like a positively charged electron.

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3
Q

What drives external circuits?

A

The generation of valence band holes through excitations.

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4
Q

What is an issue with band excitation in regards to energy?

A

Recombination with the valence band with photon emission or thermal energy generation

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5
Q

Why do absorbed crystal photons travel in random direction

A

The regularity of the structure thus must be manipulated

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6
Q

Carnot Limit

A

This sets an absolute limit on the efficiency with which heat energy can be turned into useful work.

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7
Q

What is required for electron excitaiton from valence gap to conduction gap?

A

A photon with E = hf > Egap

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8
Q

Calculating carnot effiicency of silicon…

A

6000k - 300k / 6000k = 0.95 (95%)

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9
Q

What is carnot limit inhibited by in silicon?

A

Collection efficiency and recombination

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10
Q

Collection Efficiency

A

This is the limitation of the fraction of the incident sunlight that can be transformed to energy of excited electrons.

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11
Q

Why is photon energy lost?

A

Excitation requires a defined amount of energy( more is lost as thermal)

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12
Q

Working out limit of collection efficienc of solar cell..

A

Compute total incident power…
Part of power captures by the solar cell

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13
Q

How much blackbody radiation absorbed in silicon solar cells?

A

42.9%

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14
Q

What type of band does silicon have?

A

Indirect band gap between valence and conduction band

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15
Q

Indirect Banding

A

This is the inability of photons to be emitted becuase the electrons must pass through an intermediate state and transfer momentum to the crystal lattice.

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16
Q

Consequence of indirect gap…

A

Silicon must be twice the thickness

17
Q

Mathematically describing a crystal…

A

A lattice in N dimensions (nx + my + pz) where each cartesian unit vector is 1 respectively.

18
Q

Face-Centered Cubic Lattice

A

This includes all poitns of the simple cubic lattice and also the centre point of each of the 2D faces (a = 1/2, 1/2, 0) as well as b and c

19
Q

Equating linear vectors from FCC?

A

z = a + b - c

20
Q

Structure of an FCC..

A

One diamon generated by abc vectors whilst the second offset by the first (1/4, 1/4, 1/4)

21
Q

Periodic Potential

A

Descibres electron in the lattice subject to potential due to elecromagnetic field produced by the ions in the periodic strucutre of the crystal

22
Q

First Brillouin Zone

A

A set of poitns reached from the origin without crossing a bragg plane.

23
Q

Bragg Plane

A

A plance in reciprocal space bisecting a reciprocal lattice vector at rigth angles.

24
Q

Shape of silicon diamon lattice…

A

Truncated octahedron

25
Q

Periodic Boundary Conditiosn

A

These are a set of boundary conditiosn chosen for approximating large systems through the use of unit cell.

26
Q

How to compute band structures of large solids…

A

Computing schrodinger equation in 3D in a specified potential with periodic boundary conditions

27
Q

Crystal Momenta

A

A momneum like vector associated with electrons in a crystal lattice

28
Q

Direct Band Gaps

A

Where the highest valence band has the same crystal momentum as the lowest state in the conduction band.

29
Q

How may indircet band photon excitation occur?

A

Uppermost point of the valence band and lowermost point of the conduction band, or phonons

30
Q

Indirect Binding Excitations…

A

A photon itself is not sufficient to excite an electron across the band.

31
Q

Phonons in indirect binding…

A

Phonons associated with vibration of atoms carrying lattice momentum equivalent to electron momenth,, thus able to excite electrons.

32
Q

dI/dZ = -k1

A

This describes rate of light absorption in a material where k is the absorption coefficient

Climate Change - Renewables (12 decks)

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