Generation and recombination of electron-hole pairs

Question 1

What determines the performance of a solar cell?

Answer 1

The rate of recombination

Question 2

Name the 2 types of Bandgaps

Answer 2

Direct - Indirect

Question 3

Direct Bandgap

Answer 3

The highest point of the valence band is vertically aligned with the lowest point of the conduction band

Question 4

Indirect Bandgap

Answer 4

The highest point of the valence band is not vertically aligned with the lowest point of the conduction band

Question 5

What is a phonon?

Answer 5

The vibrations coming from the crystal lattice which is use to exciting an Indirect Bandgap along with the photon (Lattice Impulse)

Question 6

Which type of band gap material is the Crystalline Silicon?

Answer 6

Indirect

Question 7

Which type of recombination will be dominated in a Crystalline Silicon?

Answer 7

Auger Recombination

Question 8

Energy of a photon Equation

Answer 8

Ep = hv

Question 9

Example of a Direct Seminconductor

Answer 9

Gallium Arsenide (GaAs)

Question 10

Describe the Photogeneration Process

Answer 10

If photon energy is higher than the bandgap energy of the semiconductor, it is sufficient to break bonds and to excite a valence electron into the conduction band, leaving a hole behind in the valence band; hence electron - hole pairs are created.

Question 11

Shockley-Read-Hall Recombination

Answer 11

• Recombination does not occur directly from bandgap to bandgap
• Recombination via a defect level (Impurity Atom / Lattice Defects)
• Recombination rates can be described by Shockley-Read-Hall Statistics
• It is the dominant recombination-generation process in semiconductors at most operational conditions
• Process typically non-radioactive
• Excess energy is dissipated into the lattice in form of heat

Question 12

Auger Recombination

Answer 12

• 3 particle process
• Momentum and energy of the recombining hole and electron is conserved by transferring energy and momentum to an another electron / or hole
• If the 3rd particle is an electron, it is excited into higher levels in the electronic band, it relaxes again, transferring its energy to vibrational energy of the lattice or phonon modes, and finally heat.
• If the 3rd particle is a hole, it is excited into deeper levels of the valence band, ir rises back to the valence band edge by transferring its energy ti phonon modes.
• The rated depends on the charge carrier densities for the electrons and holes.

Question 13

Surface Recombination

Answer 13

• Many valence electrons on the surface cannot find a partner to create a covalent bond with.
• Happens in very pure semiconductors
• Charge carrier concentration at surface ns and ps depends on fixed charge Qf
• Surface recombination velocity (cm/s) S= Rs / n differential (see presentation for correct formula)

Question 14

What happens to the extra energy that the electron-hole pairs receive from the
photons with energies larger than the band gap of the semiconductor?

Answer 14

The extra energy is released into the lattice in form of heat.

Question 15

How is Recombination classifies?

Answer 15

As Intrinsic or Extrinsic

Question 16

Intrinsic Recombination

Answer 16

• Radiative Recombination
• Auger Recombination

Question 17

Extrinsic Recombination

Answer 17

Referred as traps or defects levels
- Shockley Read Hall

Question 18

Radiative Recombination

Answer 18

The reverse process of electron-hole pair generation via the absorption of a photon. In this process, an electron in the conduction band relaxes to the valence band, recombining with an empty state, emitting all or some of its excess energy as a photon.