Solar - Solid State Physics Flashcards
What is the challenege in material-use of PV?
A material absorbs photons into specific electronic state
Electrons and degenerate states…
Fill from lowest energy first, alongside the Pauli Exclusion Principle.
Ground State
This refers to lowest energy state possible thus susecptibel to filling by electrons
Calculating electornic excitations…
Presume nuclei as fixed corresponding to ground state of the atom, solve schordinge equation for the electromagnetic potential in the electron of this state.
Imagining a solid…
A large molecule held in place by bonds similar to those of a molecule.
Crystal structure microscopically…
Periodic and predictable like diamond lattice
Phonons
The production of atomic vibration within crystal lattices in acoustiv waves.
Phonons in a lattice…
Vibration of the relative atoms vibrates the crystal structure wiht electrons adjusting configuration to the ground state.
Band Gap
Describes the energy required to promote a valence electron bound to an atom to become a conduction electron.
Structurally what is the band gap?
Diffenrece in energy betewen highest state in valence band and lowest state in conduction band
Gaps
Here there are not allowed electron states, whilst energy levels and bands of energies(electrons taking any value in a continous range)
What do bands do?
Segregatre energy levels into intervals of where the electrons energy can take any value
Band gap in silicon…
1.13ev seperating the orbital filled with electrons from that of the conduction band.
Conduction Band
The lowest empty band available for electron excitation.
What is requried of the wavelength to excite valence elextrons?
An e=hv eV equal or greater than the band gap energy.
Energy of a material at zero temperature…
All eelctron energy levels are filled to maximal energy, whilst higher energy levels are empty.
Fermi Energy
This is the energy difference between the highest and lowest occupied single particle states of non-interacting fermions at absolute temperature.
What is the highest filled state?
Fermi energy
Fermi Level
Describes the energy where the probabilty of finding the level occupied by an electron passes 1/2.
When does fermi level coincide with fermi energy?
At T=0
Determination of physical characteristics of a solid?
Positon of the fermi level relative to the band structure.
If fermi energy is within the band…
This means there are empty states with energies just above ground state, so only a small amount of energy is needed to excite an electron into conduction band.
When does a conduction band arise?
When fermi energy is in the middle of a band, being good thermal/electrical conductors.
When does a valence band arise?
When fermi energy is between two bands( meaning large energy seperation betewen the bands, being good insulator.
When does a semiconductor arise?
When fermi energy lies between two bands of small seperation.
Semiconductor band state at absolute zero…
All electrons in the valence band where romm temperature may allow excitation into conduction band,.
Structural changes of electron excitation into conduction band?
Leaves a hole in the valence band
Dislocations
Irregularities within crystial structure containing an abrupt change in the atomic arrangement.
Impurities
Where atoms are present that are different from the host crystal atoms.
What do impurities give rise to?
Quantum states within the regions between bands
