Solar - PV Cell Mechanism Flashcards
(32 cards)
Photon absorption in a semi-conducting material…
This excites valence band electrons to conduction bands with free movement in conduction bands.
What does valence band excitation create?
A hole in the valence band that can propogate like a positively charged electron.
What drives external circuits?
The generation of valence band holes through excitations.
What is an issue with band excitation in regards to energy?
Recombination with the valence band with photon emission or thermal energy generation
Why do absorbed crystal photons travel in random direction
The regularity of the structure thus must be manipulated
Carnot Limit
This sets an absolute limit on the efficiency with which heat energy can be turned into useful work.
What is required for electron excitaiton from valence gap to conduction gap?
A photon with E = hf > Egap
Calculating carnot effiicency of silicon…
6000k - 300k / 6000k = 0.95 (95%)
What is carnot limit inhibited by in silicon?
Collection efficiency and recombination
Collection Efficiency
This is the limitation of the fraction of the incident sunlight that can be transformed to energy of excited electrons.
Why is photon energy lost?
Excitation requires a defined amount of energy( more is lost as thermal)
Working out limit of collection efficienc of solar cell..
Compute total incident power…
Part of power captures by the solar cell
How much blackbody radiation absorbed in silicon solar cells?
42.9%
What type of band does silicon have?
Indirect band gap between valence and conduction band
Indirect Banding
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.
Consequence of indirect gap…
Silicon must be twice the thickness
Mathematically describing a crystal…
A lattice in N dimensions (nx + my + pz) where each cartesian unit vector is 1 respectively.
Face-Centered Cubic Lattice
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
Equating linear vectors from FCC?
z = a + b - c
Structure of an FCC..
One diamon generated by abc vectors whilst the second offset by the first (1/4, 1/4, 1/4)
Periodic Potential
Descibres electron in the lattice subject to potential due to elecromagnetic field produced by the ions in the periodic strucutre of the crystal
First Brillouin Zone
A set of poitns reached from the origin without crossing a bragg plane.
Bragg Plane
A plance in reciprocal space bisecting a reciprocal lattice vector at rigth angles.
Shape of silicon diamon lattice…
Truncated octahedron
