Photovoltaic Energetics

Photovoltaics work on the principle that when a photon of light with sufficient energy hits an atom with an extra valence electron, energy will be imparted to that electron and it will move from the valence band to the conduction band and will be free to move within the solid.

Making Photovoltaic Materials

First, pure silicon is diffused with either Boron (or Aluminum as in the picture) to make p type silicon. In this case "p" is for "positive". This is because Silicon has 4 valence electrons while Boron has only 3, leaveing a "hole" in the crystal structure.


To make n (negative) type silicon, a similiar process is used with Phosphorous. Phosphorous has 5 valence electrons and thus creates atoms with an extra valence electron in the crystal structure. Typically, both n and p type silicon is made through a diffusion process, taking two thin sheets of silicon and either boron or phosphorous together, applying heat and pressure, allowing the boron or phosphorous atoms to naturally diffuse into the silicon


The PV Junction

To then construct a PV cell, n type and p type silicon is sandwiched together and again allowed to diffuse. After some time, the diffusion force and the magnetic force due to the positive and negative charges of the two types of silicon become equal and the diffusion ceases.


Building a PV Cell

As noted above, when a photon hits an atom with an extra valence electron, that electron moves to the conduction band. Since there is a natural magnetic force created between the two types of silicon, this free electron is pushed toward the positive terminal of the cell.

Every material, however, requires a different amount of energy to free a valence electron, some of which are pictured below.


To create ever more efficient cells, several PV junctions having different bandgap energies can be stacked on top of one another as pictured below. While commercially available single junction silicon PV cells have an efficiency of approximately 15%, multi-junction experimental cells have reached efficiencies as high as 40%.

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