Photovoltaic Cells

Photo or Photovoltaic Cells

(Formerly known as Solar Cells)

*Important: The syllabus updated to reflect the new terminology of solar cells, you MUST use the term photovoltaic in order to be marked correct*

Principle

1.) Free electrons from the n-type semiconductor flow to the positive holes in the p-type
semiconductor
2.) Charges develop on the edges between the n and p type semiconductors due to excess
and deficiency of electrons. This area is known as a depletion zone and an electric
field develops
3.) A negative charge develops on the p type semi conductor and a positive charge develops on the n type conductor. HOWEVER, the electrons cannot flow from n-type to the junction and out the p-type and neither can electrons flow from the p-type through the junction (as it will just neutralise the electron hole pair).
4.) An incidence light beam (photon) hits an n-type (we will only discuss n-i-p photovoltaic cells), liberating an electron and a hole. The hole will migrate to the depletion zone whilst the electron as it has greater amounts of energy accelerates through the conductor.
5.) The electron will perform work on a load and return to the circuit through the p-type semiconductor, where it will move to the depletion zone and neutralise with a hole.

Design

Photovoltaic cells come in two types:
n-i-p type and p-i-n types. The new syllabus has not reflected the change to p-i-n types, although both work in a very similar way. (It is more about efficiency).

Junction

In a photovoltaic cell, the cell will consist of two semi-conductors, separated by a depletion zone. Be warned, although it appears there is a gap there is in fact NO GAP between the two semi-conducting materials (as seen in jacaranda). It is in fact a representation of the depletion zone that exists between the two materials. When two semiconductors of n-type and p-type join, the initial rush of electrons from the n-type move towards the p-type produces an electrical field. A depletion zone forms as the donor electrons from the n-type semi conductor move to fill in the 'holes' of the p-type semi-conductor. As the electrons move to the p-type semi-conductor it will then produce a net charge across field, resulting in a negatively charged p-type semiconductor and a positively charged type n-type semiconductor.
This field is an exertion of two states, the 'holes' of the p-type vs the free electrons of the n-type and the ions left over from donor electrons moving to the acceptor semi-conductors.
Because of this depletion zone, the movement of n-type donor electrons gets harder and harder until it forms an equilibrium between the two semi-conductors. It is then known that a voltage applied now as negative on the n-type and a positive on the p-type, a current will flow as the electrons will be liberated across the junction and flow naturally through the n-type and fill the resulting holes in the p-type after completing the circuit. This property is useful in diodes (which are used to rectify an AC current) or in photovoltaic cells, which take advantage of the photoelectric effect as well.

The photoelectric effect on photovoltaic cells

Photovoltaic cells work on much the same principal as a diode does. In fact a light emitting diode will produce a current in reverse as it uses the same principle, only working backwards.
As mentioned above it uses the n-i-p state and relies on the n-p junction.
In photovoltaic cells, the n-i-p junction works by facing the solar cell upwards so that the n-type semi conductor faces the sun. The incident photon can strike anywhere on the the silicon crystal but backwards current cannot flow past the n-p junction. This is because of the bias placed on the depletion zone. The depletion zone only allows electrons to flow out of the n-type silicon lattice whilst the holes move towards the negative p-type, if an electron hole pair is freed in the p-type semi-conductor, the electron will move the n-type lattice.

To answer questions on whether the incident photon hits the n or p type layer, the answer is that it doesn't necessarily matter and the principle applies to produce a current in the reverse bias current direction. The problem is that incident photons hitting the p-type material past the junction will meet more resistance and thus will have less energy (as it has to travel through the p and n-type layers).