A solar cell cannot properly function without p-n junctions to collect electrons generated from the sunlight .  P-n junctions are not exclusive to solar cells, but they are used in many electronics commonly in transistors, lasers, and LED’s. They are formed by joining n-type and p-type semiconductors. N-type regions on semiconductor have excess electrons, while p-type regions on semiconductors have holes. By joining the two regions together, an electric field is generated resulting to a potential forming at the two junctions.

 

Basically, the p-n junction converts the photovoltaic energy produced from the absorption of light to electricity. Doping of semiconductors is one method of producing p-n junctions. For example phosphorus  is the main dopant for silicon  its n-type region. This is because phosphorus has 1 extra valence electron as opposed to Silicon. Aluminum on the other hand is usually used to create p-doped region on the Silicon.

 

Another method is to join two semiconductors with different and unequal band gaps to produce what is commonly called a  p-n heterojunction region.  Examples of such materials include gallium (Ga) and arsenide (As).

 

The growth of heterojunctions, specifically GaAs, require the use of molecular beam epitaxy (MBE). Although harder and much more complex to manufacture, GaAs solar cells result in higher obtained efficiency compared to the common silicon solar cell. Solar Cells are then fabricated in a yellow clean room using UV photo-lithography.

 

This Figure above shows a fabricated GaAs solar cell. Due to this p-n junction formed, a solar cell is essentially a diode supplied/powered by  light. A solar cell has the following characteristics similar to that of a diode: V_OC, I_{SC ,} P_max, R_SH, R_SE , FF, and η. The open circuit voltage, V_OC, is the maximum voltage measured when no current is passing through the cell. These characteristics are shown on the table on the left.

 

In contrast, the short-circuit current, I_SC, is the maximum current obtained at zero voltage. The performance of the cell is indicated by the shift between the dark and the illuminated curves. The greater the difference between the curves infers better performance. P_max is the maximum power generated by the solar cell.  The FF or Fill Factor is the power ratio of the solar cell. The ratio is further determined by the R_SH, and R_{SE .}  The shunt resistance R_SH,  is a common error resulting from poor solar cell design usually from shorting the two contacts. The series resistance or R_SE results from contact resistance between metals (i.e. Silver and aluminium resistance), resistance of the metal, and resistance of depletion region. Finally efficiency η is obtained by determining the power generated of a solar cell given its area and the intensity of the incident light.