A homojunction is the region between an n-layer and a p-layer in a single material, photovoltaic cell. Solar cells that use crystalline silicon, for example, are examples of homojunction devices.

 

In a homojunction cell, crystalline silicon is altered so that one side is p-type, dominated by positive holes, and the other side is n-type, dominated by negative electrons. The p-n junction is located so that the maximum amount of light is absorbed near it. The free electrons and holes generated by light deep in the silicon diffuse to the p-n junction, then separate to produce a current if the silicon is of sufficient high quality.

 

In the homojunction design, several aspects of the cell may be varied to increase conversion efficiency:

 

Depth of the p-n junction below the cell's surface

Amount and distribution of dopant atoms on either side of the p/n junction

Crystallinity and purity of the silicon

 

Some homojunctions cells have also been designed with the positive and negative electrical contacts on the back of the cell. This geometry eliminates the shadowing caused by the electrical grid on top of the cell. A disadvantage is that the charge carriers, which are mostly generated near the top surface of the cell, must travel farther – all the way to the back of the cell – to reach an electrical contact. To be able to do this, the silicon must be of very high quality, without crystal defects that cause electrons and holes to recombine.