Conventional p-n Junctions
In conventional silicon photovoltaic cells there is a junction interfaced by "p"-type and "n"-type silicon each respectively generated by diffusion. In one instance an electron poor substance (for example boron) and in the other instance an electron rich substance (for example phosphorus) are used as impurities and are diffused into opposite sides of a single wafer of crystalline silicon or preferably into a single crystal of silicon. A "molecular p-n junction" is thus created. The function of the p-n junction in such a photovoltaic cell is to separate electrons and holes that are respectively created by the absorption of light near the junction.
There are two problems which arise with such p-n junction silicon photovoltaic cells. Firstly, the junction cannot be made very sharp and it usually extends in thickness to over one hundred nanometers. The second difficulty is to assure that the absorption of light will occur only at or near the junction. With materials that are presently used this precision is very difficult to achieve.