Monocrystalline p-n junctions are often used because there is no material interface at the junction; therefore, losses due to interface states can be avoided. However, there is a very limited range of materials that can be used to produce these solar cells. For efficiency greater than 30% it is necessary to have band gaps between 1-1.6 eV. Efficiencies greater than 20% require band gaps between 0.7 to 2 eV. In addition, materials must produce quantum efficiencies over a broad range of wavelengths. Diffusion lengths must be long compared to the absorption depth. Silicon is the only elemental semiconductor that has a suitable band gap and will yield high efficiency. Germanium and selenium may also be used, but their bands gaps are small and will not yield high efficiency.
Compound semiconducting crystalline materials have been developed in order to obtain band gaps within the appropriate range. Gallium arsenide and indium phosphide are notably effective materials. Not all materials with suitable band gaps can be doped both n and p. For example, amorphous silicon and similar alloys have appropriate band gaps, but the ratio between diffusion lengths and absorption depth is unsuitable. Diffusion lengths may be too short compared to absorption depth; therefore, they cannot be used for p-n homojunction cells.
Once an appropriate material has been chosen a number of design factors can be manipulated. Compsitional variations and light trapping structures can be used to controll polarity, junction depth, doping levels, doping gradients, cell thickness, surface treatments, and contact design. Determing the design of a cell is dependent upon the optical and pre-existing properties in the material. Materials with weak absorption require methods to increase optical absorption, whereas materials with high absorption require focus on minimizing recombination near the front surface .
A p-n homojunction require certain properties independent of the material.
- For efficient light absorption, thickness should exceed the absorption length.
- Junctions should be shallow compared to diffusion and absorption length.
- The emitter should be doped heavily to improve conductivity.
- Reflection of light should be minimized.
- Nelson, Jenny. The Physics of Solar Cells. London: Imperial College, 2003. Print.