Band gap or energy band gap is the minimum energy required by the electrons in the outermost shells of a substance to be able to jump free of the parent atoms (leaving a ‘hole’ in the parent atom). Such electrons then take part in conduction. Good conductors have zero band gap in that they always have free electrons available for conduction at the application of the slightest electric potential. Actually, their valence and conduction bands overlap. Insulators have very high band gaps because their electrons will need very high energy to break them loose from the parent atoms. Semiconductors have intermediate band gap energies in the order of electron volts. It requires One electron volt (1ev) to lower the potential of an electron through one volt.
Band Gap of Solar Cells
Silicon, the most popular semiconductor today, has a band gap energy of 1.11 ev (at room temperature). To knock an electron of a silicon atom at room temperature, we must provide a packet of energy greater than 1.11 ev. For solar cells made from silicon to provide PV electricity, the photons which hit a solar cell must have energy greater than 1.11 ev. Solar cells made from cadmium telluride (CdTe) the bandgap energy is 1.44 ev.
What About More Energetic Photons?
For silicon, this is just the amount of energy required to generate an electron-hole pair. What if the striking photon has energy more than this? A hole pair is generated, but the extra energy is simply converted to heat in the solar cell. This is important. Too much extra energy will simply heat the cell. Rise of temperature of a cell has negative effects.
Effect of temperature on Band Gap
The solar cell must operate in the sun. The panel will invariably heat up. Heating up makes the valence electrons of silicon more energetic, and they will need less of energy to break loose of their parent atoms. We say that the band gap energy of the atoms will decrease. Isn’t it good? So that even lower energy photons will cause generation of electron-hole pairs?
Temperature and Solar Cell Efficiency
Actually, the energy converted is the energy corresponding to the band gap. Rest of the photon energy is simply a waste in the solar cell as heat. So, the result is a loss in efficiency. It has been estimated that about 1.1% loss occurs for every 1˚C rise in temperature of the cell.
Limiting Efficiency of Solar Cells
Solar cells operate on the solar spectrum to extract energy. The Shockley–Queisser equation puts a theoretical limit on the efficiency of single-junction solar cells (meaning, a definite single value for the band gap energy). Detailed calculations yield a curve of limiting efficiency (single junction, AM=1.5), which show two peaks. The larger peak is at a band gap of 1.34 ev yielding a limiting efficiency of 33.7%. The smaller peak occurs for band gap energy of about 1.1 ev giving an efficiency limit of nearly 32%. That is close to the band gap of silicon, currently the most popular material.
Exceeding the Limit
Note that this is the theoretical limit under standard conditions for an ideal solar cell. Practical efficiencies are much lower. Readers may wonder how certain laboratories keep claiming efficiencies of better than 33%. They indeed can achieve higher efficiencies using multijunction solar cells.