Photons in the photoelectric efect

[Bruce Haawkins]

What happens to the UV photons that strike the photovoltaic cell but do not take part in the photo electric efect, do thay reflect? apparently the best comercial solar cells are only 24% eficiant.

 

[Henryk]

The process of converting photon energy into electricity involves a several steps:
1. Photon entering the solid (i.e. cell material)
2. Generation of electron-hole pair
3. Separation of the pair by p-n junction
4. Conducting current out of the cell.
Neither of these steps is 100 % efficient.
1. At the air/cell material interface, there is a partial reflection. Silicon refraction index is around 4 in the visible range, much larger than glass and the reflection is quite substantial. The partial reflection can be reduced but not quite eliminated by using an anti-reflective coating. So, the first thing is that not all the incident photons are actually entering the cell.
2. Secondly, there is loss of energy in creating electron-hole pair. Photons of energy less than the energy gap won't contribute to the photocurrent. Not all the photons of energy greater than the gap will contribute to the photo current. Furthermore, when the photon energy is much larger than the gap, the energy of the electron transition will be greater than the gap but the generated carrier will quickly thermalize, i.e. lose the excess energy (over bandgap) to heat. In other words, even if the photon energy is, say 3 or 4 eV, with a very short time the energy of the excited electron-hole pair will drop to close to the energy gap value of about 1.14 eV (for silicon).
3. Next thing is separation of the electron-photon pair. This is happening by the strong field of the p-n junction but pairs are produced everywhere in the vicinity, they must drift to the junction and on the way, a substantial fraction of the carrier will recombine and their energy will be lost.
Those that make it into the junction, will be separated provided that the junction voltage is (reversely biased) not more than around 0.7 V.
4. Photocells are semiconductors, and there are resistive losses when you draw current.
To sum up, visible spectrum contains photons from around 1.7 to 3 eV but not all of them contribute to the production of the photocurrent and out of each you can get (at best) one elementary charge of around 0.7 V that is 0.7 eV of energy.