Besides the processes already discussed, there are numerous ways absorption can occur. Quantum mechanically, there are many other possibilities for absorption of photons that may have very minor roles in the crystal lattice. However, there are some other fairly prominent processes that occur:
- As discussed briefly before, absorption can occur at high-enough photon energies by excitation across the direct forbidden band gap of an indirect-band-gap semiconductor1.
- On the other hand, the two-step process involving phonons described in the previous section can also occur in direct-band-gap semiconductors.
- Although not very significant in solar cell work, at long wavelengths of light (when carrier concentrations are large) a carrier can be excited within its band to a higher energy state by a photon, while emitting a phonon but not generating an electron-hole pair1. Though interesting, we are more interested, for solar work, in the pairing of absorption processes with the generation of electron-hole pairs.
- A rather foreign process of absorption is the process of the excitation of transition states between “allowed” and “forbidden” energy levels. This means that in a doped semiconductor, impurities can shift the allowed energy states into the forbidden gap of the intrinsic semiconductor, and thus carriers can be excited from a forbidden energy level to the conduction band with less direct energy required from a photon.
- The Franz-Keldysh effect occurs with a strong electric field, where the absorption edge is reduced to lower energies, effectively narrowing the width of the band gap and therefore the energy required to cross it. A similar effect occurs with high-dopant-impurity concentrations.
1. Goetzberger, Adolf et.al. Crystalline Silicon Solar Cells. Chichester: John Wiley & Sons Ltd., 1998.