In the charge carrier recombination mechanism, trap assisted recombination occurs which is non-radiative in nature. The lifetime is often interpreted as a trap state. Is this the lifetime where the excited electron moves from the conduction band to the trap state or the time after which the electron recombines with the holes? Also, once the electron is in trap state, will it always remain there, or we need some energy to de trap this electron? Can someone please clear this concept to me?
Hi Bibek,
Trap assisted recombination is recombination that is facilitated through a trap state, which is a localized state in the band gap. Depending on the position of the trap state it is either more likely for electrons to be trapped or for holes. This is described by Shockley-Read-Hall recombination, which also explains that a trap that is positioned in the middle of the bandgap ('deep trap') is the most effective recombination center.
Trapping and detrapping are possible, so we have a dynamic equilibrium here. The level at which trapping and detrapping are equally likely are sometimes referred to as so called demarcation levels.
The recombination lifetime (tau) is the time that a photogenerated electron or hole exists before it gets annihilated (=recombines) with a charge of the opposite sign.
The effective lifetime of charge carriers depends on both the lifetime that is associated with trap assisted recombination as well as n the lifetime associated with bimolecular (band-to-band recombination):
1/tau_eff = 1/tau_trap + 1/tau_bm
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