Trap-mediated recombination

Fig. 9.5 The drawing (left) shows trap-mediated recombination with the electron (Te) and hole (tp) lifetimes the total lifetime (t) fm movement of a carrier from the conduction band (CB) to the valence band (VB) is the sum of Te and Tp. The graph (right) plots lifetime as a function of Fermi level - i.e., doping - for the case in which the energy level of the trap (E,) is between the middle of the bandgap (E ) and the conduction band ( cb)- The ranges of Fermi levels in which t 1/N are indicated within brackets. The graph is modified from [11]...

Acceptor and donor related processes involve interfacial electron transfer from free/trapped charge carries, including back reactions 3b and 3d [22, 27]. These latter processes are essentially donor/acceptor mediated recombination reactions. 

Figure 14. Hole injection into the VB of an n-type semiconductor from an oxidant (e.g., Fe ) and the injection/recombination pathway. Both surface-state mediated and depletion layer trap mediated routes are shown for the recombination.

Surface related properties are carrier trapping on intrinsic (due to surface dangling bonds) and extrinsic (related to adsorbates, including donor and acceptor) surface states, carrier recombination mediated by surface states [26], and mass transfer of acceptor and donor and products from/to bulk solution. 

Less vigorous approaches to describe surface recombination were taken by Kelly and Memming,158 Rajeshwar,134 and Peter et al.159,160 These authors considered the possibility of charge transfer via surface states. Thus, as shown in Fig. 20, the minority carriers which are trapped by surface states would either recombine with majority carriers (surface recombination via surface states) or transfer to the electrolyte (charge transfer via surface states or surface state mediated charge transfer). There are many experimental results which support the surface state mediated charge transfer under illumination134,158-161 and in the dark.162-164... 

The effect of temperature on deep trap emission is similar to that observed for bandgap emission, with the intensity of the emission decreasing as the temperature increases. This can be explained by the involvement of nonradiative recombination processes dominating at higher temperature. Nonradiative relaxation in CdSe nanoclusters has been assigned to the involvement of a multiphonon relaxation mechanism mediated by a vibrational mode of the surface phenylse-lenolate ligands. ... 

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