Light harvesting by semiconductor band gap excitation

2 PRIMARY PROCESSES ON COLLOIDAL SEMICONDUCTORS 9.2.1 Light harvesting by semiconductor band gap excitation 

The electronic structure of semiconductors is usually comprised of a filled valence band and an empty conduction band which are energetically separated by an inter-band gap, Es. All of the reactions described above involve the initial absorption of photons by the semiconductor and, for ultra-band gap photon energies, the subsequent generation of electron-hole pairs within the material lattice (ec B J vb). as shown in Fig. 9.1a and equation (9.1). The entire process is thought to occur in 1 fs [96]. 

Semiconductor particles typically contain a high density of lattice defect sites (Te, Th), most of which are concentrated at the particle surface. The nature of these defect sites depends strongly on the constituent material of the particle and the method of particle synthesis [61]. Thus, charge carriers photogenerated in accordance with equation (9.1) may subsequently either recombine directly, re-emitting the absorbed energy as heat (A) or light (hv), 

Trapped charge carriers (e-f, At) can further participate in radiative and non-radiative recombination processes  

Femtosecond flash photolysis studies on Q-state CdS [107] indicate that reaction (4a) proceeds via two recombination processes a 50 ps decay at low excitation intensities, postulated to correspond to geminate e h+ recombination, and a faster 2 ps decay at higher flash fluences, corresponding to non-geminate or possibly three body Auger charge carrier recombination. Other studies by Nosaka and Fox [118] indicate that the second order rate coefficient for electron-hole recombination within CdS particles is of the order 9 x 10 t7 m3 s l. 

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