Electron/hole recombination pathways

The negative surface potential of the anionic micelles prevents the approach of hydrated electrons and sensitizer cations, e.g. holes, (Fig. 4.4). The preferred pathway of reaction of e q is here hydrogen formation (2ejq — H2 + 2 OH-). Hence, with such a system, the prevention of electron-hole recombination is achieved. While in pure aqueous solution the lifetime of the sensitizer cation is only several microseconds (due to the diffusion controlled back reaction with erq) the lifetime in anionic micellar systems can be as long as days or weeks. 

The light-generated electron-hole pair may undergo primary recombination through radiative and non-radiative pathways (process 1, Figure 1), or may be... 

An explanation for the observed effects can be based on the assumption that carboxyl groups in protonated form may be a good photoelectron acceptor. This may be associated with the known easier electrochemical reduction of organic acids at low pH [8]. Excited electron-hole pairs in ZnSe core may recombine in few possible ways. First, a direct recombination results in the appearance of excitonic emission band at A,=408 nm. The second possible pathway is the energy transfer to Mn ion followed by Mn emission at A.=590 nm. At the neutral and acidic pH an additional recombination channel may be realized via trapping of photoelectrons by carboxyl groups (prior to the energy transfer to Mn ions)... 

Formation of surface-active (colour) centres occurs via capture of free electrons and holes by the surface traps S. For completeness, creation of electron active centres S is indicated in reaction 5.59 (ksg). The centres interact with molecules M to yield the intermediate species M (reaction 5.61, ke ). Reaction 5.60 describes the recombination pathway for the decay of surface-active centres. 

Figure 9.4 Recombination pathways of photogenerated charge carriers in an n-type semiconductor-based photoelectrochemical cell. The electron-hole pairs can recombine through a current density in the bulk of the semiconductor, the depletion region, or through defects (trap states) at the semiconductor/liquid interface, iss- Charges can also tunnel through the electric potential barrier near the surface, 4 or can transfer across the interface, The bold arrows indicate the favourable current processes in the operation of a photoelectrochemical cell. The hollow arrows indicate the processes that oppose the excess of charge carriers generated by light absorption.

Some of these carriers may recombine within the emissive layer yielding excited electron-hole pairs, termed excitons. These excitons may be produced in either the singlet or triplet states and may radiatively decay to the ground state by phosphorescence (PL) or fluorescence (FL) pathways (Fig. 1-2). An important figure of merit for electroluminescent materials is the number of photons emitted per electron injected and this is termed the internal quantum efficiency. It is clear, therefore, that the statistical maximum internal efficiency for an EL device is 25% as only one quarter of the excitons are produced in the singlet state. In practice, this maximum value is diminished further because not all of the light generated is visi-... 

Fig. (10). Photoinduced generation of electrons and holes and their reaction cum recombination pathways. Reproduced with permission from ref [50], 1995 American Chemical Society.

Figure 26 Hole/electron recombination pathways, (a) Electron transfer from the electron to the hole can occur via pathways 1 and 3, giving the exciton (shown In green). Pathway 2 Is not typically observed, (b) The spins of the unpaired electrons In the hole and electron have a random Initial picture with no arrowheads on the electrons.

Semiconductor particles provide alternative photoactive materials for the formation of charge-separated products [48,49]. Bandgap photochemical excitation of a semiconductor particle promotes an electron from the valence band to the conduction band, thus generating an electron-hole pair (Figure 12a). Recombination of the electron-hole pair represents an analogous degradative pathway to that of diffusional back ET of photoproducts. 

Fig. n Schematic illustration of charge recombination pathways in a dye-sensitized device in dark and light conditions. In the dark, electrons in the nanocrystalline semiconductor recombine only with the hole conductor. Under illumination, some of the dye molecules are ionized and electrons may recombine either with the hole conductor or with the dye cation. The second recombination route is often neglected, but may be significant at forward bias. 

---