Photoinduced electron-hole pairs

One of the major limitations in semiconductor photocatalysis is the relatively low value of the overall quantum efficiency mainly due to the high rate of recombination of photoinduced electron-hole pairs at or near the surface. Some success in enhancing the efficiency of photocatalysts... 

Photodiodes are based on a pn junction operated at a reverse bias voltage (i.e., the opposite bias to the LED and diode laser). The reverse rather than forward bias voltage has the effect of increasing the voltage across the depletion region such that any photoinduced electron-hole pairs are rapidly swept across the junction, generating a current pulse in the external circuitry (Figure 12.24). 

It is well-known that nano-TiO is one of the suitable semiconductors for photocatalyst and has been applied in various photoeatalytic reactions (Fujishima et al., 2000). However, its properties, not only the photoefficiency or activity but also the photoresponse, are not sufficient (Kawai and Sakata, 1980). Meanwhile, the high recombination ratio of photoinduced electron-hole pairs also reduces its catalytic efficiency. Therefore various modifications have been performed on nano-TiO to promote its catalytic ability and develop new photoeatalytic functions (Ohno et al., 1996 Litter, 1999 Nawio et al., 1999 Choi et al., 1994 Nishikawa et al., 2001 Amiridis et al., 1999). 

We see therefore that photoactive semiconductor particles provide ideal environments for control of interfacial electron transfer. Photoinduced electron-hole pairs formed on irradiated semiconductor suspensions, as in photoelectrochemical cells, allow for reactivity control not available in homogeneous solution. This altered activity derives from controlled adsorption on a chemically manipula-ble surface, controlled potential afforded by the valence band edge positions, controlled kinetics by virtue of band bending effects, and controlled current flow by judicious choice of incident light intensity. 

Gold nanoparticles are virtually not luminescent, but silver nanoparticles show plasmon emissions with reasonable quantum yields. Furthermore, the non-radiative decay, resulting in electron-hole pair generation, may be used for photosensitization of wide bandgap semiconductors (see Figure 7.5) [16,17]. Similar effects may also be observed as direct photoinduced electron transfer between metal surfaces and surface-bound molecules [18]. 

In organic photoreceptors photoinduced charge generation occurs through an excited state to produce a geminate electron-hole pair bound by their coulombic attraction. These separate into free carriers by a combination of diffusion and drift... 

The timescales of the processes occurring after photo-generation of an electron hole pair can be shorter or longer than the RC time constant of the photo-electrochemical cell, Tceii- In many cases, the time scale for interfacial processes at a semiconductor/electrolyte junction is longer than Xceit, whereas electron-hole separation and electron transit through the depletion layer are much faster. The apparatus used to probe electron-hole separation is therefore different from that used to study the kinetics of photoinduced interfacial processes. 

The effect of photoinduced electron transfer can be applied in organic phototransistors, as we demonstrated recently [146]. Spiro-DPSP 115 is a hole-transport material with a low charge carrier concentration in the dark. Under illumination, electron-hole pairs are generated that increase the munber of positive charges in the film, contributing to the conductivity. As a result, the transfer characteristic curve shifts toward lower switch-on voltages. 

Therefore, to design a better molecular photodiode the distance and AG dependence should be kept in mind. In Fig. 18(b), the energy diagram of the A/S/D molecular photodiode is depicted as a function of distance across the LB film. If the forward processes indicated by arrows with solid lines are accelerated, and the backward processes with dashed lines are retarded by setting the distances and the energy levels appropriately, the photoinduced vectorial flow of electrons can be achieved, that is, the acceleration by setting AG° equal to X is assumed for the forward electron transfer processes (ii) and (iii), while the retardation, as a consequence of the inverted region, is assumed for the back-electron transfer processes (iv) and (v). Once an electron-hole pair is separated successfully, the recombination of... 

When the detector is illuminated, electron-hole pairs are created by photon absorption within the p-n junction. The electrons are driven by the diffusion voltage into the n-region, the holes into the p-region. This leads to a decrease A Vd of the diffusion voltage, which appears as the photovoltage Vph = AFd across the open electrodes of the photodiode. If these electrodes are connected through an Ampere-meter, the photoinduced current... 

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