Current doubling

Figure 16.2 depicts the processes considered, including back reactions, but not current doubling. Even if the photocatalytic process is located at the semiconductor-liquid interface [29], it was demonstrated that the dark adsorption constant of the donor does not match the adsorption constant obtained from the fitting of the photocatalytic degradation data against the L-H model [30], The L-H... 

Fig. 4. Dependence of incident photon to current efficiency (IPCE) on excitation wavelength and presence of current-doubling agent. ITO electrodes were covered with P25 (squares) or 3% [PtCl4]/P25 (circles) and biased at a constant potential of 0.7 V vs. Ag/AgCl in 0.1 M NaOH solution (a)). After addition of 0.1 M HCOONa the IPCE values increased (open circles, (b)). The inserts show the zoomed visible region.

Fig. 1.11. Schematic presentation of copper deposition on Ti02 nanoparticles in the presence of methanol as current doubling agent.

The first system exhibiting photocurrent multiplication to be studied by IMPS was the photoreduction of oxygen to H202 at p-GaP [36, 37]. The oxidation of formic acid at /i-CdS was then characterised by the same method [35]. Further examples of current doubling that have been studied by IMPS include the photo-oxidation of formic acid at Ti02 [31] and the photoanodic oxidation of /i-Si [33, 34]. 

Fig. 8.14. Theoretical plots showing competition between recombination and current doubling. Calculated for a surface state density of 5 x 10,2cm "2. The surface state is located 0.3eV below the bulk Fermi level. Donor density 1.5 x I0,ftcm. Arinj 5 x 104s" rccn = 2 x 10 7 s. Band bending values (a) 0.40 eV, (b) 0.35 eV, (c) 0.30 eV, (d) 0.2 eV. Note the transition in the IMPS response from current from current doubling control at 0.4 eV to...

Fujishima, A., T. Kato, E. Maekawa and K. Honda (1981). Mechanism of the current doubling effect. I. The ZnO photoanode in aqueous solution of sodium formate. Bulletin of the Chemical Society of Japan, 54,1671-1674. 

This leads to the so-called current-doubling -effect, as shown for p-GaP under illumination in Fig. 33. The origin of this effect is based on the result that only for the first step an excitation by light is required. At n-type electrodes, the complete reaction occurs already in the dark. The hole injection was proved by luminescence measurements in n-GaP [151, 15]. The same result has been obtained with S20i" [150] and for the reduction of quinones [152]. 

Fig. 33. Current-doubling process a cathodic photocurrent at p-GaP with 10 M H2O2 at pH 1 [47] b two-step electron transfer at p-GaP upon light excitation c the same as b for n-type in the dark...

In this case, a hole created by light excitation is transferred in the first reaction step. Similar observations have been made for the oxidation of alcohols at CdS-[158], ZnO [161] and Ti02-electrodes [159], [160]. Interestingly, it has been observed with the latter electrode material that the current-doubling effect increased with increasing doping, with respect to the oxidation of CH3OH in... 

It should be emphasized that the experiments have been performed with ZnS-particles synthesized with an excess of S in order to avoid complications by surface states (see previous section). As shown above, ZnS was a very suitable semiconductor for these investigations because of its high energy position. The particle size-effect did not occur for instance with CdS, the energy bands of which occur at relatively low energies. In this case, current doubling occurs, i.e. the radical formed by a photoexcited hole can be further oxidized at the same particle by electron injection into the conduction band of CdS (189]. 

In solutions without Cu ", the photocurrent is enhanced upon addition of CH3OH, due to the current-doubling effect (see Sect. 4.5, Fig. 34), i.e. 

Upon addition of Cu -ions, the current-doubling effect disappears. Accordingly, reaction (95b) did not take place. It has been concluded that the radical must be capable of reducing Cu ", i.e. 

Thus, the key feature of photocurrent multiplication is a majority carrier injection step (reaction 34b or 34d) from a reaction intermediate (usually a free radical) into the semiconductor CB or VB, respectively. Thus, in the examples above, each photon generates two carriers in the external circuit, affording a quantum yield (in the ideal case) of 2. This is the current-doubling process. 

Figure 29. Simulated IMPS plots showing the competition between recombination and current doubling. The IMPS response switches from current doubling control in (a) to recombination control in (d). The band-bending values are 0.40, 0.35, 0.30 and 0.20 eV in (a), (b), (c) and (d) respectively. Refer to the original work for other simulation parameters. (Reproduced with permission from Ref. [284].)...

Photoetching processes do not always consist of a simple superposition of an anodic and a cathodic partial process and may exhibit various types of complications. Firstly, even in the simple case of the photoetching of GaP single crystals in alkaline OBr solutions, the situation is actually more complex than depicted above, since at n-type crystals, it appears that the photoetching process itself induces a hole injection reaction and hence and electroless etching effect [24]. Initially, OBr is reduced at the GaP surface via the current-doubling mechanism (as is concluded from photocurrent measurements at p-type samples) ... 

At any temperature above 0°C, there will be spontaneous generation of elec-tron/hole pairs that is unrelated to incident light intensity. In addition, defects in the silicon may be sources of electrons and contribute dark electrons. Both of these processes are exponentially dependent on temperature, and CCDs must be actively cooled to reduce dark current to acceptable levels. The dark current is usually expressed as e pixel s (electrons per pixel per second) and depends both on the specific device and the temperature. A few examples are listed in Table 8.6. As an approximate rule of thumb, ihe dark current doubles for each 5°C increase in temperature. 

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