Photocathodic current, semiconductor

Figure 9.20 i-v curve for semiconductors/iedox junction in dark (dotted line) and on iUuniination (hard line) for (a) n-type semicondnctor, which on iUnmination leads to photoanodic current, and (b) p-type semiconductor, which on iUnmination leads to photocathodic current. The onset for photocurrents in both the diagrams suggests the flat-band potential, fl>-... 

Here AVmax = Vsave(max) represents the difference in voltages at the semiconductor electrode and metal electrode at the maximum power conversion point. For example, in their experiment using a p-type InP photocathode, Heller and Vadimsky [120] obtained a current 23.5 mA/cm at maximum power point. A voltage of 0.1 IV vs SCE was applied in the case of InP electrode and -0.33V vs SCE in the case of platinum electrode, to obtain this current. Thus, the maximum saved voltage AVmax= 0.11( 0.33) V= 0.43V. Therefore, Psaved=0.43 V X 23.5 mA/cm = lO.lmW/cm. As they used a solar illumination of 84.7 mW/cm, the efficiency is 11.9%. 

Bokris and Uosaki (1) have studied transient photo-assisted electrolysis current for systems including a p-type semiconductor photocathode and dark Pt anode. A set of current vs. time scans taken with a ZnTe photocathode system is shown in Figure 6. 

High-rate photoelectrolysis of CO2 was conducted in a high pressure CO2 + methanol medium using p-type semiconductor electrodes. Current densities of up to 100 mA cm 2 were achieved, with current efficiencies of up to 93 % for CO production on a p-InP photocathode. The effect of CO2 pressure on the product distributions was examined for p-InP and p-GaAs. 

Electrocatalysis at metal electrodes in aqueous (1.2) and non-aqueous ( ) solvents, phthalocyanine ( ) and ruthenium ( ) coated carbon, n-type semiconductors (6.7.8),and photocathodes (9,10) have been explored in an effort to develop effective catalysts for the synthesis of reduced products from carbon dioxide. The electrocatalytic and photocatalytic approaches have high faradaic efficiency of carbon dioxide reduction (1,6). but very low current densities. Hence the rate of product formation is low. Increasing current densities to provide meaningful amounts of product, substantially reduces carbon dioxide reduction in favor of hydrogen evolution. This reduction in current efficiency is a difficult problem to surmount in light of the probable electrostatic repulsion of carbon dioxide, or the aqueous bicarbonate ion, from a negatively charged cathode (11,12). 

The maximum power point of efficient solar cells is located close to the open circuit voltage (see Figures 2.12 and 2.89). For p-type semiconductors, the open circuit condition is the most anodic potential at which the photocathode is operated and anodic dark currents compensate the cathodic photocurrent at this potential. 

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