Open circuit photo potential

The maximum open-circuit photo voltage, attainable in a dye-sensitized solar cell, is the difference between the Fermi level of the solid under illumination and the Nemst potential of the redox mediator. However, for these devices this limitation has not been achieved and Voc is in general much smaller. It appears that Voc is kinetically limited and the diode Equation 17.12 can be applied for an n-type semiconductor in a regenerative cell.23... 

The effects on the dynamics of photo-injected electrons where not systematically studied, despite scattered reports on the influence of amines, which induce surface deprotonation, and lower surface charge with a resulting negative shift in band edge position and an increase in the open circuit potential, Voc [103], The opposite effect is induced by Li+ ions, which intercalate in the oxide structure. Guanidinium ions increase Voc when used as counterions in place of Li+. Other adsorbing molecules that influence both Voc and short circuit current are polycar-boxylic acids, phosphonic acids, chenodeoxycholate and 4-guanidinobutyric acid. 

The existence of surface states in general can lead to a variety of nonidealities in the output parameters associated with semiconductor-electrolyte junctions. Figure 28.6 provides the current-potential response for a photo-electrochemical cell containing a cadmium ferrocyanide-modified n-CdS electrode in an aqueous ferri/ferrocyanide electrolyte. Although open-circuit and... 

The overall conversion efficiency (rj) of the dye-sensitized solar cell is determined by the photo current density (/ph), the open circuit potential (Voc), the fill factor (FF) of the cell and the intensity of the incident light (Is), (Eq.8) [19]. 

In order to obtain high overall light to electric power conversion efficiencies, optimization of the short circuit photo current (z sc) and open circuit potential (Voc) of the solar cell is essential. The conduction band of the TiC>2 is known to have a Nernstian dependence on pH [55,67]. The fully protonated sensitizer 2, upon adsorption transfers most of its protons to the TiC>2 surface, charging it positively. The electric field associated with the surface dipole generated in this fashion enhances the adsorption of the anionic Ru complex and assists electron injection from the excited state of the sensitizer into the titania conduction band, favoring high photocurrents (18-19 mA cm-2). However, the open-circuit potential (0.65 V) is lower due to the positive shift of the conduction band edge induced by the surface protonation. 

The injection of oxygen to the solution resulted in an increase in the rate of formation of photogenerated surface species at open circuit . This may be explained by the fact that, in the case of O2 photo-uptake, the surface-bonded species are partly formed via cathodic reduction of dissolved molecular oxygen. As a matter of fact, parallel experiments, consisting in a controlled-potential reduction of dissolved O2 at the Ti02 electrode in the dark, revealed the presence of the surface species similar to those generated during illumination of the electrode. ... 

Silicon nanostructures can be obtained in acidic fluoride-containing media or in alkaline solution at potentials negative from open circuit potential. The (photo) current-voltage characteristic of Si in fluoride-containing electrolytes reveals a series of phenomena which have attracted considerable attention in chemistry, physics, and solar energy conversion. Figure 2.39 provides an overview of these processes for both n- and p-type Si. The I-Vcurve is characterized by two maxima and periodic variations at higher applied anodic potential. These photocurrent... 

The photoproduction and subsequent separation of electron-hole pairs in the depletion layer cause the Fermi level in the semiconductor to return toward its original position before the semiconductor-electrolyte junction was established, i.e., under illumination the semiconductor potential is driven toward its flat-band potential. Under open circuit conditions between an illuminated semiconductor electrode and a metal counter electrode, the photovoltage produced between the electrodes is equal to the difference between the Fermi level in the semiconductor and the redox potential of the electrolyte. Under close circuit conditions, the Fermi level in the system is equalized and no photovoltage exists between the two electrodes. However, a net charge flow does exist. Photogenerated minority carriers in the semiconductor are swept to the surface where they are subsequently injected into the electrolyte to drive a redox reaction. For n-type semiconductors, minority holes are injected to produce an anodic oxidation reaction, while for p-type semiconductors, minority electrons are injected to produce a cathodic reduction reaction. The photo-generated majority carriers in both cases are swept toward the semiconductor bulk, where they subsequently leave the semiconductor via an ohmic contact, traverse an external circuit to the counter electrode, and are then injected at the counter electrode to drive a redox reaction inverse to that occurring at the semiconductor electrode. 

If the electrode potential is not fixed by a potentiostat, i.e., if the electrode is at open circuit conditions, its illumination will cause photo-potentials. In this case, the electrons and holes from the photogenerated pairs will be separated in the space charge layer and will accumulate at the surface and the inner part of the semiconductor, respectively. This situation decreases the potential drop in the space charge layer and thus causes a potential change, i.e., the photo-potential. 

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