Interface CdTe-electrolyte

Figure 6. Potential dependence of imaginary impedance maximum for CdTe electrolyte interface. Conditions as in Fig. 2.

The surface state capacitance for t i CdTe-electrolyte interface is plotted as a function of electrode potential in Fig. 16 (the minimum was taken as the value at 0.2V NHE). The surface state capacitance decreases in the cathodic direction in the region -0.56 to -2.26V (NHE). Capacitance measurements at cathodic potentials less negative than -0.56V could not be carried out because of the onset of a C02 independent anodic dark current. Assuming (in consistence with other examples of pseudo capacitance behavior) that the capacitance-potential curve is symmetrical with respect to a maximum at -0.66V, the number of surface states was calculaed using the above equation. The number of surface states as a function of electrode potential, on the basis of this assumption, is shown in Fig. 17. Geometric area of the electrode was used to calculate the surface state density. Real surface area may be larger. 

Figure 16. Surface state capacitance as a function of bias potential for the CdTe electrolyte interface.

As-deposited CdTe films are n-type, so that no heterojunction is formed between the CdTe and the underlying n-type CdS. When the as-deposited CdTe film on CdS is contacted with an electrolyte and is polarized so that a depletion layer forms at the n-CdTe-electrolyte junction, electron-hole pairs generated by illumination are separated, with the holes moving towards the electrolyte interface where they... 

EQE spectrum shows a maximum near the band edge, but almost no response at higher energies. This indicates that the active junction is indeed located at the CdTe-electrolyte interface, since light can only penetrate through to the CdTe-electrolyte interface from the substrate side when the absorption coefficient of the CdTe is sufficiently low, as is the case near the band edge. 

Interest in CdTe for solar energy conversion has led to a number of studies of the CdTe/electrolyte interface (8,10-12), and development of photoelectrochemical etching (75-75). In general, the above studies focused on macroscopic etching as a microfabrication process or surface cleaning technique for CdTe. 

Highlights of research results from the chemical derivatization of n-type semiconductors with (1,1 -ferrocenediyl)dimethylsilane, , and its dichloro analogue, II, and from the derivatization of p-type semiconductors with N,N -bis[3-trimethoxysilyl)-propyl]-4,4 -bipyridinium dibromide, III are presented. Research shows that molecular derivatization with II can be used to suppress photo-anodic corrosion of n-type Si derivatization of p-type Si with III can be used to improve photoreduction kinetics for horseheart ferricyto-chrome c derivatization of p-type Si with III followed by incorporation of Pt(0) improves photoelectrochemical H2 production efficiency. Strongly interacting reagents can alter semicon-ductor/electrolyte interface energetics and surface state distributions as illustrated by n-type WS2/I-interactions and by differing etch procedures for n-type CdTe. 

Light energy may be used to reduce the necessary electrical potential in photoelectrochemical reactions. The overpotential is decreased by 700 mV for the photoelectrochemical reduction of CO on p-CdTe, compared to that on indium - the best metal electrode for CO2 reduction. For these semiconductors which involve a high concentration of surface states, the double layer at the semiconductor-electrolyte interface plays an important role in the kinetics of photoelectrochemical reactions. In this paper, we report spectroscopic and impedance aspects of the electrode-electrolyte interface as affected by reactants and radicals involved in CO reduction. 

Table II. Surface State Capacitance for Several Electrolytes at the CdTe-DMF (5% H 0) Interface...

The impedance data for the GaP-electrolyte interface can be represented by the equivalent circuit discussed for the CdTe electrode. 

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