Types of Semiconductor Electrodes

TYPES OF SEMICONDUCTOR ELECTRODES 9.5.1 Single crystal and epitaxial film electrodes... 

A thorough insight into the comparative photoelectrochemical-photocorrosion behavior of CdX crystals has been motivated by the study of an unusual phenomenon consisting of oscillation of photocurrent with a period of about 1 Hz, which was observed at an n-type CdTe semiconductor electrode in a cesium sulfide solution [83], The oscillating behavior lasted for about 2 h and could be explained by the existence of a Te layer of variable width. The dependence of the oscillation features on potential, temperature, and light intensity was reported. Most striking was the non-linear behavior of the system as a function of light intensity. A comparison of CdTe to other related systems (CdS, CdSe) and solution compositions was performed. 

Fig. 8-18. Band edge levels of semiconductor electrodes in a solution of pH 1 (n) = n-type (n, p) = n- and p types REDOX = redox reactions and the standard Fermi level. [From Gleria-Memming, 1975.]...

It follows that, when Ptk e, is greater than (e -Ev), the rate of capture of holes is greater than the rate of release of electrons (Vb>Vt). Namely, if the Fermi level ej at the surface is lower than the middle, (ej + Ev)/2, of the band gap (the p-type surface) and if p.b is close to 0.5, the valence band mechanism of Eqn. 9-24b will probably predominate over the conduction band mechanism of Eqn. 9-24a. As the band gap e, of semiconductor electrode increases, the valence band mechanism become more predominant. 

Fig. 9-10. Polarization curves of anodic dissolution and cathodic deposition of n-type and p-type covalent semiconductor electrodes n-SC (p-SC) = n-type (p-type) semiconductor electrode i (i ) = anodic dissolution (cathodic deposition) current Cp = Fermi level.

Pig. 10-19. (a) Capture of photogenerated holes in surface states to form siuface ions and (b) anodic dissolution of surface ions to form hydrated ions on an n-type semiconductor electrode Oj = rate of hole capture in surface states oqx = rate of anodic dissolution of surface ions Cn = surface state level S, = surface atom of semiconductor electrode h(vs) = hole in the valence band h(n> = hole captured in smface states h(soH-) = hole in dissolved ions. 

Nakato Y, Tsumura A, Tsubomura H (1982) Efficient photoelectrochemical conversion of solar energy with n-type silicon semiconductor electrodes surface doped with IIIA elements. Chem Lett 1071-1074... 

Our work started in the late 1960s at the University of Tokyo in research on photoelectrochemical (PEC) solar cells. I5-16) One of the first types of electrode materials we looked at was semiconducting Ti02, partly because it has a sufficiently positive valence band edge to oxidize water to oxygen. Tt is also an extremely stable material in the presence of aqueous electrolyte solutions, much more so than other types of semiconductors that have been tried. 

The possibility of solar photoelectrolysis was demonstrated for the first time with a system in which an n-type Ti02 semiconductor electrode, which was connected through an electrical load to a platinum black counter electrode, was exposed to near-UV light (Fig 2.2).l9) When the surface of the Ti02 electrode was... 

Electrochemical reactions at metal electrodes can occur at their redox potential if the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated n-type semiconductor electrode commence to occur at around the flat-band potential Ef j irrespective of the redox potential of the reaction Ergdox Efb is negative of Ere 0 (1 2,3). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively ... 

Chlorophyll-Coated Semiconductor Electrodes. Chi has first been employed by Tributsch and Calvin (55,56) in dye sensitization studies of semiconductor electrodes. Solvent-evaporated films of Chi a, Chi b, and bacteriochlorophyll on n-type semiconductor ZnO electrodes (single crystal) gave anodic sensitized photocurrents under potentiostatic conditions in aqueous electrolytes. The photocurrent action spectrum obtained for Chi a showed the red band peak at 673 nm corresponding closely to the amorphous and monomeric state of Chi a. The addition of supersensitizers (reducing agents) increased the anodic photocurrents, and a maximum quantum efficiency of 12.5% was obtained for the photocurrent in the presence of phenylhydrazine. 

It is important to note that the description of electron transfer kinetics is different in the case of semiconductor electrodes. For an n-type semiconductor electrode in the dark, the rate of electron transfer depends not only on the concentration of redox species in the solution but also on the potential dependent density of electrons in the semiconductor. Under depletion conditions, most of the potential drop is located in the solid, so that to a good approximation the activation energy for electron transfer is independent of potential. Electron transfer at semiconductor electrodes is therefore characterised in terms of a second order heterogeneous rate constant with units cm4 s-1. 

Tachibana Y., Muramoto R., Matsumoto H. and Kuwabata S. (2006), Photoelectrochemistry of p-type CU2O semiconductor electrode in ionic liquid . Res. Chem. 

Yeh L.-S. R. and Hackerman N. (1978), Electrochemistry of n-type cadmium sulfide, gallium phosphide, and gallium arsenide and p-type germanium semiconductor electrodes in N,N-dimethylformamide solutions , J. Phys. Chem. 82, 2719-26. 

In the case of semiconductor electrodes, it is impossible to obtain the same information because the energy bands are fixed at the surface and any potential variation occurs only across the space charge layer. Here the maximum rate constant is expected if the peak of the distribution curve occurs at the lower edge of the conduction band of an n-type semiconductor. Therefore, the experimental results obtained with the modified metal electrodes, are of great importance for the quantitative analysis of rate constants from current-potential curves measured with semiconductor electrodes (see e.g. Section 7.3.4). 

The main obstacle to creating liquid junction solar cells is photocorrosion of semiconductor electrodes, which reduces considerably their lifetime. In order to prevent, for example, anodic photocorrosion, a well-reversible redox couple is introduced into an electrolyte solution, so that the reaction of oxidation of the red component competes for photoholes with the reaction of photodecomposition of the electrode material (see Section IV.2). With the aid of this method, photocorrosion has been practically prevented in certain types of photocells and the duration of their continuous operation has been increased up to about one year. Yet, there are other, more subtle mechanisms of electrode degra-dation, which has hitherto prevented the lifetime of photoelectrochemical cells from becoming comparable with the 20-year lifetime of solid-state solar cells. 

Fig. 20. SERR spectrum of [Ru(bpy)3]2+ at monolayer coverage on an Ag-overlayered p-type GaAs semiconductor electrode. S is an acetonitrile solvent band. (Reproduced with permission from ref. 41.)...

A distinctive feature of semiconductor electrodes, as compared to metal ones, is possibility for both types of charge carriers (conduction band electrons, and holes) to participate in the electrochemical reactions. In other words the charge transfer between the semiconductor electrode and electrolyte solution generally proceeds via both the conduction and valence bands. 

The first application of discotic liquid crystal to organic thin film solar cells is of a binary blend of liquid crystalline phthalocyanine (Pc) as p-type of semiconductor and non- mesogenic PTCDI derivative as n-type [45], where the active layer was not a BHJ type, but had a p-i-n junction (Fig. 8.11). The Pc layer was fabricated by spin-coating and heated up to the isotropic phase followed by cooling to obtain a spontaneously formed homeotoropic alignment on ITO-coated substrate. The n-type layer was deposited by sublimation on Pc layer and finally the counter A1 electrode was deposited in vacuo. The performance was shown simply as short circuit current, Isc = 0.4 mA cm , open circuit voltage, Vqc = 0.3 V and external quantum efficiency, EQE 0.5 %. 

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