Semiconductors photoelectrochemical

Chandra Babu KS, Pandey RN, Srivastava ON (1995) Photoelectrochemical semiconductor septum (CdSe/Ti and Ti02/Ti) solar cells in relation to hydrogen production. Int J Hydrogen Energy 20 771-775... 

Photoelectrochemical semiconductor cells are used to convert photon energy into chemical substances or into electricity, the former is a photodectrolytic cell and the latter is a photovoltaic cell. A photoelectrochemical semiconductor cell consists of either a pair of metal and semiconductor electrodes or a pair of two semiconductor electrodes. 

The Ni(OH)2/NiOOH reaction is a topo-chemical type of reaction that does not involve soluble intermediates. Many aspects of the reaction are controlled by the electrochemical conductivity of the reactants and products. Photoelectrochemical measurements [86, 871 indicate that the discharged material is a p-type semiconductor with a bandgap of about 3.7eV. The charged material is an n-type semiconductor with a bandgap of about 1.75eV. The bandgaps are estimates from absorption spectra [87]. 

These three equations (11), (12), and (13) contain three unknown variables, ApJt kn and sr The rest are known quantities, provided the potential-dependent photocurrent (/ph) and the potential-dependent photoinduced microwave conductivity are measured simultaneously. The problem, which these equations describe, is therefore fully determined. This means that the interfacial rate constants kr and sr are accessible to combined photocurrent-photoinduced microwave conductivity measurements. The precondition, however is that an analytical function for the potential-dependent microwave conductivity (12) can be found. This is a challenge since the mathematical solution of the differential equations dominating charge carrier behavior in semiconductor interfaces is quite complex, but it could be obtained,9 17 as will be outlined below. In this way an important expectation with respect to microwave (photo)electro-chemistry, obtaining more insight into photoelectrochemical processes... 

As mentioned in the introduction, before an adequate theory was developed, it was difficult to understand the experimentally determined pho-toinduced PMC signals, especially the minority carrier accumulation near the onset of photocurrents.The reason was that neither conventional solid-state semiconductor theory nor photoelectrochemical theory had taken such a phenomenon into account. But we have shown that it is real and microwave (photo)electrochemical experiments clearly confirm it. 

It is well known that photoelectrochemical measurements do not indicate photocurrents in the accumulation region of an illuminated semiconductor. The reason is that majority carriers control interfacial reactions, which... 

On the basis of our theoretical considerations and preliminary experimental work, it is hoped that fast processes of charge carriers will become directly measurable in functioning photoelectrochemical cells, Typical semiconductor electrodes are not the only systems accessible to potential-dependent microwave transient measurements. This technique may also be applied to the interfacial processes of semimetals (metals with energy gaps) or thin oxide or sulfide layers on ordinary metal electrodes. 

In this chapter we have attempted to summarize and evaluate scientific information available in the relatively young field of microwave photoelectrochemistry. This discipline combines photoelectrochemical techniques with potential-dependent microwave conductivity measurements and succeeds in better characterizing the behavior ofphotoinduced charge carrier reactions in photoelectrochemical mechanisms. By combining photoelectrochemical measurements with microwave conductivity measurements, it is possible to obtain direct access to the measurement of interfacial rate constants. This is new for photoelectrochemistry and promises better insight into the mechanisms of photogenerated charge carriers in semiconductor electrodes. 

Extensive structural, optical, and electronic studies on the chalcopyrite semiconductors have been stimulated by the promising photovoltaic and photoelectrochem-ical properties of the copper-indium diselenide, CuInSe2, having a direct gap of about 1.0 eV, viz. close to optimal for terrestrial photovoltaics, and a high absorption coefficient which exceeds 10 cm . The physical properties of this and the other compounds of the family can be modulated to some extent by a slight deviation from stoichiometry. Thus, both anion and cation deficiencies may be tolerated, inducing, respectively, n- and p-type conductivities a p-type behavior would associate to either selenium excess or copper deficiency. 

A smaller class of type II alloys of II-VI binaries also exists, including the (CdS) ,(ZnSe)i (CdS) ,(ZnTe)i (CdSe) ,(ZnSe)i (CdS) ,(CdTe)i-. (CdSe)x(CdTe)i i , and (CdS) c(ZnS)i i systems, which transform at some critical composition from the W to the ZB structure. Importantly, the transition temperatures are usually well below those required to attain a thermodynamically stable wurtzite form for the binary constituents (e.g., 700-800 °C for pure CdS and > 1,020 "C for pure ZnS). The type 11 pseudobinary CdxZni jcSe is of considerable interest in thin film form for the development of tandem solar cells as well as for the fabrication of superlattices and phosphor materials for monitors. The CdSe Tei-x alloy is one of the most investigated semiconductors in photoelectrochemical applications. 

Primarily connected to corrosion concepts, Pourbaix diagrams may be used within the scope of prediction and understanding of the thermodynamic stability of materials under various conditions. Park and Barber [25] have shown this relevance in examining the thermodynamic stabilities of semiconductor binary compounds such as CdS, CdSe, CdTe, and GaP, in relation to their flat band potentials and under conditions related to photoelectrochemical cell performance with different redox couples in solution. 

The redox behavior of the SeSO -Zn-EDTA system has been discussed on the basis of Pourbaix and solubility diagrams [11], Different complexes and substrates have been employed in order to optimize the electrodeposited thin films. By the selenosulfate method it is generally possible to grow ZnSe with an almost stoichiometric composition however, issues of low faradaic efficiency as well as crystallinity and compactiveness of the product, remain to be solved. Interestingly, in most reports of photoelectrochemically characterized ZnSe electrodeposits, the semiconductor film was found to be p-type under all preparation conditions (ZnSe is normally n-type unless deliberately doped p-type). 

The ECALE deposition of ternary II-VI compound semiconductors such as CdxZni xS, CdxZni xSe, and CdSjcSei c, on Ag(lll), has been reported [51-53]. The compounds were prepared by sequential deposition of the corresponding binaries in submonolayer amounts for instance, alternate deposition of CdS and ZnS was carried out to form Cd cZni cS. The stoichiometry of the ternaries was seen to depend on the deposition sequence in a well-defined and reproducible way, with the limit that only certain discrete x values were attainable, depending on the adopted sequence profile. Photoelectrochemical measurements were consistent with a linear variation of the band gap vs. the composition parameter x of the mixed compounds. 

A novel method called pulsed light assisted electrodeposition (PLAE), involving the alternate depositions of two elements to form a compound semiconductor, one by electrochemical and the other by photoelectrochemical deposition, should be referred to in this context. Takahashi et al. [108] suggested that high-quality... 

Rajeshwar K, de Tacconi NR, Chenthamarakshan CR (2004) Spatially directed electrosynthesis of semiconductors for photoelectrochemical applications. Curr Opin Solid State Mater Sci 8 173-182... 

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. 

Junctions involving a bilayer (two-semiconductor) electrode and a liquid have been investigated as to their photoelectrochemical properties primarily in works which... 

Efficient photoelectrochemical decomposition of ZnSe electrodes has been observed in aqueous (indifferent) electrolytes of various pHs, despite the wide band gap of the semiconductor [119, 120]. On the other hand, ZnSe has been found to exhibit better dark electrochemical stability compared to the GdX compounds. Large dark potential ranges of stability (at least 3 V) were determined for I-doped ZnSe electrodes in aqueous media of pH 0, 6.3, and 14, by Gautron et al. [121], who presented also a detailed discussion of the flat band potential behavior on the basis of the Gartner model. Interestingly, a Nernstian pH dependence was found for... 

---