Z. Loizos

Electrochemical synthesis of semiconducting CdSe thin films

Abstract Semiconducting CdSe thin films were synthesized by cathodic electrodeposition from an aqueous acid solution. Alloy formation occurs when the growth rate is limited by selenium ion mass transport and when the cadmium is underpotentially deposited. The composition and the physical properties of the asgrown layers are very sensitive to the selenium concentration of the solution and to the applied potential. CdSe films always have a zinc blende structure (instead of the normal wurtzite structure) with a strong (111) preferred orientation. It is shown that the best characteristics (stoichiometry, crystallinity, band gap width, etc.) are obtained with low selenium concentrations and in a narrow potential range. By using these appropriate preparation conditions, it will be possible to reduce the chemical and physical post-treatments generally applied to these kinds of semiconducting thin films in order to increase their solar energy conversion performances in solid or liquid junction cells.

Structure and properties of CdSe and CdSexTe1–x electrolytic deposits on Ni and Ti cathodes: influence of the acidic bath pH

Abstract The structural, morphological and photoelectrochemical features of coherent CdSe and CdSexTe1−x thin film semiconductors, prepared by cathodic electrodeposition from an acid sulfate solution, were investigated. In particular, the effect of deposition potential on the properties of layers grown onto Ti and Ni substrates, in connection with the electrolyte acidity within the bath pH interval [2.2, 1.6], was studied. As verified by X-ray diffraction (XRD), scanning electron microscopy (SEM), reflectance and photoelectrochemical cell (PEC) tests, the preparation method renders compact semiconductive films, presenting a remarkably intense in the case of CdSe/Ni(111) cubic structure, without any post-thermal treatment. The passive surface of Ti is associated with weaker texture; however, deposits on Ti generally exhibit better photoelectrochemical behavior, under similar preparation conditions.

Influence of heat treatment on structure and properties of electrodeposited CdSe of Cd(Te, Se) semiconducting coatings

Abstract CdSe and mixed CdSe and CdTe semiconducting thin films, prepared by cathodic electrodeposition from an acid sulphate solution, containing selenium and tellurium oxides in Various amounts, were submitted to a to a thermal treatment at temperatures ranging between 400 and 520 °C. The crystal structure, composition, band-gap width and photoelectrochemical response of the annealed materials was investigated. It was found that all materials, rich in selenium, change their structure from cubic (zinc blende) to hexagonal (wurtzite), when annealed within the above mentioned region of temperatures. In many cases, an improvement of their semiconducting properties has been also confirmed.

Semiconducting CdSex, Te1−x thin films prepared by electrodeposition

Abstract CdTe, CdSe and CdSe x Te 1− x semiconducting compounds were prepared by cathodic electrodeposition from an acid sulphate solution containing TeO 2 and H 2 SeO 3 in various amounts. The composition of the layers, their crystal structure, morphology, band-gap width and their photoresponse in a photoelectrochemical (PEC) cell were investigated as functions of the electrolyte composition and the deposition potential. It is shown that smooth crystallized CdSe x Te 1− x thin films can be obtained only for small concentrations of H 2 SeO 3 in the bath and in a restricted potential range corresponding to the under-potential deposition of cadmium.

Influence of pulse reversed current technique on the crystalline orientation and surface morphology of nickel electrodeposits

Abstract The reversed current (RC) technique perturbs the nickel electrocrystallization process by modification of the adsorption-desorption phenomena occuring on the cathodic area and causes a radical change of the structural characteristics and properties of the deposits. The purpose of this work is to study the modifications occurring in the crystalline orientation and surface morphology of nickel deposits, prepared from an organic-free Watts bath, by application of the RC technique. The texture analysis of the deposits was realized by X-ray diffractometry and the surface morphology was studied by scanning electron microscopy techniques. It was observed that variation of the RC parameters results in a total modification of the texture and surface morphology of the deposited metal. The roughness of the metallic surface was also examined and conditions for the preparation of bright and smooth deposits were determined.

Hexagonal cadmium chalcogenide thin films prepared by electrodeposition from near-boiling aqueous solutions

Thin, n-type, CdSe and CdSexTe1−x semiconductive films were prepared by cathodic electrodeposition onto titanium electrodes. An electrochemical cell was specially designed in order to perform electrodeposition in a near-boiling aqueous-ethyleneglycol bath at a temperature of approximately 110°C. The composition of the as-grown films, their crystal structure, morphology and band-gap width were studied as a function of the deposition potential and chalcogen ion concentration. It is shown that high temperatures have a positive effect on the crystal quality and the photoresponse stability of cadmium chalcogenide thin films even by employing electrolytes rather concentrated in selenous acid. Under specific conditions, a small shift in deposition potential brings about a complete phase transformation of the CdSe layers. In this manner, the described method enables the preparation of hexagonal CdSe deposits.

On a thermodynamic description of Se(IV) electroreduction and CdSe electrolytic formation on Ni, Ti and Pt cathodes in acidic aqueous solution

Abstract A discussion on tetravalent Se electroreduction and CdSe cathodic electrodeposition in terms of thermodynamics, in association with an investigation on the voltammetric behavior of Pt, Ni and Ti working electrodes in a high temperature (85 °C) acidic aqueous bath, is presented. The underpotential co-deposition of Cd with Se is described within the frame of a known electrochemical model of compound formation. The data presented serve as an effective basis for studying the electroreduction of Se(IV) as well as for determining the relevance of theoretical predictions to experimental findings on a binary compound electrodeposition process.

Electrocrystallization of CdSe upon various substrates. Structural arrangement and photoelectrochemical performance

Abstract The growth of cadmium chalcogenide thin film semiconductors, in the context of a typical electrolytic method of formation with an aqueous bath, is to a large extent determined by the deposition substrate, together with the potential, for a given electrolyte composition and temperature. The effect of various substrates ([100]-oriented Ni, commercially pure Ni, Ti) and procedures (Ni electropolishing, Ti anodization, double-step deposition, etc.) on the structural arrangement and the resulting photoelectrochemical (PE) behavior of cathodically electroplated CdSe layers is presented. The outcome of the preparation process is analyzed in terms of structural–optical properties relation. As verified, some microcrystalline, porous samples give higher PE efficiencies than larger-grained ones, owing to the increased contact area with the PE redox electrolyte and possibly the establishment of a particular charge transfer mechanism in the solid. The latter is associated with the existence of a nanostructure.

Cadmium chalcogenide semiconducting thin films prepared by electrodeposition from boiling aqueous electrolytes

Abstract Thin n-type semiconductive films of CdTe, CdSe and their mixed compounds CdSe x Te 1− x , suitable for solar energy conversion, were prepared by catholic electrodeposition from an aqueous electrolyte onto titanium electrodes. An electrochemical cell was specially designed to perform electrodeposition in a boiling bath. The composition of the layers, their crystals structure, morphology and band-gap width were studied as functions of the electrochemical parameters and bath concentration. It is shown that the use of a high temperature improves the crystal quality of cadmium chalcogenide thin films even with solutions rather concentrated in selenous acid. CdSe or Se-rich CdSe x Te 1− x , layers tend in many cases to crystallize in the hexagonal (wurtzite) structure.

Substrate effect on the structure and properties of electrodeposited CdSe and Cd(Se, Te) coatings

CdSe, mainly, and Cd(Se, Te) alloy layers were cathodically electroplated onto titanium, nickel and (100) oriented nickel substrates from an acid sulphate solution at 85°C. The growth of the deposits, on Ti and Ni, was partly controlled by altering the mode of the nucleation first step. This procedure, though inhibiting the establishment of a strong crystal orientation, seems to positively affect the photoconversion efficiency of the CdSe films on Ni. Crystallized deposits with defined stoichiometry are obtained on (100) oriented Ni cathodes along a narrower, than the typical case, range of electrolysis potentials. The polycrystalline films are always cubic and the measured photoconversion efficiencies reach 2.6%.