Photo-electrochemical properties of Cd1−xHgxSe thin films

doi:10.1016/j.matchemphys.2007.05.043

Materials Chemistry and Physics

Volume 106, Issues 2–3, 15 December 2007, Pages 250-255

https://doi.org/10.1016/j.matchemphys.2007.05.043 Get rights and content

Abstract

The Photo-electrochemical studies on chemically deposited Cd_1−xHg_xSe thin films (0 ≤ x ≤ 1) have been carried out to assess its suitability for use in conversion of solar energy to electrical energy. The PEC cell with Cd_1−xHg_xSe thin films as photoanode and sulphide–polysulphide (0.5 M) solution as electrolyte have been constructed and investigated for various cell parameters as a function of photoelectrode composition parameter (x). The film with composition x = 0.4 (Cd_0.6Hg_0.4Se) showed enhanced solar to electrical conversion efficiency (η) and fill factor (ff). The observed enhancement has been correlated to decrease in photoelectrode resistance and band gap (E_g). The addition of Hg in CdSe, to certain limit, showed better performance and stability of the cell.

Introduction

The use of photo-electrochemical cells (PEC) utilizing thin film semiconductor material, as a direct route to harness solar energy has become a subject of intensive research due to its overriding advantages over the other conventional devices. The photo-electrochemical cells are much cheaper, simple to construct and need no complicated processing steps. Unlike semiconductor–semiconductor junction in solar cells, the semiconductor–electrolyte junction in PEC is readily established so that the study of junction energetics becomes easy [1], [2], [3]. A PEC utilizing a polycrystalline aggregate can achieve 70% efficiency of the same material in a single crystal form [4]. The efficiency and stability of PEC depends strongly on the preparation conditions of the photoelectrodes, electrolytes and the experimental conditions set during the experimentation [5]. The alloyed/mixed ternary semiconductor materials are known to function effectively in conversion of solar energy into electrical energy [6], [7]. This is because the properties of the ternary material can be easily tailored to the desired level by variation of the compositional parameter ‘x’ (i.e. the relative compositions of the two binary systems selected).

The chalcogenides, especially of cadmium group, find applications in the photovoltaic devices [8], [9], [10]. These compounds have been shown to form a solid solution over an appreciable range. However, the scope and study was limited to bulk form only and studies in thin film form are quite limited. A few of the ternary selenide series consisting of Cd_1−xZn_xSe [11], CdS_1−xSe_x [12], Cd_1−xMn_xSe [13], etc. have been prepared and studied. In this context, Cd_1−xHg_xSe seems to be promising due to following reasons (i) CdSe is a semiconductor material with a band gap of 1.7 eV while, HgSe is low band gap (∼0.6 eV in thin film form) material, so the ternary system with a proper composition can be engineered to cope with the maximum of visible spectrum (∼1.4 eV), (ii) the addition of Hg in CdSe reduces the resistance of the film, (iii) the ternary material has better absorptivity and higher spectral sensitivity over a considerable range of photon energies and (iv) CdSe and HgSe exist in cubic as well as hexagonal forms. Their lattice constants (in cubic phase) do not differ greatly, so a solid solution is possible in almost the whole range of compositions. It was, therefore, proposed to prepare Cd_1−xHg_xSe thin film photoelectrodes and study their photo-electrochemical properties with special reference to the PEC parameters namely open-circuit voltage (V_oc), short-circuit current (I_sc), efficiency (η), fill factor (ff), Barrier height (Φ_b), flat band potential (V_fb), etc.

Section snippets

Preparation of photoelectrode

The photoelectrodes of the type Cd_1−xHg_xSe with (0 ≤ x ≤ 1) were prepared onto a smooth, polished, stainless steel substrates. A well-known chemical bath deposition method utilizing AR grade cadmium acetate, mercuric nitrate and sodium selenosulphate was used. The values of composition parameter (x) were adjusted by maintaining the volume concentrations of Cd²⁺, Hg²⁺ and Se²⁻ ions in solution state. The detail of the deposition process is described elsewhere [14].

Fabrication and characterization of a PEC cell

A photo-electrochemical cell was

Results and discussion

When a semiconductor material is immersed into the solution of a redox electrolyte, the movement of charge occurs at semiconductor–electrolyte (S/E) interface in order to equilibrate the two phases, generating an electric field at the interface. The excess charge that is located on semiconductor usually extends into the bulk of electrode for a significant distance; this region is referred to as ‘space charge region’ or ‘depletion region’. When this interface is illuminated with a light having

Conclusions

The PEC cells with the photoanode of the type Cd_1−xHg_xSe have been constructed and investigated for various cell properties as a function of photoelectrode composition parameter x. The results indicated that for Cd_0.6Hg_0.4Se the efficiency and the fill factor were increased to 1.03 and 41.0, respectively. The observed enhancement is due to increased open-circuit voltage, improved grain quality and improved photoelectrode absorption. The increase in photocurrent is attributed to reduction in

Acknowledgements

The author (V.M. Bhuse) thankfully acknowledges UGC for providing financial assistance through a Minor research scheme [F. No. 47-97/2002/WRO, dated 22/03/2002].

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Photo-electrochemical properties of Cd1−xHgxSe thin films

Abstract

Introduction

Section snippets

Preparation of photoelectrode

Fabrication and characterization of a PEC cell

Results and discussion

Conclusions

Acknowledgements

Mater. Chem. Phys.

Sol. Energy Mater.

Mater. Chem. Phys.

Sol. Energy Mater. Sol. Cells

Mater. Chem. Phys.

Mater. Chem. Phys.

J. Electrochem. Soc.

Chemical and Electrochemical Energy Systems

J. Indian Chem. Soc.

Int. J. Electron.

Int. J. Ionics

Ind. J. Pure Appl. Phys.

Ind. J. Pure Appl. Phys.

Photo-electrochemical properties of Cd_1−xHg_xSe thin films