Electrical Properties of CdSe Photovoltaic Electrodeposits in a Marine Environment

Abstract

Semiconductive thin ®lms of CdSe prepared by electrodeposition have been tested under corrosive conditions in a saline atmosphere and their electrical properties (I±V curves) measured before and after exposure. The preparation conditions of CdSe electrodeposits have been studied [1±6], as have their substrate effects [7]. They are intended for photovoltaic generation of electricity, and they have a direct band gap ranging between 1.35 and 1.75 eV following the sample composition. They also present good electrical properties (such as carrier mobility and long lifetime), high absorption coef®cients in the visible and infrared part of the solar spectrum and relative stability against corrosion when used in polychalcogenite solutions. In this letter, the electrical properties of the surface of CdSe deposits exposed to corrosive conditions in a NaCl environment are presented. The deposits were prepared under potentiostatic control in a three-electrode setup in an acidic solution typically containing 1.5 mmol ly1 of selenium oxide and 0.2 mol ly1 of cadmium sulfate with a pH of 2.2. The electrolytic bath contained ethylene glycol (1=3) to obtain the required temperature of 113 8C. Rotating disc electrodes of nickel cylinders (10 mm diameter) were used. The counter electrode was a at carbon surface, and the reference was a Cd electrode. The deposition voltage used was y102.5 mV at 0.5 mA. The deposits were exposed in a salt spray chamber with a continuous spraying of a 5% NaCl solution, for 1 month, at 30 8C. The surface of the specimens was examined by scanning electron microscopy (SEM), and a typical micrograph is shown in Fig. 1. The surface stays smooth but cracks are created at speci®c sites that are found by electron dispersive analysis (EDA) a higher content of nickel than that of the unexposed surface. Oxygen and chloride compounds are also encountered on the exposed sample. The deposits were examined by X-ray diffraction (XRD) before and after exposure (Fig. 2). The crystalline deposits of CdSe mainly exhibit a cubic blende structure with pronounced (1 1 1) preferred orientation instead of the normal hexagonal wurtzite structure (1 0 0). Profound differences were not found because the thicknesses of the formed layers are beyond the sensitivity of the method. An investigation of the I±V characteristics of the fabricated samples at a given illumination level before and after surface corrosion process was performed by employing a high-precision pA-meterDC voltage source instrument (HP4140B). In all cases, the d.c. voltage hold time and step recovery time (between successive bias-setting applications) was kept at a constant value of 0.1 ms for comparison purposes. Fig. 3 presents typical experimental results. The generalized I±V relationship of a photovoltaic cell, subjected to external polarity and illumination, is given by Equation 1 [8]: