Development of CdTe Thin Film By Electrodeposition in Ionic Liquid and Studies on Its Electronic Devices

Abstract

Electrodeposition of CdTe is historically being achieved by acidic aqueous solutions with the pH in the range of 1 to 2. However, the aqueous solutions have relatively higher vapor pressure and during the process of electrodeposition they emit acidic fumes containing cadmium and/or tellurium. Elemental cadmium is highly toxic material for environment as well as mankind and living objects. On the other hand, CdTe is the ideal material to be used in electronics and solar photovoltaics. The idea behind the electrodeposition of CdTe from ionic liquid was to make use of extraordinary properties of ionic liquids such as substantially high boiling temperature and relatively low vapor pressure. It is believed that this route will be the most ‘Green’ way of producing CdTe compare to any other solution based techniques. In this study, the electrodeposition of stoichiometric CdTe on transparent conducting oxide (FTO) glass substrate was achieved by a novel ionic liquid, 1-butyl-3-methylimidazolium chloride, medium. The potential for perfect stoichiometry at 80 C was found to be -1.45 V (vs. platinum quasi reversible electrode). It was found that the change of ± 50 mV in the applied potential greatly alter the stoichiometry of CdTe thin film. The resultant films were characterized by various techniques like XRD, FE-SEM, EDX, RAMAN and UV-Vis spectroscopy for analyzing its basic material properties. The XRD, EDX and RAMAN spectroscopy confirmed high crystallinity of the films as no other impurities or even elemental cadmium or tellurium were detected. The average grain size was calculated as 44 nm and lattice parameter for cubic zinc blende CdTe was found to be 6.41 Å. After annealing the film at 400 C, the SEM images showed a compact granular CdTe thin film. The stoichiometry of Cd:Te was confirmed to be 54:46 by EDX analysis. The effect of annealing as well as deposition temperature on the structural properties of CdTe thin film was studied by XRD. It was found that appearance of (220) plane decreases as the annealing temperature is increased from 200 to 400 C and becomes invisible after 450 C. On the other hand, the increase in deposition temperature decreases the deposition time for stoichiometric CdTe. The photoelectrochemical measurements confirmed the p-type conductivity with doping density of 6.4X1014 cm-3 and flat band potential of 0.93 V in polysulphide electrolyte. The growth mechanism of this binary alloy was understood by analyzing the current-time transients on the basis of well-known Scharifker's theoretical model. It was found that the Te pre-layer deposition is necessary for the deposition of adherent, smooth and stoichiometric CdTe thin film. Experimental and fitting results showed that the formation of CdTe binary alloy progresses as co-induced deposition of cadmium into pre-deposited tellurium clusters. Various thermodynamic parameters like diffusion coefficient, number of active sites, and amount of adsorbed charge during the electrodeposition process were extracted from the theoretical fitting. iv It was found that by ionic liquid route of electrodeposition, a network of CdTe nanostructures is forming directly on the substrate. The 1:1(Te:Cd) stoichiometry of individual nanostructures as well as the overall thin film was confirmed by EDX analysis. The dimensions of CdTe nanostructures were varied by changing the molar ratio of precursors in the ionic liquid bath. It was found that the diameter and length of the CdTe nanostructures increases by increasing molar ratio of Te:Cd. After achieving the required composition and thickness of the CdTe film, the first application of this novel nanostructured thin film is shown in Schottky rectifier where the active junction was realized between FTO and p-CdTe. It was found that rectifying properties of the diode improve by increasing length and diameter of the nanostructures. Highly stable and reproducible junction between FTO and p-CdTe was obtained by low temperature (200 C) post annealing treatment. The electrical properties of the diode with nearly ideal characteristics were calculated from the dark I-V measurements. The saturation current density was found to be 2X10-5 A/cm2 , rectification ratio was calculated as 6000 and the ideality factor of the diode was 1.1. The charge transport mechanism in the diode was fully understood by analyzing the diode on the basis of thermionic emission theory. It was found that the charge transport is mainly dominated by diffusion process from reverse bias to lower forward bias region whereas the recombination current dominates at higher forward bias. The capacitance-voltage measurements revealed that junction between FTO and p-CdTe is abrupt with the built in voltage of 0.8 V. The impedance measurements showed that the diode can be electrically represented by a simple RC circuit with ideal capacitor. It was also found that the diode possesses very high series and bulk resistance. The variation of circuit parameters with the applied bias is explained in detail. A new class of filters was designed in which the selection of cut-off frequency as well as band width can be made by controlling the dimensions of CdTe nanostructures. The Schottky junction solar cells of the configuration Al:FTO:p-CdTe:Al were fabricated. The thickness of aluminum was optimized to obtain a power generating device out of developed CdTe layer. The cell generated the Voc of 805 mV, which is the highest till date among the CdTe Schottky junction solar cells. However, the high series resistance and back contact barrier hinder the performance of the solar cell and efficiency of only 0.001 % was recorded. The parameters affecting the cell performance are analyzed by impedance and temperature dependent J-V measurements. The p-n junction solar cells of the configuration Cu:FTO:n-CdS:p-CdTe:Cu were also fabricated. The required thickness of CdS layer for the deposition of stoichiometric CdTe was found to be 30 nm. The solar cell exhibited an overall efficiency of 0.02 % with the Jsc of 500 A/cm2 , Voc of 150 mV and FF of 27 %. The high series and low shunt resistance were found to be main reasons of such low performance. Finally, we have showed a way to efficiently re-use the ionic liquid for the electrodeposition of identical CdTe thin films for at-least four times.