Characteristics of Nanocrystallite-CdS Produced by Low-Cost Electrochemical Technique for Thin Film Photovoltaic Application: The Influence of Deposition Voltage

Hindawi International Journal of Photoenergy Volume 2017, Article ID 3989432, 13 pages https://doi.org/10.1155/2017/3989432 Research Article Characteristics of Nanocrystallite-CdS Produced by Low-Cost Electrochemical Technique for Thin Film Photovoltaic Application: The Influence of Deposition Voltage Obi Kingsley Echendu,1,2 Francis Birhanu Dejene,1 Imyhamy Mudiy Dharmadasa,2 and Francis Chukwuemeka Eze3 1 Department of Physics, Qwa Qwa Campus, University of the Free State, X13, Phuthaditjhaba 9866, South Africa Electronic Materials and Sensors Group, Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, UK 3 Department of Physics, Federal University of Technology, PMB 1526, Owerri, Nigeria 2 Correspondence should be addressed to Obi Kingsley Echendu; Received 12 April 2017; Accepted 14 September 2017; Published 2 November 2017 Academic Editor: Prakash Basnyat Copyright © 2017 Obi Kingsley Echendu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Electrochemical deposition and characterization of nanocrystallite-CdS thin films for thin film solar cell application are reported. The two-electrode system used provides a relatively simple and cost-effective approach for large-scale deposition of semiconductors for solar cell and other optoelectronic device application. Five CdS thin films were deposited for 45 minutes each at different cathodic deposition voltages in order to study their properties. X-ray diffraction study reveals that the as-deposited films contain mixed phases of hexagonal and cubic CdS crystallites with large amounts of internal strain and dislocation density. Postdeposition annealing results in phase transformation which leaves the films with only the hexagonal crystal phase and reduced strain and dislocation density while increasing the crystallite sizes from 21.0–42.0 nm to 31.2–63.0 nm. Photoelectrochemical cell study shows that all the CdS films have n-type electrical conductivity. Optical characterization reveals that all samples show similar transmittance and absorbance responses with the transmittance slightly increasing towards higher growth voltages. All the annealed films show energy bandgap of 2.42 eV. Scanning electron microscopy and energy dispersive X-ray analyses show that grains on the surface of the films tend to get cemented together after annealing with prior CdCl2 treatment while all the films are S-rich. 1. Introduction CdS is a wide bandgap II–VI compound semiconductor with a direct bulk bandgap of 2.42 eV [1]. Due to its desirable properties, it finds use in photovoltaic solar cells [2, 3], piezo transducers [4], photoresistors, luminescence devices [5], Schottky diodes and metal-semiconductor field effect transistors (MESFETs) [6], heterojunction diodes [7], insulated gate thin film transistors [8], and gas sensors [9]. It is also used in microelectronics, nonlinear optics, catalysis, photoelectrochemistry [10], and in photodetectors [11]. In its application in photovoltaics, CdS has been used as an n-type heterojunction partner to CdTe, CuS, and Cu(In, Ga)Se2 (CIGS) absorber materials for the fabrication of CdS/CdTe, CdS/ CuS, and CdS/CIGS solar cells [12-14]. Several growth techniques have been used for the deposition of CdS for various uses. These techniques include chemical bath deposition (CBD) [15], vacuum evaporation, chemical vapour deposition [16], spray pyrolysis [17], sputtering [18], screen printing [19], sol-gel [20], close space sublimation (CSS) [21], and electrodeposition (ED) [22]. As is common in most electrodeposition processes for semiconductors in the past, electrodeposition of CdS has always been reported in the literature based on the conventional threeelectrode configuration [23]. Report on the use of twoelectrode system for the electrodeposition of CdS is very 2 scarce in the literature, and one can only find two major reports to date [22, 24]. Diso et al. [22] grew their CdS materials using CdCl2 and Na2S2O3 as precursors but at a pH of 1.4 and temperature of 45°C for fear of precipitation. Both their reported as-deposited and heat-treated CdS films had hexagonal crystal structure without any phase transformation as a result of postdeposition heat treatment. Abdul-Manaf et al. [24] on the other hand used CdCl2 and (NH4)2S2O3 as precursors but grew CdS at a pH of 2.0 and a temperature of 85°C. The as-deposited CdS materials in this case had a mixture of hexagonal CdS, cubic CdS, and orthorhombic sulphur crystal phases which transformed to purely hexagonal phase after postgrowth heat treatment. Both Diso et al. and Abdul-Manaf et al. did not report the postdeposition heat treatment with prior CdCl2 treatment. Again, both groups did not report morphological characterization of CdS films grown at different growth voltages but only reported the morphological characterization of CdS grown at a particular voltage and annealed under different temperature conditions. Under the low temperature (45°C) and low pH (1.4) used by Diso et al., they did not observe mixed phases of CdS. It is also noted that these temperature and pH are substantially far away from those (>80°C and ~2.00, resp.) under which CdTe is usually grown for CdS/CdTe solar cell fabrication [2, 25]. However, Abdul-Manaf et al. who grew CdS at relatively higher temperature of 85°C and pH of 2.00 reported the presence of mixed cubic and hexagonal phases of CdS with the presence of orthorhombic sulphur all in their asdeposited CdS materials. These mixed phases transformed to purely hexagonal phase after postgrowth heat treatment. Then, in order to see whether the presence of these mixed phases is a function of growth temperature and pH or a function of the sulphur precursor, authors decided to grow CdS in the present work at a temperature of 80°C and pH of 1.80, both of which are in-between the values used by Diso et al. and Abdul-Manaf et al. These growth parameter values are still very close to those for the growth of CdTe on CdS for solar cell fabrication. We have also used Na2S2O3 as the sulphur precursor since Abdul-Manaf et al. have observed mixed phases with (NH4)2S2O3 as sulphur precursor. In the present work, CdCl2 and Na2S2O3 have been used as precursors and the CdS materials were grown at a pH of 1.8 and temperature of 80°C. The major reasons for the use of two-electrode system include the following: (i) to prevent any possible contamination of the deposition electrolyte by ions such as Ag+ and K+ which may eventually leak into the deposition electrolyte from the porous wick of the commonly used Ag/AgCl and Hg/HgCl (saturated calomel electrode (SCE)) reference electrodes during the electrodeposition process at elevated temperatures and (ii) to reduce the cost due to reference electrode, as reported in recent publications by the main author’s researc (...truncated)