Development and studies on the structural, optical and electrical properties of sprayed CZTS and SnS thin films for solar cell application

Abstract

Cu2ZnSnS4 and SnS compound semiconductors are emerging as an alternative absorber materials for single junction heterojunction thin film solar cell. Both materials having the favorable electrical and optical properties for photovoltaic application. These properties can be modified by changing the non-stoichiometry of these materials. All the constituent elements of CZTS and SnS are non-toxic and abundant to generate the 20 TW of non-CO2 energy to stabilize the CO2 in the atmosphere. Recently, the photo-conversion efficiency for the CZTS and SnS solar cells were obtained of 8.4% and 2.4%, respectively. To produce the solar grade CZTS and SnS using the non-vacuum process such as pray pyrolysis is highly required to reduce the production cost and increase the throughput. Usually, the material produced and deposited by spray pyrolysis process contains the defects and impurities which influences over the material process. How these impurities or defects can be minimize by optimizing the processing parameters need to be address to fabricate the efficient solar cell with large area and low cost. In this thesis, the development of earth abundant chalcogenide thin films by chemical spray pyrolysis (CSP) for solar cells and their optimization were studied, systematically. The Cu2ZnSnS4 (CZTS) and SnS chalcogenide materials were fabricated and studied by CSP for absorber layer in the thin film solar cell. Materials characterization of the fabricated films on the glass substrates were done by x-ray diffraction, scanning electron microscopy, energy dispersion spectroscopy, UV-vis spectroscopy, hot point probe and hall effect measurement techniques. The tubular quartz vacuum furnace, probe station, samples holders, shadow masks and LED light sources were developed indigenously for assisting the fabrication and characterization experiments. The pure aqueous solution and ambient air as transport medium was used though out the thesis work (for all the materials). The effect of the molar concentration of the precursor salts on the materials (CZTS and SnS) properties was studied and optimized in order to make them suitable for photovoltaic applications. As buffer layer, CdS, CdO, ZnS, SnS2, In2S3, ZnO and SnO2 compounds were identified and applied for making the heterojunction with CZTS or SnS from case to case. The CZTS thin film solar cells were fabricated in the superstrate configuration entirely by CSP process, where CdS as buffer layer and FTO coated glass as substrates were utilized. Comprehensive study on the developed device was performed, where diode properties and minority carrier lifetime was reported. Traditional model of CZTS thin film solar cell was developed for optimizing the material parameters for enhancing the conversion efficiency. One dimensional simulation approach was adopted and results were analyzed. The SnS thin film solar cells were fabricated with previous configuration. The effect of the SnS layer thickness on the device performance was studied and optimized. The I −V characteristics, C−V characteristics and open circuit voltage decay (OCVD) techniques were implemented for developed SnS solar cell. For improving the material properties of as sprayed SnS films, post annealing process was carried out. Photo-electrochemical cell study was carried out to test the photo response of large number of the CZTS and SnS absorber layers grown before they were made junction and other metal contacts. Effect of precursor concentration (Sn/S) ratio on the opto-electronic properties of SnS films was studied by PEC cell. The optimum precursor ratio of 1:1.3 (Sn:S) was identified for FTO substrate. Comprehensive review of the SnS thin film solar cell was done and major performance limiting parameters were identified. The conductance band offset and series resistance were identified crucial parameters to dictate the SnS solar cell performance. We have tried to improve the device performance by decreasing the conduction band offset and series resistance. For this In2S3 films were developed. Superstrate structure of Glass/FTO/In2S3/SnS/Graphite was developed and studied. A replacement of CdS by In2S3 for SnS solar cell was found to improve its performance by minimizing the CBO. Effect of SnS layer thickness was studied. Back contact metal selection was crucial and for this Cu and Graphite by virtue of their work function were chosen. Device was studied using J −V characteristics, C −V characteristics, impedance spectroscopy and open circuit voltage decay techniques. The effect of doping in the sprayed SnS thin film was also investigated. Thermally evaporated Cu was used as source of dopant. XRD and UV-vis was carried out. Reproducibility of the device was confirmed by fabricating the 45 numbers of devices. Based on the investigated properties of CZTS and SnS materials, traditional one dimensional model of solid state device of SLG/Mo/CZTS or SnS/CdS/ZnO/Al:ZnO configuration were studied. The material properties of CZTS and SnS materials such as acceptor carrier concentration, thickness, back metal contact has been optimized for better device performance under AM1.5 illumination. Results show that the photovoltaic conversion efficiency of 13.4% and 6.1% can be possible for CZTS and SnS thin film solar cells, respectively.