Experimental Insights of CZTS Thin Film as a Photovoltaic Absorber Deposited by RF Magnetron Sputtering and Spin Coating

Abstract

This thesis investigates the impact of deposition procedures on the properties of Cu2ZnSnS4 (CZTS) thin films and the resultant characteristics of CZTS thin film solar cells. The present study investigated CZTS absorber layer deposited by two distinct techniques namely RF magnetron sputtering utilizing an off-stoichiometric single quaternary CZTS target and sol-gel spin coating. The focus of investigation pertains to buffer materials that are free from Cd and have the potential to improve band alignment. The present study also investigates the potential of ZnO:Ga (GZO) as a transparent conducting oxide (TCO) layer for CZTS solar cell applications as an alternative to the conventional ZnO:Al (AZO). Because of its longevity, microelectronic compatibility, and efficiency, Si-based PV technology dominates. Si with low indirect band gap absorption and sophisticated production methods like high temperature treatment and ion implantation are bottlenecks. Chalcogenide-based thin film PV technologies may lower PV costs. Because of their availability and non-toxicity, kesterite semiconductors containing copper, zinc, tin, sulfur, and selenium are attractive alternatives to CdTe and CIGS. Because of its 1.5 eV direct band gap, pure sulfide CZTS seems promising among the three kesterites. Vacuum-based sputtering produces clean, homogenous kesterite thin films, whereas non-vacuum spin-coating is appealing for component control and large-scale manufacturing. Irrespective of the two fabrication processes described in this thesis, in order to explore microstructural, morphological, optical, electrical, chemical oxidation states, and photovoltaic properties, numerous characterization tools were used at the film and device levels. These are X-ray Diffractometry (XRD), Raman spectroscopy, Field Emission Scanning Electron Microscopy (FESEM), 3D profilometry, Ultraviolet-Visible Near Infra Red (UV-Vis NIR) spectroscopy, X-Ray Photoelectron Spectroscopy (XPS), Hall-effect measurements, and I-V characteristics. Some numerical studies were carried out using SCAPS-1D simulation tools in accordance with experimental results to validate them. The process of sputtering thin films has traditionally included a two-step approach. The use of elemental sulfur in conjunction with vacuum deposition for the purpose of Rapid Thermal Annealing (RTA). Firstly, RF magnetron sputtered CZTS thin films with a single off-stoichiometric quaternary target were examined for substrate temperature effects. It was found that, in-situ deposition may potentially replace the necessity of a high-temperature annealing stage. Research conducted on the literature pertaining to the alloying of CZTS with various elements such as Ag/Cu, Cd/Zn, Cd/Mg, and Ge/Sn has demonstrated the potential to enhance device efficiencies through the manipulation of band gap, control of defect size, mitigation of unwanted secondary phases, and modulation of carrier concentration. However, despite these advancements, the efficiencies achieved by these alloyed devices have not yet surpassed those of the devices currently in production. Previous research have successfully adjusted the partial replacement of Zn cation with Cd, leading to enhanced power conversion efficiency (PCE). Secondly, Ge-alloyed CZTS samples were fabricated and examined. This work hypothesized that Ge-alloying, in which Sn is partly replaced by Ge atoms, would minimize band gap fluctuations and band tailing, reducing the open-circuit voltage deficit and improving device performance. It was found that, alloyed films with G = 0.38 exhibited better morphology, crystallite size, microstrain, and dislocation density. Thirdly, a novel co-doped (Cd,Mg) CZTS thin films and superstrate structured thin film solar cells were fabricated and evaluated. Co-doped CZTS films and standard CdS heterointerface band alignments have been investigated. Numerical simulation verified both tasks. This task was predicted to lower the Cd content in the doped sample by adding Mg without compromising the benefits of Cd alone. Cadmium (Cd) and Magnesium (Mg) exhibit partial isoelectronic substitution at the Zn-site in kesterite CZTS. PCE increases significantly with 40% partial cation substitution of Zn by Cd and Mg. +>2 deficit was reduced by 23% and "cliff-like" CBO with a minimum energy of 0.12 eV was detected. In practical applications and in various academic papers, it is often observed that there is a discrepancy in the conduction band offsets between the absorber and buffer layers. Fourthly, in a study of the potential substitution of the traditional CdS buffer layer by ZnS fabricated by SILAR, results suggested that favorable "spike-like" CBO could be achieved. Finally, an alternative to the AZO window layer, GZO, was proposed, and encouraging results were found in film transmittance and resistivity.