Synthesis and characterization of lead-free cesium halide perovskite nanocrystals and their application in Schottky diodes

Akinbami, Olusola Moses
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The need for renewable energy sources cannot be overemphasized. Global warming, climate change, environmental pollution and fluctuations in crude oil prices are some of the disadvantages of utilizing non-renewable energy sources. Hence, in recent times, research has been geared towards transitioning from non-renewable energy sources to renewable energy sources. Solar energy due to its abundance, reliability and environmental friendliness is one of the renewable energy sources being explored globally. Photovoltaic cells are among the most promising technologies for clean energy production. Among the different solar cell technologies, the perovskite solar cells (PSCs) have the greatest potential due to their outstanding power conversion efficiency and their ease of fabrication. However, incorporation of lead (Pb) in the perovskite’s crystal structure has led to some disadvantages such as toxicity and stability issues in perovskites. As a result, research is ongoing on various ways of overcoming perovskite’s toxicity while improving their stability in ambient conditions. In this study, to eliminate Pb toxicity, tin (Sn), bismuth (Bi) and zinc (Zn) were used as Pb replacements in a perovskite semiconductor material with an attempt to lower toxicity and improve stability. The Pb-based perovskites are the most efficient perovskites however, Pb is toxic to humans and Pb-based perovskites are unstable, hence alternative perovskites possessing high stability and low toxicity are required for the commercialization of perovskite solar cells. Sn and Pb belong to the same group in the periodic table while also having similar ionic radii. Bi shares the same period with Pb while Zn is non-toxic and an essential mineral element. Therefore, the three cations are investigated as promising perovskite candidates. The colloidal method of synthesis was employed because of its flexibility and allowance of reaction parameter variation. Therefore, the reaction temperature and time were varied to investigate the optimum conditions for the synthesis of CsSnBr3, Cs3Bi2Br9 and Cs2ZnBr4 perovskite nanocrystals (NCs). Both temperature and time had an effect on the resultant particles. Cubic CsSnBr3 nanocrystals were successfully synthesized at the optimum conditions of 230 °C for 1 min in oleylamine (OLA) and oleic acid (OA) using SnBr2 and Cs2CO3 precursors. The CsSnBr3 NCs produced quantum dots morphology. Hexagonal Cs3Bi2Br9 NCs were synthesized at optimum conditions of 210 °C while orthorhombic Cs2ZnBr4 hybrid perovskite was formed at 160 °C, both using 1 min synthetic time. Pseudo spherical and nanorod morphologies were obtained for the Cs3Bi2Br9 and Cs2ZnBr4 NCs, respectively. The bandgaps were found to be 1.72 eV, 2.82 eV and 3.18 eV for CsSnBr3, Cs3Bi2Br9 and Cs2ZnBr4, iv respectively. The XRD and XPS showed the composition of the individual perovskite while NMR and FTIR conclusively showed that OLA successfully capped the NCs surface. The CsSnBr3 NCs were stable for three days while the Cs2ZnBr4 and Cs3Bi2Br9 were stable for over 21 days. The electrical properties of the Sn, Bi and Zn based perovskites were investigated by fabricating Schottky diodes. The thin-films were fabricated via spin-coating method and the gold (Au) contacts were thermally deposited on the thin-films. The three perovskite materials showed rectifying properties while the parameters of the diode were acquired using three different methods namely: Classical thermionic model, Cheung’s, and the Norde’s method. The Cs2ZnBr4 material showed the best electrical properties due to its lowest series resistance and impressive barrier height while both the CsSnBr3 and Cs3Bi2Br9 perovskite NCs had electrical parameters that are comparable to the ones reported in the literature. As such, CsSnBr3, Cs3Bi2Br9 and Cs2ZnBr4 are potential perovskites for solar cell applications and can be used as hole-transporting layer (wide bandgap Cs3Bi2Br9 and Cs2ZnBr4) or the active layer (CsSnBr3).
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science in Chemistry to the Faculty of Science, School of Chemistry, University of the Witwatersrand, Johannesburg, 2021