Optical and Morphological Properties of Copper Indium Disulphide Quantum Dots

Quantum dots have emerged as highly promising candidates as the next generation of photovoltaic material due to their tuneable optical properties, carrier-multiplication effects and long exciton recombination times. Recently, they have also exhibited their commercial viability as they can be fabricated using cheap and non-toxic precursors with facile and scalable production methods. One material that has displayed these features but yet to be fully understood is CuInS2. For QD materials including CuInS2, the surface chemistry plays an integral role in the properties of the quantum dots. Particularly, the presence of surface defects (or trap states) can result in non-radiative emission. Colloidal QDs were synthesised with a surrounding layer of organic molecules, which partially passivate surface defects and solubilise nanocrystals (NCs) in a solvent. A widely used approach for further passivation of the surface states is to surround the NC with an inorganic shell. The material used for passivation could have a wider band gap and comparable lattice constant. The inorganic shell also provides protection from photodegradation. Therefore, this work involves synthesising colloidal CuInS2 QDs using a solvothermal method and investigating the effect of the inorganic shell and organic ligands on their properties. In this work, optical and morphological properties of CuInS2 quantum dots were explored. Measurements of the PL (peak position, intensity and FWHM) and absorption were performed to investigate the effect of QD morphology and compare their properties. Surface passivation with ZnS resulted in a blue-shift of PL emission, possibly indicating the presence of the inorganic shell lead to an increase in the strength of carrier confinement, which is consistent with the alignment of their band structure. Conversely, the ligand replacement with shorter carbon-chain molecules lead to red-shift of the PL emission, indicating weaker confinement. Time-resolved PL measurements were also conducted which indicated the photostability of the quantum dots under ultraviolet light; photodegradation under the solar spectrum would prohibit the use of a material as a photovoltaic. In addition, time-resolved photoluminescence measurements allowed for determination of the lifetime of the exciton, from which the presence of defect states could be inferred; more defect states would provide more recombination pathways and thus a faster recombination rate, which is detrimental to photovoltaic applications. Morphological properties such as the size of the QDs were estimated using dynamic light scattering and scanning electron microscopy probed the surface of the QD. NMR was conducted in order to confirm the success of the ligand exchange