Nanotechnology for energy conversion and storage: synthesis and processing of metal sulfide nanoparticles for thin film solar cells and lithium Ion batteries

This thesis describes work carried out on the synthesis and processing of metal sulfide nanoparticles, taking advantage of distinct nano-characteristics and low-cost processing techniques to fabricate high performance lithium-ion batteries and thin film solar cells. Chapter 3 describes the synthesis and electrophoretic deposition of CZTS nanorods, and their assessment as next generation lithium-ion battery anodes in both half and full cells. This scalable deposition technique enables the formation of dense films of aligned nanorod arrays which demonstrate lithiation capacities far in excess of commercially-used graphite. Chapter 4 describes the use of these CZTS anodes as a model system with which to analyse and compare modes of lithiation in a multi-mode (combined conversion and alloying) material. This study demonstrates the inadequate nature of common testing protocols in the analysis of this promising form of material, due to the presence of multiple lithiation processes over a wide voltage range. New protocols are prescribed which will enable researchers to perform more meaningful material assessments. Chapter 5 describes the extension of the electrophoretic deposition method to tin sulfide nanocubes. The fabricated electrodes possess a high lithiation capacity at an anode-suitable voltage. Notably, capacity retention was maximized when cycling was limited to the alloying process alone. This finding determines the optimum method of utilising the well reported, but rarely examined, beneficial buffering effect of Li2S for sulfide-based anodes. Chapter 6 reports on the fabrication of two forms of CZTS solar cell, utilising high and low-temperature processing, demonstrating their suitability and flexibility for this application. A record efficiency for a low temperature fabricated CZTS cell, in addition to the first use of CZTS in the advantageous wurtzite phase in the cell, are reported. Chapter 7 describes a detailed study on the mechanisms of selenization, a key step in the fabrication of high performance CZTSSe solar cells, through ex-situ SEM, XRD and Raman analysis. Ligand modifications techniques are used to demonstrate morphological control and design optimal conditions for desirable large grain growth.