Thin-film solar cells have already reached grid parity with conventional fossil fuel energy sources, and slightly undercut traditional crystalline silicon solar cells, however they still have the potential to lower their levelized cost further. Currently, commercial thin film solar cells are still relatively expensive due to rare elemental constituents, high energy intermediate processing & fabrication methods, and scalability limitations. Cu2ZnSnSxSe(4-x) (CZTSSe) has exemplary semiconductor properties for solar energy harvesting; with earth abundant & non-toxic elements, ideal direct bandgap, and potential solution processability, which would significantly lower costs. The commercial viability of CZTSSe however, has been hindered by difficulties associated with phase control, nanocrystal ink additives such as long-chained organics, high temperature cationic disordering, and toxic selenization processes. The work presented in this thesis provides a viable pathway to surmount these barriers, utilizing solution processable Cu2ZnSnS4 (CZTS) & Cu2ZnSnSe4 (CZTSe) nanocrystal inks and an alternative selenization method. A scalable, low temperature, aqueous-based synthetic method was developed to yield stoichiometrically controlled, high-quality CZTS nanocrystal inks stabilized by tin-metal chalcogenide (Sn-MCC) ligands, [Sn2S6]4-, while minimizing organic impurities. This simultaneous redox and nanocrystal formation reaction was optimized using a combination of inductively coupled plasma tandem mass spectroscopy (ICP-MS), Raman spectroscopy, thermogravimetric analysis tandem Fourier transform infrared spectroscopy (TGA-FTIR), synchrotron powder X-ray diffraction (PXRD), in-situ UV-vis spectroscopy, and electron microscopy to assess the stoichiometric, phase, ligand, thermalization, and nanocrystal properties of the nanocrystal inks produced. The substitution of Se for S was trialled for the aqueous-based synthesis by modifying the reducing agent and Sn-MCC ligands, but o