The UK has set a legally binding target of net carbon zero by 2050 in every aspect of the economy and delivering sustainable and green solar electricity generation will be crucial in this transformation. Crystalline Si currently dominates the solar industry, representing approximately 95 % of the solar modules sold today. However, due to its low photon absorption coefficient and indirect bandgap, thicknesses up to 350 m are needed, making crystalline Si unsuitable for thin-film applications. Thin-film photovoltaic (PV) materials on flexible substrates will allow for the integration of solar energy conversion in buildings and infrastructure in urban environments, which is a key area of expansion in solar energy to meet required environmental targets. Cu2ZnSn(S,Se)4 (CZTSSe) is a non-toxic, inorganic thin-film material with desirable PV properties due to its tuneable direct bandgap between 1.0 - 1.5 eV and 20 % theoretical power conversion efficiency (PCE) under AM1.5G illumination. However, CZTSSe is currently limited by significant voltage losses, which have been linked to structural disorder including antisites defects, secondary phases, and point defect clusters, hindering the PCE. This PhD is part of a collaboration between the universities of Bristol, Northumbria and Loughborough on the SolPV project, funded by EPSRC, and industrial partners of Centre of Process Innovation, Johnson Matthey, BAE systems and M-Solv. The main purpose of this PhD is to perform interfacial engineering of the p-n junction between the CZTSSe absorber layer and buffer layer, using atomic layer deposition, to achieve record breaking efficiencies of greater than 15 %. CdS has been extensively used as a buffer layer for CZTSSe solar devices as it is an established buffer material for the most advanced technology of CuInxGa1-x(S,Se)2 (CIGS). Although CdS has worked as an efficient buffer layer in both CZTSSe and CIGS, Cd is toxic and cannot be released into the environment, therefore, a suitable replacement is required. In- and Zn-based oxysulfide alternatives will be investigated as possible replacements. Another aim of the project is to control crystallisation and composition of the absorber material and reduce the formation of detrimental defects, deep Sn trap states and carbon impurities by optimising the fabrication methodology. Bulk and surface composition and structure of the thin-films will be examined by electron microscopy techniques, Raman microscopy and Energy-Filtered Photoemission electron microscopy (EF-PEEM). EF-PEEM can provide surface electronic mapping of local effective work functions at the absorber-buffer junction, allowing for unique and vital electronic information. The focus of this research will be to confirm the PCE of CZTSSe solar cells will go beyond 15 %, while also using scalable manufacturing routes to ensure a smooth transition into industrial applications. This project falls within the EPSRC 'Solar Technology' research area.