We propose to develop cation (Ag+, Ba2+, Mn2+, Cd2+, Ge4+, Ti4+) substituted and gradient Cu2ZnSnS4 (CZTS) based absorbers to demonstrate >10% solar cell conversion efficiency by reducing the open-circuit voltage deficit, while retaining high current density. This project will be developed at two stages proposing a collaboration between Nanyang Technological University (NTU) and University of Luxembourg (UL). At the first stage, identifies suitable cations to substitute in CZTS absorber to tailor desired electrical and optical properties, and study their effect on solar cell efficiencies. During the second stage, the most promising cation substitutions will be explored further to develop a band gap gradient absorber to influence the valence and conduction band, and carrier concentration at the p-n junction to achieve high solar cell efficiencies. Further, various electrical and optical measurement techniques will be applied on these solar cells/absorbers to study quasi-Fermi level splitting, defect properties including the carrier density, spatial-doping distribution, and light-induced metastable effects. This project also intends to explore the viability of applying various buffer layers, and transparent conductive oxides developed at UL. This project will bring about the expertise of the partner in terms of solar cell device fabrication (NTU), and electrical and optical studies by UL. This project proposal is requesting one full time Post-doctoral fellow under a 24-month contract.