Earth abundant copper-zinc-tin chalcogenide (CZTSSe) is an emerging material for the development of low cost and sustainable thin film solar cells (TFSCs). A low cost, green, and up-scalable approach to the fabrication of TFSCs through a sputtering process is the main challenge to achieve high efficiency for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells for industrialization. Based on a closed isothermal chamber annealing system, we could precisely calculate and control the chalcogenide partial vapour pressure during the annealing process. We designed, developed, and optimized an environmentally friendly strategy to synthesise a high quality CZTSSe absorber thin film and to fabricate a solar cell without using toxic H2Se and H2S gases as the Se and S sources or any other volatile compounds (SnS and Sn), and a chalcogenide diffusion barrier layer. We fabricated a CZTSSe TFSC with 9.24% efficiency, which is the highest performance for sputtering processed CZTSSe TFSC prepared without using toxic gases and additional processes. Based on this green strategy, we also fabricated the integrated submodule using CZTSSe absorber layers with efficiencies as high as η = 2.76% with eight interconnected cells (active area of 22.4 cm2). Our studies on this green synthesis strategy for CZTSSe solar cells could introduce a possible pathway to green fabrication for the low cost and highly efficient TFSC industrialization field.