Abstract Organic/inorganic hybrid solar cells are an interesting type of polymer based solar cells, which combine beneficial properties of inorganic semiconductors with them of polymer based materials, in particular, the easy processability on flexible plastic substrates. Herein, we present a method to prepare polymer/copper indium sulfide hybrid solar cells on flexible PET substrates via the metal xanthate route for the in situ preparation of ligand-free inorganic nanocrystals directly in the conjugated polymer matrix. The issue that the temperatures needed for the formation of the inorganic nanoparticles were too high to apply this preparation route on flexible substrates, was solved in this study by adding n-hexylamine to the precursor solution which facilitates the formation of higher crystalline nanoparticles at lower temperatures. n-Hexylamine thereby reacts with the xanthate forming the corresponding O-2,2-dimethylpentan-3-yl-N-hexylthiocarbamate. Following this modified route, flexible hybrid solar cells with power conversion efficiencies of 1.6% could be realized using temperatures not higher than 140 °C in the whole fabrication process. Furthermore, we demonstrate that the metal xanthate route is also well suited for the fabrication of larger area solar cells and present hybrid solar cell modules on glass as well as on flexible PET substrates. In addition, the lifetime of the prepared solar cells was investigated. The devices prepared at low temperature exhibited significantly improved stability compared to devices fabricated at 195 °C.