In the development of low-cost, efficient, and environmentally friendly thin-film solar cells (TFSCs), the search continues for a suitable inorganic colloidal nanocrystal (NC) ink that can be easily used in scalable coating/printing processes. In this work, we first report on the colloidal synthesis of pure wurtzite (WZ) Cu2SnS3 (CTS) NCs using a polyol-mediated hot injection route, which is a nontoxic synthesis method. The synthesized material exhibits a random distribution of CTS nanoflakes with an average lateral dimension of ∼94 ± 15 nm. We also demonstrate that CTS NC ink can be used to fabricate low-cost and environmentally friendly TFSCs through an ethanol-based ink process. The annealing of as-deposited CTS films was performed under different S vapor pressures in a graphite box (volume; 12.3 cm3), at 580 °C for 10 min using a rapid thermal annealing (RTA) process. A comparative study on the performances of the solar cells with CTS absorber layers annealed under different S vapor pressures was conducted. The device derived from the CTS absorber annealed at 350 Torr of S vapor pressure showed the best conversion efficiency 2.77%, which is the first notable efficiency for an CTS NCs ink-based TFSC. In addition, CTS TFSCs performance degraded only slightly after 50 days in air atmosphere and under damp heating at 90 °C for 50 h, indicating their good stability. These results confirm that WZ CTS NCs may be very attractive and interesting light-absorbing materials for fabricating efficient solar-harvesting devices.