  Aqueous precursors provide an alluring approach for low-cost and
environmentally friendly production of earth-abundant Cu2ZnSn(S,Se)4 (CZTSSe)
solar cells. The key is to find an appropriate molecular agent to prepare a
stable solution and optimize the coordination structure to facilitate the
subsequent crystallization process. Herein, we introduce thioglycolic acid,
which possesses strong coordination (-SH) and hydrophilic (-COOH) groups, as
the agent and use deprotonation to regulate the coordination competition within
the aqueous solution. Ultimately, metal cations are adequately coordinated with
thiolate anions, and carboxylate anions are released to become hydrated to form
an ultrastable aqueous solution. These factors have contributed to achieving
CZTSSe solar cells with efficiency of as high as 12.2% (a certified efficiency
of 12.0%) and providing an extremely wide time window for precursor storage and
usage. This work represents significant progress in the non-toxic solution
fabrication of CZTSSe solar cells and holds great potential for the development
of CZTSSe and other metal sulfide solar cells.
