Earth abundant Cu2ZnSn(S,Se)4 (CZTSSe) has been considered as one of the most promising thin film
solar cell absorber candidates. Here, we develop a facile water-based precursor solution approach for
depositing high-efficiency Cu2ZnSn(S,Se)4 thin film solar cells. In this environmentally friendly approach,
inexpensive elemental Cu, Zn, Sn and S powders are used as the starting materials and are dissolved in
the aqueous solution of thioglycolic acid and methylamine, forming a homogeneous precursor solution
for depositing Cu2ZnSnS4 nanocrystal thin film. As-deposited CZTS nanocrystal thin films are selenized to
form the large-grain CZTSSe absorber layers. It was found that Na doping plays an important role in the
formation of the extremely dense and flat CZTSSe absorber layer, and fill factor can be significantly
improved for Na-doped CZTSSe solar cells, which lead to a photoelectric conversion efficiency of 6.96%
with an open-circuit voltage of 378 mV, a short current density of 28.17 mA cm2, and a fill factor of
65.4%.