  Cu2ZnSnS4 (CZTS) is a promising thin-film solar-cell material consisted of
earth abundant and nontoxic elements. Yet, there exists a fundamental bottle
neck that hinders the performance of the device due to complexed intrinsic
defects properties and detrimental secondary phases. Recently, it was proven
experimentally that Na and K in co-evaporation growth of CZTS can enlarge the
grain size and suppress formation of ZnS secondary phase near surface, but the
reasons are not well understood. We used first principle calculations to
investigate the kinetic processes on CZTS (1-1-2-) surface involving Group I
elements, including Na, K, and Cs, to demonstrate their surfactant effects.
Both the structure of the reconstructed surfaces involving Group I elements and
various diffusion paths of a Zn ad-atom in these reconstructed surfaces were
explored. The advantages and concerns of the surfactant effects of Na, K, Cs,
were systematically compared and discussed. Although Group I elements protect
Cu sites on the subsurface layer, a disordered metastable configuration with a
diffusion barrier of about 400meV was found. Therefore, a precise control of
growth condition is essential to avoid the metastable phase. In addition, our
studies provide a systematical design principle for surfactant effects during
the growth.
