Controlling defects and secondary phases of CZTS by surfactant potassium

$\mathrm{C}{\mathrm{u}}₂\mathrm{ZnSn}{\mathrm{S}}₄$ (CZTS) is a promising photovoltaic absorber material with earth-abundant and nontoxic elements. However, the detrimental native defects and secondary phases of CZTS will largely reduce the energy-conversion efficiencies. To understand the origin of these problems during the growth of CZTS, we investigated the kinetic processes on CZTS $(\overline{1}\overline{1}\overline{2})$ surface, using first-principles calculations. A surface Zn atom was found to occupy the Cu site near the surface easily due to a low reaction barrier, which may lead to a high $\mathrm{Z}{\mathrm{n}}_{\mathrm{Cu}}$ concentration and a Zn-rich secondary phase. These $n$-type defects may create deep electron traps near the interface and become detrimental to device performance. To reduce the population of $\mathrm{Z}{\mathrm{n}}_{\mathrm{Cu}}$ and the Zn-rich secondary phase, we propose to use K as a surfactant to alter surface kinetic processes. Improvements on crystal quality and device performance based on this surfactant are consistent with early experimental observations.