  Cu2ZnSnS4 (CZTS) is a promising photovoltaic absorber material with earth
abundant and nontoxic elements. However, the detrimental native defects and
secondary phases of CSTS 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 (-1-1-2) surface, using
first principles calculations. A surface Zn atom was found to occupy the
subsurface Cu site easily due to a low reaction barrier, which may lead to a
high ZnCu concentration and a secondary phase of ZnS. These n-type defects may
create deep electron traps near the interface and become detrimental to device
performance. To reduce the population of ZnCu and the 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.
