With the aim of exploring oxidation and selenization of the photovoltaic
material Cu2ZnSnS4,we used first principles
methods to study the
structure and stability of Cu2ZnSnSxO4-x
and
Cu2ZnSnSxSe4-x
alloys for 0 ≤ x ≤ 4. Pure Cu2ZnSnO4 was found
to have the lowest heat of formation,
followed by
Cu2ZnSnS4, and finally
Cu2ZnSnSe4. This suggests that oxidization is very
likely to occur, whereas selenization can only be accomplished under
high temperature. For the alloys, the energetically favorable chalcogen
configurations are very different for oxygen and selenium. While the
energies of the selenium alloys are insensitive to the distribution of S
and Se configurations, the lowest energy oxygen alloys have alternating S
and O sites in the a-b planes. In considering the heats of formation of
the Cu2ZnSnSxO4-x alloys,
we find that they are unstable with respect to
decomposition into binary oxides and sulfdes except for small
concentrations of O. Our results also show that it is energetically more
favorable to sulfurize Cu2ZnSnSe4
than to selenize Cu2ZnSnS4.