Defect formation and phase stability of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>Cu</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mtext>ZnSnS</mml:mtext></mml:mrow><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>photovoltaic material

First-principles studies of the phase stability of and defect formation in ${\text{Cu}}_{2}{\text{ZnSnS}}_{4}$ (CZTS) are performed. We show that CZTS is the thermodynamically stable phase for a rather small confined domain of chemical potentials. Even slight deviations from the optimal growth conditions will therefore result in the formation of other sulfidic precipitates, including ZnS, ${\text{Cu}}_{2}{\text{SnS}}_{3}$, SnS, ${\text{SnS}}_{2}$, and CuS. In particular, under the prevalent experimental Cu-poor and Zn-rich growth conditions ZnS is the main competing phase. Furthermore, the calculations unambiguously predict that Cu at the Zn site is the most stable defect in the entire stability range of CZTS. This correlates with the experimental observation that CZTS is an intrinsic $p$-type semiconductor.