  Although new photovoltaic materials, such as organic perovskites, have
attracted much attention recently,\cite{perov1,perov2,perov3} zinc-blende
semiconductors still play an important role, owing to their high efficiency and
stability in real environments. For example, Cu$_2$ZnSnSe$_4$ \cite{AM2013}
(CZTS) and Cu(In,Ga)Se$_2$ \cite{JACS2008} (CIGS), which are both based on
chalcopyrite CuGa$X_2$ ($X=$ S, Se), have been considered as potential
candidates for photovoltaics. It has been reported recently that the efficiency
of CIGS has gone beyond 20\%, which is close to that of polycrystalline s
ilicon.\cite{efficiency2014} Furthermore, photovoltaic devices related to CZTS
have gained attention because they solely contain abundant and nontoxic
materials.\cite{AM2013,CZTS}
