  The inconclusive results of the previous first-principles studies on the \S3
\ grain boundaries (GBs) in \CISe\ reveal the importance of employing a method
that can correctly describe the electronic structure of this solar-cell
material. We have employed hybrid functional calculations to study the \S3
(112) and \S3 (114) GBs in \CISe\ and \CGSe. The electronic structure changes
introduced by the formation of GBs are threefold: creation of gap states, shift
in band edges, and alteration of bandgap sizes. Gap states commonly behave as
recombination centers, but the band alignment and the change in the bandgap
size induced by GBs mitigate the destructive effect of these states in \CISe.
That means, \S3 \ GBs are not detrimental for the carrier transport in devices
based on \CISe. Conversely, these GBs are destructive for the carrier transport
in \CGSe. The different behaviors of the \S3 \ GBs in CISe and CGSe might be
considered by experimentalists to optimize the device fabrication to achieve
high-performance solar cells.
