The band alignment of bulk Y 2 Ti 2 O 5 S 2 was also calculated using the core and vacuum energies from the (001) bulk-like surface slab using the method developed by Wei and Zunger (see SI for more details) and is shown in Figure Due to the significant anisotropy in the electronic structure, it is expected that charge carrier mobility in the z direction is significantly impeded. On the other hand, the low effective masses of holes and electrons are predicted to allow for sufficiently good in-plane mobility and conductivity. While the effective masses may indicate better p-type performance, the band alignment suggests Y 2 Ti 2 O 5 S 2 favours n-type behaviour.