Finally, we model the interface between the g-C3N4 and Cs2AgBiCl6 to gain insight into the structural and electronic modifications upon interface formation. The lattice constants of g-C3N4 and Cs2AgBiCl6 nicely match along the a direction, while the b direction shows substantial differences in the lattice constants of 13%. Thus, we cut the g-C3N4 layer along the b-direction and passivate undercoordinated N atoms with H atoms, see Figure . This ensures maximal coverage without introducing artificial strain into the g-C3N4. Considering the electronic properties, we obtain a band gap of 3.22 eV for the modified g-C3N4, which is in excellent agreement with the fully periodic g-C3N4 monolayer (3.4 eV). We passivated both surfaces in the perovskite slab with the modified g-C3N4 layers. We initially performed single point calculations at different distances between the Cs2AgBiCl6 and the g-C3N4, as well as for different horizontal alignment for g-C3N4. Starting from the low energy configuration, geometry optimization was performed, resulting in a strongly bound heterostructure with a binding energy of -4.22 eV/nm 2 . Looking closer at the structure shows that each undercoordinated surface metal ion is successfully passivated (see Figure and Figure of the manuscript). The DOS shown in Figure shows a type II band alignment with the VB and CB of g-C3N4 below the ones of the perovskite layer.