Double perovskite A2BB'X6, including all-inorganic and hybrid organic-inorganic composition, show great potential applications. The role of A cations (organic molecules or inorganic ions) in the double perovskite is distinct from that in the standard perovskite. Therefore, we carried out systematic analyses of the geometric and electronic structures of Cs2AgBiBr6 and (MA)2AgBiBr6 (MA = CH3NH3) double perovskites. Cs2AgBiBr6 maintains the standard cubic double perovskite lattice. While MA molecules prefer to align in the [110] direction in (MA)2AgBiBr6 and give rise to obvious lattice distortion. The band gap of (MA)2AgBiBr6 is slightly less than that of Cs2AgBiBr6. Because of the spherical or quasi-spherical wave functions of the s/d orbitals, the lattice distortion and the transverse shift between Ag/Bi and Br induced by MA molecules do not change the composition of the band edges. But the complex bonding interactions between MA and the inorganic frameworks make the Ag-Br or Bi-Br bond lengths no longer identical values, so the bond strength and the energy level of each bonding state are dispersed and the band is expanded, which reduces the band gap of the hybrid organic-inorganic double perovskite (MA)2AgBiBr6. Making the role of A cations in the A2BB'X6 double perovskite clear, we could find an excellent double perovskite to put forward their applications.
Keywords: (MA)2AgBiBr6; Cs2AgBiBr6; band gap modulation; bonding effect; double perovskite; solar cell material.