  A variety of organic-inorganic hybrid perovskites (APbX3) consisting of mixed
center cations [A = CH3NH3+, HC(NH2)2+, Cs+] with different PbX3- cages (X = I,
Br, Cl) have been developed to realize high-efficiency solar cells.
Nevertheless, clear understanding for the effects of A and X on the optical
transition has been lacking. Here, we present universal rules that allow the
unified interpretation of the optical absorption in various hybrid perovskites.
In particular, we find that the influence of the A-site cation on the light
absorption is rather significant and the absorption coefficient (alpha) reduces
to half when CH3NH3+ is replaced with HC(NH2)2+ in the APbI3 system. Our
density functional theory (DFT) calculations reproduce all of the fine
absorption features observed in HC(NH2)2PbI3 and CH3NH3PbBr3, allowing the
unique assignment of the interband transitions in the Brillouin zone. In
contrast to general understanding that the A-site cation involves weakly in the
optical process, our theoretical calculations reveal that the center cation
plays a critical role in the interband transition and the absorption strength
in the visible region is modified by the strong A-X interaction. Furthermore,
our systematic analyses show that the variation of the absorption spectrum with
X can be described simply by the well-known sum rule. The universal rules
established in this study explain the large reduction of alpha in HC(NH2)2PbI3
and predict CsPbI3 as the highest alpha material.
