Ionic charge transfer complex induced visible light harvesting and photocharge generation in perovskite

Organometal trihalide perovskite has recently emerged as a new class of promising material for high efficiency solar cells applications. While excess ions in perovskites are recently getting a great deal of attention, there is so far no clear understanding on both their formation and relating ions interaction to the photocharge generation in perovskite. Herein, we showed that tremendous ions indeed form during the initial stage of perovskite formation when the organic methylammonium halide (MAX^a, X^a = Br and I) meets the inorganic PbX^b₂ (X^b = Cl, Br, I). The strong charge exchanges between the Pb²⁺ cations and X^a- anions result in formation of ionic charge transfer complexes (iCTC). MAX^a parties induce empty valence electronic states within the forbidden bandgap of PbX^b₂. The strong surface dipole provide sufficient driving force for sub-bandgap electron transition with energy identical to the optical bandgap of forming perovskites. Evidences from XPS/UPS and photoluminescence studies showed that the light absorption, exciton dissociation, and photocharge generation of the perovskites are closely related to the strong ionic charge transfer interactions between Pb²⁺ and X^a- ions in the perovskite lattices. Our results shed light on mechanisms of light harvesting and subsequent free carrier generation in perovskites.