title: Ionic Charge Transfer Complex Induced Visible Light Harvesting and Photocharge Generation in Perovskite.
abstract: 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 (MAXa, Xa=Br and I) meets the inorganic PbXb2 (Xb=Cl, Br, I). The strong charge exchanges between the Pb2+ cations and Xa- anions result in formation of ionic charge transfer complexes (iCTC). MAXa parties induce empty valence electronic states within the forbidden bandgap of PbXb2. 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 Xa- ions in the perovskite lattices. Our results shed light on mechanisms of light harvesting and subsequent free carrier generation in perovskites.
authors: Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.|City University of Hong Kong Shenzhen Research Institute , Shenzhen 518057, Guangdong, People's Republic of China.
authors: Tsz-Wai
authors: TW
authors: Ng
authors: Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.
authors: Hrisheekesh
authors: H
authors: Thachoth Chandran
authors: Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.
authors: Chiu-Yee
authors: CY
authors: Chan
authors: Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.|City University of Hong Kong Shenzhen Research Institute , Shenzhen 518057, Guangdong, People's Republic of China.
authors: Ming-Fai
authors: MF
authors: Lo
authors: Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.|City University of Hong Kong Shenzhen Research Institute , Shenzhen 518057, Guangdong, People's Republic of China.
authors: Chun-Sing
authors: CS
authors: Lee
keywords: charge transfer complex; energy level offsets; interface energetics; perovskite solar cells; photoemission spectroscopy