title: Lead-Free MA2CuCl(x)Br(4-x) Hybrid Perovskites.
abstract: Despite their extremely good performance in solar cells with efficiencies approaching 20% and the emerging application for light-emitting devices, organic-inorganic lead halide perovskites suffer from high content of toxic, polluting, and bioaccumulative Pb, which may eventually hamper their commercialization. Here, we present the synthesis of two-dimensional (2D) Cu-based hybrid perovskites and study their optoelectronic properties to investigate their potential application in solar cells and light-emitting devices, providing a new environmental-friendly alternative to Pb. The series (CH3NH3)2CuCl(x)Br(4-x) was studied in detail, with the role of Cl found to be essential for stabilization. By exploiting the additional Cu d-d transitions and appropriately tuning the Br/Cl ratio, which affects ligand-to-metal charge transfer transitions, the optical absorption in this series of compounds can be extended to the near-infrared for optimal spectral overlap with the solar irradiance. In situ formation of Cu(+) ions was found to be responsible for the green photoluminescence of this material set. Processing conditions for integrating Cu-based perovskites into photovoltaic device architectures, as well as the factors currently limiting photovoltaic performance, are discussed: among them, we identified the combination of low absorption coefficient and heavy mass of the holes as main limitations for the solar cell efficiency. To the best of our knowledge, this is the first demonstration of the potential of 2D copper perovskite as light harvesters and lays the foundation for further development of perovskite based on transition metals as alternative lead-free materials. Appropriate molecular design will be necessary to improve the material's properties and solar cell performance filling the gap with the state-of-the-art Pb-based perovskite devices.
authors.affiliation: Interdisciplinary Graduate School, Energy Research Institute at NTU , 639798 Singapore.|Energy Research Institute @ NTU (ERI@N), Research Technoplaza, Nanyang Technological University , Nanyang Drive, 637553 Singapore.
authors.forename: Daniele
authors.initials: D
authors.lastname: Cortecchia
authors.affiliation: Energy Research Institute @ NTU (ERI@N), Research Technoplaza, Nanyang Technological University , Nanyang Drive, 637553 Singapore.
authors.forename: Herlina Arianita
authors.initials: HA
authors.lastname: Dewi
authors.affiliation: Division of Physics and Applied Physics, Nanyang Technological University , 637371 Singapore.
authors.forename: Jun
authors.initials: J
authors.lastname: Yin
authors.affiliation: Energy Research Institute @ NTU (ERI@N), Research Technoplaza, Nanyang Technological University , Nanyang Drive, 637553 Singapore.|Division of Physics and Applied Physics, Nanyang Technological University , 637371 Singapore.
authors.forename: Annalisa
authors.initials: A
authors.lastname: Bruno
authors.affiliation: Division of Physics and Applied Physics, Nanyang Technological University , 637371 Singapore.
authors.forename: Shi
authors.initials: S
authors.lastname: Chen
authors.affiliation: Energy Research Institute @ NTU (ERI@N), Research Technoplaza, Nanyang Technological University , Nanyang Drive, 637553 Singapore.
authors.forename: Tom
authors.initials: T
authors.lastname: Baikie
authors.affiliation: Energy Research Institute @ NTU (ERI@N), Research Technoplaza, Nanyang Technological University , Nanyang Drive, 637553 Singapore.
authors.forename: Pablo P
authors.initials: PP
authors.lastname: Boix
authors.affiliation: Laboratory of Photonics and Interfaces, Department of Chemistry and Chemical Engineering, Swiss Federal Institute of Technology , Station 6, Lausanne CH-1015, Switzerland.
authors.forename: Michael
authors.initials: M
authors.lastname: Grätzel
authors.affiliation: Energy Research Institute @ NTU (ERI@N), Research Technoplaza, Nanyang Technological University , Nanyang Drive, 637553 Singapore.|School of Materials Science and Engineering, Nanyang Technological University , Nanyang Avenue, 639798 Singapore.
authors.forename: Subodh
authors.initials: S
authors.lastname: Mhaisalkar
authors.affiliation: Division of Physics and Applied Physics, Nanyang Technological University , 637371 Singapore.
authors.forename: Cesare
authors.initials: C
authors.lastname: Soci
authors.affiliation: Energy Research Institute @ NTU (ERI@N), Research Technoplaza, Nanyang Technological University , Nanyang Drive, 637553 Singapore.|School of Materials Science and Engineering, Nanyang Technological University , Nanyang Avenue, 639798 Singapore.
authors.forename: Nripan
authors.initials: N
authors.lastname: Mathews