Divalent chromium in the octahedral positions of the novel hybrid perovskites CH3NH3Pb1-xCrx(Br,Cl)3 (x=0.25, 0.5): Induction of narrow bands inside the bandgap

The hybrid organic-inorganic perovskites, MAPbX(3) (MA: methylammonium CH3-NH3+; X = halogen), are the active light absorbing materials in the new generation of solar cells. Previous theoretical quantum calculations indicated that substituting part of the Pb cations by Cr could introduce a partially filled ingap narrow band, allowing the excitation of electrons from the valence to the conduction band in two steps. This has been realized in this work, where the title compounds were prepared by ball milling from MABr, PbBr2, and CrCl2 in N-2 atmosphere. XRD patterns correspond to pure cubic phases, where Cr2+ ions have entered the octahedral sites of this perovskite framework for the first time. The valence state of Cr ions was probed by Cr K-edge X-ray absorption spectroscopy, confirming the 2+ state and possible local octahedral distortion. Magnetic susceptibility measurements indicate for the x = 0.5 compound a conspicuous antiferromagnetic ordering below T-N = 38 K and a paramagnetic moment in the high-temperature region of mu(eff) = 3.60 mu(B), corresponding to high-spin Cr2+ ions, thus confirming the incorporation of this element at the Pb sublattice. Diffuse reflectance spectra indicate, as predicted, that the presence of Cr induces a narrow band inside the bandgap of those perovskites: the new absorption at ca. 1.6 eV corresponds to the electronic transition from the valence band to the empty part of this partially filled in-gap band. Our result here may open a window of opportunity for new tools in band gap engineering, which can be used in other hybrid lead halide perovskites for improvements in solar cell devices depicting high efficiency. (C) 2019 Elsevier B.V. All rights reserved.