Charge\nTransfer Dynamics of Phase-Segregated Halide Perovskites: CH₃NH₃PbCl₃ and CH₃NH₃PbI₃ or (C₄H₉NH₃)₂(CH₃NH₃)_<i>n</i>−1Pb<i>_n</i>I_3<i>n</i>+1 Mixtures

Abstract

Lead halide perovskites\npresent a versatile class of solution-processable semiconductors with\nhighly tunable bandgaps that span ultraviolet, visible, and near-infrared\nportions of the spectrum. We explore phase-separated chloride and\niodide lead perovskite mixtures as candidate materials for intermediate\nband applications in future photovoltaics. X-ray diffraction and scanning\nelectron microscopy reveal that deposition of precursor solutions\nacross the MAPbCl₃/MAPbI₃ composition space\naffords quasi-epitaxial cocrystallized films, in which the two perovskites\ndo not alloy but instead remain phase-segregated. First-principle\ncalculations further support the formation of an epitaxial interface\nand predict energy offsets in the valence band and conduction band\nedges that could result in intermediate energy absorption. The charge\ndynamics of variable mixtures of the relatively narrow bandgap (1.57\neV) MAPbI₃ perovskite and wide bandgap (3.02 eV) MAPbCl₃ are probed to map charge and energy flow direction and kinetics.\nTime-resolved photoluminescence and transient absorption measurements\nreveal charge transfer of photoexcited carriers in MAPbCl₃ to MAPbI₃ in tens of picoseconds. The rate of quenching\ncan be further tuned by replacing MAPbI₃ with two-dimensional\nRuddlesden–Popper (BA)₂(MA)_<i>n</i>−1Pb<i>_n</i>I_3<i>n</i>+1 (<i>n</i> = 3, 2, and 1) perovskites, which also\nremain phase-separated.