Ultrafast Carrier Dynamics of the Copper(II)-Based Perovskite-Inspired Materials A₂CuCl_4-x Br _x (A = C₆H₅CH₂CH₂NH₃ or CH₃NH₃, x=0, 2, 4)

Microcrystalline powders and thin films of the copper-(II)-based layered hybrid organic-inorganic methylammonium (MA) and 2-phenylethylammonium (PEA) perovskite-inspired compounds (MA)(2)CuCl4, (MA)(2)CuCl2Br2, (MA)(2)CuBr4, (PEA)(2)CuCl4, (PEA)(2)CuCl2Br2, and (PEA)(2)CuBr4 were synthesized and structurally characterized by X-ray diffraction. They are classified by monoclinic or orthorhombic space groups. Central structural elements are two-dimensional networks of distorted corner-sharing CuX6 (X = halide) octahedra. The thin films showed pronounced texture effects, with the octahedral layers arranged parallel to the substrate's surface. The steady-state and transient optical response in the UV-vis range was investigated in detail. All compounds showed pronounced ligand-to-metal charge-transfer (LMCT) bands in the visible range and weak ligand-field (LF) absorption bands in the red to near-infrared. With increasing bromide content, the LMCT absorption bands progressively shifted to longer wavelengths, whereas the LF bands were much less affected. All of these copper(II) compounds did not show photoluminescence in the visible range. Upon excitation into the LMCT bands, transient absorption measurements found an ultrafast nonradiative relaxation channel from the initially populated LMCT to the LF states on a picosecond timescale, with the resulting excess energy being dissipated as heat. This photophysical property will be highly unfavorable for solar-cell applications as it will strongly limit the achievable open-circuit voltage. Internal conversion from the LF states to the electronic ground state occurred on a nanosecond timescale for all systems.