Measuring <i>n</i> and <i>k</i> at the Microscale in Single Crystals of CH₃NH₃PbBr₃ Perovskite

Lead-based, inorganic–organic hybrid perovskites have shown much promise in photovoltaics, and the ability to tune their band gap makes them attractive for tandem solar cells, photodetectors, light-emitting diodes, and lasers. A crucial first step toward understanding a material’s behavior in such optoelectronic devices is determining its complex refractive index, <i>n + ik</i>; however, optically smooth films of hybrid perovskites are challenging to produce, and the optical properties of films of these materials have been shown to depend on the size of their crystallites. To address these challenges, this work reports quantitative reflectance and transmittance measurements performed on individual microcrystals of CH₃NH₃PbBr₃, with thicknesses ranging from 155 to 1907 nm. The single crystals are formed by spin-coating a film of precursor solution and then stamping it with polydimethylsiloxane (PDMS) during crystallization. By measuring crystals of varying thickness, <i>n</i> and <i>k</i> values at each wavelength (405–1100 nm) have been determined, which agree with recently reported values extracted by ellipsometry on millimeter-sized single crystals. This approach can be applied to determine the optical constants of any material that presents challenges in producing smooth films over large areas, such as mixed-halide hybrid and inorganic perovskites, and to micro- or nanoplatelets