Phase Intergrowth and Structural Defects in Organic Metal Halide Ruddlesden–Popper Thin Films
Organic metal halide Ruddlesden–Popper phases combine the excellent optoelectronic properties of three-dimensional, bulk hybrid perovskites with superior material stability in ambient conditions. However, the thin film structure of these layered perovskites is still poorly understood, as phase purity is typically determined solely by specular X-ray diffraction. The thin film structure of these Ruddlesden–Popper phases was examined by increasingly local characterization techniques. It is difficult to assess the phase purity by purely bulk optical and diffraction measurements. From the comparison of grazing-incidence wide angle X-ray scattering (GIWAXS) patterns of cast films to expected scattering from single-crystal structures, significant in-plane disorder was observed. Spatially localized photoluminescence measurements show that films do not phase separate on the micrometer-scale. Selected area electron diffraction measurements show the intergrowth of different phases within the same thin film, consistent with previous observations seen in epitaxially-grown Ruddlesden–Popper complex oxides. Despite the presence of phase impurities that would typically be detrimental for device performance, fits to photothermal deflection spectroscopy measurements show relatively low Urbach energies of 33 meV for (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) 2 Pb 3 I 10 and 32 meV for (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) 3 Pb 4 I 13 , indicating that the electronic properties are insensitive to the phase impurities.