Structure-Dependent Photoluminescence in Low-Dimensional Ethyl-, Propyl-, and Butylammonium Lead Iodide Perovskites.

Abstract

Hybrid organic-inorganic perovskites have attracted great attention as the next generation material for photovoltaic and light emitting devices. However, their environment instability issue remains as the largest challenge for practical applications. Recently emerging two-dimensional (2D) perovskites with Ruddlesden-Popper (R-P) structures are found to greatly improve the stability and aging problems. Furthermore, strong confinement of excitons in these natural quantum-well structures results in the distinct and narrow light emission in the visible spectral range, enabling the development of spectrally tunable light sources. Besides the strong quasi-monochromatic emission, some 2D perovskites comprised of the specific organic cations and inorganic layer structures emit a pronounced broadband emission. Herein we report the light emitting properties and the degradation of low-dimensional perovskites consisting of the three shortest alkylammonium spacers, mono-ethylammonium (EA), n-propylammonium (PA), and n-butylammonium (BA). While (BA)2PbI4 is known to form well-oriented 2D thin films consisting of layers of corner-sharing PbI6 octahedra separated by a bilayer of BA cations, EA with shorter alkyl chains tends to form other type of lower dimensional structure. Nevertheless, optical absorption edges of as-prepared fresh EAPbI3, (PA)2PbI4, and (BA)2PbI4 are obviously blue-shifted to 2.4-2.5 eV compared to their 3D counterpart, methylammonium lead iodide (MAPbI3) perovskite, and they all emit narrow excitonic photoluminescence. Furthermore, by carefully optimizing deposition conditions, we have achieved predominantly 2D structure for (PA)2PbI4. However, unlike (BA)2PbI4, upon exposure to ambient environment (PA)2PbI4 readily transforms to a different crystal structure, exhibiting a prominently broadband light from ca. 500 nm to 800 nm and a gradual increase in intensity as structural transformation proceeds.