Structure-Dependent Photoluminescence in Low-Dimensional Ethylammonium, Propylammonium, and Butylammonium Lead Iodide Perovskites.

Hybrid organic-inorganic perovskites have attracted great attention as the next generation materials 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 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 composed 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 types of lower-dimensional structures. 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 a 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 ∼500 to 800 nm and a gradual increase in intensity as structural transformation proceeds.