Synthesis and Dual Excitonic Emission of Sn-Iodide Layered Hybrid Perovskites

Abstract

In recent years both three-dimensional (3D) and two-dimensional (2D) hybrid perovskites drew significant attention for their excellent optoelectronic properties and solution processibility. In difference to 3D perovskites like CH3NH3PbI3, 2D perovskites like ((C6H5)C2H4NH3)2PbI4 have less stringent structural requirements, which allows more flexibility in organic and inorganic compositions of 2D perovskites. Subsequently, longer hydrophobic organic molecules can be used in 2D perovskites providing better moisture stability. In this thesis, we prepared different Sn-I based 2D hybrid perovskites, which remain significantly stable. Note that in 3D Sn-I perovskites, Sn (II) rapidly oxidizes to Sn (IV) destroying the 3D perovskite structure. Two different varieties, namely Ruddlesden-Popper phase with the generic formula (R-NH3)2SnI4 and Dion-Jacobson phase with generic formula (NH3-R-NH3)SnI4 with varying hydrocarbon groups R (both alkyl and aryl) are prepared. Samples are mainly in the form of centimetre sized single crystalline flakes, along with microcrystals and some physically exfoliated samples. After the synthesis and structural characterization, the major emphasis of the thesis is in understanding their excitonic absorption and emission. The hybrid Sn-I layered perovskites show repeating quantum well structure. The charge carriers are confined in the semiconducting inorganic Sn-I layers, which are separated from each other by insulating organic layers. This quantum well structure leads to high excitonic binding energies manifesting sharp and intense excitonic absorption and emission. We find a very unusual observation where the single crystals of Sn-I layered perovskites exhibit two excitonic emissions. Comparison of these Sn-I samples with layered Pb-I perovskites shows all the samples exhibit such dual excitonic emission. These results suggest the possibility of interactions between Sn-I layers at some parts of the crystals. Understanding of this unusual property is required to optimize the optoelectronic applications of hybrid layered perovskites.