Designing Low-Dimensional Hybrid Lead Halide Perovskites for Excitonic Photophysics, Chiroptics and Water-Stability

Abstract

Three dimensional (3D) lead halide perovskites like APbX3 (A: Cs+, CH3NH3+, NH2CHNH2+; X = Cl-, Br-, I-) exhibit wonderful optoelectronic properties. A bigger A-site cation like butylammonium [CH3(CH2)3NH3+] disrupts the 3D octahedral network yielding lower-dimensional perovskite inspired structures, like two-dimensional (2D) layered perovskites (A2PbX4). Unlike 3D perovskites, the chemical compositions of low-dimensional perovskites are not limited by the Goldschmidt tolerance factor. Consequently, it is possible to think of a large variety of organic and inorganic sub-lattices. Moreover, the hybrid structure of these lower-dimensional perovskites gives rise to various types of interactions. This thesis explores the novel material design concepts, the interactions in lower-dimensional perovskites, along with their optical properties. More specifically, (i) nature of quantum well structure in layered perovskites (chapter 2), (ii) chemical insertions between Van-der-Waals layers (chapter 3), (iii) chiral hybrid perovskites (chapter 4), and (iv) water-stable low-dimensional perovskites (chapter 5) have been addressed in this thesis. Firstly, we study 2D layered lead halide perovskite single crystals using optical absorption, temperature-dependent (10 K to 300 K) photoluminescence, and spatially resolved fluorescence microscopy imaging. Surprisingly, two excitonic absorption and emission features are observed! The higher energy emission originates from individual Pb-halide quantum well layers, whereas the lower energy emission is assigned to the possible interaction between the adjacent Pb-halide layers, mainly through the layer edges. Such assignments are verified by controlling the Pb-halide inter-layer interactions through molecular intercalation and varied chain length of organic cations. Then, we used chiral organic A-site cations that induce chirality in inorganic Pb-halide sub-lattice, and therefore, showing excitonic circular dichroism. The introduced chirality splits the emission states by lifting the spin degeneracy. Moreover, these chiral-layered perovskites show higher exciton binding energies compared to the achiral one. Finally, we address the issue of water instability by using 4,4′-trimethylenedipyridinium [(4,4′-TMDP)] as A-site cation. Long-range intermolecular cation-π interactions between these A-site cations make one dimensional (1D) (4,4′-TMDP)Pb2Br6 perovskite completely water-stable. These water-stable perovskites showed a white light emission.