Photoinduce Quasi 2D to 3D Phase Transformation of Organic Halide Perovskite Nanoparticles
Mrinmoy Roy a
a IIT Indian Institute of Technology Bombay, India, Powai, Mumbai, Mumbai, India
nanoGe Perovskite Conferences
Proceedings of International Conference on Perovskite Thin Film Photovoltaics and Perovskite Photonics and Optoelectronics (NIPHO20)
Sevilla, Spain, 2020 February 23rd - 25th
Organizer: Hernán Míguez
Poster, Mrinmoy Roy, 082
Publication date: 25th November 2019

Organic halide perovskite nanoparticles (NPs) have excellent photoluminescence (PL), high PL quantum yield (QY), direct band-to-band transition with significant absorption coefficients (~105 cm-1), small excitonic binding energy (76 meV), high carrier mobility, long charge-diffusion length, etc. This makes hybrid halide perovskite nanoparticles (CH3NH3PbX3 where X=Cl, Br and I) the most promising candidate for various optoelectrical applications. Various experimental reports have illustrated that CH3NH3PbBr3 NPs belongs to an analogue of Ruddlesden–Popper phase which consists of two-dimensional perovskite-like slabs interleaved with cations[1]. General formula of Ruddlesden–Popper phase is An+1PbnBr3n+1 for a single catanionic system which can be represented as An-1A’2PbnBr3n+1­ for double catanionic compound, where A is CH3NH3+ and A’ long-chain organic cations which are taken as a surfactant during the synthesis of perovskite NPs such as oleylammonium cations (C18H35NH3+). “n” is thickness of the perovskite, i.e. the number of continuous octahedra which are stacked together in the perovskite structure. For a complete 3D perovskite form, n=∞ and for a complete transformation to 2D perovskite, n=1. In between n=2 to 4, they are known as quasi-2d perovskites; and after n ≥ 5 it starts behaving like a 3D (or bulk) perovskite[2,3]. Herein, we have shown the photo-induced phase transition from quasi-2D to 3D CH3NH3PbBr3 perovskite NPs. In this work, we successfully synthesized ­quasi-2D CH3NH3PbBr3 nanoparticles (thickness is n=2) via hot injection method[4]. This ­quasi-2D CH3NH3PbBr3 perovskite nanoparticles have cubic phase as confirmed from the XRD studies; which is also supported by the SAED investigations. Emission peak at 450 nm, 467 nm, 473 nm, and 525 nm belongs to n=2, 3, 4 and 5 thicknesses, for a layered CH3NH3PbBr NPs. Excitonic Bohr radius of CH3NH3PbBr3 nano particles is found to be 2 nm, and the thickness of a single layer of PbBr64- octahedra is 5.9 Å. As the layer thickness decreases, CH3NH3PbBr3 perovskite NPs moves to quantum confinement regime, which is governed by the blue-shift in the emission peak of CH3NH3PbBr3 nanoparticles. Room temperature steady-state emission and absorption investigations show a systematic change in the thickness of the perovskites as a result of continuous photon irradiation (340 nm, UV light source). As the phase shifts away from the quantum confinement regime, bandgap of the compound changes from 2.72 eV to 2.2 eV. XRD patterns show a continuous decrease in the full-width half maxima, which indicates that the thickness of the perovskite NPs is increasing. In order to support our claim, density functional theory (DFT) based ab-initio calculations were performed on layered CH3NH3PbBr3 structure to validate the change in band gap due to the quantum confinement effect. Due to the organic molecule surrounding of the Pb-Br octahedra, the role of van-der-Waal’s interaction becomes significant in the surface slab and this has been included in the calculations. With more accurate Heyd-Scuseria-Ernzerhof (HSE) exchange correlation functional, the band gap is found to vary from 2.79 eV to 2.29 eV, for 2-5 layered MAPbBr3 perovskite nanoparticles, which matches exceptionally well with the observed experimental trends. Our observations are both important for the understanding of spectral emission shift in hybrid perovskites and for an eventual future development of efficient perovskite LEDs.

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