Tunable Broad Light Emission from 3D “Hollow” Bromide Perovskites through Defect Engineering

Abstract

Hybrid halide perovskites consisting of corner-sharing metal halide octahedra and small cuboctahedral cages filled with counter cations have proven to be prominent candidates for many high-performance optoelectronic devices. The stability limits of their three-dimensional perovskite framework are defined by the size range of the cations present in the cages of the structure. In some cases, the stability of the perovskite-type structure can be extended even when the counterions violate the size and shape requirements, as is the case in the so-called "hollow" perovskites. In this work, we engineered a new family of 3D highly defective yet crystalline "hollow" bromide perovskites with general formula (FA)_1-<i>x</i>(<i>en</i>)_<i>x</i>(Pb)_{1-0.7<i>x</i>}(Br)_{3-0.4<i>x</i>} (FA = formamidinium (FA⁺), <i>en</i> = ethylenediammonium (<i>en</i>²⁺), <i>x</i> = 0-0.44). Pair distribution function analysis shed light on the local structural coherence, revealing a wide distribution of Pb-Pb distances in the crystal structure as a consequence of the Pb/Br-deficient nature and <i>en</i> inclusion in the lattice. By manipulating the number of Pb/Br vacancies, we finely tune the optical properties of the pristine FAPbBr₃ by blue shifting the band gap from 2.20 to 2.60 eV for the <i>x</i> = 0.42 <i>en</i> sample. A most unexpected outcome was that at <i>x</i>> 0.33 <i>en</i> incorporation, the material exhibits strong broad light emission (1% photoluminescence quantum yield (PLQY)) that is maintained after exposure to air for more than a year. This is the first example of strong broad light emission from a 3D hybrid halide perovskite, demonstrating that meticulous defect engineering is an excellent tool for customizing the optical properties of these semiconductors.