Effect of Dimensionality on Photoluminescence and Dielectric Properties of Imidazolium Lead Bromides

Abstract

Hybrid organic–inorganic lead halide perovskites have emerged as promising materials for various applications, including solar cells, light-emitting devices, dielectrics, and optical switches. In this work, we report the synthesis, crystal structures, and linear and nonlinear optical as well as dielectric properties of three imidazolium lead bromides, IMPbBr 3 , IM 2 PbBr 4 , and IM 3 PbBr 5 (IM + = imidazolium). We show that these compounds exhibit three distinct structure types. IMPbBr 3 crystallizes in the 4H-hexagonal perovskite structure with face- and corner-shared PbBr 6 octahedra (space group P 6 3 / mmc at 295 K), IM 2 PbBr 4 adopts a one-dimensional (1D) double-chain structure with edge-shared octahedra (space group P 1̅ at 295 K), while IM 3 PbBr 5 crystallizes in the 1D single-chain structure with corner-shared PbBr 6 octahedra (space group P 1̅ at 295 K). All compounds exhibit two structural phase transitions, and the lowest temperature phases of IMPbBr 3 and IM 3 PbBr 5 are noncentrosymmetric (space groups Pna 2 1 at 190 K and P 1 at 100 K, respectively), as confirmed by measurements of second-harmonic generation (SHG) activity. X-ray diffraction and thermal and Raman studies demonstrate that the phase transitions feature an order–disorder mechanism. The only exception is the isostructural P 1̅ to P 1̅ phase transition at 141 K in IM 2 PbBr 4 , which is of a displacive type. Dielectric studies reveal that IMPbBr 3 is a switchable dielectric material, whereas IM 3 PbBr 5 is an improper ferroelectric. All compounds exhibit broadband, highly shifted Stokes emissions. Features of these emissions, i.e. , band gap and excitonic absorption, are discussed in relation to the different structures of each composition.

Design, synthesis, and characterization of a new hybrid organic–inorganic perovskite with a high-Tc dielectric transition

“…The peak of fluorescence changes to 670 nm for (FTEA)PbBr 3 and 688 nm for IMrPbBr 3 . 24,26 For the above three perovskites, they all have relatively close emission spectra with a similar [PbBr 3 ] − chains. But it is worth noting that there is a big difference in the emission spectra for CEPbBr 3 (385 nm) and BEPbBr 3 (391 nm).…”

“…The single crystal X-ray diffraction data of 1 were collected at 296 K and 410 K. At 296 K, 1 crystallizes in the monoclinic crystallographic system and is a 4 H -hexagonal perovskite structure as reported in many references. 25,26,28,29 The centrosymmetric space group of 1 is P 2 1 / c and the cell parameters of 1 is a = 10.8678(4) Å, b = 35.6850(15) Å, c = 7.7853(3) Å, Z = 8, β = 100.9530(10)° and V = 2964.3(2) Å 3 .…”

“…The reported perovskite PyrPbI 3 also has a step-change in ε ′ due to a similar one-dimensional [PbX 3 ] − structure. 26 The Δ ε ′ of perovskites with [PbX 3 ] − structure is small and may not be appropriate to be used in the temperature sensors or dielectric switches. Hybrid organic–inorganic perovskite with a bigger Δ ε ′ is needed and desired.…”

“…The single crystal X-ray diffraction data of 1 were collected at 296 K and 410 K. At 296 K, 1 crystallizes in the monoclinic crystallographic system and is a 4Hhexagonal perovskite structure as reported in many references. 25,26,28,29 The centrosymmetric space group of 1 is P2 It can be seen from the packing view (Fig. 2), every Pb atom lies on a 2 1 -screw axis and bridges with six Br atoms, forming infinite one-dimension [PbBr 3 ] − chains along the c-axis.…”

“…For example, IMPbBr 3 and PyrPbI 3 have similar [PbX 3 ] − structures (X is a halogen atom), but the T c of IMPbBr 3 (221 K) and PyrPbI 3 (315 K) are much lower than that of 1. 25,26 Rather than spherical-like structure, the shape of the organic cation in IMPbBr 3 and PyrPbI 3 have a planar structure, which may decrease the barrier of disordering transition. Thus, for similar one-dimensional [PbBr 3 ] − hybrid organic-inorganic perovskite, the size, and shape of organic cation are the factors affecting T c .…”