Three-Dimensional Methylhydrazinium Lead Halide Perovskites: Structural Changes and Effects on Dielectric, Linear, and Nonlinear Optical Properties Entailed by the Halide Tuning

Abstract

Three-dimensional lead halide perovskites are promising materials for optoelectronic applications. The most famous representative comprise methylammonium (MA + ) and formamidinium (FA + ) cations, but recently, this group was enlarged by methylhydrazinium (MHy + ) analogues that crystallize in polar structures at room-temperature. Properties of threedimensional (3D) perovskites can be tuned by mixing of molecular cations or halide anions. Here, we report synthesis and physicochemical characterization of mixed-halide MHyPbBr x Cl 3−x (x = 0.40, 0.58, 0.85, 1.33, 1.95, 2.25, and 2.55) and MHyPbBr 2.8 I 0.2 perovskites. X-ray diffraction data show that all materials feature a polar monoclinic P2 1 symmetry at room temperature. With the temperature increase, all MHyPbBr x Cl 3−x perovskites undergo a displacive phase transition to another polar orthorhombic Pb2 1 m phase at T 2 ≥ 318 K. The bromine rich crystals (x ≥ 1.33) exhibit an additional order−disorder phase transition to the archetypal cubic Pm3̅ m phase at T 1 ≥ 409 K. In contrast to MHyPbBr x Cl 3−x perovskites, MHyPbBr 2.8 I 0.2 undergoes a direct P2 1 to Pm3̅ m phase transition. The temperature at which the cubic phase is stabilized, stability range of the Pb2 1 m phase, and distortion of the leadhalide octahedra decrease with the increase of Br − content. The structural changes affect dielectric, conductivity, and optical properties. In particular, the Br-rich samples show switchable dielectric behavior near 410−420 K. Furthermore, the activation energy of Cl − ionic conductivity increases with the increase of Br − content in phases Pb2 1 m and P2 1 , whereas in phase Pm3̅ m, the conductivity of Br − ions increases with the increase of Cl − content. The energy band gap narrows and the photoluminescence (PL) bands exhibit red shift when going from Cl to Br and then to I. Interestingly, whereas PL of the Br-rich and Cl-rich samples is dominated by bound exciton and self-trapped exciton bands, respectively, these bands are suppressed for 2.25 ≥ x ≥ 0.85. The PL color is strongly tuned by doping and changes from greenish-blue for the Cl-rich samples to yellowish-green for MHyPbBr 2.8 I 0.2 . SHG studies demonstrate that doping of MHyPbCl 3 with Br − ions reduces the difference between SHG signal intensities of the monoclinic and orthorhombic phases, to the extent that beyond x = 1.95, the SHG response of these phases becomes essentially the same. The relative SHG efficiencies of Br−Cl mixed materials at room temperature increase with the increase in Br content.

Temperature-Dependent Raman Studies of FAPbBr3 and MAPbBr3 Perovskites: Effect of Phase Transitions on Molecular Dynamics and Lattice Distortion

“…Hybrid lead halide perovskites crystallizing in various crystal structures, including zero-, one-, two-and three-dimensional (0D, 1D, 2D and 3D), have attracted a lot of interest in recent years due to their various functional properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Regarding 3D perovskites, the most famous ones are MAPbI 3 and FAPbI 3 (MA = methylammonium; FA = formamidinium) since they exhibit excellent photovoltaic properties with a power conversion efficiency exceeding 21% [7,8].…”

“…MAPbBr 3 was also shown to exhibit efficient multiphoton excited photoluminescence (PL) suitable for in vivo imaging applications [10]. Very recently, the family of 3D lead halide perovskites was extended by two new members, MHyPbBr 3 and MHyPbCl 3 (MHy = methylhydrazinium) [13][14][15]. Similarly to the MA and FA analogues, MHyPbBr 3 and MHyPbCl 3 and their MHyPbBr 3−x Cl x solid solutions also exhibit PL properties and MHyPbBr 3 can also be classified as a very strong two-photon absorber [13][14][15].…”

“…Very recently, the family of 3D lead halide perovskites was extended by two new members, MHyPbBr 3 and MHyPbCl 3 (MHy = methylhydrazinium) [13][14][15]. Similarly to the MA and FA analogues, MHyPbBr 3 and MHyPbCl 3 and their MHyPbBr 3−x Cl x solid solutions also exhibit PL properties and MHyPbBr 3 can also be classified as a very strong two-photon absorber [13][14][15]. It is worth adding that in contrast to the MA and FA analogues, which crystallize in centrosymmetric structures, the MHy counterparts adopt noncentrosymmetric and polar structures at room temperature (RT) [13][14][15].…”

“…Similarly to the MA and FA analogues, MHyPbBr 3 and MHyPbCl 3 and their MHyPbBr 3−x Cl x solid solutions also exhibit PL properties and MHyPbBr 3 can also be classified as a very strong two-photon absorber [13][14][15]. It is worth adding that in contrast to the MA and FA analogues, which crystallize in centrosymmetric structures, the MHy counterparts adopt noncentrosymmetric and polar structures at room temperature (RT) [13][14][15]. As a result, these compounds exhibit second-harmonic generation (SHG) activity and MHyPbCl 3 was found to show quadratic nonlinear optical (NLO) switching behaviour [13][14][15].…”

“…It is worth adding that in contrast to the MA and FA analogues, which crystallize in centrosymmetric structures, the MHy counterparts adopt noncentrosymmetric and polar structures at room temperature (RT) [13][14][15]. As a result, these compounds exhibit second-harmonic generation (SHG) activity and MHyPbCl 3 was found to show quadratic nonlinear optical (NLO) switching behaviour [13][14][15].…”