Three-Dimensional Methylhydrazinium Lead Halide Perovskites: Structural Changes and Effects on Dielectric, Linear, and Nonlinear Optical Properties Entailed by the Halide Tuning
Résumé
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 MHyPbBr2.8I0.2 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 MHyPbBrxCl3−x perovskites, MHyPbBr2.8I0.2 undergoes a direct P21 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 MHyPbBr2.8I0.2. SHG studies demonstrate that doping of MHyPbCl3 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.
Mots clés
3D perovskites
methylhydrazinium
Lead halide perovskite
Phase transition
mixed-halide perovskites
MHyPbBrxCl3−x
Second harmonic generation
hybrid organic-inorganic perovskite
HOIP
Dielectric spectroscopy
phase diagram
halide alloying
noncentrosymmetry
SHG
photoluminescence
Differential scanning calorimetry DSC
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