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 three-dimensional (3D) perovskites can be tuned by mixing of molecular cations or halide anions. Here, we report synthesis and physicochemical characterization of mixed-halide MHyPbBrxCl3–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 P21 symmetry at room temperature. With the temperature increase, all MHyPbBrxCl3–x perovskites undergo a displacive phase transition to another polar orthorhombic Pb21m phase at T2 ≥ 318 K. The bromine rich crystals (x ≥ 1.33) exhibit an additional order–disorder phase transition to the archetypal cubic Pm3̅m phase at T1 ≥ 409 K. In contrast to MHyPbBrxCl3–x perovskites, MHyPbBr2.8I0.2 undergoes a direct P21 to Pm3̅m phase transition. The temperature at which the cubic phase is stabilized, stability range of the Pb21m phase, and distortion of the lead-halide 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 Pb21m and P21, whereas in phase Pm3̅m, 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.
Highlights
Three-dimensional hybrid organic−inorganic perovskites (3DHOIPs) have been the topic of intense research efforts in recent years due to their functional properties
These properties, combined with low cost and facile synthesis, open up new perspectives for state-of-the-art applications, such as photovoltaics, light emitting devices, photodetectors, or even in photodynamic therapy.[4,14−16]. Another field of research on 3D HOIPs is the improvement of their performance via “A” site cation substitution. Such modification is an indispensable tool to allow for highly tunable modification of HOIP properties, which is usually difficult to obtain with all-inorganic counterparts
We do not know specific origins of this phenomenon, we speculate that transition between these phases generates defects that act as second-harmonic generation (SHG) active centers upon restoration of phase II
Summary
HOIPs) have been the topic of intense research efforts in recent years due to their functional properties. Numerous studies revealed technologically relevant physicochemical properties, such as high PL yields,[9] power conversion efficiencies of around 25%,10,11 broad tunability of emission colors,[2] high extinction coefficients,[12] unprecedented mobility of charge carriers,[13] and the like These properties, combined with low cost and facile synthesis, open up new perspectives for state-of-the-art applications, such as photovoltaics, light emitting devices, photodetectors, or even in photodynamic therapy.[4,14−16]. Our group has discovered 3D lead halide HOIPs comprising MHy+ cations, i.e., MHyPbBr3 and MHyPbCl3 Both these perovskites feature tolerance factors higher than 1 (TF = 1.03 and 1.05 for Br- and Cl-analogue, respectively) and crystallize in the monoclinic, polar P21 phase at room temperature (RT).[22,23] MHyPbBr3 undergoes a phase transition at 418 K to the cubic Pm-3m phase (which is archetypical of 3D perovskites) and exhibits a switchable dielectric behavior, strong second-harmonic generation (SHG). We took a closer look at effects that arise from one of the most unusual property of the MHy+-based 3D HOIPs that is SHG response, associated with structural noncentrosymmetry.[22,23] In this way, the acentric identity of studied phases is confirmed and effect of halide mixing on the relative SHG efficiency is examined
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