Mixology of MA_1–xEA_xPbI₃ Hybrid Perovskites: Phase Transitions, Cation Dynamics, and Photoluminescence

Abstract

Mixing molecular cations in hybrid lead halide perovskites is a highly effective approach to enhance the stability and performance of optoelectronic devices based on these compounds. In this work, we prepare and study novel mixed 3D methylammonium (MA)−ethylammonium (EA) MA 1−x EA x PbI 3 (x < 0.4) hybrid perovskites. We use a suite of different techniques to determine the structural phase diagram, cation dynamics, and photoluminescence properties of these compounds. Upon introduction of EA, we observe a gradual lowering of the phase-transition temperatures, indicating stabilization of the cubic phase. For mixing levels higher than 30%, we obtain a complete suppression of the low-temperature phase transition and formation of a new tetragonal phase with a different symmetry. We use broad-band dielectric spectroscopy to study the dielectric response of the mixed compounds in an extensive frequency range, which allows us to distinguish and characterize three distinct dipolar relaxation processes related to the molecular cation dynamics. We observe that mixing increases the rotation barrier of the MA cations and tunes the dielectric permittivity values. For the highest mixing levels, we observe the signatures of the dipolar glass phase formation. Our findings are supported by density functional theory calculations. Our photoluminescence measurements reveal a small change of the band gap upon mixing, indicating the suitability of these compounds for optoelectronic applications.

Differing Effects of Mixed A-Site Composition on the Properties of Hybrid Lead Iodide Perovskites

“…This agrees with that observed for the GA x MA 1– x PbI 3 system . It should also be noted that the O → T transition temperature is progressively reduced with increasing x in the EA x MA 1– x PbI 3 system . This shows that different cations can cause contrasting changes in the relative stability of different structures of a perovskite.…”

“…Hence, considering Figure a,b and the aforementioned discussion, it can be inferred that the disappearance of the orthorhombic phase is accompanied by a peak in ε″( T ) curve, while the appearance of the tetragonal phase is accompanied by a rapid increase in ε′( T ). It should be noted that this increase in ε′( T ) is much more subtle in FA0.10 than in MAPbI 3 , as evidenced by the peak’s broadening, which might be associated with the change (restriction) in the rotation of the MA + cations when mixing with other cations, as already proposed for the FA x MA 1– x PbI 3 system and others, such as EA x MA 1– x PbI 3 , FA x MA 1– x PbBr 3 and DMA x MA 1– x PbBr 3 . In these systems, it is generally shown that the activation energy for the rotation of MA + cations increases, causing a disorder at low temperatures and consequent transition broadening.…”

“…In Figure a, the abrupt change in both components of dielectric permittivity at around 160 K shows the typical behavior of the orthorhombic-tetragonal ( O → T ) phase transition for the pure MAPbI 3 , associated with the disruption of the cooperative ordering of MA + cations. ,, As for the FA 0.10 MA 0.90 PbI 3 composition, different transitions below 200 K may occur, as observed in Figure b. In the FA x MA 1– x PbI 3 system, several structures and transitions have been proposed in the literature. , It can be argued that the changes in the temperature profile of the dielectric permittivity curves compared to MAPbI 3 are partly due to the possible existence of a large-cell cubic (LC) intermediate phase between the orthorhombic and tetragonal phases .…”

“…In this sense, perhaps it can be generalized that compositions containing a high concentration of bulky cations, such as GA + , in which there is a low solubility limit, present lower thermal stability under certain circumstances. It would be interesting to test, with a similar approach, systems such as EA x MA 1– x PbI 3 and DMA x MA 1– x PbI 3 , whose predicted solubility limits are of the order of x ∼ 0.4 and x ∼ 0.2, respectively, to verify this hypothesis. Despite this, it should be noted that the TC composition did not show a formation of GAPbI 3 , which indicates that this segregation should only occur in compositions with higher concentrations of GA + .…”

Optoelectronic and stability properties of quasi-2D alkylammonium based perovskites

“…Recently, the MA 1− x EA x PbI 3 mixed perovskite system, identified in our previous study 4 as one strong candidate for improved stability and photovoltaic performance, was theoretically and experimentally confirmed. 37…”