Mixology of MA1-xEAxPbI3 Hybrid Perovskites: Phase Transitions, Cation Dynamics and Photoluminescence

Abstract

Mixing of molecular cations in hybrid lead halide perovskites is a highly effective approach to enhance stability and performance of the optoelectronic devices based on these compounds. In this work, we prepare and study novel mixed methylammonium (MA)-ethylammonium (EA) MA1-xEAxPbI3 (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 different symmetry. We use the broadband 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 signatures of the dipolar glass phase formation. Our findings are supported by the density functional theory calculations. Our photoluminescence measurements reveal a small change of the band gap upon mixing indicating suitability of these compounds for optoelectronic applications.

Influence of Bromination on the Phase Transitions, Structural, Phonon, and Optical Properties Explored for 2-Bromoethylammonium Bismuth Bromide

“…Table S6 contains tentative assignments based on the literature data for other coordination polymers with the EA + cation. 3,33 As compared to EA + , the bands seen for BEA + are somewhat displaced. Strong bands at about 570 cm −1 , resulting from the C−Br stretching vibration and other bands observed specifically for BEA + , were ascribed to the CH 2 Br groups.…”

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

“…The observed broad dielectric response for the x > 0.3 compositions is similar to other highly mixed compounds (Figures 6, 8, and 10), and, as discussed above, resembles the glassy phase (compare with Figure 2c). However, a clear Vogel−Fulcher behavior of the mean relaxation time was not detected despite broadband measurements (Hz-GHz), 166 indicating that freezing of electric dipoles may occur at very low temperatures. Note that the DFT calculations of the Guanidinium is another highly promising molecular cation used to significantly enhance the performance of the devices based on lead halide perovskites.…”

“…A detailed multitechnique study of the mixing effects on the structural phase transitions, cations dynamics and PL of the MA 1−x EA x PbI 3 (x = 0, 0.09, 0.16, 0.21, 0.31, and 0.38) compounds was recently reported by us. 166 The EA concentration was determined using solution 1 H NMR spectroscopy, and the solubility limit of about 40% was found. A set of different experimental techniques (DSC, C p , ultrasonic, SCXRD, broadband DS, and Raman spectroscopy) was used to construct the temperature−composition phase diagram of this system (Figure 12a).…”

“…A gradual evolution of the phase transitions with varying composition is in a sharp contrast to the MA-based systems, where phase transitions are typically suppressed. [164][165][166]336,381…”

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

“…Recently, the MA 1Àx EAxPbI 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 With respect to changing the large cations in the sequence (BA, PA, HXA) the modifications of the relative stability are significantly smaller. However, replacing MA with other small cations (FA, EA, GA) leads to a larger stability enhancement.…”