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

Abstract

Halide perovskites (HPs) appeared as a revolutionary class of relevant optoelectronic materials. Optimizing these materials involves developing mixed formulations currently used in many of the most efficient HP-based devices. Mixing A-site organic cations has been a widely used strategy to achieve the desired characteristics in these perovskites. Few of the reported works were devoted to understanding the effects of different organic cations on the properties of the resulting HPs. In the present work, we report a detailed study on the structural, thermal, and electrical properties and degradation assays in HPs with mixing A-site organic cations at the MAPbI 3 , FA 0.10 MA 0.90 PbI 3 , GA 0.10 MA 0.90 PbI 3 , and GA 0.05 FA 0.05 MA 0.90 PbI 3 compositions. The results show that the charge transport properties change consistently with the A-site cation size. However, the structure and phase transitions are unusually affected by the composition, suggesting that other characteristics of substituent cations are relevant to the behavior of the resulting materials. Finally, thermal degradation tests reveal that, contrary to general expectations, not all mixed compositions are more stable than their related pure HPs. More than that, the order of degradation kinetics may also depend on the type of substituent cation. The results elucidate the particularities of each type of organic cation on the properties of mixed perovskites and advance knowledge about these relevant materials.

Composition‐Property Relations for GAxFAyMA1‐x‐yPbI3 Perovskites

“…In Figure 4a, the orthorhombic-tetragonal (𝛾↔𝛽) phase transition observed for pure MAPbI 3 occurs at ≈160 K and is accompanied by an abrupt change in the peaks of the real dielectric permittivity 𝜖′(T) and in the imaginary dielectric permittivity 𝜖′′(T). [33,60] These results agree with those recently reported to describe the orthorhombic-tetragonal transition on heating for mixed organic cations HPs. [14,33] For the GA + -rich composition series (x = 2y), the introduction of substituents causes a progressive drop in the transition temperature of the 𝛾↔𝛽 phase transition, as shown in Figure 2b, so that in x = 0.08, y = 0.04 the tetragonal phase seems to remain stable even below 100 K. Figure 4c shows the same behavior occurring in the FA + = GA + series (x = y), in which it is observed that in x = 0.06, y = 0.06 the tetragonal phase also seems to be maintained even below 100 K. These results are in agreement with what was observed in compositions of the type GA x MA 1-x PbI 3 , where a point of maxima at 𝜖′(T) is already shifted down 100 K in x = 0.10.…”

“…[33,60] These results agree with those recently reported to describe the orthorhombic-tetragonal transition on heating for mixed organic cations HPs. [14,33] For the GA + -rich composition series (x = 2y), the introduction of substituents causes a progressive drop in the transition temperature of the 𝛾↔𝛽 phase transition, as shown in Figure 2b, so that in x = 0.08, y = 0.04 the tetragonal phase seems to remain stable even below 100 K. Figure 4c shows the same behavior occurring in the FA + = GA + series (x = y), in which it is observed that in x = 0.06, y = 0.06 the tetragonal phase also seems to be maintained even below 100 K. These results are in agreement with what was observed in compositions of the type GA x MA 1-x PbI 3 , where a point of maxima at 𝜖′(T) is already shifted down 100 K in x = 0.10. [33] However, for the FA + -rich series (y = 2x), the drop in the 𝜖′(T) peak in Figure 4d around the phase transition is much less intense when compared with the other series.…”

“…[14,15] Considering the results shown in Figure 4, there seems to be a trend that compositions containing high GA + contents are more efficient in stabilizing the tetragonal phase at low temperatures than those rich in FA + , in agreement with previous results. [33] This corroborates the previously found tendency that GA + is less efficient in stabilizing the cubic phases than FA + , which implies that GA + stabilizes the tetragonal phase more than FA + at high temperatures. To better visualize these trends, we constructed in Figure 5 a heat map of the 𝛾↔𝛽 transition temperature using the data organized in Note S5 (Supporting Information).…”

“…From the XRD patterns, it can be clearly seen that the compositions with lower substitution contents exhibit the typical tetragonal features, which progressively shift to the cubic features as the substitution degree increases. In this sense, substituting MA + by FA + and GA + stabilizes the cubic phase, as expected from results in similar systems, [13,21,33] meaning that the temperature of tetragonal-to-cubic (𝛽↔𝛼) phase transitions decreases.…”

“…[14,33] For the GA + -rich composition series (x = 2y), the introduction of substituents causes a progressive drop in the transition temperature of the 𝛾↔𝛽 phase transition, as shown in Figure 2b, so that in x = 0.08, y = 0.04 the tetragonal phase seems to remain stable even below 100 K. Figure 4c shows the same behavior occurring in the FA + = GA + series (x = y), in which it is observed that in x = 0.06, y = 0.06 the tetragonal phase also seems to be maintained even below 100 K. These results are in agreement with what was observed in compositions of the type GA x MA 1-x PbI 3 , where a point of maxima at 𝜖′(T) is already shifted down 100 K in x = 0.10. [33] However, for the FA + -rich series (y = 2x), the drop in the 𝜖′(T) peak in Figure 4d around the phase transition is much less intense when compared with the other series. In none of these compositions was there stabilization of the tetragonal phase below 100 K, and in addition, it is noted that between the compositions x = 0.06, y = 0.12 and x = 0.08, y = 0.16, there is a relative increase in the maximum temperature of 𝜖′(T), which grows even more with increasing degree of substitution.…”