Defect-induced local variation of crystal phase transition temperature in metal-halide perovskites

Abstract

Solution-processed organometal halide perovskites are hybrid crystalline semiconductors highly interesting for low-cost and efficient optoelectronics. Their properties are dependent on the crystal structure. Literature shows a variety of crystal phase transition temperatures and often a spread of the transition over tens of degrees Kelvin. We explain this inconsistency by demonstrating that the temperature of the tetragonal-to-orthorhombic phase transition in methylammonium lead triiodide depends on the concentration and nature of local defects. Phase transition in individual nanowires was studied by photoluminescence microspectroscopy and super-resolution imaging. We propose that upon cooling from 160 to 140 K, domains of the crystal containing fewer defects stay in the tetragonal phase longer than highly defected domains that readily transform to the high bandgap orthorhombic phase at higher temperatures. The existence of relatively pure tetragonal domains during the phase transition leads to drastic photoluminescence enhancement, which is inhomogeneously distributed across perovskite microcrystals.

Photoluminescence Tuning Through Irradiation Defects in CH3NH3PbI3 Perovskites

“…Indeed, as we can see from the dependence of the total integrated intensity represented in Figure , the intensity from the reference sample has decreased by about 50% from 10 to 150 K while it has decreased only by 30% in the case of the irradiated sample. This effect is thus very interesting, however limited to the orthorhombic phase at low temperature, as a steep intensity decrease is observed when approaching the phase transition temperature at 160 K. This step in the temperature dependence of the intensity was observed and discussed by other authors and attributed to electronic confinement in remaining tetragonal inclusions within the orthorhombic phase, as observed with micro‐photoluminescence experiments at low temperature . Very interestingly, we could interpret the intensity amplification at low‐temperature as due to a higher number of tetragonal inclusions in the orthorhombic phase after ion irradiation.…”

“…The irradiation point defects that we created may act as anchoring centers for the tetragonal inclusions. These inclusions, having a lower band‐gap than the surrounding orthorhombic phase can act as charge carrier sinks and confine efficiently the excitons . This effect may also be used on purpose to exploit more efficiently the radiative defect inclusions in the goal to realize amplified spontaneous emission and continuous‐wave lasing that were shown to be associated to the emission from tetragonal phase inclusions .…”

Effect of Thermal Cycling on the Structural Evolution of Methylammonium Lead Iodide Monitored around the Phase Transition Temperatures

“…It has been pointed out that the efficiency of the charge carrier trapping sharply decreases in the vicinity of the boundary phase . The apparent transition temperature fluctuates slightly throughout multiple cycles because it is strongly dependent on the local properties of the material and, in particular, on the type and density of structural and chemical defects.…”

“…To summarize, the structural refinement results for the thermal cycling of MAPbI 3 between 140 and 180 K indicate that the first-order phase transition, accompanied by the coexistence of the Tetr and Ort-1 phases, gives rise to domain formation. [23,24] In addition, repeatedly crossing the phase transition leads to a gradual reduction and smoothing of the jump in the average unit cell volume V(avr), which is thermodynamically favorable. The changes throughout the thermal cycling occur due to the formation of the Ort-2 phase-a boundary state between the Tetr and Ort-1 phases.…”

“…It has been pointed out that the efficiency of the charge carrier trapping sharply decreases in the vicinity of the boundary phase. [23] The apparent transition temperature fluctuates slightly throughout multiple cycles because it is strongly dependent on the local properties of the material and, in particular, on the type and density of structural and chemical defects. Indeed, the Ort-1 and Tetr phases have different symmetries, and the actual symmetry of the defects can encourage the formation of either structures; hence, the transition temperature can increase or decrease.…”