Abstract
Although structural phase transitions in single-crystal hybrid methyl-ammonium (MA) lead halide perovskites (MAPbX3, X = Cl, Br, I) as a function of temperature are common phenomena, they have never been observed in the corresponding nanocrystals. Here, we demonstrate that two-photon-excited photoluminescence (PL) spectroscopy is capable of monitoring structural phase transitions in MAPbX3 nanocrystals because nonlinear susceptibilities govern the incident light absorption rates. We provide experimental evidence that the orthorhombic-to-tetragonal structural phase transition in a single layer of 20-nm-sized 3D MAPbBr3 nanocrystals is spread out within the T ∼ 70 K–140 K temperature range. This structural phase instability is believed to arise because, unlike in single-crystal MAPbX3, free rotations of MA ions in the corresponding nanocrystals are no longer restricted by a long-range MA dipole order. The resulting configurational entropy loss can be even enhanced by the interfacial electric field arising due to charge separation at the MAPbBr3/ZnO heterointerface, extending the structural phase instability range from T ∼ 70 K–230 K. We conclude that weak sensitivity of conventional one-photon-excited PL spectroscopy to structural phase transitions in 3D MAPbX3 nanocrystals results from structural phase instability and hence from negligible distortions of PbX6 octahedra. In contrast, the intensity of two-photon-excited PL and electric-field-induced one-photon-excited PL show higher sensitivity since nonlinear susceptibilities are involved. We also show that room-temperature PL may originate from the radiative recombination of the optical-phonon vibrationally excited polaronic excitons with energies might exceed the ground-state Fröhlich polaron and Rashba energies due to optical-phonon bottleneck.
Highlights
Hybrid methyl-ammonium (MA) lead halide perovskites (MAPbX3, X = Cl, Br, I) represent a class of materials offering an illustrative platform for studying the relaxation dynamics of photoexcited carriers and their transport phenomena in novel highly efficient solar cells for solar energy harvesting technology.1–11 One of the most important specific features of these hybrid materials is that their crystalline structure can be viewed as two alternating sublattices
We conclude that a step-like scitation.org/journal/adv red-shift of the PL band with increasing temperature observed for single-crystal MAPbX3 and assigned to the structural phase transition range does not appear anymore in 3D MAPbX3 nanocrystals because of negligible distortions of PbX6 octahedra under the structural phase instability regime
One can recognize that structural phase transitions in 3D MAPbBr3 nanocrystals may occur at about the same temperatures as those in single-crystal MAPbBr3
Summary
Scitation.org/journal/adv providing an ultralow thermal conductivity being caused by a long-range MA dipole order. The structural peculiarities of these materials allow for three structural phase transitions occurring in the temperature range of T ∼140 K–240 K, which usually appear in single-crystal MAPbX3 and its polycrystalline thin film, whereas they have never been observed in MAPbX3 nanocrystals since local field fluctuations arising due to free rotations of MA ions are no longer restricted by a long-range polar order.. The second approach introduces the LO-phonon bottleneck effect dealing with the suppression of the anharmonic three-phonon LO-phonon decay (Klemens/Ridley) process involving acoustic phonon branches.34,35 This effect is controlled by thermal conductivity between the sublattices and turns the equilibrium free carrier dynamics into the non-equilibrium one since it allows the carriers in the light-emitting states to reabsorb LOphonons.. One can expect that owing to the reabsorption of multiple TO/LO-phonons, the TO/LO-phonon vibrationally excited polaronic quasiparticles will be formed, and their Fröhlich polaron energies will be progressively reduced with increasing temperature relative to the ground state Fröhlich polaron energy The latter process is expected to appear as a blue-shift of the PL band, leading to a reduction in polaron masses and to an increase in polaron radii. Because of small masses and large radii of these vibrationally excited polaronic quasiparticles, their high mobility and long-range diffusion at room temperature become possible
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