Session A63: Structure and Crystallography of Halide Perovskites

Deciphering the nature of anharmonicity in CH3NH3PbI3 by THz range Raman crystallography

Methylammonium lead iodide (CH₃NH₃PbI₃ or MAPI) shows many seemingly contradictory characteristics, like low diffusion but high carrier lifetime, small Urbach energy but defect tolerance etc., which are intimately connected to its anharmonic structural dynamics. Therefore, the fundamental understanding of the anharmonicity in MAPI is critical for harnessing its full potential in future optoelectronic devices. We combine THz-range Raman crystallography and ab initio computation to elucidate the microscopic mechanisms governing the strongly anharmonic structural dynamics in MAPI. Correlation between observed Raman intensity modulations with PbI₆ octahedral tilting patterns provides a direct evidence of highly anharmonic and anisotropic nature of such low-frequency vibrations. The anisotropy and anomalous broadening of the Raman features throughout the tetragonal phase is manifested by athermal anharmonicity. Conversely, thermally induced all-axes relaxational rotation causes anharmonicity and consequently, isotropy in cubic phase. Our study delineates the anharmonic structural dynamics of the technologically relevant tetragonal phase MAPI and offer insights to utilize anharmonicity to engineer favorable optoelectronic properties in materials with long term stability.   

Local Structure of 3D Halide Perovskites at High Pressure

Over the past decade, halide perovskites have emerged as promising materials for solar cells, due to their low-cost synthesis and robust semiconducting properties. Challenges remain in terms of improving the chemical stability of these materials, for instance to air, water, and heat, and in terms of reducing their toxicity, as the best performing halide perovskites still incorporate a lead (Pb) cation. Numerous studies have used compression to investigate structure-property relationships in these materials. However, previous structural investigations have only looked at long-range periodicity changes at high pressure. Here, we have performed in situ synchrotron X-ray total scattering at high pressure on a selection of 3D halide perovskites, including the archetypal composition, methylammonium lead iodide (MAPbI₃), to understand the evolution of their short-range, local structure. Preliminary results indicate local disorder due to octahedral tilting which has not been observable through conventional diffraction experiments. Insight into the local structural behavior has the potential to guide future synthetic efforts toward stable, non-toxic perovskite photovoltaics.   

Bias stress effects in organic-inorganic halide perovskites

The power conversion efficiencies of organic-inorganic halide perovskites (OIHP) have been improved over the last decade using a wide variety of methods, such as composition manipulation, dopant introduction, and interfacial buffers. These methods, however, have taken little regard for the electronic and interfacial effects such alterations may cause within devices under voltage bias stress. A condition required for most device operation. Using two efficient and commonly studied OIHP compositions, CH₃NH₃PbI_2.87Cl_0.13 and Cs_0.1(MA_0.17FA_0.83)_0.9Pb(I_0.83Br_0.17)₃, we investigate the effect of halide and cation substitution in OIHP structures to understand the unique current behavior while under a range of voltage bias stress in both light and dark conditions. With the use of a second device structure, without transport layers, the same bias stress effects unique for the different OIHP structures are observed. Confirming the difference in the current trends, for the two materials, is due to intrinsic behavior of the perovskite material itself, rather than the interfacial recombination and interactions with the transport layers. In this study, we continue to flush out how the changes in morphological defects and ion migration in the two OIHP materials are influencing charge transport.   

Structural phase transitions and photoluminescence mechanism in a layer of 3D hybrid perovskite nanocrystals

Although the structural phase transitions in the single-crystal hybrid methyl-ammonium (MA) lead halide perovskites (MAPbX3, X = Cl, Br, I) are common phenomena, they have never been observed in the corresponding nanocrystals. Furthermore, although these materials are promising for the future electronics, optoelectronics, and solar energy harvesting applications, the nature of quasiparticles governing their unique photoluminescence (PL) and transport properties is still unclear. Here using two-photon excited PL spectroscopy, we provide evidence that the structural phase transitions in a layer of 3D MAPbBr3 nanocrystals may occur at about the same temperatures as those in the corresponding single crystals. We also show that room-temperature PL originates from the radiative recombination of the optical-phonon vibrationally excited polaronic quasiparticles with energies might exceed the ground-state Fröhlich polaron and Rashba energies due to optical-phonon bottleneck. Because of small masses and large radii of these polaronic quasiparticles, their high mobility and long-range diffusion become possible.   

Doping of Halide Perovskites

Methylammonium lead Iodide (MAPbI3) based organic-inorganic hybrid perovskites (OIHP) have emerged as promising solar photovoltaic absorbers, with power conversion efficiency increasing from 3.8% to 23.7% in the last 10 years. Such high efficiency combined with low-temperature solution fabrication process make OIHP promising candidates for the next generation of photovoltaic materials. Current solar cell designs use semi-insulating or low carrier density layers coupled to hole and electron transport contact materials. Controlled doping, both n and p-type, would allow for tuning electrical conductivity and fabrication of p-n homojunctions, opening great opportunities in device applications that go beyond solar cells.
In this presentation we discuss possible approaches to doping of MAPbI3, exploring substitutional and interstitial impurities, on both organic and inorganic sites. Using density functional theory and hybrid functional calculations we explore all possible configurations of the various dopants, analysing formation energies, thermodynamic transition levels, and dopant solubilities.   

Anharmonic Lattice Dynamics in CsPbBr3

CsPbBr₃ is a promising material for use in solar cells and thermoelectrics. The suitability of halide perovskites as photovoltaics derives from the long lifetimes of photoexcited carriers before recombination and it has been theorized that this is due to local polar fluctuations. Furthermore, CsPbBr₃ has been shown to exhibit ultralow thermal conductivity which makes it attractive for use in thermoelectrics. CsPbBr₃ has several lattice instabilities undergoing a cubic to tetragonal transition at ~400K before transforming to an orthorhombic phase at ~360K.

We report inelastic neutron scattering (INS) studies, supported by anharmonic first-principles simulations, on CsPbBr₃ across its three phases. These momentum-resolved measurements allow us to investigate phonon behavior across the Brillouin zone and hence phonon-phonon scattering rates which directly impact thermal conductivity. We find large overall broadening of phonons, even at low temperature, but also a large temperature-dependent anisotropy in the anharmonicity.   

Structural Studies of Halide Perovskite Photovoltaic Systems

The lead halide perovskite systems have been studied intensively recently for applications as photovoltaic materials due to their very high conversion efficiency. To develop accurate models of these systems, it is critical to understand their structural phases and atomic level properties. High-resolution single crystal x-ray diffraction, Raman spectroscopy, x-ray absorption spectroscopy, x-ray pair distribution function measurements, and differential scanning calorimetry measurements are being used to determine the long-range and local structures and assess the stable phases. A discussion of the structure on different length scales over a broad temperature range will be given.   

Particle Ordering and A-site Composition Effects in Hybrid Organic-Inorganic Metal Halide Perovskite Quantum Dot Films

The promising optoelectronic properties of bulk metal halide perovskite semiconductors have motivated recent developments in nanoscale and quantum well perovskite congeners, including layered structures and nanocrystalline quantum dots (QDs). In addition to the ability to tune the optical bandgap and improve photoluminescence efficiency, confining the nanocrystalline dimensions has proven valuable in accessing compositions^[1] and crystallographic phases^[2] that are unstable in the bulk.
Herein, we report on structural studies of FA_xCs_1–xPbI₃ (FA = formamidinium) QD films by synchrotron X-ray scattering techniques. 15-nm FA_xCs_1–xPbI₃ QDs (x = 0, 0.1, 0.25, 0.5, 0.75, 1) were deposited from colloidal solution and investigated by grazing incidence wide-angle X-ray scattering (GIWAXS). GIWAXS patterns indicate coherent particle ordering on the substrate for thin spin-coated films, while the subsequent ligand exchange and particle depositions lead to isotropic ordering of the particles. The patterns also show evidence of crystallographic phase inhomogeneities and a transition from cubic (FAPbI₃) to tetragonal (Cs_0.5FA_0.5PbI₃) to orthorhombic (CsPbI₃) character in the composition series.
[1] A. Hazarika, et al., ACS Nano 2018, 12, 10327.
[2] A. Swarnkar, et al., Science 2016, 354, 92.   

Stress effects on vibrational spectra of orthorhombic and tetragonal hybrid perovskites.

Strain plays an important role in semiconductor performance and stability. Strains may develop in organic metal-halide perovskites which affect carrier mobility, non-radiative recombination, degradation and other optoelectronic properties. Measuring spatially varying strains is difficult but imperative for understanding these effects. We have used DFT to investigate effects of applied strain on the vibrations of tetragonal and orthorhombic methylammonium lead iodides (MAPI), building on our previous study of the high-temperature pseudo-cubic phase [arXiV:1907.03673]. Applying small uniaxial strains along three crystal axes, we have analyzed changes in frequency and phonon displacement patterns. We identified favorable modes for experimental measurements of local strain by Raman microscopy, and we calculated mode Grüneisen and Grüneisen parameters in different directions to connect with reports of negative thermal expansion in c-direction. Our study gives insight into the interaction between strain, structural changes and vibrational modes which may help to understand degradation   

Pressure-induced Phase Changes in Cesium Lead Bromide Perovskite Nanocrystals with Planar Defects

Lead halide perovskites have a rich landscape of structural and optical properties, which can be explored and possibly controlled by applying high pressure. CsPbBr₃ NCs with Ruddlesden-Popper (RP) faults, formed via post-synthetic fusion growth, are significantly larger in size than as-synthesized NCs and display exceptional emission stability. We compare synchrotron-based high pressure X-ray diffraction and photoluminescence (PL) properties of CsPbBr₃ (without RP) and RP-CsPbBr₃ (with RP), and resolve their crystal structure under pressure for the first time. CsPbBr₃ undergoes a phase transition from ambient orthorhombic (S.G: Pnma) to cubic (S.G: Pm-3m) phase at 1.7 GPa and RP-CsPbBr₃ transforms from Pnma to monoclinic (S.G: P2₁/m) phase at 0.74 GPa in addition to several isostructural transitions. Density-functional calculations predict a narrowing of the band gap with pressure, concomitant with the PL energies. The RP-CsPbBr₃ NCs exhibit enhanced PL intensity at 1 GPa and show band gap opening at high pressures. This study opens new strategies for tuning not just the structural properties but also tuning planar defects in alkali lead halide crystals for improved optical properties.   

Origin of Edge Emission in Two-Dimensional Ruddlesden-Popper Hybrid Perovskites

Two-dimensional (2D) Ruddlesden-Popper (RP) perovskites, with high quantum efficiency, good photostability and chemical stability, are promising for optoelectronic devices. Blancon et al. reported firstly the lower photoenergy edge emission at those (BA)₂(MA)_n-1Pb_nI_3n+1 2D RP perovskites when n≥3 in 2017[1]. Then, significant attention was attracted to explore the mechanism of edge emission. A theoretical work following them was published to assert that this edge emitting originates from interface strain[2]. After that, another work declared that the edge emission is induced by water molecules[3]. The discussion of Zhao et al. indicated that the loss of part BA at edges should be responsible for the edge emission[4]. While, a reliable and consistent explanation is still missing till now. In this work, we observe the edge emission in 2D exfoliated perovskites and clarify the mechanism of edge emission. This work reveals the nature of the edge emission and pave a way for developing the new optoelectronic devices based on the 2D RP hybrid perovskites.
[1]Blancon J. C., et al., Science, 2017, 355, 1288-1292.
[2]Kepenekian, M., et al., Nano Lett, 2018, 18, 5603-5609.
[3]Shi, E., et al., ACS Nano, 2019, 13, 1635-1644.
[4]Zhao, C., et al., J Phys Chem Lett, 2019, 10, 3950-3954.   

Stabilization of a black CsPbI3 Perovskite phase via octahedral tilting control with pressure

The air-stable, optically active CsPbI₃ perovskite phase is one of the promising candidates for applications in solar cells. However, the black CsPbI₃ perovskite is thermodynamically unstable and spontaneously converts to a yellow non-perovskite phase at room temperature. We report that a black perovskite phase can persist at room temperature by tuning the tilt of [PbI₆]^4- octahedra of high temperature perovskites with pressure, which shows improved stability and remains unchanged after releasing pressure to ambient conditions. Synchrotron X-ray diffraction, Raman spectroscopy, and photoluminescence measurements indicate that the preserved CsPbI₃ crystallizes into an orthorhombic perovskite structure and has a robust PL signal at ~702 nm. First-principles calculations reveal that the tilt of the [PbI₆]^4- octahedra play a significant role on stabilizing CsPbI₃ perovskite to room temperature. Our results present a promising approach to prepare superb stable black CsPbI₃ for perovskite solar cells.   

Monitoring Electron−Phonon Interactions in Lead Halide Perovskites Using Time-Resolved THz Spectroscopy

Lead halide perovskites have low-frequency phonon modes within the lead halide sublattice and thus are considered to be soft. The soft lattice is considered to be important in defining their interesting optoelectronic properties. Electron−phonon coupling governs hot-carrier relaxation, carrier mobilities, carrier lifetimes etc. Directly observing the interplay between free charge carriers and phonons can provide details on how phonons impact these properties. Here, we observe a delicate interplay among carriers, phonons, and excitons in mixed-cation and mixed-halide perovskite films by simultaneously resolving the contribution of free carriers and phonons in time-resolved THz photoconductivity spectra. We observe directly the increase in phonon population during carrier cooling and discuss how thermal equilibrium populations of carriers and phonons modulate the carrier transport properties, as well as reduce the population of carriers within band tails. We are also able to observe directly the formation of free carriers when excitons interact with phonons and dissociate and to describe how free carriers and exciton populations exchange through phonon interactions. Finally, we also time-resolve how the carriers are screened via the Coulomb interaction at low and room temperatures.   

Stabilizing Metal-Halide Perovskites via Nanoconfined Crystallization

Metal-halide perovskites undergo multiple polymorph transitions, with the smallest bandgap phases thermodynamically favored at elevated temperatures. We explore nanoconfinement as a strategy to shift the thermodynamics of polymorph transitions in order to stabilize high-performance phases against temperature-induced polymorph transitions and humidity-induced degradation.¹ Specifically, when crystal sizes are reduced, the surface free energy contribution to the total Gibbs free energy of the crystals becomes increasingly important. By exploiting the dependence of the surface free energy on the symmetry of the crystal structure, it is possible to shift polymorph transitions to lower temperatures under nanoconfinement compared to the bulk.^2,3 These nanoconfined crystals also exhibit excellent stability against humidity-induced degradation, with no change in their X-ray diffraction patterns over a period of at least two years of storage in air.

¹X. Kong, K. Zong, S.S. Lee. Chem. Mater., 31, 4953 (2019).
²X. Kong, K. Shayan, S. Lee, C. Ribeiro, S. Strauf, S.S. Lee. Nanoscale, 10, 8320 (2018).
³X. Kong, K. Shayan, S. Hua, S. Strauf, S.S. Lee. ACS Appl. Energy Mater., 2, 2948 (2019).   

Dimensional Confinement-Deconfinement Along the Crystallographic C Axis: A Desorption Mediated Band Engineering of Methylammonium Lead Iodide

Strong optical absorption, long diffusion length, benign intrinsic defects established organic-inorganic hybrid perovskites as one of the most important materials in the area of photovoltaics. Unfortunately, the formation of photogenerated halide rich trap centers during conventional halide exchange methods limits the use of hybrids perovskite in multi-junction solar cells.
In this report, we demonstrate band gap engineering of MAPbI₃ film due to systematic dimensional deconfinement along the crystallographic c axis during gas induced growth. There are some distinct interrelated events occurring during gas induced reaction, which involves (i) in situ formations of hexylamine from 2D (HA)₂PbI₄ upon exposure to methylamine, (ii) selective adsorption of hexylamine on the Pb sites of growing MAPbI₃ leading to the anisotropic growth, (iii) systematic desorption of hexylamine during exposure of methylamine leading to continuous band gap tuning (2.18 eV to 1.69 eV). Most importantly, this dimensional confinement-deconfinement leads to extremely smooth, homogeneous film without having the halide rich trap centres, thereby establishing the importance of this novel route in the context of hybrid perovskite photovoltaics.