Session B69: Fundamental Properties of Metal Halide Perovskites I

Carrier transfer across interfaces in metal halide perovskite

Perovskite solar cells promise to yield efficiencies beyond 30% by further improving the quality of the materials and devices. Electronic defect passivation and suppression of detrimental charge-carrier recombination at the different device interfaces has been used as a strategy to achieve high performance perovskite solar cells. However, the mechanisms that allow for carriers to be transferred across these interfaces are still unknown.

In this presentation, I will discuss the role of crystal surface structural defects on optoelectronic properties of lead halide perovskites through synchrotron-based techniques. The importance of interfaces and their contribution to detrimental recombination will also be discussed. As a result of these contributions to better understanding 2D and 3D defects, the perovskite solar cell field has been able to improve device performance. Albeit the rapid improvements in performance, there is still a need to understand how these defects affect long term structural stability and thus optoelectronic performance over the long term.

    

Molecular-orientation-dependent bandgap deformation potentials in hybrid perovskites

Different from many conventional semiconductors, the presence of dipolar organic molecules (e.g., MA: CH₃NH₃) in hybrid perovskites introduces a new degree of freedom during deformation. It remains elusive how the molecule interacts with the inorganic lattice while deforming and how this perturbs the electronic structure of hybrid perovskites. In this study, we employ first-principles density functional theory to calculate the bandgap deformation potentials in prototypical hybrid perovskites (MAPbX₃, X=I, Br) with different MA orientations under biaxial strain. We show that the molecular orientation plays a critical role in the bandgap variation during deformation, which may in turn explain an experimental puzzle of larger bandgaps at the hybrid perovskite surface than in bulk. Our comparative study of the all-inorganic counterparts (CsPbX₃, X= I, Br) further demonstrates that the anomalous bandgap deformation in hybrid perovskites is mediated by the organic molecules and is an intrinsic feature of these hybrid materials. The present work improves our understanding of the bandgap deformation potentials in novel hybrid semiconductors, and opens a new degree of freedom in bandgap engineering.   

Observation of spatially resolved Rashba states on the surface of CH3NH3PbBr3 single crystal

Hybrid organic-inorganic perovskites (HOIPs) are prime candidates for studying Rashba effects due to the heavy metal and halogen atoms in their crystal structure coupled with predicted inversion symmetry breaking. Nevertheless, observation of the Rashba effect in cubic CH₃NH₃PbBr₃ single crystals that possess bulk inversion symmetry is the subject of extensive debate due to the lack of conclusive experiments and theoretical explanations. Here, we provide experimental evidence that Rashba state in cubic CH₃NH₃PbBr₃ single crystals at room temperature occurs exclusively on the crystal surface and depends on specific surface termination that results in local symmetry breaking. We demonstrate this using a suite of spatially resolved and depth-sensitive techniques, including circular photogalvanic effect, inverse spin Hall effect, and multiphoton microscopy, that are supported by first principle calculations. Our work suggests using surface Rashba states in these materials for spintronic applications.   

Structure of ABX3: Exploring the PT Phase Diagram

Perovskite photovoltaic ABX₃ systems possess high energy-conversion efficiencies.  Understanding their atomic-level properties will lead to their optimization and utilization in real-world applications. Synchrotron-based structural measurements over a broad range of temperatures and pressures are being conducted.  The detailed studies have revealed new space group symmetries, which have revised the existing assignments. High-pressure measurements reveal multiple low-pressure phases, one of which exists as a metastable phase at ambient pressure. This work should help guide research in the perovskite photovoltaic community by providing the structural details needed to develop accurate theoretical models.   

Nonperturbative lattice effects on electron-hole recombination in lead halide perovskites

Hybrid lead halide perovskites are a class of materials that have unique photophysical properties due to their anharmonic lattices and predominately ionic bonding. Since both electrons and holes are diffusive and strongly couple to an anharmonic lattice, elucidating the nature of how photogenerated electrons and holes bind, dissociate and recombine is theoretically difficult. Subsequently, little is known about the mechanism underpinning observations of low recombination rates and small exciton binding energies. In this work, we develop a Gaussian field theory to describe the effective interactions between electrons and holes as mediated by the perovskite lattice. We find that the inclusion of a spatially dependent screening produces a repulsive exciton interaction at intermediate distances which can be responsible for the decrease in exciton binding energy and the increase in free carrier lifetimes.  We validate this theory using quasiparticle-based path integral molecular dynamics and evaluate the recombination rate and binding energies from free energy calculations. The quasiparticle-based path integral molecular dynamics framework uses an effective mass model of the charge carriers and an atomistic model for perovskite lattice, allowing us to capture all orders of anharmonicity, reducing the computational complexity associated with studying this system, which would be intractable from standard solid state methods.   

The Relationship between Structural Distortions and Spin-splitting in Perovskites

Hybrid metal halide perovskites have emerged as promising optoelectronic materials and are potential hosts of Rashba/Dresselhaus spin-splitting for spin-selective transport and spin-orbitronics. Using a first-principles approach combined with targeted experiments, we here address the microscopic factors that control the spin-splitting magnitude in perovskite systems. By investigating a broad array of chiral and achiral two-dimensional hybrid organic-inorganic perovskites (2D HOIPs), we first demonstrate that a specific bond angle disparity connected with asymmetric tilting distortion of the metal halide octahedra breaks "local" inversion symmetry and strongly correlates with the computed spin-splitting. This distortion metric can serve as a crystallographic descriptor for 2D HOIP selection with significant spin splitting. We next investigate the impact of chiral nanoligand arrangements at slab models of perovskite interfaces, indicative of perovskite nanocrystal (PNC) surfaces. Similar asymmetric tilting distortions can penetrate up to five inorganic layers into the PNCs, potentially connecting to quantum-well-like frontier orbitals located in the deeper layers of PNCs.   

Analysis of a Low T Phase Transition in Cs2AgBiBr6 via Resonance Raman Spectroscopy and Photoluminescence

Halide double perovskites are of interest due to their potential as high efficiency solar cell materials while eliminating the toxicity and stability issues of the current high efficiency perovskites[1]. Out of this family, Cs₂AgBiBr₆ has advantageous characteristics as a solar cell material including strong absorption and long carrier lifetimes[2]. While the structural phase transition at 120 K has been well-documented there appears to be another characteristic temperature near 38 K. Through polarized resonance enhanced Raman spectroscopy, the temperature dependence of the photoluminescence can be analyzed by fitting measured spectra to a series of Voight oscillators. These oscillator fitting parameters show a strong blue shift in the bandgap seen in the photoluminescence, deviating from the typical semiconducting Varshni trend. In addition, the A_1g/A_g phonon displays a hardening of the center frequency below 38 K. The investigation into this low temperature phase will aid in further developing the understanding of the optoelectronic properties of Cs₂AgBiBr₆ and similar halide double perovskites.

[1] F. Igbari et al. Advanced energy materials 2019. 9 (12), 1803150.

[2] A. H. Slavney et.al. Journal of the American Chemical Society 2016. 138 (7), 2138-2141.   

Role of hydrogen vacancies in efficiency of halide perovskites

Defect-induced nonradiative losses are limiting the performance of hybrid perovskite devices.  We use first-principles calculations based on hybrid density functional theory combined with a rigorous formalism for nonradiative recombination to identify defects that are detrimental to efficiency. For the prototypical hybrid perovskite MAPbI₃ (MA=CH₃NH₃) the results indicate that iodine interstitials are most harmful, and hence iodine-rich synthesis conditions should be avoided. Experimental reports have indicated, however, that iodine-poor conditions are also detrimental. We explain this puzzle by demonstrating that iodine-poor conditions lead to formation of hydrogen vacancies on the MA molecule, which act as very efficient nonradiative recombination centers.  By contrast, hydrogen vacancies are not a problem in FAPbI₃ [FA=CH(NH₂)₂], rationalizing why FA is essential for realizing high efficiency in hybrid perovskites.  Our findings also indicate the advantages of avoiding the organic cation altogether.  We show that the common belief that the organic cation suppresses defect-assisted nonradiative recombination is unfounded.  Our study suggests that all-inorganic halide perovskites hold great promise for high-efficiency optoelectronic applications.   

The two-dimensional nature of dynamic disorder in hybrid metal halide perovskite semiconductors

Hybrid metal halide perovskites are a novel class of semiconductor that require anharmonic structural and electronic calculations to explain the contradiction of a soft, defective lattice and remarkable optoelectronic performance of fabricated devices. One consequence of the anharmonicity is octahedral tilting instabilities driving structural phase transitions. These instabilities led to predictions of dynamic domains of the tetragonal phase persisting within the high temperature cubic phase. We utilize single crystal diffuse scattering to probe structural correlations hidden within the cubic phases of CH₃NH₃PbI₃ and CH₃NH₃PbBr₃. Rods of diffuse intensity extend along the Brillouin zone edge, intersecting at the R-points, implying the cubic phase contains 2D regions of dynamically tilted PbX₆ octahedra. Furthermore, we reproduce the diffuse scattering with MD calculations and find that the orientation of organic cations is similarly correlated. Inelastic neutron scattering suggests these regions are long-lived, and we use the MD results to visualize these regions. Finally, we discuss the impact of this 2D network on optoelectronic properties.   

A Comparative Study of Mechanical, Electronic, Optical, and Photocatalytic Properties of CsPbX3 (X= Cl, Br, I) by DFT Calculations

Metal halide perovskites are attracted for next-generation optoelectronic devices but the stability issues hurdle in the path of commercial deployment. In this context, Cs-based perovskites are founded potential candidates for different applications. In this work, a comparative study of mechanical, electronic, optical, and photocatalytic properties of CsPbX₃ (X= Cl, Br, I) are performed by DFT calculations for the applicability of these materials in photocatalytic and optoelectronic devices with better agreements of damage-tolerance. Calculated lattice parameters and bandgaps are in good agreement with experimental data and some properties are found to be much better than other theoretical reports. It is evident that CsPbX₃ are suitable for green, orange, and red emissions respectively due to the proper bandgap. CsPbI₃ is the best photocatalyst for HER, and CsPbBr₃ is the most stable photocatalyst due to its nearly balanced oxidation and reduction potentials but CaPbCl₃ is better for O₂ production. Thus, our findings would be helpful for efficient electronic and optoelectronic device applications in addition to hydrogen production, biodegradation of polluted and waste materials.