Session Q41: Synthesis and Spectroscopy

Modulating defects in metal halide perovskites using lattice strain

Defects play a pivotal role in controlling the performance of metal halide perovskite solar cells. Of all point defects in the prototypical methylammonium lead iodide perovskite, iodine interstitials have been identified as the main source of non-radiative recombination, leading to a loss of power conversion efficiency. In this work, we use first-principles calculations to investigate the modulation of defect properties, such as transition levels by the application of isotropic and epitaxial lattice strain. We also compare the properties of defects obtained using exchange-correlation functionals at the generalized gradient approximation and the hybrid level.   

Structural and Band Level Alignment of CsPbBr3 / Graphene and CsPbI3 / Graphene Heterostructures from First Principles

Heterostructures involving lead halide perovskites have shown impressive potential for many applications, primarily for high efficiency photovoltaics. This work considers graphene as an interfacial layer in contact with CsPbI₃ and CsPbBr₃ perovskites. First principles calculations are used to model the electronic structure and band level alignment of CsPbI₃ / graphene and CsPbBr₃ / graphene heterostructures. Since this analysis requires the creation of large supercells, the effect of the structural mismatch and induced strain during supercell construction is examined. This examination concludes the strain necessary to form the supercells has minimal effects on the perovskite properties. The band level alignment of eight perovskite / graphene supercells with up to 560 atoms is predicted using spin-orbit coupled hybrid density functional theory. The graphene Dirac point consistently lies inside the perovskite bandgap, validating the utility of the graphene layer for charge extraction. Furthermore, the PbX₂ (X=Br, I) terminated (010) crystal plane in both perovskites shows stronger interaction with the graphene than the CsX terminated (010) plane, as evidenced by adsorption energy calculations and confirmed by the graphene Fermi velocity. The graphene Dirac point lies closer to the perovskite CBM for the PbX₂ termination and closer to the VBM for the CsX termination.   

Electric Field Effects in MAPbI3 Perovskite via Picosecond Microscopy Techniques

We present time-resolved kerr rotation (TRKR) and photoluminescence (PL) data exhibiting electric field effects in thin film MAPbI3 (MAPI) perovskite. These data were recorded using a unique picosecond microscopy technique, with a spatial resolution of 2 microns and temporal resolution of 1 picosecond. Results show that upon application of electric fields on the order of 10^6 V/m, the amplitude of the PL decreases linearly with the applied field up to a reduction of at least 20%. Additionally, the measured lifetime T2* of the quantum beating signal in MAPI decreases by at least 33% under the influence of an applied electric field. We also further elaborate on the potential mechanisms by which the electric field may lead to the measured effects.   

First-principles investigation of Peierls-like distortions and charge disproportionation in complex halide perovskites

Halide perovskites are attractive materials for solar energy conversion applications due to their strong light-matter interactions, structural tunability, and the relative ease with which they can be synthesized and processed. Recent calculations and measurements on mixed-valent halide perovskites reveal Peierls-like distortions and charge disproportionation as a function of externally applied isotropic pressure. However, a detailed picture of the underlying microscopic mechanism, particularly the electronic states which drive distortion, as well as a characterization of material classes for which these effects occur, are still missing. In this talk, we use first principles density functional theory calculations with hybrid functionals and DFT-parameterized reduced Hamiltonians to investigate how pressure and atomic substitutions affect the electronic structure. Our findings provide insight into the precise nature of the localized states which drive charge disproportionation. Our calculations set the stage for understanding charge transport mechanisms in this important class of materials and further guide the design of next-generation halide perovskite-based photovoltaic devices.   

Magnetism of doped Metal Halide Perovskites.

We investigate the effect of transition metal doping of BaZrS3 and HfSnS3 as a prototype of new DMS material. BaZrS3, a prototypical chalcogenide perovskite semiconductor shows room temperature ferromagnetism upon doping by Mn and Fe elements. Density Functional calculations of exchange coupling in the system show substantial increase in exchange coupling strength upon electron doping due to the presence of Sulphur vacancies. We present a model of exchange coupling mediated by electrons when conduction band has mainly d-character.

    

Exciton-phonon coupling in the lead-free double-perovskite Cs2InAgCl6

The lead-free halide double perovskite Cs₂InAgCl₆ possesses a wide direct band-gap exceeding 3 eV, and exhibits very broad and strong photoluminescence (PL) in the visible region [G. Volonakis, et al.,  J. Phys. Chem. Lett. 8, 772 (2017)]. This property has been employed to realize white-light-emitting diodes with near-unity PL quantum yield [J. Luo, et al., Nature 563, 541 (2018); S. Li et al., ACS Appl. Mater. Interfaces 12, 46330 (2020)]. The atomic scale mechanisms of light emission in this compound remain a subject of debate, with proposals ranging from self-trapped excitons to defect-assisted luminescence. To shed light on this debate, in previous work [V.-A Ha, et al., J. Phys. Chem. C 125, 21689 (2021)] we investigated the electronic band structure of Cs₂InAgCl₆ using GW method, and the temperature-dependent band gap renormalization via electron-phonon interactions within special displacement method. Here, we extend this study to include excitonic effect via the Bethe-Salpeter equation, and their coupling to phonons via the special displacement method. We will discuss our results in relation to the observed PL signal.   

Ionic Transport in Perovskite Light-Emitting Electrochemical Cells and Performance Dynamics

We studied the ionic defect transport and performance dynamics of Perovskite Light-Emitting Electrochemical Cells with impedance spectroscopy and lifetime measurements. Utilizing additive ions from salts such as LiPF6 in our devices increases efficiency and lifetime. We posit that the additive ions are more mobile via a lower barrier of diffusion energy and allow the intrinsic perovskite ions to remain intact more so than without the additive ions serving as sacrificial ions that accumulate at the interfaces to produce p-type and n-type regions at the injection interfaces under electrical bias. The eventual degradation in performance is studied and is thought to be an over accumulation of ionic defects at the interface and a reduction in an optimal radiative recombination region or an unbalanced concentration of electronic charge carriers. Using Warburg diffusion impedance elements, we are able to extract the diffusion coefficients and concentrations of multiple ionic species within the perovskite layer. Using temperature dependent measurements we're able to extract diffusion barrier energies and enthalpies of formation. Diffusion elements are used in equivalent circuit models for fitting impedance spectroscopy measurements with good fits and low error of fitting parameters.   

Vapor Transport Deposition of Metal-Halide Perovskites for Photovoltaic Applications

Metal-halide perovskites are a promising semiconductor for use in optoelectronic and photovoltaic applications but require the identification of scalable manufacturing methods to achieve broad commercialization. In this work, we demonstrate solar cells based on an active layer of methylammonium lead iodide co-deposited via vapor transport deposition (VTD). VTD utilizes a hot-walled reactor operated under moderate vacuum in the range of 0.5-10 Torr. The organic and metal-halide precursors are heated and then transported by a nitrogen carrier gas to a cooled substrate where they condense and react to form a perovskite film. Our system design enables different source temperatures and carrier gas flow rates for the individual precursors, allowing for simultaneous deposition of both precursors and control over precursor ratio in the final perovskite film. This talk will discuss methods to control perovskite film morphology and phase purity by engineering VTD operating parameters including the carrier gas flow rate, chamber pressure, and substrate temperature. The impact of VTD processing conditions and film composition on solar cell performance will be described, with emphasis on tuning film composition via the substrate temperature and the precursor carrier gas flow rate ratio to optimize efficiency.   

Engineering Electrooptical Properties of 2 Dimensional Hybrid Perovskites with Pressure

Hybrid perovskites are a new class of functional materials with potential applications in various technologically important areas, such as solar cells, LED and lasing. Compared with their 3D counterparts, 2 dimensional perovskites exhibit special properties, e.g. their natural quantum-well structure yields stable excitons, able to interact more strongly with phonons, spins and defects. Layered perovskites are also more structurally stable.

High pressure induces a rich variety of phenomena in the electronic and optical properties in the hybrid Perovskite materials, including enhancement of photoluminescence (PL) by at least two orders of magnitude, narrow to white light emission, rich PL energy and lifetime changes. The optical properties of 2D perovskites are mainly governed by excitons, and the investigation of intrinsic and extrinsic, radiative and nonradiative exciton recombination pathways is essential. Lattice vibrations create spatial and temporal potential fluctuations, that governs exciton-phonon interactions. In this talk, I will illustrate how high pressure can be used to tune the electrooptical properties of the hybrid perovskite materials, and how such study can provide some guidance in the synthesis of new materials.

    

First-Principles Survey of Acceptor Dopants for p-Type Cesium Lead Bromide

All-inorganic lead halide perovskites such as cesium lead bromide (CsPbBr₃) are high-performance light emitters with applications in lighting, displays, and quantum information. However, it has proven difficult to achieve high p-type carrier concentrations in these materials. In this work [1], a wide variety of possible acceptor dopants are evaluated using first-principles calculations. Connecting the results with previous work on native defects [2], we show that p-type doping is difficult because of two related effects: the moderately high formation energies of acceptor impurities and compensation from native defects, particularly lead−cesium antisites and bromine interstitials. Sodium and silver are identified as the most promising dopants for overcoming these challenges, and optimum chemical potential conditions for acceptor doping are identified.   

Quantifying hidden symmetry in the tetragonal CH3NH3PbI3 perovskite

The assignment of an exact space group to the tetragonal CH₃NH₃PbI₃ perovskite structure is experimentally challenging and controversial in the literature. One issue is that symmetry depends on the relevant spatial and time scales due to the easy rotation of the CH₃NH₃⁺ ions within the Pb-I cage, which is not captured in a static density functional theory calculation. As a result, the structure does not have any symmetry. Nevertheless, it is clear that some kind of approximate symmetries are present, evidenced by the quasi-I4cm and quasi-I4/mcm structures commonly used in calculations. In this work we have developed a methodology to quantify the hidden symmetry of these structures using group theory, to enable use of symmetries in understanding spectroscopy. We study the approximate symmetry of vibrational modes, as well as of the dielectric, elastic, electro-optic, Born effective charge, and Raman tensors and the dynamical matrix, including analysis of degenerate representations and validity of classes. Comparing to each sub-point group, our results show that the quasi-I4cm is best described by C_2v, whereas the quasi-I4/mcm is well described by the expected C_4v. Our methodology can useful generally for any approximately symmetric material.   

A Raman Scattering Investigation of Halide Rb4Ag2BiBr6

Halide perovskites have reached photovoltaic efficiencies near the theoretical maximum but due to toxicity and stability issues other alternatives have been sought out. The double perovskite structure has been found to address these issues however the strong exciton-phonon coupling reduces carrier transport increasing electron-hole recombination rates[1]. Rb₄Ag₂BiBr₆ has been proposed as an alternative structure type to the halide double perovskites. The crystal structure is similar to that of

double perovskite Cs₂AgBiBr₆ which consists of alternating BiBr₆ octahedral groups surrounding a central cation, with Rb₄Ag₂BiBr₆ containing additional AgBr₅ square pyramids. Using a controlled cooling synthesis technique[2] Rb₄Ag₂BiBr₆ crystals have been successfully synthesized. The composition has been confirmed via X-ray powder diffraction and Energy Dispersive X-ray spectroscopy. Temperature and polarization dependent Raman scattering spectroscopy measurements will be presented to analyze the vibrational modes.

[1] C.N. Savory et al. ACS Energy Letters. 1 (5), 949−955, 2016.

[2] L. Yin et al. Advanced Optical Materials. 7 (19) 1900491, 2019.

    

Computational study of intrinsic defects in 2D hybrid perovskite phenethylammonium lead iodide

2D hybrid organic-inorganic perovskites (HOIPs) are exciting semiconductors for optoelectronic device applications due to their high measure of structural tunability. Electronic doping of HOIPs is an essential technique to control carrier concentrations in semiconductors. Conversely, intrinsic defects can adversely affect electronic doping efficiencies, e.g., by acting as carrier traps. We here present a systematic study of intrinsic point defects in phenethylammonium lead iodide (PEA₂PbI₄), both in isolation and as combined defects. Hybrid density functional theory (DFT) is used to calculate energy band structure calculations including spin-orbit coupling, as well as charge transfer between different prospective point defects, both intrinsic and in combination with Bi as a prototypical extrinsic dopant.