Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session Y41: Defects in Perovskites |
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Sponsoring Units: DMP Chair: Christopher Ciccarino, Stanford University Room: Room 319 |
Friday, March 10, 2023 8:00AM - 8:12AM |
Y41.00001: Alloy-based tuning of the bandgap and exciton binding energy in perovskites John S Colton, Kameron Hansen, Michele Eggleston, Carrie E McClure, Blake Romrell, Carter Shirley, Luisa Whittaker-Brooks We have used the technique of electroabsorption (EA) to study the exciton binding energy and other properties of 2D metal-halide perovskites (MHPs). EA is a measurement of the difference in absorption between electric field on and off, and in particular can be used as a precise measurement of the bandgap (Eg), and hence also the exciton binding energy (Eb). In this presentation we will include a short introduction to EA and then focus on changes to Eg and Eb that result in PEA2PbI4 when alloying Sn with Pb and for using combinations of I, Br, and Cl as the halogen atom. We find evidence of a co-existence of elemental-rich domains within the Pb-Sn and Br-I alloys, and are able to use the Br:Cl ratio of the Br-Cl mixtures, PEA2Pb(BrxCl1-x)4, to tune the band gap energies continuously between 3.076 and 3.643 eV and the exciton binding energies from 349 meV to 487 meV. |
Friday, March 10, 2023 8:12AM - 8:24AM |
Y41.00002: Screening of the Electron-Hole Interaction in Two-Dimensional Perovskites Kameron Hansen, John S Colton, Carrie E McClure, Andre Schleife, Cindy Wong, Luisa Whittaker-Brooks In hydrogen, the field between the electron-proton pair occurs across the vacuum. However, for an electron-hole pair within a semiconductor, the field occurs across a variety of neighboring particles such as valance electrons, phonons, and ions. A long-standing question has been, which of these neighboring particles contributes to the screening of the electron-hole interaction? Two-dimensional perovskites provide an ideal template to explore this question due to their large exciton binding energies (Eb) and highly frequency-dependent dielectric properties. Using density functional theory calculations, dielectric spectroscopy, and electroabsorption spectroscopy on over 30 distinct perovskite structures, we determine the frequency range of the dielectric spectrum that modulates Eb. Additionally, we answer several long-standing questions regarding dielectric and exciton properties in this important class of optoelectronic materials, namely, the dielectric values of the barrier and well layers, the value of Eb for the popular BA2PbI4 compound, the point where 2D MHPs transition from isolated quantum wells to superlattices, the correct model for describing exciton states in 2D MHPs, and the following effects on Eb: temperature, sample morphology, octahedral tilting, barrier polarizability, well polarizability, barrier length, and well length. |
Friday, March 10, 2023 8:24AM - 8:36AM |
Y41.00003: Ion migration in 2D and 3D Metal Halide Perovskites studied using Electrochemical Impedance Spectroscopy on an Interdigitated Electrode Geometry Carrie E McClure, Kameron Hansen, Chapman Lindsay, Carter Shirley, John S Colton Metal halide perovskites are a class of 2D and 3D materials that are currently being studied for use in solar energy and other applications. Permittivity is connected to important characteristics of these materials such as exciton binding energy, effective mass, and transition dipole moment, but has proven challenging to accurately measure. Using experimental measurements of the impedance of perovskite layers on interdigitated electrodes, we have been able to develop a model which allows us to deduce the frequency dependence of permittivity in these materials. The effects of ion migration are additionally studied through electrochemical impedance spectroscopy, and thickness and temperature effects on complex dielectric function. |
Friday, March 10, 2023 8:36AM - 8:48AM |
Y41.00004: Trends in acceptor dopants for halide perovskites John L Lyons, Michael W Swift The halide perovskites, such as cesium lead bromide (CsPbBr3) and methylammonium lead iodide (MAPI), are expected to be high-performing light emitters, with potential applications including lighting, displays, and quantum information. All of these applications would benefit from having fully controlled electrical conductivity, but it has proven difficult to achieve high p-type carrier concentrations in these materials. In this work, possible acceptor dopants are evaluated in the halide perovskites, using first-principles calculations based on a hybrid functional with spin-orbit coupling. This approach not only leads to accurate predictions for band structure and band offsets, but also provides a better description of dopant properties. We assess here whether acceptor dopants (such as Ag, Na, and Cu) can act as shallow acceptors on the proper substitutional site, and whether other configurations of these impurities might lead to compensation of potential p-type conductivity. Among the dopants considered, sodium and silver are identified as the most promising acceptors for achieving p-type conductivity, and optimum chemical potential conditions for these dopants are identified. |
Friday, March 10, 2023 8:48AM - 9:00AM |
Y41.00005: Persistent photoconductivity in perovskite oxide semiconductors Matthew D McCluskey The interaction of transparent oxide semiconductors with light is a central issue for a range of applications. Persistent effects could be exploited for holographic memory or optically defined circuits. They may also be detrimental to device operation. Large, room-temperature persistent photoconductivity (PPC) was discovered in strontium titanate (SrTiO3, STO) after annealing in a hydrogen-containing atmosphere. Barium titanate (BaTiO3, BTO), a ferroelectric material, was recently found to exhibit PPC. Recent results will also be presented on potassium tantalate (KTaO3, KTO). The findings indicate that large PPC can occur in many materials and that hydrogen plays a central role. |
Friday, March 10, 2023 9:00AM - 9:12AM |
Y41.00006: Vibrational study of hybrid organic-inorganic perovskites Haritha Sindhu Rajeev, Xiao Hu, Seunghun Lee, Joshua J Choi, Mina Yoon Metal halide perovskites (MHPs) have achieved striking success as low-cost photovoltaic and light-emitting devices owing to their long carrier lifetimes, long carrier diffusion lengths, and exceptional carrier protection from defects. Our recent study on the three-dimensional (3D) hybrid organic-inorganic MHPs (MAPbI3, MA=CH3NH3), has shown that the reorientation of the polarized molecules can assist the polaron formation, and thus prolong the charge carrier lifetime. Yet the purely inorganic MHPs without organic molecules can also achieve a moderate photovoltaic performance, which indicates that vibrations of the inorganic perovskite framework must also play indispensable roles in the optoelectronic properties of MHPs through their interactions with the charge carriers. In general, there are two scenarios for the phonon-mediated polaron formation: (1) polaron formation facilitated by optical phonons and (2) overdamping of acoustic phonons. Upon heating, acoustic phonons in the sample get overdamped which reduces their interactions with charge carriers and charge carriers can survive for a longer time. Additional question is which momentum directions such phonon melting, or overdamping would occur in MHPs. These questions call for detailed studies on phonon dispersion relations as a function of temperature on MHPs using inelastic neutron scattering. We would be presenting our results from inelastic neutron scattering measurements on partial-deuterated powder sample of MAPbI3 and non-deuterated single crystal of MAPbBr3. Both results showed that the well-defined optical phonons get overdamped in the material’s tetragonal phase (above 165 K for MAPbI3 and 145 K for MAPbBr3), which coincides with polaron formation in these materials. We would also present our results from DFT calculations to show phonon modes along specific Brillouin zone directions as a function of temperature throughout all its structural phases. |
Friday, March 10, 2023 9:12AM - 9:24AM |
Y41.00007: Direct Evidence of Polaron Formation in Halide Perovskites via Carrier Effective Mass Measurements Di Bao Carrier effective mass is a central parameter in solid-state physics. It is a measure of the strength of the coupling between a carrier and excitations arising from its surrounding medium, and features prominently in transport and optical calculations. Experimental techniques employed to determine it are steady-state ones, and so are unable to detect any change in the effective mass after a strong perturbation to the system, e.g. strong optical excitation. By combining time-resolved terahertz spectroscopy and transient absorption spectroscopy, on a mixed-cation mixed-halide perovskite thin film, we observed a large and long-lived photoinduced enhancement of the carrier effective mass, and from it deduced a two-fold increase of the carrier-phonon coupling constant, giving direct evidence of polaron formation. Our work demonstrated a new approach to track the strength and ultrafast lifetimes of photoinduced carrier-boson interactions down to picosecond timescales, that can be applied to a wide range of solid-state systems. |
Friday, March 10, 2023 9:24AM - 9:36AM |
Y41.00008: Exploring the PT Structural Phase Diagram of ABX3 Perovskite PV Materials Sizhan Liu, Stella Chariton, Vitali Prakapenka, Zhenxian Liu, Tieyan Chang, Yu-Sheng Chen, Sanjit Ghose, Yong Yang, Sandun Amarasinghe, Trevor A Tyson Perovskite photovoltaic (PV) 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. Detailed studies have revealed new space group symmetries. Structural and thermodynamic properties are related using experimental measurements. The results may help address stability issues in these materials and also provide the structural details needed to develop accurate theoretical models deeded to predict optimized materials. |
Friday, March 10, 2023 9:36AM - 9:48AM |
Y41.00009: Fine Structure of Excitons in Vacancy Ordered Double Perovskites Bruno Cucco, Claudine KATAN, Jacky Even, Mikael Kepenekian, George Volonakis A2BX6 vacancy ordered double perovskites (VODP) are air-stable Pb-free materials that have been proposed as alternatives to halide perovskites. Yet, to-date VODP are far away from the high performance of their Pb-based counterparts. In this work, we analyse the role of electron-hole (e-h) interactions in VODP by employing state-of-art G0W0 method and Bethe-Salpeter equation (BSE) calculations. We unveil key details of their electronic structure and show the strong impact of e-h interactions on their optical properties. The family is sampled by picking prototypes based on the valency of the tetravalent metal that sits at the B-site of the A2BX6 lattice. The structural properties are investigated, and we show that the size of the vacancy depends solely on the size of the halogen. We report exciton binding energies ranging from 190 to 1240 meV and analyse the exciton fine structure for each material. We perform a symmetry analysis of the band structures and exciton wavefunctions, on which a direct link between these and the metal site species is established. Finally, we identify the most and least promising candidates to act as photo-active materials for selective charge transport layers in light-emitting and solar-cell applications. Our findings provide fundamental understanding of the optical properties of the entire VODP family of materials and highlight how these are not suitable Pb-free alternatives to traditional halide perovskites. |
Friday, March 10, 2023 9:48AM - 10:00AM |
Y41.00010: One-dimensional metal halides with edge-sharing octahedra at high pressure Yu Lin Halide perovskites represent an extensive family of materials with chemical and structural versatility and display remarkable optical and electronic properties that find applications for a wide range of technologies. The structural tunability of halide perovskites is also reflected in the ability to derive a wide variety of related, low-dimensional halide perovskites and analogous systems. Here I will focus on two examples where lattice compression is used to modulate the electronic structures and search for intriguing electronic states in one-dimensional metal halides with edge-sharing octahedra. i) We demonstrate that rare electron-electron interaction enhanced by Cs-mediated electron redistribution plays a direct and prominent role in the low-temperature electrical transport of compressed δ-CsPbI3 and renders Fermi liquid-like behavior [1]. ii) We demonstrate pressure induces a highly anisotropic electronic structure and quasi-one-dimensional metallicity in δ-CsSnI3 [2]. These studies present a promising high-pressure strategy for tuning the electronic structures and achieving diverse electronic states in the broad family of low-dimensional metal halides. |
Friday, March 10, 2023 10:00AM - 10:12AM |
Y41.00011: The electronic disorder landscape of mixed halide perovskite Yun Liu, Jean-Philippe Banon, Kyle Frohna, Yu-Hsien Chiang, Ganbaatar Tumen-Ulzii, Samuel D Stranks, Marcel Filoche, Richard H Friend Bandgap tunability of lead mixed-halide perovskites makes them promising candidates for various applications in optoelectronics since they exhibit sharp optical absorption onsets despite the presence of disorder from halide alloying. Here we use localization landscape theory to reveal that the static disorder due to compositional alloying for Iodide:Bromide perovskite contributes at most 3 meV to the Urbach energy. Our modelling reveals that the reason for this small contribution is due to the small effective masses in perovskites, resulting in a natural length scale of around 20nm for the “effective confining potential” for electrons and holes, with short range potential fluctuations smoothed out. The increase in Urbach energy across the compositional range agrees well with our optical absorption measurements. We model systems of sizes up to 80 nm in three dimensions, allowing us to explore halide segregation, accurately reproducing the experimentally observed absorption spectra and demonstrating the scope of our method to model electronic structures on large length scales. Our results suggest that we should look beyond static contribution and focus on the dynamic temperature dependent contribution to the Urbach energy. |
Friday, March 10, 2023 10:12AM - 10:24AM |
Y41.00012: Halide Perovskites as Capable Thermoelectric Materials Amin Nozariasbmarz, Luyao Zheng, Bed Poudel, Kai Wang, WENJIE LI, YU ZHANG, Shashank Priya Halide perovskites have emerged as a new material class in multiple energy conversions research fields such as photovoltaics, photodetection, light emission, memristor, ferroelectric, and photothermal conversion due to their exceptional combination of optoelectronic and charge carrier transport. However, the thermoelectric properties of halide perovskites have received limited attention despite their capability. Halide perovskites are classified as having phonon glass, electron crystal. They have ultralow thermal conductivity and superior Seebeck coefficient coupled with decent mobility and charge carrier tunability. However, they suffer from instability, poor electrical conductivity, and a small power factor. Among halide perovskites, Pb- and Sn-based materials have shown better thermoelectric properties to date. For future thermoelectric applications, reliable materials synthesis methods and thermoelectric property measurements are required. In this talk, we introduce perovskites as a new thermoelectric materials system and discuss research capabilities and future perspectives on these materials systems. |
Friday, March 10, 2023 10:24AM - 10:36AM |
Y41.00013: Symmetry-informed tight-binding modeling of disordered hybrid perovskites Simon Thebaud, Claudio Quarti, Yorgos Volonakis, Mikael Kepenekian, Laurent Pedesseau, Claudine KATAN, Jacky Even In the past decade, hybrid halide perovskites have generated tremendous interest as optoelectronic materials, particularly for photovoltaïc applications due to impressive conversion efficiency values reaching over 25% in only a few years. Their large-scale industrial application, however, depends on subsequent improvements of their stability and tunability that can be achieved in both halide solid solutions (in which I, Br and Cl anions are mixed on a disordered sublattice) and two-dimensional halide perovskites (in which charge carriers are confined within inorganic atomic layers separated by organic spacers) [1,2]. Thus, it is crucial to gain an understanding of the electronic properties and carrier transport in these systems. In this talk, I will describe how a model tight-binding Hamiltonian can be extended from the archetypal CH3NH3PbI3 bulk perovskite [3-5] to mixed halide perovskites and two-dimensional perovskites using a combination of symmetry considerations and DFT calculations. This will pave the way towards a robust theoretical treatment of halide clustering effects in the former, and towards a proper description of the excitonic fine structure in the latter. |
Friday, March 10, 2023 10:36AM - 10:48AM |
Y41.00014: Macro/Mesoscopic Scale Oscillatory PL Ring Structure due to Photo-induced Ion Displacement in Mixed-Halide Perovskites (FA,MA)Pb(BrxI1-x)3 Xiaoxiao Sun, Yong Zhang, Weikun Ge Contrary to the common belief that the light-induced halide ion segregation in a mixed halide alloy occurs within the illuminated area, we discover that the anion segregation in a (FA,MA)Pb(BrxI1-x)3 thin film is a nonlocal effect of which the Br ion are expelled from the illuminated area, forming a ring with the size proportional to the illumination beam size, from μm up to well over mm, depending on the beam size. Specifically, we find that under illumination, within the illumination area, the photoluminescence (PL) peak is red shifted from the initial wavelength; while concurrently, the alloy PL peak is strongly enhanced in a ring area circling the illuminated area. Furthermore, the process is reversible, but non-monotonically, exhibiting an ultra-low-frequency, damped oscillations between the ring and center in terms of PL intensity and position. This finding may be the first observation of an ionic plasma oscillation in solids. Interestingly, right after removing the illumination, a voltage about 0.4 V between the ring and the center is established. Our findings suggest that the “ion-segregation” phenomenon, commonly viewed as an adverse effect, could have novel applications, such as ionic patterning, self-destructive memory, and energy storage (particularly a battery directly chargeable by sunlight). |
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