Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session V53: Perovskites and Oxide SemiconductorsFocus Live
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Sponsoring Units: DMP DCOMP FIAP Chair: Kirstin Alberi, National Renewable Energy Laboratory |
Thursday, March 18, 2021 3:00PM - 3:36PM Live |
V53.00001: Defect-assisted nonradiative recombination in halide perovskites Invited Speaker: Chris Van de Walle Halide perovskites offer impressively high solar conversion efficiencies and are being considered for applications as light emitters. We show that the impact of point defects on device efficiency has not been properly assessed to date. We have performed comprehensive studies for the prototypical hybrid perovskite MAPbI3 [MA=(CH3NH3)], as well as for other halide perovskites. To achieve accurate and reliable results, our first-principles calculations are based on hybrid density functional theory with spin-orbit coupling included [1]. Rigorous calculations of nonradiative recombination coefficients show the limitations of the widely adopted rule that only defects with charge-state transition levels deep in the band gap can be efficient nonradiative recombination centers. We demonstrate that the position of the level does not directly determine the capture rates, due to exceptionally strong lattice coupling and anharmonicity in the halide perovskites [2]. Our results clearly show that (1) point defects can indeed be present in relevant concentrations in the halide perovskites and (2) some of these point defects lead to nonradiative recombination rates that are just as high as in conventional semiconductors. We therefore conclude it is incorrect to call the halide perovskites “defect tolerant”. A more relevant distinction, compared to conventional semiconductors, is that halide perovskites with modest defect densities can be grown using low-cost deposition techniques; however, careful control of point defects (as well as impurities [3]) is still essential to maximize the efficiency. |
Thursday, March 18, 2021 3:36PM - 3:48PM Live |
V53.00002: Deep levels in cesium lead bromide from native defects and hydrogen Michael Swift, John Lyons Lead halide perovskites such as CsPbBr3 have achieved remarkable success in numerous optoelectronic applications, due in large part to their good performance despite high defect densities. This “defect tolerance” has often been explained by hypothesizing that there is negligible trap-assisted non-radiative recombination in these materials because none of the dominant defects give rise to deep levels in the gap. We refer to this as the “shallow defect hypothesis” (SDH). In this work, we reject the SDH for CsPbBr3. Via a thorough first-principles inventory of native defects and hydrogen impurities, we show that a number of relevant defects do in fact have deep levels, most notably the bromine interstitial and hydrogen interstitial. This adds to a growing body of evidence against the SDH, suggesting that the observed defect tolerance may be due instead to relatively low recombination rates at deep levels. Guided by the theoretical identification of these defects, experiments can take steps to mitigate trap-assisted non-radiative recombination, further boosting the efficiency of lead halide perovskite optoelectronics. |
Thursday, March 18, 2021 3:48PM - 4:00PM Live |
V53.00003: The Defect Tolerance of Chalcogenide Perovskites BaZrS3 and Ba3Zr2S7 Jiang Luo, Boyang Zhao, ZHAOHAN ZHANG, Huandong Chen, Arashdeep Thind, Steven Hartman, Bryce Sadtler, Jayakanth Ravichandran, Rohan Mishra Chalcogenide perovskites (CPs), such as BaZrS3, have been proposed as defect-tolerant materials, which maintain the electronic properties of their pristine form even in the presence of defects, with desirable characteristics such as large absorption coefficients, and long recombination lifetime for application as solar cells and optoelectronic devices. A systematic investigation of the defect tolerance of such perovskites is critical to develop a full understanding of their potential. We have used BaZrS3 and Ba3Zr2S7 as prototypical CPs and investigated their defect tolerance to intrinsic point defects using a combination of density-functional-theory calculations, spectroscopic and transport measurements. Our calculation indicates that most defects lead to shallow levels. However, we find that the sulfur vacancies have low formation energy and lead to a deep transition level that is spatially localized to act as non-radiative recombination center in BaZrS3. Our work demonstrates that control over the chalcogen stoichiometry is critical to improve the performance of CPs. We also find that sulfur vacancies act as shallow-level defects in Ba3Zr2S7, a derivative of BaZrS3 with layered structure, making Ba3Zr2S7 a promising alternative for semiconducting applications. |
Thursday, March 18, 2021 4:00PM - 4:12PM Live |
V53.00004: Persistent photoconductivity in barium titanate Christopher Pansegrau, Matthew McCluskey Barium titanate is an oxide perovskite that is known for its ferro- and piezoelectric properties. Single crystal BaTiO3 was annealed under a flowing atmosphere of humid hydrogen and systematically exposed to a series of visible-region LEDs while simultaneously taking IR spectra. Persistent photoconductivity was observed at room temperature with a factor of two change in electrical resistance and a dramatic increase in absorbance in the mid-IR. Some optical and electrical recovery was observed in the two days following exposure to the final LED. The onset of the phenomenon was observed to be approximately 2.9 eV, which is consistent with previous experiments using strontium titanate. The observation of room temperature persistent photoconductivity in a second material suggests the phenomenon is general, not restricted uniquely to strontium titanate. Furthermore, the combination of ferroelectricity, piezoelectricity, and persistent photoconductivity could lead to the creation of novel devices. |
Thursday, March 18, 2021 4:12PM - 4:24PM Live |
V53.00005: Role of defects in photocatalytic water splitting: Monodoped vs Codoped SrTiO3 Manish Kumar, Saswata Bhattacharya SrTiO3 is used as a photocatalyst due to its exceptional electronic structure, high stability, non-toxicity, and low cost. However, owing to the wide bandgap (3.2 eV), it could only utilize the UV irradiation. Here we have investigated the role of monodopants (nonmetals (N, S) and metals (Mn, Rh)) and codopants in SrTiO3 to ameliorate the photocatalytic efficiency for water splitting by reducing its bandgap using hybrid density functional theory (HSE06), many-body perturbation theory approach (viz. G0W0@HSE06) and ab initio atomistic thermodynamics. We find that substitutional defect is the most stable defect. Further, monodoping can induce visible light absorption, but not suitable to enhance the photocatalytic activity either due to the formation of recombination centers or lowering of conduction band minimum (CBm). Contrarily, the codopant can be chosen such that the recombination of photogenerated charge carriers is suppressed, and the CBm is not affected much. Our results reveal that MnSrNO (codoping of Mn at Sr site and N at O site) and MnTiSO are the potential candidates for enhancing the photocatalytic activity of SrTiO3 under visible light. |
Thursday, March 18, 2021 4:24PM - 4:36PM Live |
V53.00006: Localized Phase Transition of TiO2 Thin Films Induced by Sub-bandgap Laser Irradiation Syeed Ahmed, Violet Poole, John David Igo, Yi Gu, Matthew McCluskey Laser-induced localized phase transitions of sputtered titanium dioxide (TiO2) thin films are reported in this study. Laser-irradiation under vacuum with a 532 nm sub-bandgap continuous wave (CW) laser results in an anatase-to-rutile phase transition. Irradiating the rutile region in air changes a portion of the domain back to anatase. Raman spectroscopy and micro-Raman mapping were used to detect phase transformations and study the spatial intensity distribution of Raman-active modes. A Raman map of anatase mode Eg (144 cm-1) and rutile mode Ag (608 cm-1) revealed the formation of microstructures due to the laser treatment. The results suggest that irradiated photons are absorbed by defects, resulting in localized electronic excitation that leads to a phase transition in the treated regions. |
Thursday, March 18, 2021 4:36PM - 4:48PM Live |
V53.00007: Epitaxial stabilization of rutile germanium oxide thin film by molecular beam epitaxy Sieun Chae, Hanjong Paik, Nguyen Vu, Emmanouil Kioupakis, John Heron Ultrawide-band-gap (UWBG) semiconductors have tantalizing advantages for power electronics. Materials such as AlN/AlGaN, β-Ga2O3, and diamondhave been developed for UWBG semiconducting devices, however, they are still facing challenges, such as doping asymmetry and/or inefficient thermal conduction. Rutile GeO2 (r-GeO2) has been theoretically established to have an UWBG (4.68 eV), high electron and hole mobility (289 cm2V-1s-1 and 28cm2 V-1s-1), high thermal conductivity (51 W m–1K–1) and ambipolar dopability. The synthesis of r-GeO2 thin films has not been reported but is critical to enable microelectronics applications. Here, we report the growth of single-crystalline r-GeO2 thin films on R-plane sapphire substrates using molecular beam epitaxy. We control the competing reactions between the deeply metastable glass phase formation and rutile phase formation as well as absorption and desorption by utilizing (1) a buffer layer with reduced lattice misfit, and (2) the growth condition that allows the condensation of the preoxidized molecular precursor yet provides sufficient adatom mobility. The findings advance the synthesis of single-crystalline films of materials prone to glass formation and provide opportunities to realize promising UWBG semiconductors. |
Thursday, March 18, 2021 4:48PM - 5:00PM Live |
V53.00008: Thermal Annealing ZnO and ZnS Powders as an Alternative to Type II Heterostructure Synthesis Christopher Kuhs, Christian R Jacobson, Matthew R Reish, Jay G Simmons, N J Halas, Henry O Everitt Type II heterostructures of the wide bandgap semiconductors ZnO and ZnS spatially separate holes into ZnS and electrons into ZnO. Charge separation in such core-shell type-II heterostructure powders and nanostructures significantly enhances phosphorescence efficiency and photocatalytic activity. Consequently, there is great interest in developing low cost techniques for reproducibly producing large quantities of these powders. Here we explore a simple thermal annealing approach and examine the effect annealing has on the defect and band edge emissions of four samples: vacuum-annealed ZnO with and without sulfur, and ZnS annealed in vacuum and with oxygen. Scanning electron microscopy and cathodoluminescence spectroscopy revealed aggressive oxygen substitution for sulfur, even creating oxygen vacancy defect emission in ZnS powders, with significant particle-to-particle variability. Evidence suggesting that core-shell structures are formed will be discussed, and the limitations of this approach will be addressed. |
Thursday, March 18, 2021 5:00PM - 5:12PM Live |
V53.00009: Multipole Clustering is Responsible for the Doping Bottleneck in Hematite Valentin Urena Baltazar, Tyler Smart, Yuan Ping Optimal doping of transition metal oxides is crucial for their development in photoelectrochemical (PEC) applications to mitigate the adverse effects of polaron formation which limits carrier conductivity. However, the optimal doping concentration in hematite has been extremely low, for example less than a percent, which hinders the benefits of doping for practical applications. In this work, we investigated the underlying mechanism of low optimal doping concentration from first-principles calculations with group IV (Ti, Zr, Hf) and XIV (Si, Ge, Sn, Pb) doping. We find that novel dopant-polaron clustering can form even at low dopant concentrations and which resemble electric multipoles. These multipoles are very stable and difficult to be fully ionized compared to separate dopants, and thus detrimental to carrier concentration and conductivity. We also exclude intrinsic vacancy as a cause for dopant clustering. This allows us to resolve mysteries of the doping bottleneck in hematite and provide guidance for optimizing doping and carrier conductivity in polaronic systems towards highly efficient PEC application. |
Thursday, March 18, 2021 5:12PM - 5:24PM Live |
V53.00010: Finite-size corrections of defect energy levels involving ionic polarization Stefano Falletta, Julia Wiktor, Alfredo Pasquarello We develop a scheme for finite-size corrections of vertical transition energies and single-particle energy levels involving defect states with built-in ionic polarization in supercell calculations. The method accounts on an equal footing for the screening of the electrons and of the ionic polarization charge arising from the lattice distortions. We demonstrate the accuracy of our corrections for various defects in MgO and in water by comparing with the dilute limit achieved through the scaling of the system size. The general validity of our formulation is also confirmed through a sum rule that connects vertical transition energies with the formation energies of structurally relaxed defects. |
Thursday, March 18, 2021 5:24PM - 5:36PM Live |
V53.00011: Donor doping of CsPbBr3 John Lyons The inorganic lead halide perovskite CsPbBr3 exhibits many outstanding properties, in addition to potentially offering environmental stability better than its hybrid perovskite counterparts. The full utilization of this material in optoelectronic applications would be aided by gaining the ability to control the electrical conductivity via impurity doping. One possible donor, bismuth, has been found to enhance solar cell efficiency, and an increase in the position of the Fermi level upon doping. Here I examine how bismuth incorporates into CsPbBr3 using first-principles hybrid density functional theory. The stability of different configurations is considered as a function of chemical potential and Fermi level. Although bismuth prefers to substitute for lead under most conditions, it introduces a deep donor level ~500 meV from the conduction-band minimum, indicating it will not efficiently generate free carrier concentrations. Based on these results, donor-doping strategies for CsPbBr3 are reconsidered. |
Thursday, March 18, 2021 5:36PM - 5:48PM Live |
V53.00012: First-principles calculations of native defects in beryllium oxide YUBI CHEN, Mark Turiansky, Chris Van de Walle Beryllium is the main plasma-facing material used in the International Thermonuclear Experimental Reactor (ITER) and is likely accompanied by its oxide, BeO. BeO is a ceramic and is used as a transparent coating on optical components. BeO is stable in the wurtzite phase and boasts several useful properties, including high thermal conductivity and an ultra-wide band gap exceeding 10 eV. The large band gap makes BeO promising as a host for quantum defects. The technological relevance for reactors and coatings and potential for quantum information applications highlight the importance of a thorough understanding of the defect chemistry of BeO. We study these defects using first-principles density functional theory calculations with a hybrid functional. We examine the energetics and electronic structure, as well as the atomic geometries and spin properties of the native defects of BeO. We compare the properties of the native defects of BeO with those in other group-II oxides, which generally prefer the rocksalt phase, and with diamond, a tetrahedrally coordinated material known to host quantum defects. |
Thursday, March 18, 2021 5:48PM - 6:00PM On Demand |
V53.00013: Resolving the puzzle of solvated electrons’ location in alkali metal doped zeolites Debalaya Sarker, Maria Troppenz, Santiago Rigamonti, Claudia Draxl, Sergey Levchenko, Matthias Scheffler Doping faujasite Y (FAU-Y), a nanoporous aluminosilicate zeolite, with alkali metal atoms (M) is a promising way to produce outstanding catalysts. The dopants, along with extra-framework metal atoms, often form M4+3 clusters inside zeolite pores, leaving the valence electron of the dopant solvated and available for catalysis. Despite extensive experimental efforts1,2, the distribution of dopants and solvated electrons remains debated to date. Combining a cluster-expansion model, parameterized with density-functional theory calculations, with ab initio atomistic thermodynamics, we address this issue. The electronic structure for low-energy configurations is calculated with hybrid functional HSE06. We find that even at room temperature, Na atoms in NaY zeolites with two extra-framework atoms per unit cell on average redistribute such that areas with lower and higher local concentrations emerge. The redistribution is driven by increased configurational disorder, mainly at higher concentrations. This explains why solvated electrons can be located inside both small and large cages in NaY, reconciling experiments that assign the solvated electrons to a particular pore type. |
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