Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K34: Hybrid Organic-Inorganic Halide Perovskites IFocus
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Sponsoring Units: DMP Chair: Feliciano Giustino, University of Oxford Room: 297 |
Wednesday, March 15, 2017 8:00AM - 8:36AM |
K34.00001: Theory and modeling of correlated ionic motions in hybrid organic-inorganic perovskites Invited Speaker: Andrew Rappe The perovskite crystal structure hosts a wealth of intriguing properties, and the renaissance of interest in halide (and hybrid organic-inorganic) perovskites (HOIPs) has further broadened the palette of exciting physical phenomena. Breakthroughs in HOIP synthesis, characterization, and solar cell design have led to remarkable increases in reported photovoltaic efficiency. However, the observed long carrier lifetime and PV performance have eluded comprehensive physical justification. The hybrid perovskites serve as an enigmatic crossroads of physics. Concepts from crystalline band theory, molecular physics, liquids, and phase transitions have been applied with some success, but the observations of HOIPs make it clear that none of these conceptual frameworks completely fits. In this talk, recent theoretical progress in understanding HOIPs will be reviewed and integrated with experimental findings. The large amplitude motions of HOIPs will be highlighted, including ionic diffusion, anharmonic phonons, and dynamic incipient order on various length and time scales. The intricate relationships between correlated structural fluctuations, polar order, and excited charge carrier dynamics will also be discussed. [Preview Abstract] |
Wednesday, March 15, 2017 8:36AM - 8:48AM |
K34.00002: Local polar fluctuations in lead halide perovskites Liang Tan, Omer Yaffe, Yinsheng Guo, Louis Brus, Andrew Rappe, David Egger, Leeor Kronik The lead halide perovskites have recently attracted much attention because of their large and growing photovoltaic power conversion efficiencies. However, questions remain regarding the temporal and spatial correlations of the structural fluctuations, their atomistic nature, and how they affect electronic and photovoltaic properties. To address these questions, we have performed a combined ab initio molecular dynamics (MD) and density functional theory (DFT) study on CsPbBr$_{\mathrm{3}}$. We have observed prevalent anharmonic motion in our MD trajectories, with local polar fluctuations involving head-to-head motion of A-site Cs cations coupled with Br window opening. We calculate Raman spectra from the polarizability auto-correlation functions obtained from these trajectories and show that anharmonic A-site cation motion manifests as a broad central peak in the Raman spectrum, which increases in intensity with temperature. A comparison of the experimental Raman spectrum of hybrid organometallic MAPbBr$_{\mathrm{3}}$ and fully inorganic CsPbBr$_{\mathrm{3}}$ suggests that structural fluctuations in lead-halide perovskites is more general than rotation of polar organic cations and is intimately coupled to the inorganic framework. [Preview Abstract] |
Wednesday, March 15, 2017 8:48AM - 9:00AM |
K34.00003: Local polar fluctuations in lead halide perovskite crystals Yinsheng Guo, Omer Yaffe, Liang Tan, David Egger, Trevor Hull, Constantinos Stoumpos, Fan Zheng, Tony Heinz, Leeor Kronik, Mercouri Kanatzidis, Jonathan Owen, Andrew Rappe, Marcos Pimenta, Louis Brus Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in both hybrid (CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbBr$_{\mathrm{3}})$ and all-inorganic (CsPbBr$_{\mathrm{3}})$ lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations show that head-to-head Cs motion coupled to Br face expansion, on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr$_{\mathrm{3}}$. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K34.00004: The dynamics and structure of methylammonium ions in hybrid halide perovskites; a combined neutron scattering and first-principles study Linda Hung, Bahar Ipek, Craig Brown, Taner Yildirim, Benjamin Foley, Tianran Chen, Joshua Choi, Seunghun Lee We study the influence of the methylammonium cations on the structure and dynamics of CH$_3$NH$_3$PbI$_3$ through a combined first-principles computation and neutron scattering approach. Temperature-dependent powder diffraction indicates phase transitions near 160 K and 300 K, and the lattice parameters, bond angles, and atomic positions are determined. Density-functional theory (DFT) calculations confirm that the most stable form of the orthorhombic phase is non-polar, although the energy difference compared to ferroelectric orientations is only a few meV per atom. Inelastic neutron scattering (INS) spectra are compared to first-principles computations, and significant multi-phonon scattering is observed. Computations predict that lattice dynamics are sensitive to the orientation of the dipoles, and suggest that vibrational spectroscopy may be used as a probe of the local dipole ordering. In particular, the twisting mode along the C-N bond is a single sharp peak near 39 meV for the non-polar orientation and in our INS measurements, but splits into separate peaks for certain ferroelectric orientations. [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:24AM |
K34.00005: Computational Discovery of Two Lead Free Halide Double Perovskites with Band Gaps in the Visible Range: Cs2BiAgCl6 and Cs2BiAgBr6 Marina Filip, George Volonakis, Amir Abbas Haghighirad, Samuel Hillman, Nobuya Sakai, Bernard Wenger, Henry Snaith, Feliciano Giustino The perovskite solar cell is emerging as one of the most promising solution processable photovoltaic technologies, with an efficiency that now exceeds the performance of thin-film silicon devices. This performance is exclusively due to the optimum optoelectronic properties of the prototypical methylammonium lead-iodide perovskite (MAPI). However, the presence of lead in MAPI, and its problematic stability in ambient conditions poses concerns for its potential environmental impact. These concerns are motivating the search for novel non-toxic halide perovskites with similar optoelectronic properties to MAPI. In this work we will present the computational search for the homovalent and the heterovalent replacement of Pb in lead-halide perovskites. This search has lead to the computational discovery and experimental synthesis of two stable lead-free halide double perovskites based on Bi and Ag: Cs2BiAgCl6 and Cs2BiAgBr6. These new compounds are highly stable, they are semiconducting and absorb light in the visible range. In this talk we will present the electronic and optical properties of Cs2BiAgCl6 and Cs2BiAgBr6 calculated within DFT and GW and discuss the stability and formability of the entire Cs2BB'X6 family of semiconductors (B = Bi, Sb, B’ = Cu, Ag, Au, X = Cl, Br, I). [Preview Abstract] |
Wednesday, March 15, 2017 9:24AM - 9:36AM |
K34.00006: Electronic structure and stability of RbGeCl$_3$ Santosh kumar Radha, Walter R . L Lambrecht Metal halide pervoskites have recently received significant attention, including recently the Ge based AGeX$_3$. First-principles calculations of RbGeCl$_3$ stability and structure were performed using the full-potential linearized muffin-tin orbital (FP-LMTO) method. The band structures were calculated using the quasi-particle self-consistent (QS)$GW$ method. We find that the perovskite structure exhibits octahedral rotations rather than the ferro-electric distortion, in contrast to CsGeCl$_3$, as expected from the tolerance factor $\approx 1$. The relative stability of the perovskite form and the monoclinic form is found to be very sensitive to the exchange-correlation functional used. LDA predicts the Pervoskite to be the lower energy while GGA predicts the monoclinic. GGA is significantly better at predicting the lattice constants of these materials than LDA. The band structure in the perovskite phase has significantly lower band gap than the monoclinic phase. [Preview Abstract] |
Wednesday, March 15, 2017 9:36AM - 9:48AM |
K34.00007: Ion Migration in Lead-Halide Perovskites: Insights from First-Principles Calculations David A. Egger, Subham Dastidar, Liang Z. Tan, Samuel B. Cromer, Andrew D. Dillon, Shi Liu, Aaron T. Fafarman, Andrew M. Rappe, Leeor Kronik Lead-halide perovskites (LHPs) are promising semiconductors especially for efficient solar cells. However, LHP cells show hysteresis in the current-voltage curves and their stability with respect to water exposure is problematic. These issues are possibly related to defect migration occurring in the perovskite material [1]. Here, we present our first-principles results on defect migration phenomena in hybrid and all-inorganic LHPs.[2,3] Our calculations are based on density functional theory and nudged-elastic band methods to optimize minimum energy pathways of defect species. First, the migration of hydrogen interstitials is discussed,[1] where we find that migration barriers are small, indicating mobile hydrogen interstitials at room temperature. Second, results on migration and anion-exchange reactions of interstitial Cl in all-inorganic CsPbI$_3$ are presented.[2] Cl migration is found to facilitate Cl doping at levels near the solubility limit for chloride in CsPbI$_3$. At these incorporation levels the stability of a CsPbI$_3$ film is strongly improved compared to an undoped film. References: [1] Egger et al., Acc. Chem. Res. 49, 573 (2016), and references therein. [2] Egger et al., Angew. Chem. Int. Ed. 54, 12437 (2015). [3] Dastidar et al., Nano Lett. 16, 3563 (2016). [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K34.00008: Evaluating density functionals: case study for organic perovskites Menno Bokdam, Jonathan Lahnsteiner, Benjamin Ramberger, Tobias Sch\"afer, Christoph Dellago, Georg Kresse The precise ordering of the molecules in organometal halide perovskites at elevated temperatures is difficult to determine. Density functional theory (DFT) calculations proposes ordering patterns, but they depend on the type of exchange-correlation functional used. Van der Waals interactions are important in this system and require an accurate description, which goes beyond choosing the "best" density functional based on common reasoning. Here we use the random phase approximation (RPA) to evaluate various density functionals for the perovskites, specifically MAPbI$_3$. The evaluation is done by first creating finite temperature ensembles for small supercells of MAPbI$_3$ using a beyond DFT approach, and then evaluating the variance between the random phase approximation and various approximate density functionals for these ensembles. The finite temperature ensembles are generated using hybrid Monte-Carlo techniques as well as finite temperature RPA molecular dynamics. We will present an overview of various density functionals and how well they correspond with the random phase approximation applied to the perovskites. [Preview Abstract] |
Wednesday, March 15, 2017 10:00AM - 10:12AM |
K34.00009: Study of Excitons in MAPbI$_{\mathrm{3}}$ by optical Faraday rotation P. Odenthal, Y. Yao, N. Gundlach, W. Talmadge, C. Zhaeng, D. Sun, ZG Yu, Z. V. Vardeny, Y. S. Li Recently metal halide perovskites have gained much interest due to their rapid rise in solar cell efficiency. Their large Rashba effect and anomalously long spin lifetime at low temperature reveal the perovskites great promise for spintronic applications. Exciton physics should dominate the (spin dependent) optoelectronic properties of the MAPbI$_{\mathrm{3}}$ at low temperatures. A complete understanding of the exciton physics in metal halide perovskites is therefore needed. Faraday rotation measurement in a pump-probe scheme provides unique insight into the perovskites' exciton physics, as it is sensitive to the quantum beating between spin-polarized exciton states. By varying the energy of the probe beam or the fluence of the pump beam, we probe difference species including possibly free excitons, bounded excitons, and free carriers. Our recent results of unusual energy and fluence dependent spin dynamics will be presented and discussed by considering the exciton physics. We acknowledge funding from the University of Utah and the Department of Energy Office of Science (DE-SC0014579). [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K34.00010: Local and Long Range Structural Studies of Hybrid Perovskites T. A. Tyson, Y. Yan, W. Gao, Y.-S. Chen, S. Ghose In this work we study a class of hybrid$_{\mathrm{\thinspace }}$perovskites with high photovoltaic conversion efficiency which exhibits an anomalous transition near \textasciitilde 330 K (T*). Local structural measurements over a broad temperature range have been conducted and compared with single crystal diffraction measurements. The detailed motion of atoms is extracted and the response to chemical modification is explored. Extensive modeling of the temperature dependent structure is conducted. The connection of the structure to the novel transport properties will be discussed. This work is supported by DOE Grant DE-FG02$-$07ER46402. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K34.00011: Entropy-driven structural transition and kinetic trapping in formamidinium lead iodide perovskite Tianran Chen, Benjamin Foley, Changwon Park, Craig Brown, Leland Harriger, Jooseop Lee, Jacob Ruff, Mina Yoon, Joshua Choi, Seung-Hun Lee A challenge of hybrid perovskite solar cells is device instability, which calls for an understanding of the perovskite structural stability and phase transitions. Using neutron diffraction and first-principles calculations on formamidinium lead iodide (FAPbI$_{\mathrm{3}})$, we show that the entropy contribution to the Gibbs free energy caused by isotropic rotations of the FA$^{\mathrm{+}}$ cation plays a crucial role in the cubic-to-hexagonal structural phase transition. Furthermore, we observe that the cubic-to-hexagonal phase transition exhibits a large thermal hysteresis. Our first-principles calculations confirm the existence of a potential barrier between the cubic and hexagonal structures, which provides an explanation for the observed thermal hysteresis. By exploiting the potential barrier, we demonstrate kinetic trapping of the photovoltaic pseudo-cubic phase at low temperatures by thermal quenching $^{\mathrm{[1]}}$. [1] Chen \textit{et al. Sci. Adv.} 2016;2: e1601650 21 October 2016 [Preview Abstract] |
Wednesday, March 15, 2017 10:36AM - 10:48AM |
K34.00012: Crystal Structures of a Photovoltaic Two-Dimensional Perovskite Depei Zhang, Tianran Chen, Alexander Chen, Craig Brown, Leland Harriger, Mina Yoon, Joshua Choi, Seung-Hun Lee Arguably the biggest challenge of the high-efficiency perovskite solar cells, such as CH$_3$NH$_3$PbI$_3$ and CH(NH$_2$)$_2$PbI$_3$, is their device instability. A recent study\footnote{Tsai, Hsinhan, et al. "High-efficiency two-dimensional Ruddlesden–Popper perovskite solar cells." \textit{Nature} 536.7616 (2016): 312-316.} of two dimensional (2D) perovskite compounds, (CH$_3$(CH$_2$)$_3$NH$_3$)$_2$(CH$_3$NH$_3$)$_{n-1}$Pb$_{n}$I$_{3n+1}$, proposed a solution to this problem. This class of materials shows a maximum photovoltaic efficiency of 12.52\%, without any degradation over 2250 hrs of standard light illumination, or 650 hrs of 65\% relative humidity test. In this talk, we present our neutron scattering experiments to study the crystal structure of the 2D 3-layer perovskite (n = 3) as a function of temperature. We have observed two phase transitions between 12 K and 370 K. Rietveld refinements as well as the first principle calculations were used to determine the structures of all three phases. [Preview Abstract] |
Wednesday, March 15, 2017 10:48AM - 11:00AM |
K34.00013: Neutron Scattering Studies of Hybrid Perovskites for Photovoltaic Applications Michael Crawford, Pamela Whitfield, Niina Jalarvo, Georg Ehlers, Madhusudan Tyagi, Norman Herron, Lynda Johnson, William Guise, Ivan Milas, Yongqiang Cheng, Luke Daemen, Anibal Ramirez-Cuesta, Katharine Page, Xiaoping Wang, Feng Ye Hybrid perovskites (ABX$_{\mathrm{3}})$ have attracted a great deal of attention as light absorbers for photovoltaics. Here the A site is occupied by organic cations, for example methyl ammonium, CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$; the B site is occupied by metal cations, for example Pb$^{\mathrm{2+}}$; and the X site is occupied by halogen anions: I$^{\mathrm{-}}$, Br$^{\mathrm{-}}$, or Cl$^{\mathrm{-}}$. Typical of perovskites, these materials exhibit structural phase transitions involving rotations or tilts of the BX$_{\mathrm{6}}$ octahedra, but with the added complexity that the inorganic framework is coupled to order-disorder transitions of the organic cations. We have used neutron scattering techniques to characterize the vibrations and dynamics of several of these compounds as a function of temperature, including samples where the hydrogen atoms are partially or fully substituted by deuterium. Combined with the results of density functional theory calculations, these studies contribute to our understanding of the excellent photovoltaic properties of these materials. [Preview Abstract] |
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