Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y12: Computational Materials Design - Solar Cells and Solid State Lighting MaterialsFocus
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Sponsoring Units: DMP DCOMP Chair: Anderson Janotti, Univ of Delaware Room: LACC 303B |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y12.00001: Accelerated discovery of lead-free organic-inorganic halide double perovskites for optoelectronics Kesong Yang Hybrid organohalide perovskites have emerged as one class of most promising semiconductor materials for the next-generation optoelectronics because of their exceptional properties, though the demand of lead-free perovskites is still a great challenge. Here we show that, by using high-throughput first-principles electronic structure calculations and efficient screening algorithm, we have generated the quantum data of about 2000 organic-inorganic double perovskites, and successfully uncovered about 40 promising lead-free candidate materials with promising materials properties for photovoltaic applications. This work paves the way to an accelerated design of novel halide double perovskites for optoelectronics. |
Friday, March 9, 2018 11:27AM - 11:39AM |
Y12.00002: Enhancing Ferroelectric Dipole Ordering in the Organic-Inorganic Hybrid Perovskite CH3NH3PbI3: Strain and Doping Engineering Yuheng Li, Maziar Behtash, Joseph Wong, Kesong Yang Hybrid halide perovskites have shown astonishing power conversion efficiencies (PCE) due to their superb optoelectronic properties. However, the fundamental reasons for their high efficiencies compared to their inorganic counterparts are unclear. There has been great controversy regarding one of the most possible reasons, the ability of organic cations to form ferroelectric (FE) domains in hybrid perovskites. Here we study FE dipole ordering of methylammonium (CH3NH3+, i.e., MA) cations in MAPbI3. The results indicate that FE ordering exists in tetragonal MAPbI3. We further explored two approaches to enhancing the FE dipole ordering: (1) application of bi-axial compressive strain (strain engineering), and (2) substitutional doping with smaller halogen anions (doping engineering). These two approaches are based on an increase of the unit cell aspect ratio c/a. We propose the strain and doping engineering as practical means to effectively improve PCE of hybrid-perovskite-based solar cells. |
Friday, March 9, 2018 11:39AM - 12:15PM |
Y12.00003: Discovery of Halide Perovskites and superlattices as a Design Problem Invited Speaker: Alex Zunger This presentation will offer the author’s perspective on a few central themes in the emerging field of Design and Discovery (D&D) of functional materials, using halide perovskites as an example. Materials have traditionally been introduced into inorganic photovoltaics (PV) either by incremental improvements on long-known substances (Si, CuInSe2, CdTe, In2O3) or via chemical substitution of known materials . This time-honored tradition leaves out the possibility of yet undiscovered but potentially useful materials. Inspection of databases of all inorganic materials previously made reveals that a few thousands are simply missing. These are combinations of chemical elements that are analogous to those appearing listed compounds but represent unreported compounds. One wonders if they are missing for a good reason (such as some inherent instability), or the community has simply not gotten around to try and make them, yet they might be interesting. Systematic laboratory synthesis of all such candidate-missing materials would seem a horrendous project. A possible alternative is to perform initial screening of hundreds of Missing Compounds by using first-principles thermodynamics and property calculation, and then attempt laboratory realization of narrower lists of missing materials. I will describe in this talk the idea behind such functionality-directed use of Quantum Thermodynamics and property screening applied to (i) traditional inorganic PV materials (ii) single halide perovskites and (iii) double halide perovskites and (iv) Design of artificial supe |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y12.00004: NaSbSe2 as a Promising Light-Absorber Semiconductor in Solar Cells: First-Principles Insights Chenmin Dai, Menglin Huang, Zenghua Cai, Dan Han, Shiyou Chen Using density functional theory calculations, we studied the electronic, optical and defect properties of NaSbSe2 systematically. It has an indirect gap (1.11 eV) which is slightly smaller than the direct gap (1.18 eV). The density of states analysis shows that the high optical absorption coefficient of NaSbSe2 is attributed to the p-p transition from the valence band composed of Sb s/p + Se p states to the conduction bands composed of Sb p + Se p. Additionally, NaSbSe2 exhibits intrinsic p-type conductivity due to the acceptor defects NaSb and VSb which always have the lowest formation energies under different chemical potential conditions. The ionization energy level (-2/0) of NaSb is very shallow and VSb can be ionized spontaneously once formed, so it is challenging to dope NaSbSe2 to n-type. VSe is a deep-level donor defect and may have high concentration under the Se-poor condition, so it may act as a recombination center and thus a Se-rich condition is required for achieving high solar cell efficiency. Our calculated results indicate that NaSbSe2 may be a potential light-absorber semiconductor in thin-film solar cells. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y12.00005: Computational Generation of Hydrogenated Amorphous Silicon for Tandem Solar Cells: Strain, Voids, and Interfaces Enrique Guerrero, David Strubbe We present computational generation of hydrogenated amorphous silicon (a-Si:H) for use in studying the electronic properties of "heterojunction with intrinsic thin-layer" (HIT) tandem solar cells, in which a-Si:H passivates c-Si at interfaces and enables efficiencies up to 27%. We generate structures using the Wooten-Winer-Weaire classical-potential Monte Carlo method using our CHASSM (Computational Hydrogenated Amorphous Semiconductor Structure Maker) code. We create hydrogenated structures of varying densities and study the effect on the bond-angle and bond-length distributions. At low densities, voids form which are likely to constitute defects limiting carrier mobilities and may be implicated in the light-induced degradation of the Staebler-Wronski effect. We mathematically characterize the size and position of voids using Voronoi polyhedra and persistent homology. We generate interfaces with c-Si in different crystal orientations and investigate the effect of the interface on local amorphous structure. To ensure physical structures, we monitor total energy and bond-angle deviation during the Monte Carlo process to avoid ending in crystalline structures or a class of excessively coordinated artifact structures. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y12.00006: Coupling between Octahedral Tilting, A-site Displacements, and Strain in Inorganic Halide Perovskites. Jonathon Bechtel, Anton Van der ven Dynamic instabilities, stabilized by anharmonic interactions in cubic and tetragonal halide perovskites at high temperature, play a role in the optoelectronic properties of halide perovskites. Inorganic and hybrid perovskite materials undergo structural phase transitions associated with octahedral tilts of the metal halide octahedra. We investigate the structural instabilities present in inorganic CsMX3 perovskites with Pb or Sn on the metal site and Br or I on the X site. Defining primary order parameters in terms of symmetry adapted collective displacement modes and secondary order parameters in terms of symmetrized Hencky strain components, we unravel the coupling between octahedral tilt modes and strains as well as the role of A-site displacements in perovskite phase stability. Symmetry allowed strain order parameters are enumerated for the 14 unique perovskite tilt systems. Using first principles calculations to explore the Born-Oppenheimer energy surface in terms of symmetrized order parameters, we find coupling between octahedral tilting and A-site displacements is necessary to stabilize Pnma ground states. Also, we show that the relative stability of a perovskite tilt system correlates with the volume decrease from the cubic phase to the low symmetry distorted phase. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y12.00007: First-Principles Investigation of Cubic Tin and Germanium Sulfides and Selenides for Photovoltaic and Thermoelectric Applications Michael Waters, Emmanouil Kioupakis Chalcogenide compound semiconductors offer a wide range of band gaps with many prospective applications in photovoltaics, near-infrared devices, and thermoelectrics. In this work, we investigate the electronic and optical properties of the recently synthesized, cubic polymorph π-tin sulfide, as well as the germanium and selenide analogs, with predictive calculations based on density functional and many-body perturbation theory. Structural relaxation calculations provide the equilibrium geometry and thermodynamic properties. The electronic structure is determined with the GW method and the optical absorption spectrum is obtained with the Bethe-Salpeter equation. Our calculations provide insights into the applications of cubic chalcogenides in the π structure for photovoltaic and thermoelectric applications. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y12.00008: BInGaN alloys lattice-matched to GaN for high-power high-efficiency visible LEDs Logan Williams, Kevin Greenman, Emmanouil Kioupakis InGaN-based visible LEDs are used commercially for solid-state lighting and displays, but lattice mismatch with GaN limits the thickness of high crystalline quality InGaN quantum wells. Since narrower wells operate at a higher carrier density for a given current, more carriers are lost to Auger recombination and lower the LED efficiency. Alloying InGaN with boron, a smaller group-III element, is a promising method to create nitride alloys lattice-matched to GaN with band gaps in the visible range. In this work, we apply calculations based on hybrid density functional theory to predict the structural, electronic, and thermodynamic properties of BInGaN alloys. Our results show that BInGaN alloys with a B:In ratio of 2:3 are better lattice matched to GaN compared to InGaN. Varying the Ga mole fraction while keeping the B:In ratio constant enables the adjustment of the (direct) gap in the 1.75-3.39 eV range, which covers the entire visible spectrum. Our results indicate that BInGaN alloys are promising for fabricating nitride heterostructures with thick active regions for high-power, high-efficiency LEDs (article in press in Appl. Phys. Lett.). |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y12.00009: Thermodynamic Routes to Novel Stable and Metastable Nitride Semiconductors Wenhao Sun, Aaron Holder, Bernardo Orvananos, Elisabetta Arca, Andriy Zakutayev, Stephan Lany, Gerbrand Ceder Compared to oxides, the nitrides are relatively unexplored, making them a promising chemical space for novel materials discovery. Of particular interest are nitrogen-rich nitrides, which often possess useful semiconducting properties for electronic and optoelectronic applications. However, nitrogen-rich nitrides are typically metastable, meaning they are rare in nature and difficult to synthesize. Here, we design rational thermodynamic routes to novel stable and metastable nitrogen-rich nitrides, and map these strategies onto a predictive computational search to identify stabilizable nitride materials across broad chemical spaces. Our search revealed dozens of new metastable nitrogen-rich binary nitrides which can be synthesized using reactive nitrogen precursors, and 87 previously unreported M1-M2-N ternary nitride systems with stable ternary nitrides. Using machine-learning algorithms, we cluster and map the ternary metal nitride space, elucidating the chemical families in this space to guide further materials exploration. |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y12.00010: First-Principles High-Throughput Analysis of Wide Band Gap Semiconductors for Optical Applications Francesco Naccarato, Francesco Ricci, Geoffroy Hautier, Ludger Wirtz, Gian-Marco Rignanese Relying on first-principles calculations, a database with the static refractive index and the bandgap of more than 4000 inorganic semiconductors is created. The inverse relationship between these two quantities is confirmed, but outliers are identified that combine both a wide band gap and a large refractive index. Focusing on these outliers, we explore the requirements for a wide band gap semiconductor to achieve a large refractive index. Our analysis reveals the importance of the density of states allowing for transitions between the top of the valence and the bottom of the conduction bands. The large availability of states for energy transitions somehow balances the effect of the band gap on the refractive index. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y12.00011: A'A"TeBiO6: A new family of inorganic double perovskite oxides containing bismuth for photovoltaic applications Arashdeep Thind, Shalinee Kavadiya, Sung Cho, Liang-Yi Lin, Ghanshyam Pilania, Pratim Biswas, Rohan Mishra Recently, halide double perovskites containing bismuth, such as Cs2AgBiBr6, have emerged as benign and promising alternatives to lead-organohalide perovskite photovoltaics. However, these halide double perovskites have large indirect band gaps and degrade over a period of weeks on exposure to ambient air and light. We use regression analysis and high-throughput density-functional theory (DFT) calculations to explore the vast composition space of double perovskite oxides containing bismuth to identify promising photovoltaics. We predict a stable family of compounds with a general formula of A'A"TeBiO6, where A' and A" are alkali and alkaline-earth metal cations respectively. We predict an indirect band gap of 1.94 eV and 1.99 eV for KBaTeBiO6 and RbBaTeBiO6, respectively, which is similar to that of Cs2AgBiBr6 (2.06 eV). The effective mass of holes and electrons for KBaTeBiO6 is 0.25me and 0.28me, respectively, which is comparable to those (0.14me and 0.37me) for Cs2AgBiBr6. We have successfully synthesized KBaTeBiO6 using wet-chemistry synthesis, with preliminary UV-vis measurements showing an indirect band gap of 1.7 eV. We will discuss the effect of composition on the electronic structure of these double perovskites. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y12.00012: Improved Tolerance Factor for Classifying the Formability of Perovskite Oxides and Halides Christopher Bartel, Christopher Sutton, Bryan Goldsmith, Runhai Ouyang, Charles Musgrave, Luca Ghiringhelli, Matthias Scheffler The Goldschmidt tolerance factor (t) has been used for decades to predict whether ABX3 solids are likely to crystallize into perovskite or nonperovskite structures. We find that t correctly classifies only 80% of 555 experimentally characterized ABX3 solids (X = O, F, Cl, Br, I) using a decision tree model fit to all examples to identify the optimal range for t. Higher structure classification accuracy is desired for the high-throughput screening of perovskite formability, especially for perovskite halides, which are generally predicted less accurately than perovskite oxides using t. In this work, Sure Independence Screening and Sparsifying Operator (SISSO) is used to identify the highest performing 2D descriptors among ~1011 candidate features derived from basic elemental and atomic properties (e.g., radii, mass, electronegativity, etc. of each A, B and X element). The result is an interpretable and linear descriptor that correctly classifies 93% of the 555 oxides and halides, including 95% on an excluded test set of 111 compounds. This descriptor is implemented on ~8,000 ABX3 compositions to construct a map of perovskite formability with respect to each of the elements, A, B, and X, and identify new chemical spaces that are likely to form perovskite structures. |
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