Instilling defect tolerance in new compounds

Walsh, Aron,Zunger, Alex Nature Publishing Group 2017 Nature materials Vol.16 No.10

Self-Regulation Mechanism for Charged Point Defects in Hybrid Halide Perovskites

Walsh, Aron,Scanlon, David O,Chen, Shiyou,Gong, X G,Wei, Su-Huai WILEY-VCH Verlag 2015 Angewandte Chemie Vol.54 No.6

<P>Hybrid halide perovskites such as methylammonium lead iodide (CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>) exhibit unusually low free-carrier concentrations despite being processed at low-temperatures from solution. We demonstrate, through quantum mechanical calculations, that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self-regulate the concentration of charge carriers through ionic compensation of charged point defects. The equilibrium charged vacancy concentration is predicted to exceed 0.4 % at room temperature. This behavior, which goes against established defect conventions for inorganic semiconductors, has implications for photovoltaic performance.</P>

Oxidation states and ionicity

Walsh, Aron,Sokol, Alexey A.,Buckeridge, John,Scanlon, David O.,Catlow, C. Richard A. Springer Science and Business Media LLC 2018 NATURE MATERIALS Vol.17 No.11

What Is Moving in Hybrid Halide Perovskite Solar Cells?

Frost, Jarvist M.,Walsh, Aron American Chemical Society 2016 Accounts of chemical research Vol.49 No.3

<P>Organic-inorganic semiconductors, which adopt the perovskite crystal structure, have perturbed the landscape of contemporary photovoltaics research. High-efficiency solar cells can be produced with solution-processed active layers. The materials are earth abundant, and the simple processing required suggests that high-throughput and low-cost manufacture at scale should be possible. While these materials bear considerable similarity to traditional inorganic semiconductors, there are notable differences in their optoelectronic behavior. A key distinction of these materials is that they are physically soft, leading to considerable thermally activated motion. In this Account, we discuss, the internal motion of methylammonium lead iodide (CH3NH3PbI3) and formamidinium lead iodide ([CH(NH2)(2)]PbI3), covering: (i) molecular rotation-libration in the cuboctahedral cavity; (ii) drift and diffusion of large electron and hole-polarons; (iii) transport of charged ionic defects. These processes give rise to a range of properties that are unconventional for photovoltaic materials, including frequency-dependent permittivity, low electron-hole recombination rates, and current-voltage hysteresis. Multiscale simulations, drawing from electronic structure, ab initio molecular dynamic and Monte Carlo computational techniques, have been combined with neutron diffraction measurements, quasi-elastic neutron scattering, and ultrafast vibrational spectroscopy to qualify the nature and time scales of the motions. Electron and hole motion occurs on a femtosecond time scale. Molecular libration is a sub-picosecond process. Molecular rotations occur with a time constant of several picoseconds depending on the cation. Recent experimental evidence and theoretical models for simultaneous electron and ion transport in these materials has been presented, suggesting they are mixed-mode conductors with similarities to fast-ion conducting metal oxide perovskites developed for battery and fuel cell applications. We expound on the implications of these effects for the photovoltaic action. The temporal behavior displayed by hybrid perovskites introduces a sensitivity in materials characterization to the time and length scale of the measurement, as well as the history of each sample. It also poses significant challenges for accurate materials modeling and device simulations. There are large differences between the average and local crystal structures, and the nature of charge transport is too complex to be described by common one-dimensional drift-diffusion models. Herein, we critically discuss the atomistic origin of the dynamic processes and the associated chemical disorder intrinsic to crystalline hybrid perovskite semiconductors.</P>

Stacking-dependent energetics and electronic structure of ultrathin polymorphicV2VI3topological insulator nanofilms

Li, Can,Winzer, Torben,Walsh, Aron,Yan, Binghai,Stampfl, Catherine,Soon, Aloysius American Physical Society 2014 Physical review. B, Condensed matter and materials Vol.90 No.7

Frontier Orbital Engineering of Metal–Organic Frameworks with Extended Inorganic Connectivity: Porous Alkaline-Earth Oxides

Hendon, Christopher H.,Walsh, Aron,Dincă,, Mircea American Chemical Society 2016 Inorganic Chemistry Vol.55 No.15

<P>The development of conductive metal-organic frameworks is challenging owing to poor electronic communication between metal clusters and the organic ligands that bridge them. One route to overcoming this bottleneck is to extend the inorganic dimensionality, while using the organic components to provide chemical functionality. Using density functional theory methods, we demonstrate how the properties of the alkaline-earth oxides SrO and BaO are ttansformed upon formation of porous solids with organic oxygen sources (acetate and trifluorocetate). The electron affinity is significantly enhanced in the hybrid materials, while the ionization potential can be tuned over a large range with the polarity of the organic moiety. Furthermore, because of their high-vacuum fraction, these materials have dielectric, properties suitable for low-kappa applications.</P>

Stability and flexibility of heterometallic formate perovskites with the dimethylammonium cation: pressure-induced phase transitions

Ptak, Maciej,Svane, Katrine Louise,Walsh, Aron,Paraguassu, Waldeci Royal Society of Chemistry 2019 Physical chemistry chemical physics Vol.21 No.8

<P>We report the high-pressure properties of two heterometallic perovskite-type metal-organic frameworks (MOFs) templated by dimethylammonium (NH2(CH3)2, DMA<SUP>+</SUP>) with the general formula [DMA]MI0.5CrIII0.5(HCOO)3, where M<SUP>I</SUP> = Na<SUP>+</SUP> (DMANaCr) and K<SUP>+</SUP> (DMAKCr). The high-pressure Raman scattering studies show crystal instabilities in the 4.0-4.4 GPa and 2.0-2.5 GPa ranges for DMANaCr and DMAKCr, respectively. The mechanism is similar in the two compounds and involves strong deformation of the metal-formate framework, especially pronounced for the subnetwork of CrO6 octahedra, accompanied by substantial compressibility of the DMA<SUP>+</SUP> cations. Comparison with previous high-pressure Raman studies of sodium-chromium heterometallic MOFs show that the stability depends on the templated cation and increases as follows: ammonium < imidazolium < DMA<SUP>+</SUP>. Density functional theory (DFT) calculations are performed to get a better understanding of the structural properties leading to the existence of phase transitions. We calculate the energy of the hydrogen bonds (HBs) between the DMA<SUP>+</SUP> cation and the metal formate cage, revealing a stronger interaction in the DMAKCr compound due to a HB arrangement that primarily involves the energetically preferred bonding to KO6 octahedra. This material however also has a smaller structural tolerance factor (TF) and a higher vibrational entropy than DMANaCr. This indicates a more flexible crystal structure, explaining the lower phase transition pressure, as well as the previously observed phase transition at 190 K, which is absent in the DMANaCr compound. The DFT high-pressure simulations show the largest contraction to be along the trigonal axis, leading to a minimal distortion of the HBs formed between the DMA<SUP>+</SUP> cations and the metal-formate sublattice.</P>

Relativistic electronic structure and band alignment of BiSI and BiSeI: candidate photovoltaic materials

Ganose, Alex M.,Butler, Keith T.,Walsh, Aron,Scanlon, David O. The Royal Society of Chemistry 2016 Journal of Materials Chemistry A Vol.4 No.6

<P>Bismuth-based solar absorbers are of interest due to similarities in the chemical properties of bismuth halides and the exceptionally efficient lead halide hybrid perovskites. Whilst they both experience the same beneficial relativistic effects acting to increase the width of the conduction band, bismuth is non-toxic and non-bioaccumulating, meaning the impact of environmental contamination is greatly reduced. Here, we use hybrid density functional theory, with the addition of spin orbit coupling, to examine two candidate bismuth containing photovoltaic absorbers, BiSI and BiSeI, and show that they possess electronic structures suitable for photovoltaic applications. Furthermore, we calculate band alignments against commonly used hole transporting and buffer layers, which indicate band misalignments are likely to be the source of the poor efficiencies reported for devices containing these materials. Based on this we have suggested alternative device architectures expected to result in improved power conversion efficiencies.</P>

Solid-state chemistry of glassy antimony oxides

Kim, Chang-Eun,Skelton, Jonathan M.,Walsh, Aron,Soon, Aloysius The Royal Society of Chemistry 2015 Journal of Materials Chemistry C Vol.3 No.43

<▼1><P>Using hybrid density-functional theory (DFT) and <I>ab initio</I> molecular dynamics, we calculate and characterize glassy amorphous antimony oxides in elevated oxygen environments, and provide information on how their atomic and electronic structures change as a function of their oxygen environment.</P></▼1><▼2><P>The amorphous phases of antimony oxide at three different oxidation levels have been investigated by <I>ab initio</I> molecular-dynamics calculations using a simulated ‘melt-quench’ approach. The atomic and electronic structures of the amorphous phases are analyzed and compared to their crystalline counterparts. The amorphous structure of the trioxide (a-Sb2O3) resembles the β-phase of the crystalline form (valentinite), in agreement with previous observations. In the relaxed athermal structure, however, more senarmontite-like structural features are apparent. The phase diagram of the amorphous phases with respect to the oxygen chemical potential has been calculated within the framework of <I>ab initio</I> thermodynamics. At elevated oxidation levels, the resulting tetraoxide and pentoxide materials show distinct variation in electronic structure compared to the trioxide, with the electronic density of states indicating a narrowing of the electronic gap with increasing oxidation level.</P></▼2>

Analysis of electrostatic stability and ordering in quaternary perovskite solid solutions

Caetano, Clovis,Butler, Keith T.,Walsh, Aron American Physical Society 2016 Physical Review B Vol.93 No.14

<P>There are three distinct classes of perovskite structured metal oxides, defined by the charge states of the cations: A(I)B(V)O(3), A(II)B(IV)O(3), and A(III)B(III)O(3). We investigated the stability of cubic quaternary solid solutions ABO(3)-A'B'O-3 using a model of point-charge lattices. The mixing enthalpies were calculated and compared for the three possible types of combinations of the compounds, both for the random alloys and the ground-state-ordered configurations. The mixing enthalpy of the (I,V)O-3-(III,III)O-3 alloy is always larger than the other alloys. We found that, different from homovalent alloys, for these heterovalent alloys a lattice constant mismatch between the constituent compounds could contribute to stabilize the alloy. At low temperatures, the alloys present a tendency to spontaneous ordering, forming superlattices consisting of alternated layers of ABO(3) and A'B'O-3 along the [110] direction.</P>