Defects in Halide Perovskites: Does It Help to Switch from 3D to 2D?

Ruddlesden-Popper hybrid iodide 2D perovskites are put forward as stable alternatives to their 3D counterparts. Using first-principles calculations, we demonstrate that equilibrium concentrations of point defects in the 2D perovskites PEA$_2$PbI$_4$, BA$_2$PbI$_4$, and PEA$_2$SnI$_4$ (PEA: phenethyl ammonium, BA: butylammonium), are much lower than in comparable 3D perovskites. Bonding disruptions by defects are more detrimental in 2D than in 3D networks, making defect formation energetically more costly. The stability of 2D Sn iodide perovskites can be further enhanced by alloying with Pb. Should, however, point defects emerge in sizable concentrations as a result of nonequilibrium growth conditions, for instance, then those defects hamper the optoelectronic performance of the 2D perovskites, as they introduce deep traps. We suggest that trap levels are responsible for the broad sub-bandgap emission in 2D perovskites observed in experiments

Compound Defects in Halide Perovskites: A First-Principles Study of CsPbI3_3

Lattice defects affect the long-term stability of halide perovskite solar cells. Whereas simple point defects, i.e., atomic interstitials and vacancies, have been studied in great detail, here we focus on compound defects that are more likely to form under crystal growth conditions, such as compound vacancies or interstitials, and antisites. We identify the most prominent defects in the archetype inorganic perovskite CsPbI$_3$, through first-principles density functional theory (DFT) calculations. We find that under equilibrium conditions at room temperature, the antisite of Pb substituting Cs forms in a concentration comparable to those of the most prominent point defects, whereas the other compound defects are negligible. However, under nonequilibrium thermal and operating conditions, other complexes also become as important as the point defects. Those are the Cs substituting Pb antisite, and, to a lesser extent, the compound vacancies of PbI$_2$ or CsPbI$_3$ units, and the I substituting Cs antisite. These compound defects only lead to shallow or inactive charge carrier traps, which testifies to the electronic stability of the halide perovskites. Under operating conditions with a quasi Fermi level very close to the valence band, deeper traps can develop

In Situ IR Spectroscopy Studies of Atomic Layer-Deposited SnO₂ on Formamidinium-Based Lead Halide Perovskite

Perovskite photovoltaics has achieved conversion efficiencies of 26.0% by optimizing the optoelectronic properties of the absorber and its interfaces with charge transport layers (CTLs). However, commonly adopted organic CTLs can lead to parasitic absorption and device instability. Therefore, metal oxides like atomic layer-deposited (ALD) SnO2 in combination with fullerene-based electron transport layers have been introduced to enhance mechanical and thermal stability. Instead, when ALD SnO2 is directly processed on the absorber, i.e., without the fullerene layer, chemical modifications of the inorganic fraction of the perovskite occur, compromising the device performance. This study focuses on the organic fraction, particularly the formamidinium cation (FA+), in a CsFAPb(I,Br)3 perovskite. By employing in situ infrared spectroscopy, we investigate the impact of ALD processing on the perovskite, such as vacuum level, temperature, and exposure to half and full ALD cycles using tetrakis(dimethylamido)-Sn(IV) (TDMA-Sn) and H2O. We observe that exposing the absorber to vacuum conditions or water half-cycles has a negligible effect on the chemistry of the perovskite. However, prolonged exposure at 100 °C for 90 min results in a loss of 0.7% of the total formamidinium-related vibrational features compared to the pristine perovskite. Supported by density functional theory calculations, we speculate that FA+ deprotonates and that formamidine desorbs from the perovskite surface. Furthermore, the interaction between TDMA-Sn and FA+ induces more decomposition of the perovskite surface compared to vacuum, temperature, or H2O exposure. During the exposure to 10 ALD half-cycles of TDMA-Sn, 4% of the total FA+-related infrared features are lost compared to the pristine perovskite. Additionally, IR spectroscopy suggests the formation and trapping of sym-triazine, i.e., a decomposition product of FA+. These studies enable to decouple the effects occurring during direct ALD processing on the perovskite and highlight the crucial role of the Sn precursor in affecting the perovskite surface chemistry and compromising the device performance

Complete genome sequence of Shigella flexneri 5b and comparison with Shigella flexneri 2a

BACKGROUND: Shigella bacteria cause dysentery, which remains a significant threat to public health. Shigella flexneri is the most common species in both developing and developed countries. Five Shigella genomes have been sequenced, revealing dynamic and diverse features. To investigate the intra-species diversity of S. flexneri genomes further, we have sequenced the complete genome of S. flexneri 5b strain 8401 (abbreviated Sf8401) and compared it with S. flexneri 2a (Sf301). RESULTS: The Sf8401 chromosome is 4.5-Mb in size, a little smaller than that of Sf301, mainly because the former lacks the SHI-1 pathogenicity island (PAI). Compared with Sf301, there are 6 inversions and one translocation in Sf8401, which are probably mediated by insertion sequences (IS). There are clear differences in the known PAIs between these two genomes. The bacteriophage SfV segment remaining in SHI-O of Sf8401 is clearly larger than the remnants of bacteriophage SfII in Sf301. SHI-1 is absent from Sf8401 but a specific related protein is found next to the pheV locus. SHI-2 is involved in one intra-replichore inversion near the origin of replication, which may change the expression of iut/iuc genes. Moreover, genes related to the glycine-betaine biosynthesis pathway are present only in Sf8401 among the known Shigella genomes. CONCLUSION: Our data show that the two S. flexneri genomes are very similar, which suggests a high level of structural and functional conservation between the two serotypes. The differences reflect different selection pressures during evolution. The ancestor of S. flexneri probably acquired SHI-1 and SHI-2 before SHI-O was integrated and the serotypes diverged. SHI-1 was subsequently deleted from the S. flexneri 5b genome by recombination, but stabilized in the S. flexneri 2a genome. These events may have contributed to the differences in pathogenicity and epidemicity between the two serotypes of S. flexneri

Genome dynamics and diversity of Shigella species, the etiologic agents of bacillary dysentery

The Shigella bacteria cause bacillary dysentery, which remains a significant threat to public health. The genus status and species classification appear no longer valid, as compelling evidence indicates that Shigella, as well as enteroinvasive Escherichia coli, are derived from multiple origins of E.coli and form a single pathovar. Nevertheless, Shigella dysenteriae serotype 1 causes deadly epidemics but Shigella boydii is restricted to the Indian subcontinent, while Shigella flexneri and Shigella sonnei are prevalent in developing and developed countries respectively. To begin to explain these distinctive epidemiological and pathological features at the genome level, we have carried out comparative genomics on four representative strains. Each of the Shigella genomes includes a virulence plasmid that encodes conserved primary virulence determinants. The Shigella chromosomes share most of their genes with that of E.coli K12 strain MG1655, but each has over 200 pseudogenes, 300∼700 copies of insertion sequence (IS) elements, and numerous deletions, insertions, translocations and inversions. There is extensive diversity of putative virulence genes, mostly acquired via bacteriophage-mediated lateral gene transfer. Hence, via convergent evolution involving gain and loss of functions, through bacteriophage-mediated gene acquisition, IS-mediated DNA rearrangements and formation of pseudogenes, the Shigella spp. became highly specific human pathogens with variable epidemiological and pathological features

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Traveling Wave Solutions of a Delayed Cooperative System

This paper deals with the dynamics of a delayed cooperative system without quasimonotonicity. Using the contracting rectangles, we obtain a sufficient condition on the stability of the unique positive steady state of the functional differential system. When the spatial domain is whole R , the existence and nonexistence of traveling wave solutions are investigated, during which the asymptotic behavior is investigated by the contracting rectangles

The delta C-13 changes in four plant species of the Loess Plateau over the last 70 years

The relative abundance of carbon isotope (delta C-13) was measured in four C-3 species (Sophora viccifolia. Quercus liaotungensis. Ostryopsis davidiana and Zizyphus jujuba var. spinosa) of the Loess Plateau in China from the 1930's to 2002. The results showed that the delta C-13 values in the four species varied from -25.05 parts per thousand to -29.75 parts per thousand with their average at -27.04 parts per thousand. A decrease in the delta C-13 value with time was found in all the four species, which indicating that the water use efficiencies (WUEs) of all the measured species declined during 70 years. However. the decrease in delta C-13 value differed among the four species with its significant decreases measured in two of the species, Sophora viciifolia and Quercus liaotungensis, its relatively significant decrease found in Ostryopsis davidiana, and its slight decrease appearing in Zizyphus,jujuba var. spinosa. in the delta C-13 values in the four species decreased by 14.65 parts per thousand, 14.46 parts per thousand, 11.99 parts per thousand and 2.44 parts per thousand, respectively. The different species were shown to have different sensitivities to climatic change, and Zizyphus jujuba var. spinosa was found to be the most drought-tolerant species of the four, which had a high WUE

Intrinsic defects in primary halide perovskites: A first-principles study of the thermodynamic trends

Defects in halide perovskites play an essential role in determining the efficiency and stability of the optoelectronic devices based on these materials. We present a systematic study of intrinsic point defects in six primary metal halide perovskites, MAPbI3, MAPbBr3, MAPbCl3, FAPbI3, CsPbI3, and MASnI3 (where MA denotes methylammonium and FA denotes formamidinium), based upon density functional theory calculations. Within a single computational scheme, using the SCAN+rVV10 functional, we compare the impact of changing anions and cations on the defect formation energies and the charge state transition levels in the six compounds, and identify the physical origins underlying the observed trends. Dominant defects in the lead iodide compounds are the A+ cation interstitials (A=Cs, MA, FA), charge-compensated by I- interstitials or lead (2-) vacancies. In the lead bromide and lead chloride compounds, halide interstitials are most prominent, and for MAPbCl3, the chlorine vacancy also becomes important. These trends can be explained in terms of the changes in electrostatic interactions and chemical bonding upon replacing cations and anions. Defect physics in MASnI3 is strongly dominated by tin (2-) vacancies, promoted by the easy oxidation of the tin. Intrinsically, all compounds are mildly p doped, except for MASnI3, which is strongly p doped. All acceptor levels created by defects in the six perovskites are shallow. Some defects, halide vacancies and Pb or Sn interstitials in particular, can create deep donor traps. Although such traps might hamper the electronic behavior of MAPbCl3, in bromine- and iodine-based perovskites their equilibrium concentrations are too small to affect the materials' properties