1,720,968 research outputs found
Anharmonic motion-induced self-passivation of hydrogen defects in hybrid organic–inorganic perovskites: Ab initio quantum dynamics
No full textHydrogen defects in hybrid organic–inorganic perovskites have garnered increasing attention due to their impact on device performance. Using nonadiabatic molecular dynamics on CH3NH3PbI3, we demonstrate that anharmonic motions at room temperature facilitate the formation of a stable Pb–N coordination bond between the Pb2+ ion and the CH2NH3 molecule in the presence of a negatively charged hydrogen interstitial defect. This bond eliminates the midgap hole trap state arising from the lone-pair electrons on the nitrogen of the dissociated CH3NH3+ cation at 0 K, enabling defect self-healing. It also reduces nonadiabatic coupling by decreasing the electron–hole wave function overlap and slows decoherence by suppressing thermal fluctuations. The reduced coupling, combined with a slight increase in the bandgap, dominates over the slower decoherence, extending the charge carrier lifetime to over 1.5 times that of the pristine system. This study establishes a self-passivation mechanism for hydrogen defects in perovskites under operational conditions.This work was supported by the National Natural Science Foundation of China Grant no. 92372121) and the Fundamental Research Funds for the Central Universities. A.S.V. acknowledges support from the project “International academic cooperation” of HSE University.Peer reviewe
Schottky defects suppress nonradiative recombination in CH3NH3PbI3 through charge localization
No full textHybrid lead halide perovskites are promising materials for photovoltaic applications due to their exceptional optoelectronic properties. Here, we investigate the impact of Schottky defects─specifically PbI2(VPbI2) and CH3NH3I (VMAI) vacancies─on nonradiative recombination in CH3NH3PbI3 using time-dependent density functional theory and nonadiabatic (NA) molecular dynamics. Our results reveal that Schottky defects do not alter the fundamental bandgap or introduce trap states but instead distort the surrounding lattice, localizing the hole distribution. This reduces the spatial overlap of electron and hole wave functions, weakening NA coupling and increasing intensitieis of high-intensity phonon modes that accelerate dephasing. Consequently, nonradiative recombination lifetimes extend to 2.1 and 2.6 ns for VPbI2 and VMAI, respectively─over double that of pristine CH3NH3PbI3. This work demonstrates the potential of Schottky defects to enhance perovskite solar cell performance by suppressing nonradiative recombination.This work was supported by the National Natural Science Foundation of China (grant no. 92372121) and the Fundamental Research Funds for the Central Universities. L.Q. acknowledges funding from the Alexander von Humboldt Foundation.Peer reviewe
Formation and recombination dynamics of polarons in goethite: A time-domain Ab initio study
No full textThe temperature and the coordination environment significantly affect polaron dynamics. Using goethite (FeOOH) as a model, our study examines polaron formation and recombination behavior under various conditions, including electron injection, photoexcitation, and heterovalent doping. Ab initio and nonadiabatic molecular dynamics (NAMD) simulations reveal that polaron formation in FeOOH is dependent on temperature via an adiabatic mechanism with higher temperatures leading to shorter formation times. Only electron polarons form in FeOOH, regardless of the formation method. NAMD simulations indicate that photoexcited electron polaron recombination is significantly faster in FeOOH than in Fe2O3. This difference arises from the distinct coordination environments, resulting in higher inelastic charge–phonon scattering and stronger nonadiabatic coupling in FeOOH. Our findings highlight the crucial roles of temperature and coordination environment in polaron dynamics, offering valuable insights for designing materials to optimize carrier dynamics.This work was supported by the National Natural Science Foundation of China (92372121) and the Fundamental Research Funds for the Central Universities.Peer reviewe
Compression eliminates charge traps by stabilizing perovskite grain boundary structures: An Ab initio analysis with machine learning force field
No full textGrain boundaries (GBs) play an important role in determining the optoelectronic properties of perovskites, requiring an atomistic understanding of the underlying mechanisms. Strain engineering has recently been employed in perovskite solar cells, providing a novel perspective on the role of perovskite GBs. Here, we theoretically investigate the impact of axial strain on the geometric and electronic properties of a common CsPbBr3 GB. We develop a machine learning force field and perform ab initio calculations to analyze the behavior of GB models with different axial strains on a nanosecond time scale. Our results demonstrate that compressing the GB efficiently suppresses structural fluctuations and eliminates trap states originating from large-scale distortions. The GB becomes more amorphous under compressive strain, which makes the relationship between the electronic structure and axial strain nonmonotonic. These results can help clarify the conflicts in perovskite GB experiments.This research was supported in part through computational resources of HPC facilities at HSE University. A.S.V. and M.R.S. acknowledge support from the Academic Fund Program at HSE University in 2024 (grant no. 24-00-007 New materials for solar energetics). Y.W. and O.V.P. acknowledge support of the US National Science Foundation, grant CHE-2154367.Peer reviewe
Ferroelectric polarization and single-atom catalyst synergistically promoting CO2 photoreduction of CuBiP2Se6
No full textFurther improving the activity and selectivity of photocatalytic CO2 reduction remains a challenge. Herein, we propose a new strategy for synergistically promoting photocatalytic CO2 reduction by combining two-dimensional (2D) ferroelectric polarization and single-atom catalysis. Our calculations showed that the ferroelectric polarization of CuBiP2Se6 provides the internal driving force for the separation and migration of photogenerated carriers, which provides a prerequisite for enhancing the photocatalytic efficiency. In addition, the introduction of single Ag atoms can act as an electron reservoir to significantly modify the bonding configurations on the surface through proper static electron transfer, thus effectively promoting the adsorption and activation of CO2 molecules. More importantly, we found that switching the ferroelectric polarization can synergistically optimize the limiting potential as well as control the final products. This study provides a new approach for enhancing the catalytic activity and selectivity of photocatalytic CO2 reduction.This work is supported by the Scientific Research Fund of Hunan Provincial Education Department (No. 23B0679), the Science Initiation Fund of Hengyang Normal University (No. 2023QD32), and the Basic Research Program of HSE University. The computational resources for this research were provided by the TianHe supercomputer in Changsha, China, and the HPC facilities at HSE University.Peer reviewe
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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