The Role of Grain Boundaries on Ion Migration and Charge Recombination in Halide Perovskites.

Grain boundaries (GBs) have a significant role in polycrystalline perovskite solar cells (PSCs). However, there is ongoing debate regarding the impact of GBs on the performance and long-term stability of PSCs. Employing the first-principles molecular dynamics for perovskites, the iodine vacancy defect migrations both in bulk and at GBs are investigated. i) The positive iodine vacancy (VI +) is found that have both lower formation energy (1.4 eV) and activation energy (0.18 eV) than those of neutral iodine vacancy (VI), statistically. It indicated the VI + acts as the dominant migrated iodine vacancy rather than VI; ii) the iodine vacancy at GBs has ≈0.48 eV higher activation energy than those in bulk, which leads to the accumulation of iodine vacancy at GBs; iii) the presence of VI + result in a 3-fold increase in charge recombination ratio at GBs, compared to pristine PSCs. Based on quantum molecular dynamics statistical results, which are consistent with experimental measurements, insights into iodine vacancy migration both at GBs and in the bulk are gained. This understanding can be valuable for defects engineering related to ion migration, in order to improve the long-term stability and promote the performance of PSCs.

RbPbI3 Seed Embedding in PbI2 Substrate Tailors the Facet Orientation and Crystallization Kinetics of Perovskites.

High power conversion efficiencies (PCEs) in perovskite solar cells (PSCs) have always been awe-inspiring, but perovskite films scalability is an exacting precondition for PSCs commercial deployment, generally unachievable through the antisolvent technique. On the contrary, in the two-step sequential method, the perovskite's uncontrolled crystallization and unnecessary PbI2 residue impede the device's performance. These two issues motivated to empower the PbI2 substrate with orthorhombic RbPbI3 crystal seeds, which act as grown nuclei and develop orientated perovskites lattice stacks, improving the perovskite films morphologically and reducing the PbI2 content in eventual perovskite films. Thence, achieving a PCE of 24.17% with suppressed voltage losses and an impressive life span of 1140 h in the open air.

A Universal Microscopic Patterned Doping Method for Perovskite Enables Ultrafast, Self-Powered, Ultrasmall Perovskite Photodiodes.

Novel metal halide perovskite is proven to be a promising optoelectronic material. However, fabricating microscopic perovskite devices is still challenging because the perovskite is soluble with the photoresist, which conflicts with conventional microfabrication technology. The size of presently reported perovskite devices is about 50 µm. Limited by the large size of perovskite optoelectronic devices, they cannot be readily adopted in the fields of imaging, display, etc. Herein a universal microscopic patterned doping method is proposed, which can realize microscale perovskite devices. Rather than by the conventional doping method, in this study the local Fermi level of perovskite is modulated by the redistributing intrinsic ion defects via a polling voltage. A satisfactorily stable polarized ion distribution can be achieved by optimization of the perovskite material and polling voltage, resulting in ultrafast (40 µs), self-powered microscale (2 µm) photodiodes. This work sheds light on a route to fabricate integrated perovskite optoelectronic chips.

Theoretical investigation of electrochromic mechanism in D-A conjugated polymers in visible and infrared bands.

Electrochromic materials have been widely-applied in military camouflage and intelligent materials, in consideration of the multicolor display and infrared absorption. However, most of them have a narrow width of absorption spectra, and the electrochromic mechanism is still not well understood, especially in materials based on a copolymer structure in visible and infrared bands. Therefore, based on the polaron model, in order to enhance polarizability, we designed an "electronic donors-electronic acceptor" (D-A) type π-conjugated electrochromic polymer, which has an abundant color (wavelengths from 450 nm to 750 nm) with voltage range (from -0.2 V to 1.0 V). Employing first-principle calculations, we investigated the electrochromism of the polymer, which has a strong connection with the introduced new molecular orbital in the polaron (or cation), comparing with those in the neutral molecule. This study addressed the underlying mechanism for the electrochromic phenomenon and the behavior of the cation. It indicated the polaron molecular orbitals provide the photon absorption, whose energies are in the visible range and result in the electrochromic abundant color. In this work, we provide a molecular design for the adjustment of visible and infrared band absorption, which could have broad application in multicolor and infrared electrochromic materials.

Design of Intramolecular Dihedral Angle between Electronic Donor and Acceptor in Thermally Activated Delayed Fluorescence Molecules.

In order to improve the exciton utilization efficiency (ηexc) of organic light-emitting materials, we addressed the ideal donor-acceptor dihedral angle (θD-A) in the TADF molecule by striking a balance between two photophysical processes. One is the conversion of triplet excitons into singlet excitons, and the other is the radiative process from a low-lying excited state to the ground state. Using a combination of first-principles calculations and molecular dynamics simulations, we investigated the impact of θD-A on the splitting energy and spin-orbit coupling between singlet and triplet excitons as well as the transition dipole moment for carbazole benzonitrile (CzBN) derivatives. By comparison with the reverse intersystem crossing rate (krISC), fluorescence emission rate (kr), and ηexc, we proposed a potential highest ηexc (of 94.4%) with the ideal θD-A of 77° for blue light CzBN derivatives; the calculated results have a good agreement with experimental measurement. The structure-efficiency physical connection between the molecular structure (θD-A) and efficiency provided an ideal parameter for a potential candidate for blue TADF-OLED materials.

Strain-Tunable Electronic and Transport Properties of One-Dimensional Fibrous Phosphorus Nanotubes.

The one-dimensional van der Waals (1D vdW) material fibrous red phosphorus (FRP) nanotubes are a promising direct-bandgap semiconductor with high carrier mobility and anisotropic optical responses because of low deformation potential and dangling-bond-free anisotropic interface. Employing first-principles calculations, we captured the potential of 1D FRP nanotubes. The thermal stability of 1D FRP nanotubes was confirmed by phonon calculation. Meanwhile, Raman spectroscopy indicated the strong vibration mode (366 cm-1) is along the phosphorus nanotube. Interestingly, spatial anisotropy bandgaps were found along with various stacking orientations. The charge transport calculations showed that the 1D FRP nanotube has a high hole mobility (499.2 cm2 V-1 s-1), considering the weak acoustic phonon scattering. More importantly, we found that the hole mobility changes dramatically (down to 7.1 cm2 V-1 s-1) under the strain, and the strain-dependent charge transport property of 1D FRP nanotubes could be considered to have many potential applications for electronics, optoelectronics, and switching devices.

A Novel Hybrid Approach for Risk Evaluation of Vehicle Failure Modes.

This paper addresses the problem of evaluating vehicle failure modes efficiently during the driving process. Generally, the most critical factors for preventing risk in potential failure modes are identified by the experience of experts through the widely used failure mode and effect analysis (FMEA). However, it has previously been difficult to evaluate the vehicle failure mode with crisp values. In this paper, we propose a novel hybrid scheme based on a cost-based FMEA, fuzzy analytic hierarchy process (FAHP), and extended fuzzy multi-objective optimization by ratio analysis plus full multiplicative form (EFMULTIMOORA) to evaluate vehicle failure modes efficiently. Specifically, vehicle failure modes are first screened out by cost-based FMEA according to maintenance information, and then the weights of the three criteria of maintenance time (T), maintenance cost (C), and maintenance benefit (B) are calculated using FAHP and the rankings of failure modes are determined by EFMULTIMOORA. Different from existing schemes, the EFMULTIMOORA in our proposed hybrid scheme calculates the ranking of vehicle failure modes based on three new risk factors (T, C, and B) through fuzzy linguistic terms for order preference. Furthermore, the applicability of the proposed hybrid scheme is presented by conducting a case study involving vehicle failure modes of one common vehicle type (Hyundai), and a sensitivity analysis and comparisons are conducted to validate the effectiveness of the obtained results. In summary, our numerical analyses indicate that the proposed method can effectively help enterprises and researchers in the risk evaluation and the identification of critical vehicle failure modes.

Improved efficiency and stability of flexible perovskite solar cells by a new spacer cation additive.

Flexible perovskite solar cells (PSCs) have attracted tremendous attention due to their potential application in portable and wearable electronics. However, the photoelectric conversion efficiency (PCE) of flexible PSCs is still far lower than that of usual rigid PSCs. Moreover, the mechanical stability of flexible PSCs cannot meet the needs of commercial applications because of the cracking of perovskite grains caused by bending stress. Here, we introduced a spacer cation additive (2-(chloromethyl) pyridine hydrochloride, CPHC) within the perovskite organic precursor to improve the device PCE and its mechanical stability. We observed that the CPHC spacer cation additive could simultaneously facilitate the crystallization of perovskite and stitch the grain boundaries to improve the flexibility. Compared to the 17.64% PCE of the control devices, the target flexible PSCs achieved a more highly efficiency over 19% with an improved mechanical stability (87.2% of the initial PCE after the 1000 cycles with the bending radius R = 6 mm). In addition, compared to methylammonium or formamidinium cation, due to the stronger hydrophobic and larger activation energy barrier for the ion migration of the CPHC spacer cation, the device retained over 80% of the initial PCE after 30 days storage in an ambient environment.

Understanding liquefaction in halide perovskites upon methylamine gas exposure.

Methylamine (CH3NH2, MA) gas-induced fabrication of organometal CH3NH3PbI3 based perovskite thin films are promising photovoltaic materials that transform the energy from absorbed sunlight into electrical power. Unfortunately, the low stability of the perovskites poses a serious hindrance for further development, compared to conventional inorganic materials. The solid-state perovskites are liquefied and recrystallized from CH3NH2. However, the mechanism of this phase transformation is far from clear. Employing first principles calculations and ab initio molecular dynamics simulations, we investigated the formation energy of primary defects in perovskites and the liquefaction process in CH3NH2 vapor. The results indicated that defect-assisted surface dissolution leads to the liquefaction of perovskite thin films in CH3NH2 vapor. Two primary defects were studied: one is the Frenkel pair defect (including both negatively charged interstitial iodide ion (Ii -) and iodide vacancy (VI +) at the PbI2-termination surface, and the other is the Schottky defects (methylammonium vacancy, VMA) at the MAI-termination surface. Moreover, the defect-induced disorder in the microstructure reduces the degeneration of energy levels, which leads to a blue shift and broader absorption band gap, as compared to the clean perovskite surface. The mechanism of how defects impact the surface dissolution could be applied for the further design of high-stability perovskite solar cells.

Surface-Anchored Metal-Organic Frameworks as Versatile Resists for Gas-Assisted E-Beam Lithography: Fabrication of Sub-10 Nanometer Structures.

We demonstrate that surface-anchored metal-organic frameworks (SURMOFs) are extraordinary well-suited as resists for high-resolution focused electron beam induced processing (FEBIP) techniques. The combination of such powerful lithographic protocols with the huge versatility of MOF materials are investigated in respect to their potential in nanostructures fabrication. The applied FEBIP methods rely on the local decomposition of Fe(CO)5 and Co(CO)3NO as precursors, either by the direct impact of the focused electron beam (electron beam induced deposition, EBID) or through the interaction of the precursor molecules with preirradiated/activated SURMOF areas (electron beam induced surface activation, EBISA). We demonstrate the huge potential of the approach for two different types of MOFs (HKUST-1 and Zn-DPDCPP). Our "surface science" approach to FEBIP, yields well-defined deposits with each investigated precursor/SURMOF combination. Local Auger electron spectroscopy reveals clean iron deposits from Fe(CO)5; deposits from Co(CO)3NO contain cobalt, nitrogen, and oxygen. EBISA experiments were successful with Fe(CO)5. Remarkably EBISA with Co(CO)3NO does not result in deposit formation on both resists, making the process chemically selective. Most importantly we demonstrate the fabrication of "nested-L" test structures with Fe(CO)5 on HKUST-1 with extremely narrow line widths of partially less than 8 nm, due to reduced electron proximity effects within the MOF-based resists. Considering that the actual diameter of the electron beam was larger than 6 nm, we see a huge potential for significant reduction of the structure sizes. In addition, the role and high potential of loading and transport of the precursor molecules within the porous SURMOF materials is discussed.

High Antimicrobial Activity of Metal-Organic Framework-Templated Porphyrin Polymer Thin Films.

Development of surface coatings with high antimicrobial activity is urgently required to fight bacteria and other microorganisms on technical and hygiene relevant surfaces. Control over structure and topology of the surface coatings, combined with the ability to include functional molecules within the structure, is crucial for optimizing their performance. Herein, we describe a novel strategy to synthesize structurally well-defined porphyrin polymer thin films via a template approach. In this approach, bisazido-functionalized porphyrin molecules are preorganized within a metal-organic framework (MOF) structure. Afterward, porphyrin units within the MOF are covalently connected via a secondary linker. Removal of the metal ions of the MOF results in water-stable porphyrin polymer thin films that demonstrate high antibacterial activity against pathogens via visible-light-promoted generation of reactive oxygen species. In addition, this approach offers the inherent possibility to incorporate guest molecules within the structures, to functionalize the surface with biomolecules, and to create hierarchically structured materials.

Linear Chains of Magnetic Ions Stacked with Variable Distance: Ferromagnetic Ordering with a Curie Temperature above 20†K.

We have studied the magnetic properties of the SURMOF-2 series of metal-organic frameworks (MOFs). Contrary to bulk MOF-2 crystals, where Cu(2+) ions form paddlewheels and are antiferromagnetically coupled, in this case the Cu(2+) ions are connected via carboxylate groups in a zipper-like fashion. This unusual coupling of the spin 1/2 ions within the resulting one-dimensional chains is found to stabilize a low-temperature, ferromagnetic (FM) phase. In contrast to other ordered 1D systems, no strong magnetic fields are needed to induce the ferromagnetism. The magnetic coupling constants describing the interaction between the individual metal ions have been determined in SQUID experiments. They are fully consistent with the results of ab initio DFT electronic structure calculations. The theoretical results allow the unusual magnetic behavior of this exotic, yet easy-to-fabricate, material to be described in a detailed fashion.

Electrochromic switching of monolithic Prussian blue thin film devices.

Monolithic, crystalline and highly oriented coordination network compound (CNC) Prussian blue (PB) thin films have been deposited though different routes on conductive substrates. Characterization of the monolithic thin films reveals a long-term stability, even after many redox cycles the crystallinity as well as the high orientation remain intact during the electrochromic switching process.

Genetic Map Construction and Detection of Genetic Loci Underlying Segregation Distortion in an Intraspecific Cross of Populus deltoides.

Based on a two-way pseudo-testcross strategy, high density and complete coverage linkage maps were constructed for the maternal and paternal parents of an intraspecific F2 pedigree of Populus deltoides. A total of 1,107 testcross markers were obtained, and the mapping population consisted of 376 progeny. Among these markers, 597 were from the mother, and were assigned into 19 linkage groups, spanning a total genetic distance of 1,940.3 cM. The remaining 519 markers were from the father, and were also were mapped into 19 linkage groups, covering 2,496.3 cM. The genome coverage of both maps was estimated as greater than 99.9% at 20 cM per marker, and the numbers of linkage groups of both maps were in accordance with the 19 haploid chromosomes in Populus. Marker segregation distortion was observed in large contiguous blocks on some of the linkage groups. Subsequently, we mapped the segregation distortion loci in this mapping pedigree. Altogether, eight segregation distortion loci with significant logarithm of odds supports were detected. Segregation distortion indicated the uneven transmission of the alternate alleles from the mapping parents. The corresponding genome regions might contain deleterious genes or be associated with hybridization incompatibility. In addition to the detection of segregation distortion loci, the established genetic maps will serve as a basic resource for mapping genetic loci controlling traits of interest in future studies.

Photoinduced Charge-Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap?

For inorganic semiconductors crystalline order leads to a band structure which gives rise to drastic differences to the disordered material. An example is the presence of an indirect band gap. For organic semiconductors such effects are typically not considered, since the bands are normally flat, and the band-gap therefore is direct. Herein we show results from electronic structure calculations demonstrating that ordered arrays of porphyrins reveal a small dispersion of occupied and unoccupied bands leading to the formation of a small indirect band gap. We demonstrate herein that such ordered structures can be fabricated by liquid-phase epitaxy and that the corresponding crystalline organic semiconductors exhibit superior photophysical properties, including large charge-carrier mobility and an unusually large charge-carrier generation efficiency. We have fabricated a prototype organic photovoltaic device based on this novel material exhibiting a remarkable efficiency.

Synthesis and electromagnetic, microwave absorbing properties of core-shell Fe3O4-poly(3, 4-ethylenedioxythiophene) microspheres.

Highly regulated core-shell Fe(3)O(4)-poly(3, 4-ethylenedioxythiophene) (PEDOT) microspheres were successfully synthesized by a two-step method in the presence of polyvinyl alcohol (PVA) and p-toluenesulfonic acid (p-TSA). And their morphology, microstructure, electromagnetic and microwave absorbing properties were subsequently characterized. By simply adjusting the molar ratio of 3, 4-ethylenedioxythiophene (EDOT) to Fe(3)O(4) (represented by (EDOT)/(Fe(3)O(4))), the thickness of the polymer shell can be tuned from tens to hundreds of nanometers. Moreover, it was found that the composite exhibited excellent microwave absorbing property with a minimum reflection loss (RL) of about -30 dB at 9.5 GHz with a (EDOT)/(Fe(3)O(4)) ratio of 20.