28,067 research outputs found

    Mitigating Detrimental Effect of Self‐Doping Near the Anode in Highly Efficient Organic Solar Cells

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    Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been one of the most established hole transport layers (HTL) in organic solar cells (OSCs) for several decades. However, the presence of PSS− ions is known to deteriorate device performance via a number of mechanisms including diffusion to the HTL-active layer interface and unwanted local chemical reactions. In this study, it is shown that PSS− ions can also result in local p-doping in the high efficiency donor:non-fullerene acceptor blends – resulting in photocurrent loss. To address these issues, a facile and effective approach is reported to improve the OSC performance through a two-component hole transport layer (HTL) consisting of a self-assembled monolayer of 2PACz ([2-(9H-Carbazol-9-yl)ethyl]phosphonic acid) and PEDOT:PSS. The power conversion efficiency (PCE) of 17.1% using devices with PEDOT:PSS HTL improved to 17.7% when the PEDOT:PSS/2PACz two-component HTL is used. The improved performance is attributed to the overlaid 2PACz layer preventing the formation of an intermixed p-doped PSS− ion rich region (≈5–10 nm) at the bulk heterojunction-HTL contact interface, resulting in decreased recombination losses and improved stability. Moreover, the 2PACz monolayer is also found to reduce electrical shunts that ultimately yield improved performance in large area devices with PCE enhanced from 12.3% to 13.3% in 1 cm2 cells

    Highly Accurate Quantum Chemical Property Prediction with Uni-Mol+

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    Recent developments in deep learning have made remarkable progress in speeding up the prediction of quantum chemical (QC) properties by removing the need for expensive electronic structure calculations like density functional theory. However, previous methods learned from 1D SMILES sequences or 2D molecular graphs failed to achieve high accuracy as QC properties primarily depend on the 3D equilibrium conformations optimized by electronic structure methods, far different from the sequence-type and graph-type data. In this paper, we propose a novel approach called Uni-Mol+ to tackle this challenge. Uni-Mol+ first generates a raw 3D molecule conformation from inexpensive methods such as RDKit. Then, the raw conformation is iteratively updated to its target DFT equilibrium conformation using neural networks, and the learned conformation will be used to predict the QC properties. To effectively learn this update process towards the equilibrium conformation, we introduce a two-track Transformer model backbone and train it with the QC property prediction task. We also design a novel approach to guide the model's training process. Our extensive benchmarking results demonstrate that the proposed Uni-Mol+ significantly improves the accuracy of QC property prediction in various datasets. We have made the code and model publicly available at \url{https://github.com/dptech-corp/Uni-Mol}

    Halogenation of polypnictogen ligand complexes

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    After that a previous investigation from our group demonstrated that the iodination and the “classical” oxidation of En ligand complexes can afford different results and are therefore being considered as complementary tools for the synthesis of new polypnictogen complexes, we were interested in extending the investigation to other complexes as well as to other halogens or halogen sources. Therefore, the first object of this work was the investigation of the reactivity of [{CpMo(CO)2}2(μ,ɳ2:ɳ2-P2)] (A) towards halogens (I2, Br2) and halogen sources (PBr5, PCl5). Based on the obtained results, we were interested in expanding the investigation of the reactivity of halogens towards different Pn ligand complexes, whose redox properties have already been elucidated. Accordingly, the next object was the investigation of the reactivity of the triple-decker complex [(Cp*Mo)2(μ,ɳ6:ɳ6-P6)] (B) towards halogens (I2, Br2) and halogen sources (PBr5, PCl5). Finally, we were interested in how the nature of the En ligand and of the pnictogen atom involved could affect the outcome of the reaction. Thus, we wanted to explore a possible alternative way to obtain E-X bonds without using the harsh conditions required for halogenation reactions. Therefore, the next objectives were the investigation of the halogenation of the triple decker complexes [(Cp’’’Co)2(μ,ɳ2:ɳ2-E2)2] (E = As (C), P (D)) bearing two independent E2 units and the exploration of the possibility of quenching the cations of [(Cp’’’Co)2(μ,ɳ4:ɳ4-E4)][TEF]2 (E = P, As) with nucleophilic halides. In conclusion, the investigation of the reactivity of the heterobimetallic triple-decker complexes [(Cp*Fe)(Cp’’’Co)(μ,ɳ5:ɳ4-E5)] (E = P (E), As (F)) towards halogens (I2, Br2) and halogen sources (PCl5) was exploited. The results of these investigation show that the En ligand involved in the halogenation reactions was the variable with the highest influence on the different products obtained. It is not possible to find a general trend based on the halogen used because the outcome was always different from one polypnictogen complex to the other. While in some cases the reactivity of the respective En ligand compound was similar towards Br2 and Cl2 sources but completely different with I2 (e.g. with A and D), in other cases it was comparable to I2 and Br2 and different towards PCl5 (e.g. with E and F), or the same with all the halogens (e.g. with C). On the other hand, with B the reactivity was different towards all the halogen sources, with the formation of similar products among the iodinated or chlorinated derivatives or among the brominated and chlorinated ones. The halogenation of the tetrahedrane complex A, compared to B, has a higher chemoselectivity. The halogenation of the triple decker complexes led in general to a large number of products, especially when PCl5 was involved. Specifically, for B, it was observed that the chlorination reaction requires lower temperatures to isolate some of the products. The investigation of this reactivity for compounds C-F showed that the nature of the pnictogen ligand affects the final products, contrarily to what was observed for the one- or two-electron oxidation of the same compounds. In conclusion, the halogenation can be considered an additional tool for the synthesis of new functionalized En ligand complexes, whose related difficulties (high number of products, low yields) can be partly “balanced” by the opportunity of further functionalization of the products obtained

    Synthesis and Characterization of Atropisomeric Bis-arylboryl-Carbazoles for use in CP-OLEDs

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    This work presents the synthesis and characterization of two functionalized atropisomeric amino-boranes. The two products are composed of a boron atom bonded with three different aryl groups: a mesityl substituent, a 2-trifluoromethyl-toluyl substituent and an asymmetrically substituted carbazole moiety. A methacrylate group was installed on the carbazole scaffold with and without an alkyl “spacer” in between. We performed an extensive computational analysis to gather predictions on the energies and peculiar geometries of the ground states, on the energies related to the many transition states that allow stereoisomerization, on the fluorescent and solvatochromic properties and lastly on circularly polarized luminescence. The synthesis was followed by CSP-HPLC, separating the four atropisomers of each product. The single atropisomers underwent two kinetic studies to experimentally determine the two energy barriers for stereoisomerization. Through Dynamic HPLC we studied the racemization kinetics and via Dynamic NMR the E-Z interconversion kinetics. For each atropisomer we determined the absolute configuration by matching its ECD spectrum with the ones predicted from the computational study (TD-DFT). Fluorescence was visually verified by placing the products under a 366 nm UV light

    Temperature-dependent interphase formation and Li+ transport in lithium metal batteries

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    Abstract High-performance Li-ion/metal batteries working at a low temperature (i.e., <−20 °C) are desired but hindered by the sluggish kinetics associated with Li+ transport and charge transfer. Herein, the temperature-dependent Li+ behavior during Li plating is profiled by various characterization techniques, suggesting that Li+ diffusion through the solid electrolyte interface (SEI) layer is the key rate-determining step. Lowering the temperature not only slows down Li+ transport, but also alters the thermodynamic reaction of electrolyte decomposition, resulting in different reaction pathways and forming an SEI layer consisting of intermediate products rich in organic species. Such an SEI layer is metastable and unsuitable for efficient Li+ transport. By tuning the solvation structure of the electrolyte with a lower lowest unoccupied molecular orbital (LUMO) energy level and polar groups, such as fluorinated electrolytes like 1 mol L−1 lithium bis(fluorosulfonyl)imide (LiFSI) in methyl trifluoroacetate (MTFA): fluoroethylene carbonate (FEC) (8:2, weight ratio), an inorganic-rich SEI layer more readily forms, which exhibits enhanced tolerance to a change of working temperature (thermodynamics) and improved Li+ transport (kinetics). Our findings uncover the kinetic bottleneck for Li+ transport at low temperature and provide directions to enhance the reaction kinetics/thermodynamics and low-temperature performance by constructing inorganic-rich interphases

    Wonderful varieties with a view towards Poisson geometry

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    These are expanded lecture notes from the author's minicourse at the 2022 Poisson Geometry Summer School, which took place at the Centre de Recerca Matematica in Barcelona, Spain. After giving a general introduction to wonderful varieties, and an explicit construction of the wonderful compactification of a semisimple adjoint group, we outline several connections to Poisson geometry and to varieties of Lagrangian subalgebras. This survey is intended to be accessible to readers without an extensive background in algebraic geometry.Comment: 23 pages, comments welcom

    Ruthenium-rhenium and ruthenium-palladium supramolecular photocatalysts for photoelectrocatalytic CO2 and H+ reduction.

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    Photoelectrocatalysis offers the opportunity to close the carbon loop and convert captured CO2 back into useful fuels and feedstocks, mitigating against anthropogenic climate change. However, since CO2 is inherently stable and sunlight is a diffuse and intermittent energy source, there are considerable scientific challenges to overcome. In this paper we present the integration of two new metal–organic photocatalysts into photocathodes for the reduction of CO2 using ambient light. The two molecular dyads contained a rhenium carbonyl or palladium-based catalytic centre bridged to a ruthenium bipyridyl photosensitizer functionalised with carboxylic acid groups to enable adsorption onto the surface of mesoporous NiO cathodes. The photocathodes were evaluated for photoelectrochemical reduction of CO2 to CO or H+ to H2 and the performances were compared directly with a control compound lacking the catalytic site. A suite of electrochemical, UV-visible steady-state/time-resolved spectroscopy, X-ray photoelectron spectroscopy and gas chromatography measurements were employed to gain kinetic and mechanistic insight to primary electron transfer processes and relate the structure to the photoelectrocatalytic performance under various conditions in aqueous media. A change in behaviour when the photocatalysts were immobilized on NiO was observed. Importantly, the transfer of electron density towards the Re–CO catalytic centre was observed, using time resolved infrared spectroscopy, only when the photocatalyst was immobilized on NiO and not in MeCN solution. We observed that photocurrent and gaseous photoproduct yields are limited by a relatively low yield of the required charge-separated state across the NiO|Photocatalyst interface. Nonetheless, the high faradaic efficiency (94%) and selectivity (99%) of the Re system towards CO evolution are very promising

    Synthesis, crystal structure, spectroscopic characterisation, and photophysical properties of iridium(iii) complex with pyridine-formimidamide ancillary

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    Two-dimensional MoS2 has been speculated to be the best material to replace graphene due to its peculiar structural-electronic properties. The MoS2 with size smaller than its exciton Bohr radius (ca. 1.61 nm) would favor multi exciton generation upon absorption of photon with sufficient energy, Ephoton ≫ Egap (1.89 eV); which would increase the efficiency of an excitonic solar cell greater than 60%. Despite promising properties of the MoS2, however an excitonic solar cell with high efficiency is yet to be exhibited. In this work, the MoS2 thin films were fabricated using vacuum thermal evaporation technique and characterized. Four objectives have been outlined i.e., to study the effect of heating rate (steady, and rapid) on the (i) morphology, (ii) size, (iii) optoelectronic and (iv) crystal properties of the fabricated thin films. The MoS2 precursor was heated at the rate 2.027 A/s (steady), and 18.75 A/s (rapid), 1.5 × 10−3 Torr, 1.48 A, and 4.58 V. The deposited films later were characterized using Field Emission Scanning Electron Microscope with Energy Dispersive X-ray attachment, photoluminescence spectrometer, UV–vis-NIR spectrometer, and X-ray Diffractometer. The fabricated thin films exhibited nanosphere morphology with different size distributions i.e., wide (steady heating), and narrow (rapid heating). Two hypotheses were made based on the optoelectronic properties i.e., the basic building block of the MoS2 thin film fabricated under steady heating is (i) experiencing stronger quantum confinement effect, and (ii) dominated by nanocrystals which are smaller than that of the rapid heating. Similar energy loss could be expected in both MoS2 thin films i.e., ca. 0.15 to 0.17 eV, indicating the existence of shallow trap states. The MoS2 thin films were dominated by (0 0 2), (0 0 4), and (1 0 6) crystal planes. Therefore, the vacuum thermal evaporation technique would offer materials with unique size, crystal arrangement, and optoelectronic properties upon change of heating rate

    Exploring the Influence of Engineering the Linker between the Donor and Acceptor Fragments on Thermally Activated Delayed Fluorescence Characteristics

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    We have expounded the unique molecular design architecture for efficient thermally activated delayed fluorescence (TADF) materials based on a donor–linker–acceptor–linker–donor (D–L–A–L–D) framework, which can be employed as predecessors of organic light-emitting diode (OLED) devices. Different from traditional donor–acceptor-type (D–A-type) TADF scaffolds, the D–L–A–L–D structural design avoids direct coupling amid the D and A fragments allowing the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) to be spatially separated. It results in a reduced overlap between HOMOs and LUMOs, thus realizing fairly a slight singlet–triplet energy gap (ΔEST) and higher photoluminescence quantum yield (Φ). We revealed that manipulating a linker between D and A fragments in intramolecular charge transfer compounds is an auspicious approach for realizing small ΔEST. Herein, we report a group of organic electroluminescent D–L–A–L–D-type molecules with different electron-donating and electron-accepting moieties using density functional theory calculations and time-dependent density functional theory calculations. Two types of linkers, the π-conjugated phenylene (−C6H4−) and aliphatic alkyl chains or σ-spacer (−CH2– and −CH2–CH2−), were exploited between D and A fragments. In principle, the conjugation in D−π–A−π–D-type molecules and hyperconjugation in D−σ–A−σ–D type molecules encourage the spatial separation of the HOMO–LUMO causing a reduction in the ΔEST. All the designed molecules show a blue-shift in the emission wavelengths (λem) over the directly linked parent molecules except DPA-DPS-C6H4 and BTPA-DPS-C6H4 which show a red-shift. Violet-blue to green-yellow (376–566 nm) λem was observed from all of the investigated molecules. Other important properties that affect the efficiency of emission quantum yields like frontier molecular orbital analysis, natural population analysis, electron excitation analysis, exciton binding energies, ionization potentials, electronic affinities, and reorganization energies of the designed molecules were also inspected. We are confident that our work will effectively give a straightforward and distinctive approach to building incredibly effective TADF-OLEDs and a new perspective on their structural design
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