Showing papers by "Tobin J. Marks published in 2018"

High- k Gate Dielectrics for Emerging Flexible and Stretchable Electronics

Abstract: Recent advances in flexible and stretchable electronics (FSE), a technology diverging from the conventional rigid silicon technology, have stimulated fundamental scientific and technological research efforts. FSE aims at enabling disruptive applications such as flexible displays, wearable sensors, printed RFID tags on packaging, electronics on skin/organs, and Internet-of-things as well as possibly reducing the cost of electronic device fabrication. Thus, the key materials components of electronics, the semiconductor, the dielectric, and the conductor as well as the passive (substrate, planarization, passivation, and encapsulation layers) must exhibit electrical performance and mechanical properties compatible with FSE components and products. In this review, we summarize and analyze recent advances in materials concepts as well as in thin-film fabrication techniques for high-k (or high-capacitance) gate dielectrics when integrated with FSE-compatible semiconductors such as organics, metal oxides, quantum...

Understanding Film Formation Morphology and Orientation in High Member 2D Ruddlesden–Popper Perovskites for High-Efficiency Solar Cells

Abstract: 2D Ruddlesden–Popper (RP) perovskites have recently emerged as promising candidates for hybrid perovskite photovoltaic cells, realizing power-conversion efficiencies (PCEs) of over 10% with technologically relevant stability. To achieve solar cell performance comparable to the state-of-the-art 3D perovskite cells, it is highly desirable to increase the conductivity and lower the optical bandgap for enhanced near-IR region absorption by increasing the perovskite slab thickness. Here, the use of the 2D higher member (n = 5) RP perovskite (n-butyl-NH3)2(MeNH3)4Pb5I16 in depositing highly oriented thin films from dimethylformamide/dimethylsulfoxide mixtures using the hot-casting method is reported. In addition, they exhibit superior environmental stability over thin films of their 3D counterpart. These films are assembled into high-efficiency solar cells with an open-circuit voltage of ≈1 V and PCE of up to 10%. This is achieved by fine-tuning the solvent ratio, crystal growth orientation, and grain size in the thin films. The enhanced performance of the optimized devices is ascribed to the growth of micrometer-sized grains as opposed to more typically obtained nanometer grain size and highly crystalline, densely packed microstructures with the majority of the inorganic slabs preferentially aligned out of plane to the substrate, as confirmed by X-ray diffraction and grazing-incidence wide-angle X-ray scattering mapping.

Dopant-Free Tetrakis-Triphenylamine Hole Transporting Material for Efficient Tin-Based Perovskite Solar Cells

Abstract: Developing dopant-free hole transporting layers (HTLs) is critical in achieving high-performance and robust state-of-the-art perovskite photovoltaics, especially for the air-sensitive tin-based perovskite systems. The commonly used HTLs require hygroscopic dopants and additives for optimal performance, which adds extra cost to manufacturing and limits long-term device stability. Here we demonstrate the use of a novel tetrakis-triphenylamine (TPE) small molecule prepared by a facile synthetic route as a superior dopant-free HTL for lead-free tin-based perovskite solar cells. The best-performing tin iodide perovskite cells employing the novel mixed-cation ethylenediammonium/formamidinium with the dopant-free TPE HTL achieve a power conversion efficiency as high as 7.23%, ascribed to the HTL’s suitable band alignment and excellent hole extraction/collection properties. This efficiency is one of the highest reported so far for tin halide perovskite systems, highlighting potential application of TPE HTL materi...

A Chemically Doped Naphthalenediimide-Bithiazole Polymer for n-Type Organic Thermoelectrics

Abstract: The synthesis of a novel naphthalenediimide (NDI)-bithiazole (Tz2)-based polymer [P(NDI2OD-Tz2)] is reported, and structural, thin-film morphological, as well as charge transport and thermoelectric properties are compared to the parent and widely investigated NDI-bithiophene (T2) polymer [P(NDI2OD-T2)]. Since the steric repulsions in Tz2 are far lower than in T2, P(NDI2OD-Tz2) exhibits a more planar and rigid backbone, enhancing p-p chain stacking and intermolecular interactions. In addition, the electron-deficient nature of Tz2 enhances the polymer electron affinity, thus reducing the polymer donor-acceptor character. When n-doped with amines, P(NDI2OD-Tz2) achieves electrical conductivity (approximate to 0.1 S cm(-1)) and a power factor (1.5 mu W m(-1) K-2) far greater than those of P(NDI2OD-T2) (0.003 S cm(-1) and 0.012 mu W m(-1) K-2, respectively). These results demonstrate that planarized NDI-based polymers with reduced donor-acceptor character can achieve substantial electrical conductivity and thermoelectric response.

Closely packed, low reorganization energy π-extended postfullerene acceptors for efficient polymer solar cells

Abstract: New organic semiconductors are essential for developing inexpensive, high-efficiency, solution-processable polymer solar cells (PSCs). PSC photoactive layers are typically fabricated by film-casting a donor polymer and a fullerene acceptor blend, with ensuing solvent evaporation and phase separation creating discrete conduits for photogenerated holes and electrons. Until recently, n-type fullerene acceptors dominated the PSC literature; however, indacenodithienothiophene (IDTT)-based acceptors have recently enabled remarkable PSC performance metrics, for reasons that are not entirely obvious. We report two isomeric IDTT-based acceptors 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-benz-(5, 6)indanone))-5,5,11,11-tetrakis(4-nonylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]di-thiophene (ITN-C9) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-benz(6,7)indanone))-5,5,11,11-tetrakis(4-nonylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITzN-C9) that shed light on the exceptional IDTT properties vis-a-vis fullerenes. The neat acceptors and blends with fluoropolymer donor poly{[4,8-bis[5-(2- ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]]} (PBDB-TF) are investigated by optical spectroscopy, cyclic voltammetry, thermogravimetric analysis, differential scanning calorimetry, single-crystal X-ray diffraction, photovoltaic response, space-charge-limited current transport, atomic force microscopy, grazing incidence wide-angle X-ray scattering, and density functional theory-level quantum chemical analysis. The data reveal that ITN-C9 and ITzN-C9 organize such that the lowest unoccupied molecular orbital-rich end groups have intermolecular π−π distances as close as 3.31(1) A, with electronic coupling integrals as large as 38 meV, and internal reorganization energies as small as 0.133 eV, comparable to or superior to those in fullerenes. ITN-C9 and ITzN-C9 have broad solar-relevant optical absorption, and, when blended with PBDB-TF, afford devices with power conversion efficiencies near 10%. Performance differences between ITN-C9 and ITzN-C9 are understandable in terms of molecular and electronic structure distinctions via the influences on molecular packing and orientation with respect to the electrode.

Hole-Transfer Dependence on Blend Morphology and Energy Level Alignment in Polymer: ITIC Photovoltaic Materials

Abstract: Bulk-heterojunction organic photovoltaic materials containing nonfullerene acceptors (NFAs) have seen remarkable advances in the past year, finally surpassing fullerenes in performance. Indeed, acceptors based on indacenodithiophene (IDT) have become synonymous with high power conversion efficiencies (PCEs). Nevertheless, NFAs have yet to achieve fill factors (FFs) comparable to those of the highest-performing fullerene-based materials. To address this seeming anomaly, this study examines a high efficiency IDT-based acceptor, ITIC, paired with three donor polymers known to achieve high FFs with fullerenes, PTPD3T, PBTI3T, and PBTSA3T. Excellent PCEs up to 8.43% are achieved from PTPD3T:ITIC blends, reflecting good charge transport, optimal morphology, and efficient ITIC to PTPD3T hole-transfer, as observed by femtosecond transient absorption spectroscopy. Hole-transfer is observed from ITIC to PBTI3T and PBTSA3T, but less efficiently, reflecting measurably inferior morphology and nonoptimal energy level alignment, resulting in PCEs of 5.34% and 4.65%, respectively. This work demonstrates the importance of proper morphology and kinetics of ITIC → donor polymer hole-transfer in boosting the performance of polymer:ITIC photovoltaic bulk heterojunction blends.

Photoactive Blend Morphology Engineering through Systematically Tuning Aggregation in All-Polymer Solar Cells

Abstract: Polymer aggregation plays a critical role in the miscibility of materials and the performance of all-polymer solar cells (APSCs). However, many aspects of how polymer texturing and aggregation affect photoactive blend film microstructure and photovoltaic performance are poorly understood. Here the effects of aggregation in donor– acceptor blends are studied, in which the number-average molecular weights of both an amorphous donor polymer, poly[4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PBDTT-FTTE) and a semicrystalline acceptor polymer, poly{[N,N′bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)) are systematically varied. The photovoltaic performance is correlated with active layer microstructural and optoelectronic data acquired by in-depth transmission electron microscopy, grazing incidence wide-angle X-ray scattering, thermal analysis, and optical spectroscopic measurements. Coarse-grained modeling provides insight into the effects of polymer aggregation on the blend morphology. Notably, the computed average distance between the donor and the acceptor polymers correlates well with solar cell photovoltaic metrics such as short-circuit current density and represents a useful index for understanding/predicting active layer blend material intermixing trends. Importantly, these results demonstrate that for polymers with different texturing tendencies (amorphous/semicrystalline), the key for optimal APSC performance, photovoltaic blend morphology can be controlled via both donor and acceptor polymer aggregation.

Hyperbolic Dispersion Arising from Anisotropic Excitons in Two-Dimensional Perovskites.

Abstract: Excitations of free electrons and optical phonons are known to permit access to the negative real part of relative permittivities (ϵ^{'}<0) that yield strong light-matter interactions. However, negative ϵ^{'} arising from excitons has been much less explored. Via development of a dielectric-coating based technique described herein, we report fundamental optical properties of two-dimensional hybrid perovskites (2DHPs), composed of alternating layers of inorganic and organic sublattices. Low members of 2DHPs (N=1 and N=2) exhibit negative ϵ^{'} stemming from the large exciton binding energy and sizable oscillator strength. Furthermore, hyperbolic dispersion (i.e., ϵ^{'} changes sign with directions) occurs in the visible range, which has been previously achieved only with artificial metamaterials. Such naturally occurring, exotic dispersion stems from the extremely anisotropic excitonic behaviors of 2DHPs, and can intrinsically support a large photonic density of states. We suggest that several other van der Waals solids may exhibit similar behaviors arising from excitonic response.

Surface Carbon as a Reactive Intermediate in Dry Reforming of Methane to Syngas on a 5% Ni/MnO Catalyst

Abstract: A 5% Ni/MnO catalyst has been tested for the dry reforming of methane at different temperatures and reactant partial pressures. Changing the reactant ratio with time on stream results in a decrease in the deactivation rate of the catalyst. Graphitic carbon growth and metal particle sintering have been observed by applying in situ transmission XRD using synchrotron radiation under actual reaction conditions. Both methane and carbon monoxide separately result in graphitic surface carbon, which can then be oxidized by carbon dioxide. The morphology of the surface carbon has been analyzed by TEM, and the reactions of both methane and carbon monoxide result in the same graphitic multiwalled carbon nanotubes. The present combination of catalytic experiments and in situ techniques suggests that surface carbon acts as an intermediate in the formation of CO and that catalyst deactivation happens via metallic particle sintering. These results enable a more rational choice of reaction conditions to ensure high catal...

Enhancing Polymer Photovoltaic Performance via Optimized Intramolecular Ester-Based Noncovalent Sulfur···Oxygen Interactions

Abstract: Head-to-head (HH) bithiophenes are typically avoided in polymer semiconductors since they engender undesirable steric repulsions, leading to a twisted backbone. While introducing electron-donating alkoxy chains can lead to intramolecular noncovalent S···O interactions, this comes at the cost of elevating the HOMOs and compromising polymer solar cell (PSC) performance. To address the limitation, a novel HH bithiophene featuring an electron-withdrawing ester functionality, 3-alkoxycarbonyl-3′-alkoxy-2,2′-bithiophene (TETOR), is synthesized. Single crystal diffraction reveals a planar TETOR conformation (versus highly twisted diester bithiophene), showing distinctive advantages of incorporating alkoxy on promoting backbone planarity. Compared to first-generation 3-alkyl-3′-alkoxy-2,2′-bithiophene (TRTOR), TETOR contains an additional planarizing (thienyl)S···O(carbonyl) interaction. Consequently, TETOR-based polymer (TffBT-TETOR) has greatly lower-lying FMOs, stronger aggregation, closer π-stacking, and bett...

High-performance and scalable metal-chalcogenide semiconductors and devices via chalco-gel routes

Abstract: We report a general strategy for obtaining high-quality, large-area metal-chalcogenide semiconductor films from precursors combining chelated metal salts with chalcoureas or chalcoamides. Using conventional organic solvents, such precursors enable the expeditious formation of chalco-gels, which are easily transformed into the corresponding high-performance metal-chalcogenide thin films with large, uniform areas. Diverse metal chalcogenides and their alloys (MQ x : M = Zn, Cd, In, Sb, Pb; Q = S, Se, Te) are successfully synthesized at relatively low processing temperatures ( 2 Se 3 active layers. The CdSe TFTs exhibit a maximum field-effect mobility greater than 300 cm 2 V −1 s −1 with an on/off current ratio of >10 7 and good operational stability (threshold voltage shift 10 13 Jones and seven-stage ring oscillators operating at a speed of ~2.6 MHz (propagation delay of

Synergistic Boron Doping of Semiconductor and Dielectric Layers for High-Performance Metal Oxide Transistors: Interplay of Experiment and Theory.

Abstract: We report the results of a study to enhance metal oxide (MO) thin-film transistor (TFT) performance by doping both the semiconductor (In2O3) and gate dielectric (Al2O3) layers with boron (yielding IBO and ABO, respectively) and provide the first quantitative analysis of how B doping affects charge transport in these MO dielectric and semiconducting matrices The impact of 1–9 atom % B doping on MO microstructure, morphology, oxygen defects, charge transport, and dielectric properties is analyzed together, in detail, by complementary experimental (microstructural, electrical) and theoretical (ab initio MD, DFT) methods The results indicate that B doping frustrates In2O3 crystallization while suppressing defects responsible for electron trapping and carrier generation In the adjacent Al2O3 dielectric, B doping increases the dielectric constant and refractive index while reducing leakage currents Furthermore, optimized solution-processed TFTs combining IBO channels with 6 atom % B and ABO dielectrics with

Metal Composition and Polyethylenimine Doping Capacity Effects on Semiconducting Metal Oxide-Polymer Blend Charge Transport.

Abstract: Charge transport and film microstructure evolution are investigated in a series of polyethylenimine (PEI)-doped (0.0–6.0 wt%) amorphous metal oxide (MO) semiconductor thin film blends. Here, PEI doping generality is broadened from binary In2O3 to ternary (e.g., In+Zn in IZO, In+Ga in IGO) and quaternary (e.g., In+Zn+Ga in IGZO) systems, demonstrating the universality of this approach for polymer electron doping of MO matrices. Systematic comparison of the effects of various metal ions on the electronic transport and film microstructure of these blends are investigated by combined thin-film transistor (TFT) response, AFM, XPS, XRD, X-ray reflectivity, and cross-sectional TEM. Morphological analysis reveals that layered MO film microstructures predominate in PEI-In2O3, but become less distinct in IGO and are not detectable in IZO and IGZO. TFT charge transport measurements indicate a general coincidence of a peak in carrier mobility (μpeak) and overall TFT performance at optimal PEI doping concentrations. O...

Epitaxial Growth of γ-Cyclodextrin-Containing Metal-Organic Frameworks Based on a Host-Guest Strategy.

Abstract: A class of metal-organic frameworks (MOFs)-namely CD-MOFs-obtained from natural products has been grown in an epitaxial fashion as films on the surfaces of glass substrates, which are modified with self-assembled monolayers (SAMs) of γ-cyclodextrin (γ-CD) molecules. The SAMs are created by host-guest complexation of γ-CD molecules with surface-functionalized pyrene units. The CD-MOF films have continuous polycrystalline morphology with a structurally out-of-plane ( c-axial) orientation, covering an area of several square millimeters, with a thickness of ∼2 μm. Furthermore, this versatile host-guest strategy has been applied successfully in the growth of CD-MOFs as the shell on the curved surface of microparticles as well as in the integration of CD-MOF films into electrochemical devices for sensing carbon dioxide. In striking contrast to the control devices prepared from CD-MOF crystalline powders, these CD-MOF film-based devices display an enhancement in proton conductance of up to 300-fold. In addition, the CD-MOF film-based device exhibits more rapid and highly reversible CO2-sensing cycles under ambient conditions, with a 50-fold decrease in conductivity upon exposure to CO2 for 3 s which is recovered within 10 s upon re-exposure to air.

Suppressing Defect Formation Pathways in the Direct C–H Arylation Polymerization of Photovoltaic Copolymers

Abstract: Direct C–H arylation polymerization (DARP) holds great promise for the green, atom-efficient synthesis of π-conjugated copolymers for use in high-performance polymer solar cells (PSCs). However, C–...

Nitroacetylacetone as a Cofuel for the Combustion Synthesis of High-Performance Indium–Gallium–Zinc Oxide Transistors

Abstract: Thin-film combustion synthesis has been utilized for the fabrication of solution processed high-performance metal-oxide thin-film transistors (MOTFTs) at lower temperatures than conventional sol–gel processes. The fuel-oxidizer ensemble in the MO precursor solution/film plays an important role in achieving high-efficiency and low-residual combustion byproducts. However, unlike conventional bulk combustion, only a very limited number of thin-film fuels have been investigated. Here we report the use of an efficient new cofuel, 3-nitroacetylacetone (NAcAcH), incorporating a −NO2 group, for the combustion synthesis of display-relevant indium–gallium–zinc-oxide (IGZO) thin films. Compared to the traditional acetylacetone (AcAcH) fuel, a higher enthalpy of combustion (988.6 vs 784.4 J/g) and a lower ignition temperature (107.8 vs 166.5 °C) are achieved for NAcAcH-based formulations. The resulting NAcAcH-derived IGZO TFTs exhibit far higher average electron mobilities (5.7 cm2 V–1 s–1) than AcAcH-derived TFTs (2...

Chain Length Dependence of the Dielectric Constant and Polarizability in Conjugated Organic Thin Films

Abstract: Dielectric materials are ubiquitous in optics, electronics, and materials science. Recently, there have been new efforts to characterize the dielectric performance of thin films composed of molecular assemblies. In this context, we investigate here the relationship between the polarizability of the constituent molecules and the film dielectric constant, using periodic density functional theory (DFT) calculations, for polyyne and saturated alkane chains. In particular, we explore the implication of the superlinear chain length dependence of the polarizability, a specific feature of conjugated molecules. We show and explain from DFT calculations and a simple depolarization model that this superlinearity is attenuated by the collective polarization. However, it is not completely suppressed. This confers a very high sensitivity of the dielectric constant to the thin film thickness. This latter can increase by a factor of 3–4 at reasonable coverages, by extending the molecular length. This significantly limits...

Unprecedented Large Hyperpolarizability of Twisted Chromophores in Polar Media.

Abstract: Twisted intramolecular charge transfer (TICT) chromophores exhibit uniquely large second-order optical nonlinearities (μβ). However, their promise as electro-optic (E-O) materials is yet untapped, reflecting a strong tendency to aggregate in low-polarity media, leading to a dramatic fall in μβ. Until now, TICT chromophores in deaggregating polar solvents suffered decreased response due to polarity-driven changes in electronic structure. Here we report a new series of benzimidazolium-based TICT chromophores with interaryl torsional angles in the range of 64–77°. The most twisted, B2TMC-2, exhibits a large μβvec = −26,000 × 10–48 esu (at 1907 nm) in dilute nonpolar CH2Cl2 solution, which is maintained in polar DMF (μβvec= −20,370 × 10–48 esu) as measured by DC electric field-induced second harmonic generation (EFISH). Sterically enforced interaryl torsional angles are confirmed by single-crystal X-ray diffraction and solution phase Nuclear Overhauser Effect (NOE) NMR, and spectroscopic characterization reve...

Catalyst Nuclearity Effects on Stereo- and Regioinduction in Pyridylamidohafnium-Catalyzed Propylene and 1-Octene Polymerizations

Abstract: In comparison to monometallic controls, bimetallic olefin polymerization catalysts often exhibit superior performance in terms of higher polyolefin Mw, higher comonomer incorporation, and higher polar comonomer tolerance. However, using cooperating catalyst centers to modulate stereoselectivity in α-olefin polymerizations is relatively unexplored. In this contribution, the monometallic Hf(IV) complex, L1-HfMe2 (catalyst A, L1 = 2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1-yl)pyridin-2-yl]methyl}aniline), and homo-bimetallic Hf(IV) complexes, L2-Hf2Me5 (catalyst B) and L2-Hf2Me4 (catalyst C) (L2 = N,N′-{[naphthalene-1,4-diylbis(pyridine-6,2-diyl)]bis[(2-isopropylphenyl)methylene)]bis(2,6-diisopropylaniline}), are activated with Ph3C+B(C6F5)4– and investigated in propylene and 1-octene homopolymerizations. In propylene polymerizations, the conformationally flexible catalyst B-derived bimetallic dicationic catalyst produces higher Mw polypropylene (up to 7.8×), higher total stereo- and regiodefect ...

Atomic Layer Deposition of Molybdenum Oxides with Tunable Stoichiometry Enables Controllable Doping of MoS2

Abstract: Metal oxides are a ubiquitous class of materials that have recently attracted interest as dopants for two dimensional (2D) materials. While distinct metal oxides have been used to achieve discrete doping changes, a metal oxide with a tunable oxidation state would allow engineering of carrier concentrations and band alignments in 2D devices. Herein we demonstrate new low-temperature atomic layer deposition (ALD) processes for molybdenum oxides and show that electronic properties can be tuned continuously with composition. We exploit this control to tune the carrier concentration in MoS2 field-effect transistors over a range spanning from depletion to accumulation (i.e., relatively p-type to n-type). Such doping should be broadly useful for 2D semiconductors due to uniform, nondestructive van der Waals nucleation and tunability of carrier concentration. More generally, scalable ALD of molybdenum oxide thin films with controlled oxidation states may have many other applications in catalysis and electronic ma...

Cationic Pyridylamido Adsorbate on Brønsted Acidic Sulfated Zirconia: A Molecular Supported Organohafnium Catalyst for Olefin Homo- and Co-Polymerization

Abstract: Here we report the combined application of high-resolution solid-state 13C–CPMAS-NMR and FT-IR spectroscopy, elemental analysis, kinetic poisoning/active site counting, variable dielectric constant medium, and DFT computation to characterize the surface chemistry of a pyridylamido hafnium complex (Cat1, L1-HfMe2, L1 = 2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1-yl)pyridin-2-yl]methyl}aniline) adsorbed on Bronsted acidic sulfated zirconia (ZrS). The spectroscopic and DFT results indicate protonolytic formation of organohafnium cations having a largely electrostatic pyridylamido-Hf-CH3+···ZrS– interaction with elongated Hf···OZrS distances of ∼2.14 A. High-molecular-weight polyethylenes and ethylene/1-octene copolymers are obtained with this supported catalyst without an activator/cocatalyst. The DFT calculations reveal that the first ethylene insertion into the Hf-methyl bond has a lower barrier than the corresponding insertion into the Hf-aryl bond of this single-site heterogeneous catalyst.

Performance, Morphology, and Charge Recombination Correlations in Ternary Squaraine Solar Cells

Abstract: Ternary bulk-heterojunction organic solar cells (BHJ-OSCs) are demonstrated by combining two squaraine donors (USQ3OH and IDPSQ) having complementary optical absorption and PC71BM as the acceptor. ...

Novel unsymmetrical squaraine-based small molecules for organic solar cells

Abstract: A new unsymmetrical squaraine dye (USQ) series for organic solar cells (OSC) is synthesized by a simple route without using highly toxic reagents. These USQs exhibit a low bandgap of 1.57 eV and HOMO energies of ∼−5.2–5.3 eV. Despite the very simple molecular structures, OSC devices based on these USQs exhibit an impressive open-circuit voltage of 0.92 V and power conversion efficiencies (PCEs) approaching 5%, achieved without using any additives or post film deposition treatment. The results indicate that USQs are promising materials for sustainable energy applications.

Boundary Lubrication Mechanisms for High-Performance Friction Modifiers.

Abstract: We recently reported a new molecular heterocyclic friction modifier (FM) that exhibits excellent friction and wear reduction in the boundary lubrication regime. This paper explores the mechanisms b...

Low-Loss Near-Infrared Hyperbolic Metamaterials with Epitaxial ITO-In2O3 Multilayers

Abstract: Artificial metamaterials with hyperbolic dispersions exhibit unusual optical properties not found in Nature. Such hyperbolic metamaterials (HMMs) permit the access to and control of electromagnetic waves with large wave vectors. An important criterion for multilayer-based HMMs is whether the thickness of each individual layer can be far below the operating wavelength while still maintaining the material and interfacial quality. Herein, we report heteroepitaxial growth of HMMs composed of multilayers of ultrathin indium tin oxide (ITO) and indium oxide (In2O3) films. The disparate metallic and dielectric properties of the individual building blocks, in conjunction with the good carrier mobility and film morphology enable a low-loss infrared HMM platform on which we demonstrate ultrafast optical switching and the enhancement of the radiative decay rate of PbS quantum dots in the telecommunication wavelength regime.

Synthesis of Supported Pd0 Nanoparticles from a Single-Site Pd2+ Surface Complex by Alkene Reduction

Abstract: A surface metal–organic complex, (-AlOx)Pd(acac) (acac = acetylacetonate), is prepared by chemically grafting the precursor Pd(acac)2 onto γ-Al2O3 in toluene at 25 °C. The resulting surface complex is characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and dynamic nuclear polarization surface-enhanced solid-state nuclear magnetic resonance spectroscopy (DNP SENS). This surface complex is a precursor in the direct synthesis of size-controlled Pd nanoparticles under mild reductive conditions and in the absence of additional stabilizers or pretreatments. Indeed, upon exposure to gaseous ethylene or liquid 1-octene at 25 °C, the Pd2+ species is reduced to form Pd0 nanoparticles with a mean diameter of 4.3 ± 0.6 nm, as determined by scanning transmission electron microscopy (STEM). These nanoparticles are catalytically relevant using the aerobic 1-phenylethanol oxidation as a probe reaction, with rates ...

Synthesis, Characterization, and Thermal Properties of N-alkyl β-Diketiminate Manganese Complexes.

Abstract: A series of N,N′-dialkyl-β-diketiminato manganese(II) complexes was synthesized and characterized by single crystal X-ray diffraction, UV–vis and FTIR spectroscopy, and then assayed for volatility,...

Effects of 1,8-diiodooctane on domain nanostructure and charge separation dynamics in PC71BM-based bulk heterojunction solar cells

Abstract: Processing additives are commonly used to optimize phase separation morphology and power conversion efficiency (PCE) in the bulk heterojunction (BHJ) organic photovoltaics (OPVs), however their exact effects are not well understood. A bulk-heterojunction OPV system containing the model bithiophene imide-benzodithiophene copolymer (PBTIBDT):phenyl-C71-butyric-acid-methyl ester (PC71BM) exhibits a maximum PCE of 5.4% with addition of 3.0 vol% 1,8-diiodooctane (DIO). Here the effects of increasing DIO concentrations (0 → 5 vol%) on the PBTIBDT:PC71BM solutions and resulting thin films are studied by X-ray scattering methodologies and transient optical absorption spectroscopy with 0.1 ps time resolution. As the DIO concentration increases, the radius of gyration of the PC71BM aggregates falls from 17 A to 9 A in solution, and TEM indicates the formation of increasing PC71BM charge percolation pathways in the thin BHJ films. Increased PBTIBDT + PC71BM intermixing not only affects BHJ film charge transport, but also enhances the initial exciton splitting yield in the PBTIBDT cation population. In contrast, the hole carrier population (as represented by the polymer cation) detected several nanoseconds after the photoexcitation is greatest with 3.0 vol% DIO, agreeing well with the corresponding BHJ composition for maximum OPV short circuit current density (Jsc) and fill factor (FF). The increase in initial polymer cation yield with DIO concentration is attributed to enhanced donor–acceptor interfacial area while the increase in long-lived cation population is attributed to formation of a bicontinuous donor polymer–PC71BM acceptor network that promotes large spatial separation of free charges in the device active layer. These results demonstrate the importance of OPV function for the correct balance, as tuned by processing additives, between a high initial donor cation formation yield and high carrier transport efficiency with minimized charge recombination rate.