The Pivotal Role of Hot Carriers in Plasmonic Catalysis of C-N Bond Forming Reaction of Amines.

Here, we demonstrate the simultaneous utilization of both the hot carriers (electrons and holes) in the photocatalytic transformation of benzylamine to N-benzylidenebenzylamine and the scope of reaction has also been successfully demonstrated with catalytic oxidation of 4-methoxybenzylamine. The wavelength-dependent excitation of AuNP allows us to tune the potential energy of charge carriers relative to the redox potential of the reactants which leads to energetically favorable product formation on the nanoparticle surface. We capture the formation of reaction intermediates and products by using in situ Raman spectroscopy, complemented by NMR spectroscopy and GC-MS. Based on the experimental substantiations, a plausible reaction mechanism has been proposed.

Photophysics of a ruthenium 4H-imidazole panchromatic dye in interaction with titanium dioxide.

The photophysics of bis(4,4'-di-tert-butyl-2,2'-bipyridine-κ(2)N,N')[2-(4-carboxyphenyl)-4,5-bis(p-tolylimino-κN)imidazolato]ruthenium(II) hexafluorophosphate is investigated, both in solution and attached to a nanocrystalline TiO2 film. The studied substitution pattern of the 4H-imidazole ligand is observed to block a photoinduced structural reorganization pathway within the 4H-imidazole ligand that has been previously investigated. Protonation at the 4H-imidazole ring decreases the excited-state lifetime in solution. When the unprotonated dye is anchored to TiO2, photoinduced electron injection occurs from thermally nonrelaxed triplet metal-to-ligand charge transfer ((3)MLCT) states with a characteristic time constant of 0.5 ps and an injection efficiency of roughly 25%. Electron injection from the subsequently populated thermalized (3)MLCT state of the dye does not take place. The energy of this state seems to be lower than the conduction band edge of TiO2.

A promising candidate with D-A-A-A architecture as an efficient sensitizer for dye-sensitized solar cells.

A series of metal-free organic dyes with electron-rich (D) and electron-deficient units (A) as π linkers have been studied theoretically by means of density functional theory (DFT) and time-dependent DFT calculations to explore the effects of π spacers on the optical and electronic properties of triphenylamine dyes. The results show that Dye 1 with a structure of D-A-A-A is superior to the typical C218 dye in various key aspects, including the maximum absorption (λmax =511 nm), the charge-transfer characteristics (D/Δq/t is 5.49 Å/0.818 e(-) /4.41 Å), the driving force for charge-carrier injection (ΔGinject =1.35 eV)/dye regeneration (ΔGregen =0.27 eV), and the lifetime of the first excited state (τ=3.1 ns). It is thus proposed to be a promising candidate in dye-sensitized solar cell applications.

Direct Imaging of Highly Anisotropic Photogenerated Charge Separations on Different Facets of a Single BiVO4 Photocatalyst.

Spatially resolved surface photovoltage spectroscopy (SRSPS) was employed to obtain direct evidence for highly anisotropic photogenerated charge separation on different facets of a single BiVO4 photocatalyst. Through the controlled synthesis of a single crystal with preferentially exposed {010} facets, highly anisotropic photogenerated hole transfer to the {011} facet of single BiVO4 crystals was observed. The surface photovoltage signal intensity on the {011} facet was 70 times stronger than that on the {010} facets. The influence of the built-in electric field in the space charge region of different facets on the anisotropic photoinduced charge transfer in a single semiconductor crystal is revealed.

A bipolar transporter as an efficient green fluorescent emitter and host for red phosphors in multi- and single-layer organic light-emitting diodes.

Multifunctional donor-acceptor compound 4,4'-bis(dibenzothiophene-S,S-dioxide-2-yl)triphenylamine (DSTPA) was obtained by linking a strongly electron-withdrawing core and a strongly electron-donating core with a biphenyl bridge in linear spatial alignment. DSTPA not only has suitable HOMO and LUMO levels for easily accepting both holes and electrons, it was also demonstrated to have a high fluorescence quantum yield of 0.98 and a high triplet energy level of 2.39 eV. Versatile applications of DSTPA for bipolar transport, green fluorescent emission, and sensitizing a red phosphor were systematically investigated in a series of multi- and single-layer organic light-emitting devices. In traditional multilayer devices, it shows excellent performance both in an undoped fluorescent device (used as a green emitter and achieving maximum current and power efficiencies (CE and PE) of 12.6 cd A(-1) and 9.4 Lm W(-1) , respectively) and in a red phosphorescent device (used as a host and achieving maximum CE and PE of 26.4 cd A(-1) and 26.3 Lm W(-1) , respectively). Furthermore, DSTPA was also simultaneously used as an emitter, a hole transporter, and an electron transporter in a single-layer device showing CE and PE of 5.1 cd A(-1) and 4.7 Lm W(-1) , respectively. A single-layer red phosphorescent device with efficiencies of 11.7 cd A(-1) and 12.6 Lm W(-1) was obtained by doping DSTPA with a red phosphor. The performances of all of the devices in this work are comparable to the best of their corresponding classes in the literature.

Cocatalyst Modified Polymeric Carbon Nitride Photoanode for Enhanced Photoelectrochemical Properties.

As a new organic photocatalyst, polymeric carbon nitride (CN) has shown good application potential in the field of photoelectrochemistry due to its unique physical and chemical properties, but its application has been seriously hindered due to its inherent characteristics such as the difficulty in charge separation. In this study, FeOOH modified CN photoanode (CN-Fe) was constructed to investigate the effect of the cocatalyst on the charge injection capacity of organic semiconductor photoelectrodes. The experimental results demonstrate significant improvement in the charge injection efficiency of the photoanode due to the introduction of FeOOH cocatalyst, leading to enhanced photoelectrochemical performance with approximately 2.4 times increase in photocurrent density. By thoroughly investigating the mechanism behind the loading of FeOOH on the polymeric carbon nitride photoanode, we gained profound insights into the behavior of charge carriers and reaction kinetics during the photoelectrocatalytic process.

Effects of oriented surface dipole on photoconversion efficiency in an alkane/lipid-hybrid-bilayer-based photovoltaic model system.

When a phospholipid monolayer containing a zinc-coordinated porphyrin species formed atop a self-assembled monolayer of heptadecafluoro-1-decanethiol (CF3(CF2)7(CH2)2SH) is subjected to photoelectrochemical current generation, a significant modulation effect is observed. Compared with devices that contain similar photoactive lipid monolayers but formed on 1-dodecanethiol SAMs, these fluorinated hybrid bilayers produce a >60% increase in cathodic currents and a similar decrease in anodic currents. Photovoltages recorded from these hybrid bilayers are found to vary in the same fashion. The modulation of photovoltaic responses in these hybrid-bilayer-based devices is explained by the opposite surface dipoles associated with the thiols employed in this study, which in one case (fluorothiol) increase and in another (alkanethiol) decrease the work function of the underlying gold substrates. A similar trend of photovoltage/photocurrent modulation is also observed if fullerene is used as the photoagent in these devices. Our results reveal the intricacy of orientated surface dipole in influencing the photovoltaic processes, and its subtle interplay with other factors related to the photoagents, such as their location and orientation within the organic matrix.

Carbazole-Containing Polymer-Assisted Trap Passivation and Hole-Injection Promotion for Efficient and Stable CsCu2 I3 -Based Yellow LEDs.

Perovskite light-emitting diodes (LEDs) are emerging light sources for next-generation lighting and display technologies; however, their development is greatly plagued by difficulty in achieving yellow electroluminescence, environmental instability, and lead toxicity. Copper halide CsCu2 I3 with intrinsic yellow emission emerges as a highly promising candidate for eco-friendly LEDs, but the electroluminescent performance is limited by defect-related nonradiative losses and inefficient charge transport/injection. To solve these issues, a hole-transporting poly(9-vinlycarbazole) (PVK)-incorporated engineering into CsCu2 I3 emitter is proposed. PVK with carbazole groups is permeated at the grain boundaries of CsCu2 I3 films by interacting with the uncoordinated Cu+ , reducing the CuCs and CuI antisite defects to increase the radiative recombination and enhancing the hole mobility to balance the charge transport/injection, resulting in substantially enhanced device performances. Eventually, the yellow LEDs exhibit an 8.5-fold enhancement of external quantum efficiency, and the half-lifetime reaches 14.6 h, representing the most stable yellow LEDs based on perovskite systems reported so far.

Limiting Factors of Detectivity in Near-Infrared Colloidal Quantum Dot Photodetectors.

Lead sulfide colloidal quantum dots (PbS CQDs) have shown great potential in photodetectors owing to their promising optical properties, especially their strong and tunable absorption. However, the limitation of the specific detectivity (D*) in CQD near-infrared (NIR) photodetectors remains unknown due to the ambiguous noise analysis. Therefore, a clear understanding of the noise current is critically demanded. Here, we elucidate that the noise current is the predominant factor limiting D*, and the noise is highly dependent on the trap densities in halide-passivated PbS films and the carriers injected from the Schottky contact (EDT-passivated PbS films/metal). It is found that the thickness of CQDs is proportional to their interface trap density, while it is inversely proportional to their minimal bulk trap density. A balance point can be reached at a certain thickness (136 nm) to minimize the trap density, giving rise to the improvement of D*. Utilizing thicker PbS-EDT films broadens the width of the tunneling barrier and thereby reduces the carrier injection, contributing to a further enhancement of D*. The limiting factors of D* determined in this work not only explain the physical mechanism of the influence on detection sensitivity but also give guidance to the design of high-performance CQD photodetectors.

Efficient Cadmium-Free Inverted Red Quantum Dot Light-Emitting Diodes.

Here, we report an efficient inverted red indium phosphide (InP) comprising QD (InP/ZnSe/ZnS, core/shell structure) light-emitting diode (QLED) by modulating an interfacial contact between the electron transport layer and emissive InP-QDs and applying self-aging approach. The red InP-QLED with optimized interfacial contact exhibits a significant improvement in maximum external quantum efficiency and current efficiency from 4.42 to 10.2% and 4.70 to 10.8 cd/A, respectively, after 69 days of self-aging, which is an almost 2.3-fold improvement compared to the fresh device. The analysis indicates the consecutive reduction in electron injection and accumulation in the emissive QD due to changes in the conduction band minimum of ZnMgO (0.1 eV after 10 days of storage) through a downward vacuum-level shift according to the aging times. During the device aging periods, the oxygen vacancy of ZnMgO reduces, which leads to lower the conductivity of ZnMgO. As a result, charge balance of the device is improved with the suppression of exciton quenching at the interface of ZnMgO and InP-QD.

The Effect of the Dielectric Constant and Ion Mobility in Light-Emitting Electrochemical Cells.

Light-emitting electrochemical cells (LEECs) are a promising low-cost option for display and solid-state lighting. In these devices, the interplay of mobile ions, electrons, and holes makes for rich physics that can be leveraged for high performance. One example of this interplay is in the formation and radiative decay of excitons-bound electron and hole pairs. Considerations from exciton binding and Langevin recombination suggest that a low dielectric constant (ϵ) would enhance emission. However, emission is also enhanced by the product of the bulk hole and electron concentrations, which in LEECs are enhanced by the motion of small mobile ions yielding high dielectric constants. These competing effects make it difficult to predict whether active layers with low or high dielectric constants will optimize device performance. Here, the effect of varying the dielectric constant on the performance of LEEC devices from ionic transition-metal complexes was studied by systematically exchanging the negative counterions paired with an iridium complex emitter. Electrochemical impedance spectroscopy, constant voltage and constant current device studies, and drift-diffusion simulations were performed. The results clarify the competing effects of Langevin bimolecular recombination and ion-assisted injection processes occurring in LEECs.

Boosting Perovskite Light-Emitting Diode Performance via Tailoring Interfacial Contact.

Solution-processed perovskite light-emitting diodes (LEDs) have attracted wide attention in the past several years. However, the overall efficiency and stability of perovskite-based LEDs remain inferior to those of organic or quantum dot LEDs. Nonradiative charge recombination and the unbalanced charge injection are two critical factors that limit the device efficiency and operational stability of perovskite LEDs. Here, we develop a strategy to modify the interface between the hole transport layer and the perovskite emissive layer with an amphiphilic conjugated polymer of poly[(9,9-bis(3'-( N, N-dimethylamino)propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)] (PFN). We show evidences that PFN improves the quality of the perovskite film, which effectively suppresses nonradiative recombination. By further improving the charge injection balance rate, a green perovskite LED with a champion current efficiency of 45.2 cd/A, corresponding to an external quantum efficiency of 14.4%, is achieved. In addition, the device based on the PFN layer exhibits improved operational lifetime. Our work paves a facile way for the development of efficient and stable perovskite LEDs.

Improving the Back Surface Field on an Amorphous Silicon Carbide Thin-Film Photocathode for Solar Water Splitting.

Amorphous silicon carbide (a-SiC:H) is a promising material for photoelectrochemical water splitting owing to its relatively small band-gap energy and high chemical and optoelectrical stability. This work studies the interplay between charge-carrier separation and collection, and their injection into the electrolyte, when modifying the semiconductor/electrolyte interface. By introducing an n-doped nanocrystaline silicon oxide layer into a p-doped/intrinsic a-SiC:H photocathode, the photovoltage and photocurrent of the device can be significantly improved, reaching values higher than 0.8 V. This results from enhancing the internal electric field of the photocathode, reducing the Shockley-Read-Hall recombination at the crucial interfaces because of better charge-carrier separation. In addition, the charge-carrier injection into the electrolyte is enhanced by introducing a TiO2 protective layer owing to better band alignment at the interface. Finally, the photocurrent was further enhanced by tuning the absorber layer thickness, arriving at a thickness of 150 nm, after which the current saturates to 10 mA cm-2 at 0 V vs. the reversible hydrogen electrode in a 0.2 m aqueous potassium hydrogen phthalate (KPH) electrolyte at pH 4.

Phenothiazine-Sensitized Solar Cells: Effect of Number of Cyanocinnamic Acid Anchoring Groups on Dye-Sensitized Solar Cell Performance.

Dye-sensitized solar cells (DSSCs) were built to probe the effect of the number of cyanocinnamic acid anchoring groups on the ring periphery of phenothiazine dyes. Two kinds of dyes, one with substitution on the N-terminal (1) and the other at the C-3 aromatic ring (2-5), have been synthesized for this purpose. Additionally, the number of cyanocinnamic acid groups have been increased to two in the case of compound 6. In the case of compounds 2-6, the N terminus was functionalized to carry methyl, 2-hydroxyethane, tris(ethylene oxide), or hexyl groups. Evidence for charge injection from the excited singlet state of phenothiazine to TiO2 was secured from femtosecond transient absorption spectral studies, wherein an ultrafast charge injection process (kCI ≈1010 -1011  s-1 ) was witnessed. The DSSCs were built on nanocrystalline TiO2 by using the traditional two-electrode method. Cell performance was better for carbon-functionalized phenothiazine dyes compared with that of nitrogen-functionalized dyes. The second cyanocinnamic group also increased the efficiency to 5.8 %.

High Triplet Energy Level Achieved by Tuning the Arrangement of Building Blocks in Phosphorescent Polymer Backbones for Furnishing High Electroluminescent Performances in Both Blue and White Organic Light-Emitting Devices.

A high triplet energy level (ET) of ca. 2.83 eV has been achieved in a novel polymer backbone through tuning the arrangement of two kinds of building blocks, showing enhanced hole injection/transporting capacity. Based on this new polymer backbone with high ET, both blue and white phosphorescent polymers were successfully developed with a trade-off between high ET and enhanced charge-carrier transporting ability. In addition, their photophysical features, electrochemical behaviors, and electroluminescent (EL) properties have been characterized in detail. Benefitting from the advantages associated with the novel polymer backbone, the blue phosphorescent polymers show top-ranking EL performances with a maximum luminance efficiency (ηL) of 15.22 cd A-1, corresponding to a power efficiency (ηP) of 12.64 lm W-1, and external quantum efficiency (ηext) of 6.22% and the stable Commission Internationale de L'Eclairage (CIE) coordinates of (0.19, 0.38). Furthermore, blue-orange (B-O) complementary-colored white phosphorescent polymers based on this novel polymer backbone were also obtained showing encouraging EL efficiencies of 12.34 cd A-1, 9.59 lm W-1, and 4.10% in the optimized WOLED together with exceptionally stable CIE coordinates of (Δx = 0.014, Δy = 0.010) in a wide driving voltage range from 4 to 16 V. All of these attractive EL results achieved by these novel phosphorescent polymers show the great potential of this new polymer backbone in developing highly efficient phosphorescent polymers.

Air-tolerant Fabrication and Enhanced Thermoelectric Performance of n-Type Single-walled Carbon Nanotubes Encapsulating 1,1'-Bis(diphenylphosphino)ferrocene.

The thermally-triggered n-type doping of single-walled carbon nanotubes is demonstrated using 1,1'-bis(diphenylphosphino)ferrocene, a novel n-type dopant. Through a simple thermal vacuum process, the phosphine compounds are moderately encapsulated inside single-walled carbon nanotubes. The encapsulation into SWNTs is carefully characterized using Raman/X-ray spectroscopy and transmission electron microscopy. This easy-to-handle doping with air-stable precursors for n-type SWNTs enables the large-scale fabrication of thermoelectric materials showing an excellent power factor exceeding approximately 240 µW mK(-2) .

Microwave-assisted synthesis of dual-conducting Cu2O@Cu-graphene system with improved electrochemical performance as anode material for lithium batteries.

Two by two: A low-cost, fast, and green microwave-assisted approach has been developed for the controlled synthesis of core-shell Cu2O@Cu/graphene hybrids. With the unique dual-conducting system consisting of graphene support and Cu layer on the surface of the Cu2O nanoparticles, the as-obtained hybrids exhibited significantly enhanced lithium ion battery performance as anode materials.