Highly Efficient and Thermally Stable Electro‐Optical Dendrimers for Photonics

After a brief review on electro‐optical (EO) polymers, the recent development of EO dendrimers is summarized. Both single‐ and multiple‐dendron‐modified nonlinear optical (NLO) chromophores in the guest–host polymer systems showed a very significant enhancement of poling efficiency (up to a three‐fold increase) due to the minimization of intermolecular electrostatic interactions among large dipole moment chromophores through the dendritic effect. Moreover, multiple NLO chromophore building blocks can also be placed into a dendrimer to construct a precise molecular architecture with a predetermined chemical composition. The site‐isolation effect, through the encapsulation of NLO moieties with dendrons, can greatly enhance the performance of EO materials. A very large EO coefficient (r₃₃ = 60 pm/V at 1.55 μm) and high temporal stability (85 °C for more than 1000 h) were achieved in a NLO dendrimer (see Figure) through the double‐end functionalization of a three‐dimensional phenyl‐tetracyanobutadienyl (Ph‐TCBD)‐containing NLO chromophore with thermally crosslinkable trifluorovinylether‐containing dendrons.     

Optical intensity modulator based on a novel electrooptic polymerincorporating high μβ chromophore

We have synthesized a novel electrooptic (EO) polymer based on a high μβ chromophore incorporating tricyanobutadiene acceptors. A crosslinked polyurethane network was also adopted to enhance its thermal stability. In order to find the optimum poling condition for the polymer, the influence of the electric poling profile on optical characteristics such as the EO effect, thermal stability, and damage was investigated. Then a high-speed intensity modulator using the EO polymer was designed and fabricated. The measured half-wave voltage V_π was 4.5 V at the wavelength of 1.31 μm. Accordingly, the achieved EO coefficient r₃₃ was as high as 25 pm/V, and the thermal stability of the poled polymer was as high as 95°C. Finally, the modulator was successfully operated up to 40 GHz     

Poled Sol–Gel Materials With Heterocycle Push–Pull Chromophores that Confer Enhanced Second‐Order Optical Nonlinearity

Hybrid organic–inorganic materials doped with zwitterionic push–pull chromophores with high hyperpolarizability have been synthesized by a sol–gel procedure. A large chromophore concentration was reached by using N‐(hydroxyethyl)carbazole as a physical spacer (preventing the dye aggregation). Spin‐coated doped films were electrically poled and second harmonic generation measurements performed in situ. During the thermally assisted poling under a N₂ atmosphere, only the carbazole molecules degraded. Second‐harmonic generation measurements gave an estimation of the nonlinear coefficient, r₃₃, of 38 pm V^–1 at 1064 nm.     

Solution‐Processible Carbazole Dendrimers as Host Materials for Highly Efficient Phosphorescent Organic Light‐Emitting Diodes

A group of dendrimers with oligo‐carbazole dendrons appended at 4,4′‐ positions of biphenyl core are synthesized for use as host materials for solution‐processible phosphorescent organic light‐emitting diodes (PHOLEDs). In comparison with the traditional small molecular host 4,4′‐N,N′‐dicarbazolebiphenyl (CBP), the dendritic conformation affords these materials extra merits including amorphous nature with extremely high glass transition temperatures (ca. 376 °C) and solution‐processibility, but inherent the identical triplet energies (2.60–2.62 eV). In comparison with the widely‐used polymeric host polyvinylcarbazole (PVK), these dendrimers possess much higher HOMO levels (–5.61 to –5.42 eV) that facilitate efficient hole injection and are favorable for high power efficiency in OLEDs. The agreeable properties and the solution‐processibility of these dendrimers makes it possible to fabricate highly efficient PHOLEDs by spin coating with the dendimers as phosphorescent hosts. The green PHOLED containing Ir(ppy)₃ (Hppy = 2‐phenyl‐pyridine) dopant exhibits high peak efficiencies of 38.71 cd A^?1 and 15.69 lm W^?1, which far exceed those of the control device with the PVK host (27.70 cd A^?1 and 9.6 lm W^?1) and are among the best results for solution‐processed green PHOLEDs ever reported. The versatility of these dendrimer hosts can be spread to orange PHOLEDs and high efficiencies of 32.22 cd A^?1 and 20.23 lm W^?1 are obtained, among the best ever reported for solution‐processed orange PHOLEDs.     

Optical intensity modulators using diazo-dye-substituted polymerchannel waveguides

The electrooptic (EO) modulation of 1.32 μm laser light is measured in Mach-Zehnder channel waveguides fabricated with diazo-dye-substituted polymers with coplanar-waveguide (CPW) or microstrip (MS) electrodes. Two types of channel waveguide fabricated by photobleaching or O₂ reactive-ion etching (RIE) exhibit the same EO coefficients r₃₃ under the same poling condition. However, the photobleached waveguides show a lower half-wave voltage than the RIE-fabricated ones because of both the optical power concentration in a core layer and the existence of an EO active cladding layer. These tendencies are well explained by considering “effective” overlap integrals between the optical and electric fields, including the distribution of EO-active regions. The maximum r₃₃ value (26 pm/V) of the poled diazo-dye-substituted polymer is obtained with an RIE-fabricated MS-modulator     

Large‐Size Bulk and Thin‐Film Stilbazolium‐Salt Single Crystals for Nonlinear Optics and THz Generation

We present new stilbazolium salt DSTMS (4‐N,N‐dimethylamino‐4′‐N′‐methyl‐stilbazolium 2,4,6‐trimethylbenzenesulfonate) with both high second‐order nonlinear optical properties and very favorable crystal growth characteristics. We are able to obtain very large area bulk single crystals of more than 3 × 3 × 0.2 cm³ with a high optical quality without using seed crystals by using low‐temperature solution growth. We also demonstrate the growth of single crystalline thin films of DSTMS with an area of up to 6 × 5 mm² and a thickness between 5–30 μm. Nonlinear optical measurements reveal that DSTMS possesses large nonlinear optical susceptibilities with χ₁₁₁⁽²⁾ = (430 ± 40) pm V^–1 at 1.9 μm. Highly efficient generation of broadband THz waves with THz electric field strengths of more than 4 kV cm^–1 using 160 fs laser pump pulses at a wavelength λ = 1.45 μm and DSTMS crystals has been demonstrated.     

Octupolar Films with Large Second Harmonic Generation and Electro‐Optical Effects

Thin films that benefit from efficient octupolar molecular packing are prepared for second harmonic generation (SHG) and electro‐optic (EO) applications. The films are composed of 1,3,5‐tricyano‐2, 4,6‐tris(p‐diethylaminostyryl)benzene (TTB) in a ploymethylmetacrylate (PMMA) matrix on aluminum/BK7 glass (Al/BK7) and polyimide/indium tin oxide (PI/ITO) substrates. Octupolar films prepared on both substrates display polycrystalline and cylindrical domains. The molecular orientation, SHG efficiencies, and EO coefficients of the crystalline domains are measured. In the cylinders, the molecular crystal planes are oriented perpendicularly to the major cylinder axis, whereas in the polycrystals, the planes are randomly oriented. While both structures exhibit high and stable SHG and EO efficiencies, the cylinders, in particular, exhibited a very large SHG, a large EO coefficient, and high thermal stability; these characteristics will be useful in second order nonlinear optical applications.     

Coreless Fiber‐Based Whispering‐Gallery‐Mode Assisted Lasing from Colloidal Quantum Well Solids

Whispering gallery mode (WGM) resonators are shown to hold great promise to achieve high‐performance lasing using colloidal semiconductor nanocrystals (NCs) in solution phase. However, the low packing density of such colloidal gain media in the solution phase results in increased lasing thresholds and poor lasing stability in these WGM lasers. To address these issues, here optical gain in colloidal quantum wells (CQWs) is proposed and shown in the form of high‐density close‐packed solid films constructed around a coreless fiber incorporating the resulting whispering gallery modes to induce gain and waveguiding modes of the fiber to funnel and collect light. In this work, a practical method is presented to produce the first CQW‐WGM laser using an optical fiber as the WGM cavity platform operating at low thresholds of ≈188 µJ cm^?2 and ≈1.39 mJ cm^?2 under one‐ and two‐photon absorption pumped, respectively, accompanied with a record low waveguide loss coefficient of ≈7 cm^?1 and a high net modal gain coefficient of ≈485 cm^?1. The spectral characteristics of the proposed CQW‐WGM resonator are supported with a numerical model of full electromagnetic solution. This unique CQW‐WGM cavity architecture offers new opportunities to achieve simple high‐performance optical resonators for colloidal lasers.     

New Family of Ruthenium‐Dye‐ Sensitized Nanocrystalline TiO2 Solar Cells with a High Solar‐Energy‐Conversion Efficiency

A new type of ruthenium complexes 6 – 8 with tridentate bipyridine–pyrazolate ancillary ligands has been synthesized in an attempt to elongate the π‐conjugated system as well as to increase the optical extinction coefficient, possible dye uptake on TiO₂, and photostability. Structural characterization, photophysical studies, and corresponding theoretical approaches have been made to ensure their fundamental basis. As for dye‐sensitized solar cell applications, it was found that 6 – 8 possess a larger dye uptake of 2.4 × 10^–7 mol cm^–2, 1.5 × 10^–7 mol cm^–2, and 1.3 × 10^–7 mol cm^–2, respectively, on TiO₂ than that of the commercial N3 dye (1.1 × 10^–7 mol cm^–2). Compound 8 works as a highly efficient photosensitizer for the dye‐sensitized nanocrystalline TiO₂ solar cell, producing a 5.65 % solar‐light‐to‐electricity conversion efficiency (compare with 6.01 % for N3 in this study), a short‐circuit current density of 15.6 mA cm^–2, an open‐circuit photovoltage of 0.64 V, and a fill factor of 0.57 under standard AM 1.5 irradiation (100 mW cm^–2). These, in combination with its superior thermal and light‐soaking stability, lead to the conclusion that the concomitant tridentate binding properties offered by the bipyridine‐pyrazolate ligand render a more stable complexation, such that extended life spans of DSSCs may be expected.     

High‐Performance and Stable Organic Thin‐Film Transistors Based on Fused Thiophenes

A series of new organic semiconductors for organic thin‐film transistors (OTFTs) using dithieno[3,2‐b:2′,3′‐d]thiophene as the core are synthesized. Their electronic and optical properties are investigated using scanning electron microscopy (SEM), X‐ray diffraction (XRD), UV‐vis and photoluminescence spectroscopies, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The compounds exhibit an excellent field‐effect performance with a high mobility of 0.42 cm² V^–1 s^–1 and an on/off ratio of 5 × 10⁶. XRD patterns reveal these films, grown by vacuum deposition, to be highly crystalline, and SEM reveals well‐interconnected, microcrystalline domains in these films at room temperature. TGA and DSC demonstrate that the phenyl‐substituted compounds possess excellent thermal stability. Furthermore, weekly shelf‐life tests (under ambient conditions) of the OTFTs based on the phenyl‐substituted compounds show that the mobility for the bis(diphenyl)‐substituted thiophene was almost unchanged for more than two months, indicating a high environmental stability.     

Self‐Powered Water‐Splitting Devices by Core–Shell NiFe@N‐Graphite‐Based Zn–Air Batteries

Development of highly efficient and low‐cost multifunctional electrocatalysts for the oxygen evolution reaction (OER), the oxygen reduction reaction (ORR), and the hydrogen evolution reaction is urgently required for energy storage and conversion applications, such as in Zn–air batteries and water splitting to replace very expansive noble metal catalysts. Here, the new core–shell NiFe@N‐graphite electrocatalysts with excellent electrocatalytic activity and stability toward OER and ORR are reported and the Ni_0.5Fe_0.5@N‐graphite electrocatalyst is applied as the air electrode in Zn–air batteries. The respective liquid Zn–air battery shows a large open‐circuit potential of 1.482 V and a small charge–discharge voltage gap of 0.12 V at 10 mA cm⁻², together with excellent cycling stability even after 40 h at 20 mA cm⁻². Interestingly, the all‐solid‐like Zn–air battery thus derived shows a highly desired mechanical flexibility, whereby little change is observed in the voltage when bent into different angles. Using the same Ni_0.5Fe_0.5@N‐graphite electrode, a self‐driven water‐splitting device, which is powered by two Zn–air batteries in‐series, is constructed. The present study opens a new opportunity for the rational design of metal@N‐graphite‐based catalysts of core–shell structures for electrochemical catalysts and renewable energy applications.     

Photorefractive Performance of Hyperstructured Cyclotriphosphazene Molecular Glasses containing Carbazole Moieties

Two hyperstructured photorefractive (PR) molecular glasses ( M1 and M2 ) with a cyclotriphosphazene core are synthesized via nucleophilic substitution and an azo‐coupling reaction. These molecules exhibit excellent solubility in common organic solvents and maintain a complete amorphous state in spite of their high glass‐transition temperature. The nonlinear optical effects, two‐beam coupling and four‐wave mixing, respectively, are used to prove the PR performance in the optically transparent films of M1 and M2 doped with 30 % N‐ethyl‐carbazole. With no external electric field, a gain coefficient of 102 cm^–1 and diffraction efficiency of 24 % are obtained in the composite made from M1 , and an even higher gain coefficient of 214 cm^–1 and diffraction efficiency of 31 % are obtained in the composite made from M2 owing to its higher chromophore loading.     

Design of Hierarchical NiCo@NiCo Layered Double Hydroxide Core–Shell Structured Nanotube Array for High‐Performance Flexible All‐Solid‐State Battery‐Type Supercapacitors

A novel hierarchical nanotube array (NTA) with a massive layered top and discretely separated nanotubes in a core–shell structure, that is, nickel–cobalt metallic core and nickel–cobalt layered double hydroxide shell (Ni?Co@Ni?Co LDH), is grown on carbon fiber cloth (CFC) by template‐assisted electrodeposition for high‐performance supercapacitor application. The synthesized Ni?Co@Ni?Co LDH NTAs/CFC shows high capacitance of 2200 F g^?1 at a current density of 5 A g^?1, while 98.8% of its initial capacitance is retained after 5000 cycles. When the current density is increased from 1 to 20 A g^?1, the capacitance loss is less than 20%, demonstrating excellent rate capability. A highly flexible all‐solid‐state battery‐type supercapacitor is successfully fabricated with Ni?Co LDH NTAs/CFC as the positive electrode and electrospun carbon fibers/CFC as the negative electrode, showing a maximum specific capacitance of 319 F g^?1, a high energy density of 100 W h kg^?1 at 1.5 kW kg^?1, and good cycling stability (98.6% after 3000 cycles). These fascinating electrochemical properties are resulted from the novel structure of electrode materials and synergistic contributions from the two electrodes, showing great potential for energy storage applications.     

Soft‐Lithographed Up‐Converted Distributed Feedback Visible Lasers Based on CdSe–CdZnS–ZnS Quantum Dots

The development of a solution‐deposited up‐converted distributed feedback laser prototype is presented. It employs a sol–gel silica/germania soft‐lithographed microcavity and CdSe–CdZnS–ZnS quantum dot/sol–gel zirconia composites as optical gain material. Characterization of the linear and nonlinear optical properties of quantum dots establishes their high absorption cross‐sections in the one‐ and two‐ photon absorption regimes to be 1 × 10^?14 cm² and 5 × 10⁴ GM, respectively. In addition, ultrafast transient absorption dynamics measurements of the graded seal quantum dots reveal that the Auger recombination lifetime is 220 ps, a value two times higher than that of the corresponding CdSe core. These factors enable the use of such quantum dots as optically pumped gain media, operating in the one‐ and two‐photon absorption regime. The incorporation of CdSe–CdZnS–ZnS quantum dots within a zirconia host matrix affords a quantum‐dot ink that can be directly deposited on our soft‐lithographed distributed feedback grating to form an all‐solution‐processed microcavity laser.     

Unexpected Propeller‐Like Hexakis(fluoren‐2‐yl)benzene Cores for Six‐Arm Star‐Shaped Oligofluorenes: Highly Efficient Deep‐Blue Fluorescent Emitters and Good Hole‐Transporting Materials

Grafting six fluorene units to a benzene ring generates a new highly twisted core of hexakis(fluoren‐2‐yl)benzene. Based on the new core, six‐arm star‐shaped oligofluorenes from the first generation T1 to third generation T3 are constructed. Their thermal, photophysical, and electrochemical properties are studied, and the relationship between the structures and properties is discussed. Simple double‐layer electroluminescence (EL) devices using T1–T3 as non‐doped solution‐processed emitters display deep‐blue emissions with Commission Internationale de l'Eclairage (CIE) coordinates of (0.17, 0.08) for T1 , (0.16, 0.08) for T2 , and (0.16, 0.07) for T3 . These devices exhibit excellent performance, with maximum current efficiency of up to 5.4 cd A^?1, and maximum external quantum efficiency of up to 6.8%, which is the highest efficiency for non‐doped solution‐processed deep‐blue organic light‐emitting diodes (OLEDs) based on starburst oligofluorenes, and is even comparable with other solution‐processed deep‐blue fluorescent OLEDs. Furthermore, T2‐ and T3‐ based devices show striking blue EL color stability independent of driving voltage. In addition, using T0–T3 as hole‐transporting materials, the devices of indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS)/ T0–T3 /tris(8‐hydroxyquinolinato)aluminium (Alq₃)/LiF/Al achieve maximum current efficiencies of 5.51–6.62 cd A^?1, which are among the highest for hole‐transporting materials in identical device structure.     

Fluorene‐Carbazole Dendrimers: Synthesis,Thermal, Photophysical and Electroluminescent Device Properties

Novel hole‐transporting dendrimeric molecules containing dioctylfluorene, spirobi(fluorene) and spiro(cylododecane‐fluorene) as the core unit and different numbers of carbazole and thiophene moieties as the peripheral groups are synthesized. All the dendrimers are characterized by ¹H NMR, ¹³C NMR, FTIR, UV–vis, PL spectroscopy, and MALDI‐TOF. They are thermally stable with high glass transition and decomposition temperatures and exhibit chemically reversible redox processes. They are used as the hole‐transporting layer (HTL) material for multilayer organic light emitting diodes (OLEDs) with a low turn‐on voltage of around 2.5 V and a bright green emission with a maximum luminance of around 25400 cd m^?2.     

High‐Performance Wearable Micro‐Supercapacitors Based on Microfluidic‐Directed Nitrogen‐Doped Graphene Fiber Electrodes

Fiber‐shaped micro‐supercapacitors (micro‐SCs) have attracted enormous interest in wearable electronics due to high flexibility and weavability. However, they usually present a low energy density because of inhomogeneity and less pores. Here, we demonstrate a microfluidic‐directed strategy to synthesize homogeneous nitrogen‐doped porous graphene fibers. The porous fibers‐based micro‐SCs utilize solid‐state phosphoric acid/polyvinyl alcohol (H₃PO₄/PVA) and 1‐ethyl‐3‐methylimidazolium tetrafluoroborate/poly(vinylidenefluoride‐co‐hexafluoropropylene) (EMIBF₄/PVDF‐HFP) electrolytes, which show significant improvements in electrochemical performances. Ultralarge capacitance (1132 mF cm^?2), high cycling‐stability, and long‐term bending‐durability are achieved based on H₃PO₄/PVA. Additionally, high energy densities of 95.7–46.9 µWh cm^?2 at power densities of 1.5–15 W cm^?2 are obtained in EMIBF₄/PVDF‐HFP. The key to higher performances stems from microfluidic‐controlled fibers with a uniformly porous network, large specific surface area (388.6 m² g^?1), optimal pyridinic nitrogen (2.44%), and high electric conductivity (30785 S m^?1) for faster ion diffusion and flooding accommodation. By taking advantage of these remarkable merits, this study integrates micro‐SCs into flexible and fabric substrates to power audio–visual electronics. The main aim is to clarify the important role of microfluidic techniques toward the architecture of electrodes and promote development of wearable electronics.     

Interface Engineering by Thiazolium Iodide Passivation Towards Reduced Thermal Diffusion and Performance Improvement in Perovskite Solar Cells

Interface engineering has become one of the most facile and effective approaches to improve solar cells performance and its long‐term stability and to retard unwanted side reactions. Three passivating agents are developed which can functionalize the surface and induce hydrophobicity, by employing substituted thiazolium iodide (TMI) for perovskite solar cells fabrication. The role of TMI interfacial layers in microstructure and electro‐optical properties is assessed for structural as well as transient absorption measurements. TMI treatment resulted in V_OC and fill factor enhancement by reducing possible recombination paths at the perovskite/hole selective interface and by reducing the shallow as well as deep traps. These in turn allow to achieve higher performance as compared to the pristine surface. Additionally, the TMI passivated perovskite layer considerably reduces CH₃NH₃⁺ thermal diffusion and degradation induced by humidity. The un‐encapsulated perovskite solar cells employing TMI exhibit a remarkable stability under moisture levels (≈50% RH), retaining ≈95% of the initial photon current efficiency after 800 h of fabrication, paving the way towards a potential scalable endeavor.     

Multiphoton‐Pumped Highly Polarized Polariton Microlasers from Single Crystals of a Dye‐Coordinated Metal–Organic Framework

Room‐temperature, low‐threshold, photostable, cost‐effective, efficient, miniaturized, and all‐solid‐state lasers are highly desirable in many technological and medicinal applications. Here, an archetypical dye laser is introduced, with the above attributes, based on single‐crystalline microplates of a dye‐coordinated metal–organic framework (MOF) without an external cavity, holding a potential to be the next‐generation laser. With an exciton–polariton lasing mechanism combined with large multiphoton absorption cross sections, the lasing thresholds of the three microplates are in the range from 0.34 to 0.12 µJ cm^?2 under various optical pumping schemes. The lasing threshold is observed to be reduced with an increment in the order of optical nonlinearity involved in the pumping scheme. Lasing at an extreme‐red region is demonstrated, with a high photostability (with a drop in lasing output as low as 25% after 1.8 × 10⁶ cycles), a large degree of polarization (up to 92%), and an excellent conversion efficiency (up to 12%), thereby realizing a crucial milestone in the field of laser technology.     

Multibranched Octupolar Module Embedded Covalent Organic Frameworks Enable Efficient Two‐Photon Fluorescence

Covalent organic frameworks (COFs) have emerged as potential light emitting polymers for optoelectronic and optical devices, but their nonlinear optical properties, particularly two‐photon absorption and fluorescence (TPA/TPF), have seldom been explored. Herein, to construct octupolar three‐branched modules (e.g., acceptor 3‐(donor‐core), triphenylbenzene core) within a 2D cyano‐sp²c‐conjugated framework is proposed that results in two‐photon luminescent COFs, combining a large TPA cross section and high quantum yield (QY). Such octupolar module‐embedded sp²c‐conjugated COFs emit not only intense one‐photon fluorescence with QY of 27.2% in the solid state and 38.1% in tetrahydrofuran—superior to almost all reported COFs, but also efficient two‐photon fluorescence with large TPA cross section of 1225 GM—remarkably surpassing the corresponding cyano‐sp²c‐linked model compounds (104 GM). The finding highlights the synergy between sp²c‐conjugated framework and octupolar modules that leads to markedly improved TPA response owing to extended conjugated length, enhanced planarity and multidimensional intramolecular interaction. In view of the versatility of the branched chromophore, the proposed design idea is expected to be used to exploit more two‐photon active COF materials for a range of applications. Multiple uses of the COF in information encryption and warm white light‐emitting diodes are also exemplified.