Highly Efficient Electron‐Transporting/Injecting and Thermally Stable Naphthyridines for Organic Electrophosphorescent Devices

A series of 1,8‐naphthyridine derivatives is synthesized and their electron‐transporting/injecting (ET/EI) properties are investigated via a multilayered electrophosphorescent organic light‐emitting device (OLED) using fac‐tris(2‐phenylpyridine)iridium [Ir(ppy)₃] as a green phosphorescent emitter doped into a 4,4′‐N,N′‐dicarbazolebiphenyl (CBP) host with 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (a‐NPD) as the hole‐transporting layer, and poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped with tris(4‐bromophenyl)ammonium hexachloroantimonate (TBPAH) as the hole‐injecting layer. The turn‐on voltage of the device is 2.5 V using 2,7‐bis[3‐(2‐phenyl)‐1,8‐naphthyridinyl]‐9,9‐dimethylfluorene (DNPF), lower than that of 3.0 V for the device using a conventional ET material. The maximum current efficiency (CE) and power efficiency (PE) of the DNPF device are much higher than those of a conventional device. With the aid of a hole‐blocking (HB) and exciton‐blocking layer of bathocuproine (BCP), 13.2–13.7% of the maximum external quantum efficiency (EQE) and a maximum PE of 50.2–54.5 lm W^?1 are obtained using the naphthyridine derivatives; these values are comparable with or even higher than the 13.6% for conventional ET material. The naphthyridine derivatives show high thermal stabilities, glass‐transition temperatures much higher than that of aluminum(III) bis(2‐methyl‐8‐quinolinato)‐4‐phenylphenolate (BAlq), and decomposition temperatures of 510–518 °C, comparable to or even higher than those of Alq₃.     

Design of Multilayered Nanostructures and Donor–Acceptor Interfaces in Solution‐Processed Thin‐Film Organic Solar Cells

Multilayered polymer thin‐film solar cells have been fabricated by wet processes such as spin‐coating and layer‐by‐layer deposition. Hole‐ and electron‐transporting layers were prepared by spin‐coating with poly(3,4‐ethylenedioxythiophene) oxidized with poly(4‐styrenesulfonate) (PEDOT:PSS) and fullerene (C₆₀), respectively. The light‐harvesting layer of poly‐(p‐phenylenevinylene) (PPV) was fabricated by layer‐by‐layer deposition of the PPV precursor cation and poly(sodium 4‐styrenesulfonate) (PSS). The layer‐by‐layer technique enables us to control the layer thickness with nanometer precision and select the interfacial material at the donor–acceptor heterojunction. Optimizing the layered nanostructures, we obtained the best‐performance device with a triple‐layered structure of PEDOT:PSS|PPV|C₆₀, where the thickness of the PPV layer was 11 nm, comparable to the diffusion length of the PPV singlet exciton. The external quantum efficiency spectrum was maximum (ca. 20%) around the absorption peak of PPV and the internal quantum efficiency was estimated to be as high as ca. 50% from a saturated photocurrent at a reverse bias of ?3 V. The power conversion efficiency of the triple‐layer solar cell was 0.26% under AM1.5G simulated solar illumination with 100 mW cm^?2 in air.     

Flash‐Memory Effect for Polyfluorenes with On‐Chain Iridium(III) Complexes

Polyfluorenes containing Ir(III ) complexes in the main chain are demonstrated to have promising application in a polymer memory device. A flash‐memory device is shown whereby a polymer solution is spin‐coated as the active layer and is sandwiched between an aluminum electrode and an indium tin oxide electrode. This device exhibits very good memory performance, such as low reading, writing, and erasing voltages and a high ON/OFF current ratio of more than 10⁵. Both ON and OFF states are stable under a constant voltage stress of ?1.0 V and survive up to 10⁸ read cycles at a read voltage of ?1.0 V. Charge transfer and traps in polymers are probably responsible for the conductance‐switching behavior and the memory effect. The fluorene moieties act as an electron donor and Ir(III ) complex units as the electron acceptor. Furthermore, through the modification of ligand structures of Ir(III ) complex units, the resulting polymers also exhibit excellent memory behavior. Alteration of ligands can change the threshold voltage of the device. Hence, conjugated polymers containing Ir(III ) complexes, which have been successfully applied in light‐emitting devices, show very promising application in polymer memory devices.     

Tuning the Au‐Free InSb Nanocrystal Morphologies Grown by Patterned Metal–Organic Chemical Vapor Deposition

A thorough study of direct InSb nanocrystal formations on patterned InAs (111)B substrates is provided. These nanostructures are created without the use of Au catalysts or initial InAs segments. Under the growth conditions generally used for selective‐area, catalyst‐free epitaxy, a wide range of InSb nanocrystal morphologies are observed. This is because the low‐energy InSb surfaces, studied by first‐principles calculations, are the {111} facets as opposed to the {110} facets. By controlling the V/III ratio during growth, different InSb nanostructures can be achieved. Using low V/III growth conditions, In droplets start to form and InSb nucleation takes place at the droplet–semiconductor interface only, resulting in vertical, self‐catalyzed InSb nanopillars.     

A Light‐Blue Phosphorescent Dendrimer for Efficient Solution‐Processed Light‐Emitting Diodes

We describe the preparation of a dendrimer that is solution‐processible and contains 2‐ethylhexyloxy surface groups, biphenyl‐based dendrons, and a fac‐tris[2‐(2,4‐difluorophenyl)pyridyl]iridium(III ) core. The homoleptic complex is highly luminescent and the color of emission is similar to the heteroleptic iridium(III ) complex, bis[2‐(2,4‐difluorophenyl)pyridyl]picolinate iridium(III ) (FIrpic). To avoid the change in emission color that would arise from attaching a conjugated dendron to the ligand, the conjugation between the dendron and the ligand is decoupled by separating them with an ethane linkage. Bilayer devices containing a light‐emitting layer comprised of a 30 wt.‐% blend of the dendrimer in 1,3‐bis(N‐carbazolyl)benzene (mCP) and a 1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene electron‐transport layer have external quantum and power efficiencies, respectively, of 10.4 % and 11 lm W^–1 at 100 cd m^–2 and 6.4 V. These efficiencies are higher than those reported for more complex device structures prepared via evaporation that contain FIrpic blended with mCP as the emitting layer, showing the advantage of using a dendritic structure to control processing and intermolecular interactions. The external quantum efficiency of 10.4 % corresponds to the maximum achievable efficiency based on the photoluminescence quantum yield of the emissive film and the standard out‐coupling of light from the device.     

A New Understanding of Au‐Assisted Growth of III–V Semiconductor Nanowires

Semiconductor nanowires of III–V materials have generated much interest in recent years. However, the growth mechanisms by which these structures form are not well understood. The so‐called vapor–liquid–solid (VLS) mechanism has often been proposed for III–V systems, with a chemically inert, liquid metal particle (typically Au) acting as a physical catalyst. We assert here that Au is, in fact, not inert with respect to the semiconductor material but rather interacts with it to form a variety of intermetallic compounds. Moreover, we suggest that III–V nanowire growth can best be understood if the metallic particle is not a liquid, but a solid‐phase solution or compound containing Au and the group III material. The four materials GaP, GaAs, InP, and InAs will be considered, and growth behavior related to their particular temperature‐dependent interaction with Au.     

Orientation‐Controlled Selective‐Area Epitaxy of III–V Nanowires on (001) Silicon for Silicon Photonics

Monolithic integration of III–V nanowires on silicon platforms has been regarded as a promising building block for many on‐chip optoelectronic, nanophotonic, and electronic applications. Although great advances have been made from fundamental material engineering to realizing functional devices, one of the remaining challenges for on‐chip applications is that the growth direction of nanowires on Si(001) substrates is difficult to control. Here, catalyst‐free selective‐area epitaxy of nanowires on (001)‐oriented silicon‐on‐insulator (SOI) substrates with the nanowires aligned to desired directions is proposed and demonstrated. This is enabled by exposing {111} planes on (001) substrates using wet chemical etching, followed by growing nanowires on the exposed planes. The formation of nanowire array‐based bottom‐up photonic crystal cavities on SOI(001) and their coupling to silicon waveguides and grating couplers, which support the feasibility for on‐chip photonic applications are demonstrated. The proposed method of integrating position‐ and orientation‐controllable nanowires on Si(001) provides a new degree of freedom in combining functional and ultracompact III–V devices with mature silicon platforms.     

A New Carbazole‐Constructed Hyperbranched Polymer: Convenient One‐Pot Synthesis,Hole‐Transporting Ability,and Field‐Effect Transistor Properties

A new hyperbranched polymer ( HB‐car ), constructed fully by carbazole moieties, is successfully synthesized through a one‐pot Suzuki coupling reaction. The resultant polymer is well‐characterized, and its hole‐transporting ability is studied carefully. The device, in which HB‐car is utilized as a hole‐transporting layer and tris‐(8‐hydroxyquinoline) aluminum as an electron‐emitting layer as well as electron‐transporting layer, gives a much higher efficiency (3.05 cd A^–1), than that of a poly(N‐vinylcarbazole) based device (2.19 cd A^–1) under similar experimental conditions. The remarkable performance is attributed to its low energy barrier and enhanced hole‐drifting ability in the HB‐car based device. In addition, for the first time, a field‐effect transistor (FET) based on the hyperbranched polymer is fabricated, and the organic FET device shows that HB‐car is a typical p‐type FET material with a saturation mobility of 1 × 10^–5 cm² V^–1 s^–1, a threshold voltage of –47.1 V, and an on‐to‐off current ratio of 10³.     

A Hole‐Transporting Material with Controllable Morphology Containing Binaphthyl and Triphenylamine Chromophores

An organic compound with two triphenylamine moieties linked with binaphthyl at the 3,3′‐positions (2,2′‐dimethoxyl‐3,3′‐ di(phenyl‐4‐yl‐diphenyl‐amine)‐[1,1′]‐binaphthyl, TPA–BN–TPA) can be synthesized by Suzuki coupling. Amorphous and homogeneous films are obtained by either vacuum deposition or spin‐coating from solution in good solvents, while single crystals are grown in an appropriate polar solvent. X‐ray crystallography showed that a TPA–BN–TPA crystal is a multichannel structure containing solvent molecules in the channels. The intramolecular charge‐transfer state resulting from amino conjugation effects is observed by solvatochromic experiments. The high glass‐transition temperature (130 °C) and decomposition temperature (439 °C) of this material, in combination with its reversible oxidation property, make it a promising candidate as a hole‐transport material for light‐emitting diodes. With TPA–BN–TPA as the hole‐transporting layer in an indium tin oxide/TPA–BN–TPA/aluminum tris(8‐hydroxyquinoline)/Mg:Ag device, a brightness of about 10 100 cd m^–2 at 15.6 V with a maximum efficiency of 3.85 cd A^–1 is achieved, which is superior to a device with N,N′‐di(1‐naphthyl)‐N,N′‐diphenyl‐[1,1′‐biphenyl]‐4,4′‐diamine as the hole‐transporting layer under the same conditions. Other devices with TPA–BN–TPA as the blue‐light‐emitting layer or host for a blue dye emitter are also studied.     

1‐Methyl‐2‐(anthryl)‐imidazo[4,5‐f][1,10]‐phenanthroline: A Highly Efficient Electron‐Transport Compound and a Bright Blue‐Light Emitter for Electroluminescent Devices

A new organic blue‐light emitter 1‐methyl‐2‐(anthryl)‐imidazo[4,5‐f][1,10]‐phenanthroline ( 1 ) has been synthesized and fully characterized. The utility of compound 1 as a blue‐light emitter in electroluminescent (EL) devices has been evaluated by fabricating a series of EL devices A where compound 1 functions as an emitter. The EL spectrum of device series A has the emission maximum at 481 nm with the CIE (Commission Internationale de l'Eclairage) color coordinates 0.198 and 0.284. The maximum luminance of devices in series A is 4000 cd m^–2 and the best external quantum efficiency of device series A is 1.82 %. The utility of compound 1 as an electron injection–electron transport material has been evaluated by constructing a set of EL devices B where 1 is used as either the electron‐injection layer or the electron injection–electron transport layer. The performance of device series B is compared to the standard device in which Alq₃ (tris(8‐hydroxyquinoline) aluminum) is used as the electron injection–electron transport layer. The experimental results show that the performance of 1 as an electron injection–electron transport material is considerably better than Alq₃. The stability of device series B is comparable to that of the standard Alq₃ device. The excellent performance of 1 as an electron injection/transport material may be attributed to the strong intermolecular interactions of 1 in the solid state as revealed by single‐crystal X‐ray diffraction analysis. In addition, compound 1 is a colorless material with a much larger highest occupied molecular orbital–lowest unoccupied molecular (HOMO–LUMO) gap than Alq₃, which renders it potentially useful for a wide range of applications in EL devices.     

High Responsivity and Response Speed Single‐Layer Mixed‐Cation Lead Mixed‐Halide Perovskite Photodetectors Based on Nanogap Electrodes Manufactured on Large‐Area Rigid and Flexible Substrates

Mixed‐cation lead mixed‐halide perovskites are employed as the photoactive material in single‐layer solution‐processed photodetectors fabricated with coplanar asymmetric nanogap Al–Au and indium tin oxide–Al electrodes. The nanogap electrodes, bearing an interelectrode distance of ≈10 nm, are patterned via adhesion lithography, a simple, low‐cost, and high‐throughput technique. Different electrode shapes and sizes are demonstrated on glass and flexible plastic substrates, effectively engineering the device architecture, and, along with perovskite film and material optimization, paving the way toward devices with tunable operational characteristics. The optimized coplanar nanogap junction perovskite photodetectors show responsivities up to 33 A W^?1, specific detectivity on the order of 10¹¹ Jones, and response times below 260 ns, while retaining a low dark current (0.3 nA) under ?2 V reverse bias. These values outperform the vast majority of perovskite photodetectors reported so far, while avoiding the complicated fabrication steps involved in conventional multilayer device structures. This work highlights the promising potential of the proposed asymmetric nanogap electrode architecture for application in the field of flexible optoelectronics.     

Optimal Structure for High‐Performance and Low‐Contact‐Resistance Organic Field‐Effect Transistors Using Contact‐Doped Coplanar and Pseudo‐Staggered Device Architectures

A low contact resistance achieved on top‐gated organic field‐effect transistors by using coplanar and pseudo‐staggered device architectures, as well as the introduction of a dopant layer, is reported. The top‐gated structure effectively minimizes the access resistance from the contact to the channel region and the charge‐injection barrier is suppressed by doping of iron(III)trichloride at the metal/organic semiconductor interface. Compared with conventional bottom‐gated staggered devices, a remarkably low contact resistance of 0.1–0.2 kΩ cm is extracted from the top‐gated devices by the modified transfer line method. The top‐gated devices using thienoacene compound as a semiconductor exhibit a high average field‐effect mobility of 5.5–5.7 cm² V^?1 s^?1 and an acceptable subthreshold swing of 0.23–0.24 V dec^?1 without degradation in the on/off ratio of ≈10⁹. Based on these experimental achievements, an optimal device structure for a high‐performance organic transistor is proposed.     

Novel Heterogeneous Integration Technology of III–V Layers and InGaAs FinFETs to Silicon

Heterogeneous integration of III–V compound semiconductors to Si substrates is regarded as a necessary step for advancing high‐speed electronics and hybrid optoelectronic systems for data processing and communications, and is extensively being pursued by the semiconductor industry. Here, an innovative fab‐compatible, hybrid integration process of III–V materials to Si, namely InGaAs thin films to insulator‐on‐Si, is reported, and the first III–V FinFET devices on Si are demonstrated. Transfer of crystalline InGaAs layers with high quality to SiO2/Si is accomplished by the formation of a robust interfacial nickel‐silicide (NiSi) bonding interface, marking the first report for using silicides in III–V hybrid integration technology. The performance of optimally fabricated InGaAs FinFETs on insulator on Si is systematically investigated for a broad range of channel lengths and Fin perimeters with excellent switching characteristics. This demonstrates a viable approach to large‐scale hybrid integration of active III‐V devices to mainstream Si CMOS technology, enabling low‐power electronic and fully‐integrated optoelectronic applications.     

Low‐Temperature‐Grown Transition Metal Oxide Based Storage Materials and Oxide Transistors for High‐Density Non‐volatile Memory

An effective stacked memory concept utilizing all‐oxide‐based device components for future high‐density nonvolatile stacked structure data storage is developed. GaInZnO (GIZO) thin‐film transistors, grown at room temperature, are integrated with one‐diode (CuO/InZnO)–one‐resistor (NiO) (1D–1R) structure oxide storage node elements, fabricated at room temperature. The low growth temperatures and fabrication methods introduced in this paper allow the demonstration of a stackable memory array as well as integrated device characteristics. Benefits provided by low‐temperature processes are demonstrated by fabrication of working devices over glass substrates. Here, the device characteristics of each individual component as well as the characteristics of a combined select transistor with a 1D–1R cell are reported. X‐ray photoelectron spectroscopy analysis of a NiO resistance layer deposited by sputter and atomic layer deposition confirms the importance of metallic Ni content in NiO for bi‐stable resistance switching. The GIZO transistor shows a field‐effect mobility of 30 cm² V⁻¹ s⁻¹, a V_th of +1.2 V, and a drain current on/off ratio of up to 10⁸, while the CuO/InZnO heterojunction oxide diode has forward current densities of 2 × 10⁴ A cm⁻². Both of these materials show the performance of state‐of‐the‐art oxide devices.     

Novel Type‐II InAs/AlSb Core–Shell Nanowires and Their Enhanced Negative Photocurrent for Efficient Photodetection

The control of optical and transport properties of semiconductor heterostructures is crucial for engineering new nanoscale photonic and electrical devices with diverse functions. Core–shell nanowires are evident examples of how tailoring the structure, i.e., the shell layer, plays a key role in the device performance. However, III–V semiconductors bandgap tuning has not yet been fully explored in nanowires. Here, a novel InAs/AlSb core–shell nanowire heterostructure is reported grown by molecular beam epitaxy and its application for room temperature infrared photodetection. The core–shell nanowires are dislocation‐free with small chemical intermixing at the interfaces. They also exhibit remarkable radiative emission efficiency, which is attributed to efficient surface passivation and quantum confinement induced by the shell. A high‐performance core–shell nanowire phototransistor is also demonstrated with negative photoresponse. In comparison with simple InAs nanowire phototransistor, the core–shell nanowire phototransistor has a dark current two orders of magnitude smaller and a sixfold improvement in photocurrent signal‐to‐noise ratio. The main factors for the improved photodetector performance are the surface passivation, the oxide in the AlSb shell and the type‐II bandgap alignment. The study demonstrates the potential of type‐II core–shell nanowires for the next generation of photodetectors on silicon.     

An Investigation into III–V Compounds to Reach 20% Efficiency with Minimum Cell Thickness in Ultrathin-Film Solar Cells

III–V single-junction solar cells have already achieved very high efficiency levels. However, their use in terrestrial applications is limited by the high fabrication cost. High-efficiency, ultrathin-film solar cells can effectively solve this problem, as their material requirement is minimum. This work presents a comparison among several III–V compounds that have high optical absorption capability as well as optimum bandgap (around 1.4 eV) for use as solar cell absorbers. The aim is to observe and compare the ability of these materials to reach a target efficiency level of 20% with minimum possible cell thickness. The solar cell considered has an n-type ZnSe window layer, an n-type Al_0.1Ga_0.9As emitter layer, and a p-type Ga_0.5In_0.5P back surface field (BSF) layer. Ge is used as the substrate. In the initial design, a p-type InP base was sandwiched between the emitter and the BSF layer, and the design parameters for the device were optimized by analyzing the simulation outcomes with ADEPT/F, a one-dimensional (1D) simulation tool. Then, the minimum cell thickness that achieves 20% efficiency was determined by observing the efficiency variation with cell thickness. Afterwards, the base material was changed to a few other selected III–V compounds, and for each case, the minimum cell thickness was determined in a similar manner. Finally, these cell thickness values were compared and analyzed to identify more effective base layer materials for III–V single-junction solar cells.     

Assessing material qualities and efficiency limits of III–V on silicon solar cells using external radiative efficiency

The paper presents a quantitative approach to the investigation and comparison of the material qualities of III–V on silicon (III–V/Si) solar cells by using external radiative efficiencies. We use this analysis to predict the limiting efficiencies and evaluate the criteria of material quality in order to achieve high‐efficiency III–V/Si solar cells. This result yields several implications for the design of high‐efficiency III–V/Si solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

Multi‐Site Functionalization of Protein Scaffolds for Bimetallic Nanoparticle Templating

The use of biological scaffolds to template inorganic material offers a strategy to synthesize precise composite nanostructures of different sizes and shapes. Proteins are unique biological scaffolds that consist of multiple binding regions or epitope sites that site‐specifically associate with conserved amino acid sequences within protein‐binding partners. These binding regions can be exploited as synthesis sites for multiple inorganic species within the same protein scaffold, resulting in bimetallic inorganic nanostructures. This strategy is demonstrated with the scaffold protein clathrin, which self‐assembles into spherical cages. Specifically, tether peptides that noncovalently associate with distinct clathrin epitope sites, while initiating simultaneous synthesis of two inorganic species within the assembled clathrin protein cage, are designed. The flexibility and diversity of this unique biotemplating strategy is demonstrated by synthesizing two types of composite structures (silver–gold mixed bimetallic and silver–gold core–shell nanostructures) from a single clathrin template. This noncovalent, Template Engineering Through Epitope Recognition, or TEThER, strategy can be readily applied to any protein system with known epitope sites to template a variety of bimetallic structures without the need for chemical or genetic mutations.     

Perovskite Photovoltaics for Dim‐Light Applications

The use of photovoltaic cells with an organometallic perovskite as the active layer for indoor dim‐light energy harvesting is evaluated. By designing the electron‐transporting materials and fabrication processes, the traps in the perovskite active layers and carrier dynamics can be controlled, and efficient devices are demonstrated. The best‐performing small‐area perovskite photovoltaics exhibit a promising high power conversion efficiency up to ≈27.4%, no hysteresis behavior, and an exceptionally low maximum power point voltage variation of ≈0.1 V under fluorescent lamp illumination at 100–1000 lux. The 5.44 cm² large‐area device also shows a high efficiency of 20.4% and a promising long‐term stability. Compared with the most efficient inorganic and organic solar cells nowadays, the competitive efficiency, low fabrication cost, and low raw material costs make perovskite photovoltaics ideal for indoor light harvesting and as Internet of Things power provider.     

Polymeric Light‐Emitting Diodes Based on Poly(p‐phenylene ethynylene), Poly(triphenyldiamine), and Spiroquinoxaline

In this paper polymeric light‐emitting diodes (LEDs) based on alkoxy‐substituted poly(p‐phenylene ethynylene) EHO‐OPPE as emitter material in combination with poly(triphenyldiamine) as hole transport material are demonstrated. Different device configurations such as single‐layer devices, two‐layer devices, and blend devices were investigated. Device improvement and optimization were obtained through careful design of the device structure and composition. Furthermore, the influence of an additional electron transporting and hole blocking layer (ETHBL), spiroquinoxaline (spiro‐qux), on top of the optimized blend device was investigated using a combinatorial method, which allows the preparation of a number of devices characterized by different layer thicknesses in one deposition step. The maximum brightness of the investigated devices increased from 4 cd/m² for a device of pure EHO‐OPPE to 260 cd/m² in a device with 25 % EHO‐OPPE + 75 % poly(N,N′‐diphenylbenzidine diphenylether) (poly‐TPD) as the emitting/hole‐transporting layer and an additional electron‐transport/hole‐blocking spiro‐qux layer of 48 nm thickness.