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We present a remarkable improvement in the efficiency of AlGaN deep-ultraviolet light-emitting diodes (LEDs) enabled by the coupling of localized surface plasmon resonance (LSPR) mediated by a high-density array of Al nanoparticles (NPs). The Al NPs with an average diameter of ∼40 nm were uniformly distributed near the Al

Three-dimensional (3D) plasmonic structures have attracted great attention because abnormal wetting behavior of plasmonic nanoparticles (NPs) on 3D nanostructure can enhance the localized surface plasmons (LSPs). However, previous 3D plasmonic nanostructures inherently had weak plasmonic light absorption, low electrical conductivity, and optical transmittance. Here, we fabricated a novel 3D plasmonic nanostructure composed of Ag NPs as the metal for strong LSPs and 3D nano-branched indium tin oxide (ITO BRs) as a transparent and conductive framework. The Ag NPs formed on the ITO BRs have a more dewetted behavior than those formed on the ITO films. We experimentally investigated the reasons for the dewetting behavior of Ag NPs concerning the geometry of ITO BRs. The spherical Ag NPs are spatially separated and have high density, thereby resulting in strong LSPs. Finite-domain time-difference simulation evidenced that spatially-separated, high-density and spherical Ag NPs formed on ITO BRs dramatically boost the localized electric field in the active layer of organic solar cells (OSCs). Photocurrent of PTB7:PCBM OSCs with the ITO BRs/Ag NPs increased by 14%.

Subwavelength-scale nanorods were implemented on the hexagonal pyramid of photochemically etched light-emitting diodes (LEDs) to improve light extraction efficiency (LEE). Sequential processes of Ag deposition and inductively coupled plasma etching successfully produce nanorods on both locally unetched flat surface and sidewall of hexagonal pyramids. The subwavelength-scale structures on flat surface offer gradually changed refractive index, and the structures on side wall of hexagonal pyramid reduce backward reflection, thereby enhancing further enhancement of the light extraction efficiency. Consequently, the nanorods implemented LED shows a remarkable enhancement in the light output power by 14% compared with that of the photochemically etched LEDs which is known to exhibit the highest light output power. Theoretical calculations using a rigorous coupled wave analysis method reveal that the subwavelength-scale nanorods are very effective in the elimination of TIR as well as backward reflections, thereby further enhancing LEE of the LEDs.

Red, green, and blue top-emission organic light-emitting diodes (RGB TOLEDs) suffer from white color change with viewing angle due to the microcavity effect, called white angular dependence (WAD). Great efforts are devoted by applying various kinds of hazy films, but they suffer from poor mechanical stability and optical transmittance. Herein, we introduce an air-gap-embedded hazy film (AEHF) to solve these problems and suppress WAD in RGB TOLEDs. The AEHF is designed with optical simulation to realize high haze with transparency. By tuning geometries of the air gap inside the polymer, the AEHF realizes high haze of more than 90% in all RGB colors while maintaining high transparency. To experimentally demonstrate the AEHF, the O2 plasma is treated on a polymer film with AgCl as an etching mask to fabricate microstructures with high aspect ratios. Afterward, PDMS is coated on the patterned surface; air gaps develop spontaneously in the valleys between microstructures during the coating process. Using these processes, an air gap with 1.2 μm size and 400 nm period is formed inside the film and ∼100% haze is achieved while maintaining a high transmittance of 88%; these results agree well with rigorous coupled wave analysis results. By utilizing the AEHF into TOLEDs, the WAD can be drastically suppressed by 95.2% compared with that of a device without AEHF.

Cavity top emission organic light-emitting diodes (TOLEDs) suffer from an angular color shift due to the microcavity effect, called white angular dependence (WAD). The hazy films embedding air-gaps exhibit high transparency as well as enhanced scratch resistance. The haze was systemically controlled by changing the size of the air-gaps, determined by the size of the nanostructure and the viscosity of the coated resin. The optimized hazy film significantly reduced WAD about 92 % in a multi-color TOLED without complicated design for red, green and blue colors. Optical simulations using FDTD were conducted to interpret the effect of haze on WAD.

By virtue of their semiconducting and electrolytic characteristics, hybrid organic–inorganic perovskites have received intense research attention in various applications, which include energy, electronics, and display technologies. While research studies on the semiconducting or electronic properties of perovskite materials in solar cells and light-emitting diodes have been actively investigated, studies on their electrolytic or ionic behavior have rarely been conducted. To probe the electrolyte properties of the metal halide perovskite, we have fabricated solution-processed zinc oxide (ZnO) thin-film transistors using a methylammonium lead iodide (CH3NH3PbI3) perovskite thin film as a gate insulator material. The resulting perovskite film revealed ionic characteristics, with an ionic conductivity of about 10–8 S/cm. The perovskite-gated ZnO transistors exhibited typical n-type characteristics with an average field-effect mobility of 0.047 cm2/V s at a low applied voltage below 2 V because of the electric...

We present an innovative approach to fabricate an extremely flat (EF) metal film which was done by depositing metal on an extremely flat mother substrate, then detaching the metal from the substrate. The detached flexible metal films had a roughness that was within 2% of the roughness of the mother substrate, so EFs with Ra < 1 nm could be fabricated using the surface roughness transfer method. With quantitative analysis using in situ synchrotron XPS, it was concluded that the chemical reaction of oxygen atoms with the metal film played a critical role in designing a peel-off system to get extremely flat metal films from the mother substrate. The OLED was successfully implemented on the metal film. The OLED's luminance could be increased from 15 142 to 17 100 cd m−2 at 25 mA m−2 by replacing the glass substrate with an EF copper (Cu) substrate, due to the enhanced heat dissipation during the operation. This novel method can be very useful for mass production of large scale, low-cost and high quality metal films using roll-to-roll process.

For flexible organic light emitting diodes (OLEDs), roll-to-roll production enables low-cost fabrication and wide-ranging applications. Choosing an appropriate substrate material is one of the critical issues in the fabrication of flexible OLEDs. We demonstrated top-emitting OLEDs with a highly reflective distributed Bragg reflector (DBR) using a metal foil substrate. The DBR, made of seven pairs of SiO2/ZrO2, was formed by electron-beam evaporation on metal foil and showed high reflectivity of 90.5% at λ = 500 nm. The DBR served not only as the optical reflector, but also the substrate insulating layer which enabled the electrical isolation and prevented crosstalk. The OLEDs showed an operation voltage of 6.5 V at a current density of J = 10 mA cm−2 and maximum luminance of 17 400 cd m−2 at J = 225 mA cm−2. The electroluminescence property of the device could be maintained under the tensile bending condition.

Implementing nanostructures on plastic film is indispensable for highly efficient flexible optoelectronic devices. However, due to the thermal and chemical fragility of plastic, nanostructuring approaches are limited to indirect transfer with low throughput. Here, we fabricate single-crystal AgCl nanorods by using a Cl2 plasma on Ag-coated polyimide. Cl radicals react with Ag to form AgCl nanorods. The AgCl is subjected to compressive strain at its interface with the Ag film because of the larger lattice constant of AgCl compared to Ag. To minimize strain energy, the AgCl nanorods grow in the [200] direction. The epitaxial relationship between AgCl (200) and Ag (111) induces a strain, which leads to a strain gradient at the periphery of AgCl nanorods. The gradient causes a strain-induced diffusion of Ag atoms to accelerate the nanorod growth. Nanorods grown for 45 s exhibit superior haze up to 100% and luminance of optical device increased by up to 33%., Rapid, scalable, low-temperature processes to fabricate nanostructures on flexible surfaces will be important for future device technologies. Park et al., develop a rigorous methodology to grow nanorods on polymer films by plasma-assisted strain relaxation.

Energy-saving window that selectively blocks near-infrared (NIR) is a promising technology to save energy consumption. However, it is hard to achieve both high transmittance in visible light and high reflectance in NIR for the energy-saving windows. Here, a TiO2 /Ag/TiO2 /SiO2 /TiO2 multilayer is demonstrated on a glass substrate to selectively block NIR while maintaining high transmittance to visible light. The thickness of a TiO2 /Ag/TiO2 structure is first design and optimized; the metal layer reflects NIR and the dielectric layers increase transmittance of visible light with zero reflection condition. To further enhance NIR-blocking capability, a TiO2 back reflector is implemented with a SiO2 spacer to TiO2 /Ag/TiO2 structure. The back reflector can induce additional Fresnel reflection without sacrificing transmittance to visible light. The optimal TiO2 (32 nm)/Ag (22 nm)/TiO2 (30 nm)/SiO2 (100 nm)/TiO2 (110 nm)/glass shows solar energy rejection 89.2% (reflection 86.5%, absorption 2.7%) in NIR, visible transmittance 69.9% and high long-wave (3 ≤ λ ≤ 20 µm) reflectance > 95%. This proposed visible-transparent, near-infrared-reflecting multilayer film can be applied to the windows of buildings and automobiles to reduce the energy consumption.

Two-dimensional transition metal dichalcogenides (TMDs) have been intensively researched due to their excellent physical, chemical, and mechanical properties, which make them essential for various electronic devices owing to several disadvantages of conventional hole-transport layers (HTLs) such as hygroscopic effect and highly acidic nature, which can induce low stability of the fabricated devices. Especially, they have been considered as hole-transport layers (HTLs) in organic light-emitting diodes (OLEDs) and organic photovoltaics due to its chemical stability. Despite of its adequate work-function value and chemical stability, the device stability could be enhanced but, device performance of pristine TMD-HTL-based has been reported lower than conventional devices. In this work, we report a facile route to synthesize alloyed transition metal disulfides (TMD) thin films and their application as hole transport layers in OLEDs. Polycrystalline, large-area, and uniform Mo1−XWXS2 thin films are synthesized via simple thermal disproportionation methods by chemical vapour deposition. The physical and chemical properties of the synthesized alloyed TMD layers are controlled by varying the precursor concentrations. The device performance of alloyed TMD-layer-based OLED is comparable to that of conventional poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) and device stability in air is significantly improved. Thus, a novel approach to synthesize alloyed Mo1−XWXS2 thin films and their application in optoelectronic devices are presented herein.

Flexible inverted top-illuminated polymer solar cells (IT-PSCs) are fabricated by wetting of polyelectrolyte and designing a microcavity structure by laying an indium-tin-oxide (ITO) interlayer on top of an Ag reflector. The ITO-coated Ag makes the surface hydrophilic, thereby improving wettability of polyethyleneimine (PEIE). This increased wettability of PEIE yields a reflective cathode with low work function of 3.73 eV. The ITO layer also tunes the light absorption spectrum in the active layer. Finite-domain time-difference simulation provides evidence that the ITO layer played a role in both the shift in resonant wavelength in the microcavity and confinement of the electric field to the active layer. Time-dependent simulation suggests that the time to reach steady-state light absorption is longer (6.6 fs) when a microcavity is present than when it is not present (3.8 fs); i.e., the microcavity increases light absorption in the active layer. The designed IT-PSCs show a maximum photo-conversion efficiency of 6.4% on plastic film and 6.1% on opaque copper foil; these are the highest values obtained by top-illuminated PSCs on a metallic substrate. The IT-PSCs have excellent mechanical flexibility and more stable in air than conventional normal structured devices.

Graphene has shown strong potential to occupy transparent electrodes, replacing indium tin oxide (ITO). However, the commercialization of graphene is still limited because of its poor chemical and electrical stability from reaction with environmental factors or essential materials such as poly[3,4-(ethylenedioxy)thiophene]:poly(styrenesulfonate) (PSS). Here, we have demonstrated a multilayered electrode in which graphene is sandwiched between metal oxides (MOs) that have high stability and optical properties. The MOs overcoated graphene, and thereby protected it from desorption of chemical dopants. Because of the resulting chemical and electrical stability, the electrodes maintain low sheet resistance 2.4 times longer than bare graphene and 36 times longer thanPSS-coated graphene. On the basis of optical simulations, we derive the design rules for highly transparent MO/graphene/MO stacks and demonstrate an optimized structure with a TiO2 and WO3 electrode that has high transmittance (96%) which exceeds those of ITO (87%) and graphene (90%). Using a TiO2/graphene/WO3 electrode in organic light-emitting diodes (λ = 520 nm) instead of ITO or graphene anodes increases the cavity resonance and thereby increases power efficiencies by up to 30%. The MO/graphene/MO stacks designed will provide opportunities for commercialization of flexible electronics with graphene electrodes.

Wavelength-scale inverted pyramid structures with low reflectance and excellent haze have been designed for application to polymer solar cells (PSCs). The wavelength-scale structured haze films are fabricated on the back surface of glass without damages to organic active layer by using a soft lithographic technique with etched GaN molds. With a rigorous coupled-wave analysis of optical modeling, we find the shift of resonance peaks with the increase of pattern's diameter. Wavelength-scale structures could provide the number of resonances at the long wavelength spectrum (λ = 650-800 nm), yielding enhancement of power conversion efficiency (PCE) in the PSCs. Compared with a flat device (PCE = 7.12%, Jsc = 15.6 mA/cm(2)), improved PCE of 8.41% is achieved in a haze film, which is mainly due to the increased short circuit current density (Jsc) of 17.5 mA/cm(2). Hence, it opens up exciting opportunities for a variety of PSCs with wavelength-scale structures to further improve performance, simplify complicated process, and reduce costs.

Improving device deformability under mechanical stress is an essential requirement for the realization of next-generation wearable electronic applications. Herein, we report a new concept of stretchable thin-film transistors (TFTs) that can be fabricated without substrate components. Instead of using plastic substrates, we employ an elastic (and also free-standing) solid-state electrolyte as a gate insulator. The strategy not only realizes stretchable TFTs, but also enhances the device performance (e.g. mobility, operation voltage) by the formation of high-capacitance electric double layers. As source-drain electrodes, semi-circular shaped gold (Au) thin-film was employed to enhance the device stretchability. By the combination of a solid electrolyte (gate insulator), metal film (electrodes), and polymer semiconductor (channel layer), the device is stretchable up to around 10%, and shows typical p-channel switching performance with low operation voltage of 1 V and TFT mobility of about 0.81 cm2/V s. The devices were processed by a simple solution process and thermal evaporation under low temperature (

This paper describes two highly-sensitive flexible motion sensors (piezoelectric nanogenerators and ultraviolet light emitting diodes based flexible GaN) based on asymmetric polarization created across flexible GaN film and very high correlations of the sensors for precise motion measurement. The operation mechanism is based on piezotronic effect using polarization charges at the interface for tuning the electronic and optoelectronic processes. The nanogenerator showed very high sensitivity (S = 93 at 0.45% under concave bending) and the convex bending led to the decrease of the sensitivity to 24. The electroluminescence intensity also almost linearly decreased with the change of the bending direction, clearly shown in the confocal scanning electroluminescence microscopy images with the bending motion. The two sets of measures showed very high correlations (R2 = 0.98 and 0.96 under concave and convex bending, respectively) with very sensitive directional information. This approach makes to recognize the direction of bending as well as to measure the magnitude of the strain for realizing multi-functional, motion detection sensing devices.

Refractive-index (RI)-matched nanostructures are implemented in GaN-based light-emitting diodes (LEDs) for enhancing light output efficiency. The RI-matched indium tin oxide (ITO) nanostructures are successfully implemented in GaN-based lateral LEDs by using ITO sol–gel and nanoimprint lithography. The ITO sol–gel nanostructures annealed at 300 °C have RI of 1.95, showing high transparency of 90% and high diffused transmittance of 34%. Consequently, the light output power in LEDs with the RI-matched nanostructures increases by 8% in comparison with that in LEDs containing flat ITO. Ray tracing and finite-difference time-domain (FDTD) simulations show that the RI-matched nanostructures on the transparent current spreading layer dramatically reduce Fresnel reflection loss at the interface of the current spreading layer with the nanostructure and extract confined waveguide lights in LEDs.

This paper presents a method to produce subwavelength-scale (

A novel method to synthesize a hydrogenated molybdenum oxide (HyMoO3−x) thin film by irradiation of photons using a KrF laser (λ = 248 nm) on an ammonium heptamolybdate ((NH4)6Mo7O24·4H2O) precursor layer is demonstrated. The laser-assisted synthesis is simple, and can be conducted in an ambient atmosphere without damaging the underlying bottom electrode and plastic substrate. The exposure time (30 ns) is extremely short compared to thermal annealing (>3 min). Because the high-energy photons are absorbed by the MoO3 layer and provide the activation energy for the reaction, the hydrogen atoms that dissociate from the ammonium molecules bond to the MoO3; this process yields a HyMoO3−x thin-film. By controlling the laser energy, the stoichiometry of the HyMoO3−x layer can be manipulated to simultaneously obtain advantageous electrical properties of both high work function (5.6 eV) and electrical conductivity (9.9 μS cm−1). The HyMoO3−x hole transport layer (HTL) is successfully demonstrated on flexible top-illuminated PTB7:PCBM organic solar cells (OSCs). This OSC has good mechanical flexibility, and 75% higher short-circuit current than the device with a PEDOT:PSS HTL. Finite-domain time-difference simulations were conducted to verify the enhancement of the photocurrent. The thin layer of HyMoO3−x was proven to be suitable for the microcavity condition which allows a resonant wavelength match to the PTB7:PCBM active layer.

We demonstrated visible color tunable three-dimensional (3D) pyramidal light emitting diodes by depositing the MgO on and near the tip of the pyramid as an insulating layer. Here, we show that the degradation of the materials (i.e., p-GaN) crystallinity and the built-in electric field due to the nanoscale geometry of the tip region is responsible for the large leakage current observed in LEDs. Confocal scanning electroluminescence microscopy images clearly showed that the intensity of the light emitted out of the side facet of the pyramid is much higher than that of the light extracted out of the tip surface, indicating that the MgO layer prohibited the carrier injection to the MQWs layer, suppressing the leakage occurring at or near the tip region of the pyramids. The color range of the LEDs can be also tuned by using the MgO layer, a blue-shift by 10.3 nm in the wavelength. This technique is simple and scalable, providing a promising solution for developing 3D pyramidal LEDs with low leakage current and controllable light emission.

We report the use of a continuous 1D-metallic microfibers web (MFW) as transparent electrode for organic solar cells (OSCs). The MFW electrode can be produced with a process that involves simple electrospinning and wet etching of metal thin film. Au MFW exhibits a maximum optical transmittance of 90.8% (at 15 Ω/sq of the sheet resistance) and excellent mechanical flexibility. The MFW structure has an average width in the range from 4 to 6 μm and a junction-free structure, resulting in very smooth surface roughness. The OSCs with Au MFW electrode exhibited a higher power conversion efficiency (PCE) of 3.50% than the device with an indium tin oxide electrode (PCE = 3.20%). The optical modeling calculation showed that the Au MFW electrode induced light scattering and improved the light absorption in the active layer, resulting in an improved PCE in the OSCs.

Top-illuminated flexible organic solar cells with a high power conversion efficiency (≈6.75%) are fabricated using a dielectric/metal/polymer (DMP) electrode. Employing a polymer layer (n = 1.49) makes it possible to show the high transmittance, which is insensitive to film thickness, and the excellent haze induced by well-ordered nanopatterns on the DMP electrode, leading to a 28% of enhancement in efficiency compared to bottom cells.

Metal sulfides (MeS2) such as MoS2 and WS2 were used as charge transport layers in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells in order to enhance the stability in air comparing to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PSS). MeS2 layers with a polycrystalline structure were synthesized by a chemical deposition method using uniformly spin-coated (NH4)MoS4 and (NH4)WS4 precursor solutions. The ultraviolet-ozone (UV-O3) treatment on MeS2 leads to the removal of the surface contaminants produced by the transfer process, resulting in a uniform surface and an increase of the work function. The maximum luminance efficiencies of the OLEDs with UV-O3-treated MoS2 and WS2 were 9.44 and 10.82 cd/A, respectively. The power conversion efficiencies of OPV cells based on UV-O3-treated MoS2 and WS2 were 2.96 and 3.08%, respectively. These values correspond to over 95% of those obtained with (PSS) based devices. Furthermore, OLEDs and OPV cells based on MeS2 showed two to six times longer stability in air compared withPSS based devices. These results suggest that UV-O3-surface-treated MeS2 could be a promising candidate for a charge transport layer in optoelectronic devices.

We report on the enhanced thermal reliability of vertical-LEDs (VLEDs) using novel reflective p-type ohmic contacts with good thermal stability. The reflective p-type ohmic contacts with Ni/Ag-Cu alloy multi-layer structure shows low contact resistivity, as low as 9.3 × 10−6 Ωcm2, and high reflectance of 86% after annealing at 450°C. The V-LEDs with Ni/Ag-Cu alloy multi-layer structure show good thermal reliability with stress time at 300°C in air ambient. The improved thermal stability of the reflective ohmic contacts to p-type GaN is believed to play a critical role in the thermal reliability of V-LEDs. Open image in new window

The simple and cost-effective method to form vanadium oxides (VO x ) with high density of gap-states and role of gap-states for organic electronics were investigated. The percentage of V 2 O 5 decreased from 82.7% to 29.3% as the deposition rate increased from 0.1 to 1.0 A/s in thermal evaporation. Synchrotron radiation photo emission spectroscopy revealed that a number of gap states relating to oxygen vacancies was induced near the Fermi level, transporting charges and lowering the hole injection barrier from 0.92 to 0.67 eV. As a result, the operation voltage at 1 mA/cm 2 was reduced from 8.3 to 5.2 V

We present a simple approach to improving the quality of CVD grown graphene, exploiting a Cu(111) foil catalyst. The catalyst is epitaxially grown by evaporation on a single crystal sapphire substrate, thickened by electroplating, and peeled off. The exposed surface is atomically flat, easily reduced, and exclusively of (111) orientation. Graphene grown on this catalyst under atmospheric CVD conditions and without wet chemical prereduction produces single crystal domain sizes of several hundred micrometers in samples that are many centimeters in size. The graphene produced in this way can easily be transferred to other substrates using well-established techniques. We report mobilities extracted using field-effect (as high as 29 000 cm(2) V(-1) s(-1)) and Hall bar measurement (up to 10 100 cm(2) V(-1) s(-1)).

Although rapid progress has been made recently in bulk heterojunction organic solar cells, systematic studies on an ultrathin interfacial layer at the electron extraction contact have not been conducted in detail, which is important to improve both the device efficiency and the lifetime. We find that an ultrathin BaF2 layer at the electron extraction contact strongly influences the open-circuit voltage (Voc ) as the nanomorphology evolves with increasing BaF2 thickness. A vacuum-deposited ultrathin BaF2 layer grows by island growth, so BaF2 layers with a nominal thickness less than that of single-coverage layer (≈3 nm) partially cover the polymeric photoactive layer. As the nominal thickness of the BaF2 layer increased to that of a single-coverage layer, the Voc and power conversion efficiency (PCE) of the organic photovoltaic cells (OPVs) increased but the short-circuit current remained almost constant. The fill factor and the PCE decreased abruptly as the thickness of the BaF2 layer exceeded that of a single-coverage layer, which was ascribed to the insulating nature of BaF2 . We find the major cause of the increased Voc observed in these devices is the lowered work function of the cathode caused by the reaction and release of Ba from thin BaF2 films upon deposition of Al. The OPV device with the BaF2 layer showed a slightly improved maximum PCE (4.0 %) and a greatly (approximately nine times) increased device half-life under continuous simulated solar irradiation at 100 mW cm(-2) as compared with the OPV without an interfacial layer (PCE=2.1 %). We found that the photodegradation of the photoactive layer was not a major cause of the OPV degradation. The hugely improved lifetime with cathode interface modification suggests a significant role of the cathode interfacial layer that can help to prolong device lifetimes.

The effect of inductively coupled plasma (ICP) etching on the performance of light emitting diodes (LEDs) with doped graphene (D-G) electrodes was investigated. The sheet resistance decreased from 220 Ω sq−1 to 105–140 Ω sq−1, and the value of the work function increased from 4.2 eV to the 4.9–5.1 eV range after four-layer graphene was doped with metal chlorides. The voltage at a 10 mA current was 5.5 V for LEDs with pristine graphene (P-G) and D-G electrodes when graphene was treated using the ICP etching step. The ICP etching process heated the D-G, recovering P-G properties in D-G and depositing residual photoresist on the graphene surface. Therefore, no difference was found in the electrical and luminance characteristics of P-G and D-G LEDs. The voltage at a 10 mA current was lowered to 4.25–4.5 V for LEDs using D-G electrodes without ICP etching. The luminance properties of these LEDs were improved compared to the LED with a P-G electrode. The decrease in the sheet resistance and increase in the value of the work function in D-G enhanced the electrical properties of the LEDs. Therefore, avoiding the ICP etching step is better for D-G electrodes in GaN-based LEDs.

We developed a method to control the alignment and density of indium tin oxide (ITO) nanowires by using ion beam assisted deposition (IBAD). During electron beam evaporation, IBAD changed the randomly oriented branch type nanowires to aligned nanowires without branches. This is due to the energetic ion beams which play a role in nucleating tin containing indium nanodots at the initial stage of growth. The improved alignment of the ITO nanowires reduced the sensing time for ethanol gas.

We report the effect of yellow Sr 2 SiO 4 :Eu 2+ and green SrGa 2 S 4 :Eu 2+ phosphors on the efficiency of organic photovoltaic (OPV) cells. Each phosphor was coated on the back side of indium tin oxide (ITO)/glass substrates by spin coating with poly(methyl methacrylate) (PMMA). The maximum absorption wavelength of the active layer in the OPV cells was ∼512 nm. The emission peaks of Sr 2 SiO 4 :Eu 2+ and SrGa 2 S 4 :Eu 2+ were maximized at 552 nm and 534 nm, respectively. The short circuit current density ( J sc ) and power conversion efficiency (PCE) of the OPV cells with Sr 2 SiO 4 :Eu 2+ (8.55 mA/cm 2 and 3.25%) and with SrGa 2 S 4 :Eu 2+ (9.29 mA/cm 2 and 3.3%) were higher than those of the control device without phosphor (7.605 mA/cm 2 and 3.04%). We concluded that phosphor tuned the wavelength of the incident light to the absorption wavelength of the active layer, thus increasing the J sc and PCE of the OPV cells.

A simple device structure of bulk hetero-junction polymer solar cells (PSCs) with poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM) is demonstrated without poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS). Ultraviolet (UV)–ozone treatment alters the surface atomic compositions making it oxygen rich and carbon deficient. These chemical changes increase the workfunction of indium tin oxide (ITO) by 0.2 eV. The ITO anode with high workfunction results in efficient hole collection at ITO/P3HT interface. This method makes it possible to eliminate PEDOT:PSS with high efficiency of 3.35%, identical to the device with PEDOT:PSS (3.35%). In addition, the PSC with UV–ozone treated ITO anode shows 3.6 times longer life-time than that with PEDOT:PSS.

Organic photovoltaic (OPV) cells with a longer lifetime than that of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)-based OPV cells were fabricated using UV/ozone-treated graphene sheets as hole extraction layers (HELs). The sheet resistance increased from 1.1 kΩ/sq to infinity and the transmittance at 550 nm decreased from 95.2% to 93.8% after UV–ozone treatment for 9 min due to the formation of carbon–oxygen functionalization layers. The intensity of the C–C, C–O, and C=O bonds in X-ray photoemission spectroscopy spectra increased with increasing UV–ozone treatment time, indicating the bonding formation between carbon and oxygen. Ultraviolet photoemission spectroscopy data showed that the work function of graphene increased from 4.3 eV to 4.85 eV after UV/ozone treatment for 9 min. However, the OPV cell with 5 min-treated graphene HEL displayed the best efficiency of 3.0% because the graphene sheet was damaged by UV–ozone exposure after 7 min. The efficiency of the PEDOT:PSS-based device rapidly decreased to 0% after 14 h exposure in humid conditions while the UV/ozone-treated device continued to operate for 26 h. These results suggested that UV/ozone-treated graphene under optimized condition is a good HEL candidate in OPV cells as a replacement for the conventional PEDOT:PSS layer.

Transparent electrodes to enhance external quantum efficiency (EQE) in optoelectronic devices are proposed based on the suppression of surface plasmons (SPs) at the metal–dielectric (or metal–organic) interface using a frequency conversion layer. Plasmonic absorption at metal-based electrodes causes severe optical losses in the planar stacks of optoelectronic devices. Even though Ag is suitable for transparent electrodes owing to its lowest absorption coefficient compared to other metals, the surface plasmon resonant frequency (SPRF) of Ag is located in the visible region (i.e., ωSP ∼ 3.9 eV, λSP = 500–550 nm). Thus, incident light is absorbed by surface plasmon resonance (SPR) at the interface between Ag and dielectric materials. These plasmonic resonances could be dramatically suppressed by adding a 2 nm-thick Al interlayer with resonance frequency out of the visible region (i.e., ωSP ∼ 15 eV, λSP = 250–300 nm), which results in an extreme enhancement of the optical transmittance of Ag-based electrodes from 68% to 91% at 470 nm. These approaches for highly transparent and conductive multilayer stacks are applicable to universal optoelectronics because they are straightforward, cost-effective and reliable even in large area fabrication.

A flexible hazy substrate (FHS) with embedded air bubbles to increase light extraction efficiency of organic light-emitting diodes (OLEDs) is reported. In order to embed the air bubbles in the flexible substrate, micropatterned substrates are fabricated by plasma treatment, and then coated with a planarization layer. During the planarization layer coating, air bubbles are trapped between the substrate and the planarization layer. The haze of the FHS can be controlled from 1.7% to 68.4% by changing the size of micropatterns by adjusting the plasma treatment time. The FHS shows average haze of 68.4%, average total transmittance of 90.3%, and extremely flat surface with average roughness (R a) of 1.2 nm. Rigorous coupled-wave analysis and finite-difference time-domain simulations are conducted to demonstrate that the air bubbles in the substrate can effectively extract photons that are trapped in the substrate. The FHS increases the power efficiency of OLEDs by 22% and further increases by 91% combined with an external extraction layer. Moreover, the FHS has excellent mechanical flexibility. No defect has been observed after 10 000 bending cycles at bending radius of 4 mm.

Parallel aligned mesopore arrays in pyramidal-shaped GaN are fabricated by using an electrochemical anodic etching technique, followed by inductively coupled plasma etching assisted by SiO2 nanosphere lithography, and used as a promising photoelectrode for solar water oxidation. The parallel alignment of the pores of several tens of micrometers scale in length is achieved by the low applied voltage and prepattern guided anodization. The dry etching of single-layer SiO2 nanosphere-coated GaN produces a pyramidal shape of the GaN, making the pores open at both sides and shortening the escape path of evolved gas bubbles produced inside pores during the water oxidation. The absorption spectra show that the light absorption in the UV range is ∼93% and that there is a red shift in the absorption edge by 30 nm, compared with the flat GaN. It also shows a remarkable enhancement in the photocurrent density by 5.3 times, compared with flat GaN. Further enhancement (∼40%) by the deposition of Ni was observed due to the generation of an electric field, which increases the charge separation ratio.

Three primary colors, cyan, yellow, and green, are obtained from Ag nano-dot embedded organic light emitting diodes (OLEDs) by localized surface plasmon resonance (LSPR). By changing the thickness of the Ag film, the size and spacing of Ag nano-dots are controlled. The generated light from the emissive layer in the OLEDs interacts with the free electrons near the surface of the Ag nano-dots, which leads to LSPR absorption and scattering. The UV-visible absorption spectra of glass/ITO/Ag nano-dot samples show intense peaks from 430 nm to 520 nm with an increase of Ag nano-dot size. And also, the Rayleigh scattering spectra results show the plasmon resonance wavelength in the range of 470–550 nm. The effect of the LSPR of Ag nano-dots on the change of emission color in OLEDs is demonstrated using 2 dimensional finite-difference time-domain simulations. The intensity of the electro-magnetic field in the sample with 5 nm-thick Ag is low at the incident wavelength of 500 nm, but it increases with the incident wavelength. This provides evidence that the emission color change in OLEDs originates from LSPR at the Ag nano-dots. As a result, the emission peak wavelength of OLEDs shifted toward longer wavelengths, from cyan to yellow-green, with the increase of Ag nano-dot size.

Cone-shaped nanostructures with controllable side-wall angle are success- fully fabricated with a SiO(2) nanosphere lithography (NSL) etching mask. Vertical LEDs with cone-shaped nanostructures with a 24.1° side-wall angle provide 6% more light output power compared to those using hexagonal pyramids formed by photochemical etching. This achievement is attributed to effective elimination of total internal reflection by angle-controlled nanostructures.

The mechanism of charge generation in metal oxide-based charge generation layers (CGLs) in tandem organic light emitting diodes (OLEDs) was studied via in situ synchrotron radiation photoelectron spectroscopy (SRPES) and in situ ultraviolet photoemission spectroscopy (UPS). The energy band structure and interface dipole energy of a CGL architecture comprising Ca doped tris(8-hydroxyquinoline) aluminum (Alq3), 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (α-NPD), and various kinds of metal oxides are studied. The charge generation property is contributed to the amount of work function and interface dipole energy of metal oxide CGLs. The hole injection barrier at the metal oxide/α-NPD interface decreased as a function of the work function of the metal oxide. However, contrary to common belief, the large interface dipole resulted in a small hole injection barrier and low operation voltage of the device. Using data on interface energetics measured by in situ SRPES and UPS, it is shown that the work function ...

Nanostructured vertical light-emitting diodes (V-LEDs) with a very dense forest of vertically aligned ZnO nanowires on the surface of N-face n-type GaN are reported with a dramatic improvement in light extraction efficiency (∼3.0×). The structural transformation (i.e., dissociation of the surface nitrogen atoms) at the nanolevel by the UV radiation and Ozone treatments contributes significantly to the initial nucleation for the nanowires growth due to the interdiffusion of Zn into GaN, evident by the scanning photoemission microscopy (SPEM), high-resolution transmission electron microscopy (HR-TEM), and ultraviolet photoelectron spectroscopy (UPS) measurements. This enables the growth of densely aligned ZnO nanowires on N-face n-type GaN. This approach shows an extreme enhancement in light extraction efficiency (>2.8×) compared to flat V-LEDs, in good agreement with the simulation expectations (∼3.01×) obtained from 3D finite-difference time-domain (FDTD) tools, explained by the wave-guiding effect. The further increase (∼30%) in light extraction efficiency is also observed by optimized design of nanogeometry (i.e., MgO layer on ZnO nanorods).

We fabricated a self-connected and habitually tilted ZnO nanorod (NR) array, which is free of any patterning process for the connection of the NRs and is easily bent by a normal force. The vertically well-aligned ZnO NRs were grown by a strain relaxation process on MgO-buffered C-plane sapphire, and the remaining epitaxial ZnO wetting layer acted as a self-connecting layer of NRs. The epitaxial ZnO film on the step-terrace structured substrate caused the tilting angle from the surface normal direction (~0.2°) to match the step between the ZnO film and MgO-buffered C-plane sapphire, resulting in easy bending of the ZnO NRs by normal force. The unsymmetrical strain between the tensile and compressive stressed region in the habitually tilted ZnO NRs caused a gradient in the piezoelectric potential, resulting in an electrical field along the lateral direction of NR growth, resulting in the control of the current direction and level to be about 0.1 μA/cm(2) at 2 kgf normal force.

Organic light emitting diodes (OLEDs) have rapidly progressed in recent years due to their potential applications in flat panel displays and solid-state lighting. In spite of the commercialization of OLEDs, they still have a low out-coupling efficiency of about 20% due to factors such as the total internal reflection, absorption, and surface plasmon coupling. This light out-coupling efficiency is a major limitation on the high efficiency levels of OLEDs. Hence, enhancing the light out-coupling efficiency of OLEDs offers the greatest potential for achieving a substantial increase in the external quantum efficiency and power efficiency of OLEDs. Accordingly, significant advancements in OLEDs have driven the development of light extraction technologies as well as highly transparent conducting electrode materials. Recent efforts to combine light extraction structures with the improved out-coupling efficiency of OLEDs have produced OLEDs with an efficiency level that matches the efficiency of a fluorescent tube (>100 lm/W). This paper reviews the technical issues and recent progress in light extraction technologies and discusses ways of enhancing the out-coupling efficiency of OLEDs.

We investigated the optical properties of a dielec-tric−metal−dielectric multilayer for the transparent top cathode in top-emitting organic light emitting diodes (TOLEDs). The optical transmittance of the metal layer was enhanced by depositing a dielectric material which had a high refraction index n below and above the metal (Ag) layer. Due to multiple reflections and interferences, the Ag layer sandwiched between dielectric materials with a high value of n can show improved transmittance. Because the WO3 had a high value of n (>2.0), a thin WO3 layer could fulfill the optimum zero-reflection condition with an Ag metal layer. Thus, a WO3/Ag/WO3 multilayer should have high transmittance with a low sheet resistance. The optimum thicknesses of both Ag and WO3 to obtain the best transmittance value were determined by theoretical calculation, and they agreed well with the experimental results. The best results were obtained for the thermally evaporated WO3 (300 A)/Ag (120 A)/WO3 (300 A) structure, a high tran...

The enhancement of quantum efficiency using a suface modified Ag anode in top-emitting organic light emitting diodes (TEOLEDs) is reported. The operation voltage at the current density of 1 mA/cm 2 of TEOLEDs decreased from 9.3 V to 4.3 V as the surface of anode coated with CuOx layer. The work function of these structures were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra revealed that the work function of the Ag/CuOx structure is higher by 0.6 eV than that of Ag. Thus, the CuOx structure acts as a role in reducing the hole injection barrier by about 0.6 eV, resulting in a decrease of the turn-on voltage of top-emitting light emitting diodes.

We report enhanced hole injection using an RuOx layer between indium tin oxide anodes and 4,4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl in organic light emitting diodes (OLEDs). The operation voltage of OLEDs at a current density of 100 mA/cm2 decreased from 17 V to 14 V and the maximum luminance value increased from 120 cd/m2 to 2500 cd/m2 upon transformation of the Ru layer to RuOx by surface treatment using O2 plasma. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.4 eV as the Ru layer was transformed to RuOx. Thus, the hole injection energy barrier was lowered, reducing the turn-on voltage and increasing the quantum efficiency of the OLEDs.

We report the change of surface electronic structure of indium–tin–oxide (ITO) as a function of ultraviolet (UV)–ozone treatment time. The voltage of organic light emitting diodes at a current density of 100 mA/cm2 was reduced as the surface treatment time using UV–ozone was lengthened. X-ray photoelectron spectroscopy results showed that the relative concentration of carbon atoms decreased, but oxygen concentration increased relatively with UV–ozone treatment. This led to the increase in the ITO work function via the reduction of operation voltage.