Piezoelectric and ferroelectric properties of [(K0.4725Na0.4725)Li0.055]NbO3–(Ag0.5Li0.5)TaO3 lead‐free ceramics

New lead‐free piezoelectric (1 – x)[(K_0.4725Na_0.4725)Li_0.055]NbO₃– x (Ag_0.5Li_0.5)TaO₃ [(1 – x)KNNL–x ALT] ceramics were prepared by conventional sintering. Piezoelectric and ferroelectric properties and Curie temperature of the ceramics were studied. The (1 – x)KNNL–x ALT (x = 0.04) ceramics exhibit good properties (d₃₃ ～ 252 pC/N, k_p ～ 41%, T_C ～ 471 °C, T_o–t = 47 °C, P_r = 33.1 μC/cm², E_c = 10.6 kV/cm). These results show that (1 – x)KNNL–x ALT (x = 0.04) ceramic is a promising lead‐free piezoelectric material for high temperature application. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fast‐response organic–inorganic hybrid light‐emitting diode

We demonstrated important changes produced on the modulation frequency of hybrid organic–inorganic light‐emitting diodes to examine the applicability as a light source for visible optical communications. The fabricated device structure was 4,4′‐bis[N ‐(1‐napthyl)‐N ‐phenyl‐amino]biphenyl/4,4′‐(bis(9‐ethyl‐3‐carbazovinylene)‐1,1′‐biphenyl:4,4′‐bis[9‐dicarbazolyl]‐2,2′‐biphenyl/ZnS/LiF/MgAg. This device showed an improvement in the modulation frequency using ZnS instead of an organic material, tris(8‐hydroxyquinoline)aluminum. A maximum cutoff frequency of 20.6 MHz was achieved.

     

Microcrystalline silicon‐carbon alloys as anti‐reflection window layers in high efficiency thin film silicon solar cells

Microcrystalline silicon‐carbide (μc‐SiC:H) films were prepared using hot wire chemical vapor deposition at low substrate temperature. The μc‐SiC:H films were employed as window layers in microcrystalline silicon (μc‐Si:H) solar cells. The short‐circuit current density (J_SC) in these n‐side illuminated n–i–p cells increases with increasing the deposition time t_W of the μc‐SiC:H window layer from 5 min to 60 min. The enhanced J_SC is attributed to both the high transparency and an anti‐reflection effect of the μc‐SiC:H window layer. Using these favourable optical properties of the μc‐SiC:H window layer in μc‐Si:H solar cells, a J_SC value of 23.8 mA/cm² and cell efficiencies above 8.0% were achieved with an absorber layer thickness of 1 μm and a Ag back reflector. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microstructure,dielectric, and piezoelectric properties of 0.38Bi(Gax Sc1–x )O3–0.62PbTiO3 high temperature piezoelectric ceramics

0.38Bi(Ga_x Sc_1–x )O₃–0.62PbTiO₃ (BGSPTx) ceramics have been prepared by using the conventional mixed oxide method. X‐ray diffraction analysis revealed that BGSPTx has a pure perovskite structure, and the crystal symmetry of BGSPTx changed from rhombohedral to tetragonal with increasing Ga content (x). The Curie temperature (T_C) of BGSPTx ceramics is in the range of 448–467 °C for different x. The ferroelectric phase transition of BGSPTx was found to be of the first order type according to the Curie–Weiss law. For x = 0.125, BGSPTx ceramics show enhanced piezoelectric properties: piezoelectric constant d₃₃ = 420 pC/N and d₃₁ = –142 pC/N, planar and thickness electromechanical coupling factors k_p = 56.27% and k_t = 56.00%, respectively. The high‐T_C of BGSPTx coupled with its excellent piezoelectric properties suggests those future high‐temperature applications. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Plasmon enhanced light absorption in bulk heterojunction organic solar cells

Silver nanospheres (Ag NSs) buffer layers were introduced via a solution casting process to enhance the light absorption in poly (3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM) bulk heterojunction organic solar cells. These Ag NSs, as surface plasmons, could increase the optical electric field in the photoactive layer whilst simultaneously improving the light scattering. As a result, this buffer layer improves the light absorption of P3HT:PCBM blend and consequently improves the external quantum efficiency (EQE) of organic solar cells. In this work, different sizes of Ag NSs plasmon‐enhanced layer were investigated, with the aim of optimizing the performance of devices. UV‐vis spectrometer measurement demonstrates that the total optical absorption of P3HT:PCBM blend films in the spectral range of 350–650 nm is increased by ～4 and 6% with incorporation of the 20 and 40 nm Ag NSs, respectively. The J_sc was shown to increase by ～21 and 24% for 20 and 40 nm Ag NSs, respectively. This is due to the extra photogenerated excitons by the plasmonic resonance of Ag NSs. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoinduced C70 radical anions in polymer:fullerene blends

Photoinduced polarons in solid films of polymer:fullerene blends were studied by photoluminescence (PL), photoinduced absorption (PIA) and electron spin resonance (ESR). The donor materials used were P3HT and MEH‐PPV. As acceptors we employed PC₆₀BM as reference and various soluble C₇₀‐derivates: PC₇₀BM, two different diphenylmethano‐[70]fullerene oligoether (C₇₀‐DPM‐OE) and two dimers, C₇₀–C₇₀ and C₆₀–C₇₀. Blend films containing C₇₀ revealed characteristic spectroscopic signatures not seen with C₆₀. Light‐induced ESR showed signals at g ≥ 2.005, assigned to an electron localized on the C₇₀ cage. The formation of C₇₀ radical anions also leads to a subgap PIA band at 0.92 eV, hidden in the spectra of C₇₀‐based P3HT and MEH‐PPV blends, which allows for more exact studies of charge separated states in conjugated polymer:C₇₀ blends. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Er‐ and Eu‐doped GaP‐oxide porous composites for optoelectronic applications

We demonstrate the controlled preparation of Er‐ and Eu‐doped GaP‐oxide porous composites. The fabrication procedure entails the use of porous semiconductor templates and the impregnation of rare earth ions from a rare earth salt solution in alcohol and thermal treatment. The composites exhibit strong green and red emission that comes from finely dispersed ErPO₄ and EuPO₄ oxide submicron phases in the composite. These materials may prove useful in future generations of optoelectronic and photonic devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Room‐temperature epitaxial growth of high‐quality m ‐plane InGaN films on ZnO substrates

The authors have grown high‐quality m ‐plane In_0.36Ga_0.64N (1 00) films on ZnO (1 00) substrates at room temperature (RT) by pulsed laser deposition (PLD) and have investigated their structural properties. m ‐plane InGaN films grown on ZnO substrates at RT possess atomically flat surfaces with stepped and terraced structures, indicating that the film growth proceeds in a two‐dimensional mode. X‐ray diffraction measurements have revealed that the m ‐plane InGaN films grow without phase separation reactions at RT. The full‐width at half‐maximum values of the 1 00 X‐ray rocking curves of films with X‐ray incident azimuths perpendicular to the c ‐ and a‐axis are 88 arcsec and 78 arcsec, respectively. Reciprocal space‐mapping has revealed that a 50 nm thick m ‐plane In_0.36Ga_0.64N film grows coherently on the ZnO substrate, which can probably explain the low defect density that is observed in the film. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

HfO2/AlGaN/GaN structures with HfO2 deposited at ultra low pressure using an e‐beam

We show that HfO₂/AlGaN/GaN structures with HfO₂ layer deposited using an e‐beam in ultra high vacuum are suitable for field effect transistors. The dielectric constant of the HfO₂ was found ε_HfO > 23–24, which is close to the highest re‐ ported values for this material. The leakage current did not exceed 10^–4 A/cm² at the threshold voltage. The comparison of the losses in the samples with and without HfO₂ indicates low concentration of the interface traps. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Field-effect transistor structures on the basis of poly(3-hexylthiophene), fullerene derivatives [60]PCBM, [70]PCBM,and nickel nanoparticles

Organic field-effect transistor (OFET) structures with the active layers on the basis of composite films of semiconductor polymer poly(3-hexylthiophene) (P3HT), fullerene derivatives [60]PCBM, [70]PCBM, and nickel (Ni) nanoparticles are obtained, and their optical, electrical, and photoelectrical properties are studied. It is shown that introducing Ni nanoparticles into P3HT: [60]PCBM and P3HT: [70]PCBM films leads to an increase in the absorption and to quenching of photoluminescence of the composite in the 400–600 nm spectral band due to the plasmon effect. In P3HT: [60]PCBM: Ni and P3HT: [70]PCBM: Ni OFET structures at the P3HT: [60]PCBM and P3HT: [70]PCBM concentrations of ~1: 1 and Ni concentrations of ~3–5 wt %, current–voltage (I–V) characteristics typical of ambipolar OFETs with the dominant hole conduction are observed. The charge-carrier (hole) mobilities calculated from the I–V characteristic at V_G =–10 V were found to be ~0.46 cm²/(V s) for P3HT: [60]PCBM: Ni and ~4.7 cm²/(V s) for P3HT: [70]PCBM: Ni, which means that the mobility increases if [60]PCBM in the composition is replaced with [70]PCBM. The effect of light on the I–V characteristics of P3HT: [60]PCBM: Ni and P3HT: [70]PCBM: Ni OFETs is studied.     

Mobility Enhancement of P3HT‐Based OTFTs upon Blending with Au Nanorods

The mobility enhancement of organic thin‐film transistors based on poly(3‐hexylthiophene) (P3HT) by incorporating gold nanorods (Au NRs) is reported. Through varying the doping concentration and surface modifier of the Au NRs in P3HT matrix, the P3HT/Au composite with 0.5 mg mL^?1 pyridine‐capped Au NRs exhibits a hole mobility of 0.059 cm² V^?1 s^?1, this value is seven times higher than that of pristine P3HT. This remarkable improvement of mobility originates from the enhanced crystallinity and optimized orientation of P3HT after doping with Au NRs. In addition, the appropriate surface modification can produce more‐efficient hole conduction of Au NRs.     

Homoepitaxy of ZnO by pulsed‐laser deposition

ZnO thin films were grown homoepitaxially on O‐face ZnO single crystals by pulsed‐laser deposition. The ZnO substrates grown by the hydrothermal method were heat‐treated in oxygen ambient at 1000 °C for 2 h prior to deposition. After the thermal treatment the substrates show bilayer steps between 200–400 nm wide terraces and a considerably improved crystalline structure. Thin film surfaces exhibit closed loop spirals and show steps of c /2 or c. The FWHM of the (0002) rocking curve of the best sample is 29″. Similar to the substrates used, Al is contained in the thin films (<10¹⁴ cm^–3) as photoluminescence (PL) and thermal admittance spectroscopy suggest. However, deep levels between 200 and 400 meV below the conduction band are the dominant donors at room temperature. Low temperature PL is dominated by (Al⁰,X) (I6, FWHM: 200 µeV) and extremely homogeneous (σ ≈ 1%).

     

Effect of solvent evaporation on the self‐assembly of poly(3‐hexylthiophene‐2,5‐diyl) and on the film morphology during electrospray deposition

A high‐voltage rectangular pulse was applied to the electro‐spray deposition (ESD) to control the evaporation‐induced self‐assembly of poly(3‐hexylthiophene‐2,5‐diyl) (P3HT). Two groups of P3HT thin films were deposited by a series of high‐voltage rectangular pulses. Compared with the ESD driven by a constant voltage, the pulse‐driven ESD enables to probe the effect of solvent evaporation on the self‐assembly of P3HT molecules. The droplet size and the evaporation of residual solvent in the droplet determine the film morphology. Ultraviolet–visible absorption spectroscopy was used to identify the ordering of P3HT molecules in the films. The self‐assembly of P3HT molecules took place during the solvent evaporation which can be controlled by a combination of the pulse amplitude and the pulse interval. With an appropriate combination of the amplitude and the interval, the ESD produced a P3HT thin film with high chain ordering.

     

Electron transport with mobility above 10–3 cm2/Vs in amorphous film of co‐planar bipyridyl‐substituted oxadiazole

We demonstrate that a bipyridyl substituted oxadiazole (Bpy‐OXD) shows high electron mobility that reached above 10^–3 cm²/Vs. We believe that the high mobility results from both the hybrid molecular structure of the two electron‐accepting units: bipyridyl and oxadiazole, and the planar molecular structure based on its lack of sterically hindered bulky substituent. The computational analysis elucidates that the amorphous nature of Bpy‐OXD in thin‐film state probably results from the polymorphic effect in isolated state and the volume effect in solid state. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and application of the solar cell of poly(3-hexylthiophene)/Fe N/titanium dioxide nanocomposite material

A series of nanocomposites of poly(3-hexylthiophene) with Fe N-doped TiO₂ (P3HT/Fe N/TiO₂) were synthesized by the chemical method in situ. The structure of the prepared composites was characterized using X-ray diffraction patterns (XRD), infrared spectroscopy (IR), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Optical and electrochemical properties were determined using UV-vis spectroscopy, fluorescence spectroscopy, and cyclic voltammetry. These tests indicated that P3HT/Fe N/TiO₂ is a new p-n semiconductor. Two solar cells based on P3HT/Fe N/TiO₂ were manufactured and studied.     

Polaron and ion diffusion in a poly(3-hexylthiophene) thin-film transistor gated with polymer electrolyte dielectric

Electrolytes are finding applications as dielectric materials in low-voltage organic thin-film transistors (OTFT). The presence of mobile ions in these materials (polymer electrolytes or ion gels) gives rise to very high capacitance (>10 μF/cm²) and thus low transistor turn-on voltage. In order to establish fundamental limits in switching speeds of electrolyte gated OFETs, we carry out in situ optical spectroscopy measurement of a poly(3-hexylthiophene) (P3HT) OTFT gated with a LiClO₄:poly(ethyleneoxide) (PEO) dielectric. Based on spectroscopic signatures of molecular vibrations and polaron transitions, we quantitatively determine charge carrier concentration and diffusion constants. We find two distinctively different regions: at V _G≥−1.5 V, drift-diffusion (parallel to the semiconductor/dielectric interface) of hole-polarons in P3HT controls charging of the device; at V _G<−1.5 V, electrochemical doping of the entire P3HT film occurs and charging is controlled by drift/diffusion (perpendicular to the interface) of ClO₄ ⁻ counter ions into the polymer semiconductor.     

Raman spectroscopic identification of fullerene inclusions in polymer/fullerene blends

Thin films of the conjugated polymer poly(3‐hexylthiophene) (P3HT) and blends of the soluble fullerene derivative[6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) with P3HT—a well studied but not completely understood donor–acceptor system for organic solar cells—have been studied by means of UV–visible absorption and resonant Raman spectroscopy. Additionally, we have employed atomic force microscopy phase imaging to characterize the nanomorphology of the P3HT : PCBM thin film, revealing a close intermixing of two phases with domain sizes ranging from a few to several tens of nanometers. A systematic analysis of pristine polymer and blend Raman spectra provides evidence that features attributable to PCBM, possibly even depending on the charge state of the fullerene molecule, can be observed. Hence our results suggest that fullerene inclusions in polymer/fullerene blends can be identified via Raman spectroscopy. Copyright © 2011 John Wiley & Sons, Ltd.     

Dielectric behavior of TiC–PVDF nanocomposites

TiC/PVDF nanocomposite is prepared via simple blending and hot pressing route. Percolation theory was employed to explain the dielectric behavior of the TiC/PVDF composites. The dependence of the dielectric properties of the composite on both volume fraction of the filler and frequency is investigated. High dielectric constant (? = 540) and low loss (tan δ = 0.48) of the composites at 100 Hz have been observed near the percolation threshold (0.12). The composites show a weak frequency dependence towards the high frequency range (10–100 kHz), regardless of the TiC content. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In‐situ neutron reflectometry probing competitive swelling and de‐swelling of thin polystyrene films

The competitive penetration of toluene with respect to oxygen, nitrogen and carbon dioxide into a thin and initially dry film of deuterated polystyrene (dPS) is compared to the reverse process of de‐swelling. In‐situ neutron reflectometry is used to follow the swelling and de‐swelling kinetics, allowing the determination of the total dPS thickness, roughness and solvent penetration. Swelling cannot be described in terms of case II diffusion due to the small film thickness and undersaturation results in a small swelling rate of 0.03 nm s^–1. De‐swelling is slower by more than two orders in magnitude, thereby underlining the difficulty to remove once incorporated solvent molecules. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)