Effects of incorporating salts with various alkyl chain lengths on carrier balance of light-emitting electrochemical cells

Recently, solid-state light-emitting electrochemical cells (LECs) have attracted much attention since they have advantages such as low operation voltages, simple device structure and balanced carrier injection. Salts are commonly added in the emissive layer of LECs to provide additional mobile ions and thus to accelerate device response. However, in addition to modified ionic property, carrier balance of LECs would also be tailored by salt additives. In this work, we improve device efficiency of LECs by incorporating imidazole-based salts bearing various alkyl chain lengths. As the alkyl chain length of the added salt increases, the device current decreases and the recombination zone approaches the anode. These results reveal that hole transport in the emissive layer of LEC containing a salt with a larger size would be impeded more significantly than electron transport. When doped with a salt possessing a proper size, nearly doubled device efficiency as compared to that of the neat-film device can be obtained due to improved carrier balance. This work demonstrates a feasible strategy to improve device performance of LECs and clarifies the physical insights of the effect of salt size on carrier balance of LECs.     

White light-emitting devices based on star-shape like polymers with diarylmaleimde fluorophores on the side chain of polyfluorene arms

A series of novel star-shape like white polymers were synthesized by employing the benzene as the core, and incorporating different amount of bis (4-methoxyphenyl) maleimide guest onto the side-chain of the polyfluorene (PF) arm host. Due to the large feed ratio of core, such a construction strategy could efficiently avoid or suppress the formation of linear PF which always existed in the previous star-like polymers employing guest as core. When applied to the single emitting layer devices fabricated by solution spin-coating method, the efficiency of the devices enhanced gradually with the amount of guest increasing from 0.01% to 2%. The device performance was further improved substantially by annealing at 140 °C. A typical device base on P1 containing 1% dopant exhibited maximal luminous efficiency of 6.13 cd/A. Interestingly, the increase of efficiency is not originated from the self-dopant effect of α or β crystalline phase, which was always produced by thermal treatment at 120 °C in linear or previous star-like polyfluorenes. Morphology analysis disclosed that annealing made the polymeric molecules stack become closer, consequently resulting in a more effectively charge transfer and energy transfer from host to guest. As a result, the compact molecular stacking of the star-shape like polymers should be responsive for the high efficiency of annealed devices. The results would be conducive to design new star-shape like polymers with more arms, and improve the performance of white devices.     

Effect of doping with nitrogen on electrical properties and erbium electroluminescence of a-Si:H(Er) films

The effect of nitrogen doping on the electrical and electroluminescence properties of amorphous hydrogenated silicon films doped with erbium has been studied. The parameters of the material, the characteristics of structures on its base, and the efficiency of Er electroluminescence (λ=1.54 μm) are determined by the excess of the nitrogen doping level over the background value depending on the Er concentration. It is shown that effectively luminescing structures can be obtained by reducing the background concentration, with nitrogen doping remaining at the level of ∼10²¹ cm⁻³. A possible mechanism is proposed, accounting for this effect in terms of two possible forms of nitrogen incorporation into an Er-doped a-Si:H structure: with either an Er-N complex or a Na ₄ ⁺ -Si ₃ ⁻ charged defect pair formed. In this case, the electroluminescence efficiency is determined by the number of these pairs. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 10, 2001, pp. 1250–1255. Original Russian Text Copyright ? 2001 by Kon’kov, Terukov, Granitsyna.     

In situ deposition of polythiophene nanoparticles on flexible transparent films: Effect of the process conditions

Polythiophene (PTh) was deposited as a thin film and nanoparticles on polyethylene naphthalate (PEN) films via ultrasonication or via magnetic stirring with or without a cationic (cetyltrimethylammonium bromide, CTAB) or non-ionic (Triton X-100) surfactant. The resulting conductive flexible films were characterized by UV-Vis spectroscopy, fluorescence spectrometry, field-emission scanning electron microscopy, contact angle measurements, the four-point-probe technique, X-ray diffraction, and cyclic voltammetry. The highest conductivity was obtained for PTh-CTAB nanoparticles, which had the most compact surface morphology among nanoparticles on PEN films. In general, the surface morphology, electrical conductivity, and whether PTh nanoparticles had been precipitated or deposited depended on the surfactant used, the surfactant/monomer ratio, the oxidant/monomer ratio, and the monomer concentration.     

Effect of a novel self-assembly based on coordination polymer with zinc porphyrin in supramolecular solar cells

Within this work, we firstly report the self-assemblies of zinc porphyrin coordination polymers (CPs) appended isonicotinic acid ligands by metal–ligand axial coordination approach immobilized on the nanostructured TiO₂ electrode surfaces in photoelectrochemical devices. Compared to the assemblies based on zinc porphyrins integrated isonicotinic acid ligands via metal–ligand axial coordination or metal–ligand edged binding approach, the CPs-based assemblies exhibit significantly improved photovoltaic performances. Especially, the assembly based on iminazole-substituted zinc porphyrin coordination polymer exhibits an excellent photovoltaic performance with a short circuit photocurrent density (J_sc) of 3.8 mA cm⁻², an open circuit voltage (V_oc) of 0.31 V, a fill factor (FF) of 0.67 and an overall conversion efficiency (η) of 0.48% under AM 1.5 conditions. The results serve as another good testing ground for the fabrication of supramolecular devices techniques in future.     

Conjugated Polymer Based on Polycyclic Aromatics for Bulk Heterojunction Organic Solar Cells: A Case Study of Quadrathienonaphthalene Polymers with 2% Efficiency

Polycyclic aromatics offer great flexibility in tuning the energy levels and bandgaps of resulting conjugated polymers. These features have been exploited in the recent examples of benzo[2,1‐b:3,4‐b']dithiophene ( BDT )‐based polymers for bulk heterojunction (BHJ) photovoltaics (ACS Appl. Mater. Interfaces 2009 , 1, 1613). Taking one step further, a simple oxidative photocyclization is used here to convert the BDT with two pendent thiophene units into an enlarged planar polycyclic aromatic ring— q uadra t hieno n aphthalene ( QTN ). The reduced steric hindrance and more planar structure promotes the intermolecular interaction of QTN‐ based polymers, leading to increased hole mobility in related polymers. As‐synthesized homopolymer ( HMPQTN ) and donor–acceptor polymer ( PQTN ‐ BT ) maintain a low highest occupied molecular orbital (HOMO) energy level, ascribable to the polycyclic aromatic ( QTN ) moiety, which leads to a good open‐circuit voltage in BHJ devices of these polymers blended with PCBM ([6,6]‐phenyl‐C₆₁‐butyric acid methyl ester; HMPQTN : 0.76 V, PQTN ‐ BT : 0.72 V). The donor–acceptor polymer ( PQTN ‐ BT ) has a smaller optical bandgap (1.6 eV) than that of HMPQTN (2.0 eV), which explains its current (5.69 mA cm^?2) being slightly higher than that of HMPQTN (5.02 mA cm^?2). Overall efficiencies over 2% are achieved for BHJ devices fabricated from either polymer with PCBM as the acceptor.     

Effect of deposition conditions on charging processes in SiNx: Application to RF-MEMS capacitive switches

The paper presents a systematic investigation of dielectric charging in low temperature silicon nitride for RF-MEMS capacitive switches. The dielectric charging is investigated with the aid of Metal-Insulator-Metal (MIM) capacitors with different thickness dielectric film and symmetric and asymmetric metal contacts. The experimental results demonstrate that the charging process is almost symmetric in low temperature deposited silicon nitride. Experiments performed in both MIM and MEMS reveal that the charging process is strongly affected by temperature. Specifically at high temperatures the charging rate increases exponentially with temperature.     

Effect of electronic flame off parameters on copper bonding wire: Free-air ball deformability, heat affected zone length, heat affected zone breaking force

Cu bonding wire is more and more used for interconnections to integrated circuits (ICs) to reduce cost and increase performance compared to Au wire. To eliminate underpad damage for Cu wire applications, it is worthwhile to reduce the hardness of the free-air ball (FAB). Short heat affected zone (HAZ) and high HAZ breaking load are often required for advanced microelectronics packaging in order to decrease the loop height and thereby the package thickness.Online measurements of deformability and HAZ breaking force at temperatures close to the bonding temperature of 220 °C are new tools used in this study to evaluate the effects of electronic flame off (EFO) current and firing time on the Cu FAB deformability and the HAZ length and tensile strength. FABs with 50 μm diameter formed from a 25 μm diameter Cu wire with a breaking load of 118.6 mN were used. EFO currents and firing times ranged from 40 to 250 mA and 0.11 to 0.90 ms, respectively. Average FAB deformability factors, HAZ breaking forces, and HAZ lengths were in the rounded ranges of 36.64–44.09% (with a deformation force of 0.60 N), 107.7–116.8 mN, and 167–215 μm, respectively. When produced with 250 mA current during 0.11 ms, the FABs are 7.01–7.89% more deformable than when produced with 45 mA during 0.9 ms, the HAZ breaking force is 7.53–9.37% higher, and the HAZ length is 7–90 μm shorter.