Identifying the Impact of Surface Recombination at Electrodes in Organic Solar Cells by Means of Electroluminescence and Modeling

This work reports on combining current‐voltage characteristics, electroluminescence (EL) measurements, and modeling to identify the selectivity of the electrodes in bulk‐heterojunction organic solar cells. Devices with the same photoactive layer but different contact materials are compared and the impact of surface recombination at the contacts on their performance is determined. The open‐circuit voltage, V _OC, depends strongly on the selectivity of the electrodes and it is observed that the EL signal of cells with lower V _OC is dramatically reduced. This is ascribed to an enhanced rate of surface recombination, which is a non‐radiative recombination pathway and does therefore not contribute to the EL yield. In addition, these cells have a lower current in forward direction despite the fact that the surface recombination occurs in addition to the recombination in the bulk. A theoretical model was set up and in the corresponding numerical simulations all three findings (lower V _OC, strongly reduced EL signal and lower forward current) could be clearly reproduced by varying just one single parameter which determines the selectivity of the electrode.     

Quantum‐Dot Sensitized Solar Cells Employing Hierarchical Cu2S Microspheres Wrapped by Reduced Graphene Oxide Nanosheets as Effective Counter Electrodes

Hierarchical Cu₂S microspheres wrapped by reduced graphene oxide (RGO) nanosheets are prepared via a one‐step solvothermal process. The amount of graphene oxide used in the synthesis process has a remarkable effect on the features of Cu₂S microspheres. Compared to Pt and Cu₂S electrodes, RGO‐Cu₂S electrodes show better electrocatalytic activity, greater stability, lower charge‐transfer resistance, and higher exchange current density. As expected, RGO‐Cu₂S electrodes exhibit superior performance when functioning as counter electrodes in CdS/CdSe quantum dot‐sensitized solar cells (QDSSCs) using a polysulfide electrolyte. A power conversion efficiency up to 3.85% is achieved for the QDSSC employing an optimized RGO‐Cu₂S counter electrode, which is higher than those of the QDSSCs featuring Pt (2.14%) and Cu₂S (3.39%) counter electrodes.     

Intracellular measurement of ammonium in Chara corallina using ion-selective microelectrodes

The study of ammonium (NH₄ ⁺) transport across plant cell membranes requires accurate measurement of NH₄ ⁺ gradients across subcellular gradients. We have developed an ammonium-selective microelectrode based on the ionophore nonactin. This electrode can detect NH₄ ⁺ activities (a_NH4) in vivo in the millimolar range in the presence of cytosolic levels of potassium, the main interfering ion. The electrode was used to measure intracellular a_NH4 in internodal cells of the giant alga Chara corallina. Results from cells incubated in media supplemented with 1 mM NH₄ ⁺ produced two populations, with means of 7.3 and 30.8 mM, respectively. HPLC analysis of vacuolar sap suggests the higher population represents vacuolar impalements, and the lower population can thus be assumed to be cytosolic. These results suggest a four-fold accumulation of NH₄ ⁺ in the vacuolar compartment of Chara. This revised version was published online in June 2006 with corrections to the Cover Date.     

V2O5 Nano‐Electrodes with High Power and Energy Densities for Thin Film Li‐Ion Batteries

Nanostructured V₂O₅ thin films have been prepared by means of cathodic deposition from an aqueous solution made from V₂O₅ and H₂O₂ directly on fluorine‐doped tin oxide coated (FTO) glasses followed by annealing at 500°C in air, and studied as film electrodes for lithium ion batteries. XPS results show that the as‐deposited films contained 15% V⁴⁺, however after annealing all the vanadium is oxidized to V⁵⁺. The crystallinity, surface morphology, and microstructures of the films have been investigated by means of XRD, SEM, and AFM. The V₂O₅ thin film electrodes show excellent electrochemical properties as cathodes for lithium ion intercalation: a high initial discharge capacity of 402 mA h g^?1 and 240 mA h g^?1 is retained after over 200 cycles with a discharging rate of 200 mA g^?1 (1.3 C). The specific energy density is calculated as 900 W h kg^?1 for the 1^st cycle and 723 W h kg^?1 for the 180^th cycle when the films are tested at 200 mA g^?1 (1.3 C). When discharge/charge is carried out at a high current density of 10.5 A g^?1 (70 C), the thin film electrodes retain a good discharge capacity of 120 mA h g^?1, and the specific power density is over 28 kW kg^?1.     

A Triboelectric Generator Based on Checker‐Like Interdigital Electrodes with a Sandwiched PET Thin Film for Harvesting Sliding Energy in All Directions

A triboelectric generator based on checker‐like interdigital electrodes (TEGC) with a sandwiched polyethylene terephthalate (PET) thin film that can convert translation kinetic energy in all directions to electricity is reported. The design of the sandwiched PET thin film can effectively avoid direct wear between metal electrodes and sliding panel. The mechanism of the TEGC is described in detail. The performance of the TEGC in different sliding directions is studied, indicating a maximum output power density of 1.9 W m^‐2 and open‐circuit voltage of 210 V achieved in the X or Y sliding direction. The TEGC is used to charge a 110 μF commercial capacitor to 5 V in 35 s and light up two light‐emitting diodes (LEDs) connected with the capacitor simultaneously. The TEGC based mouse pad and sliding panel are fabricated to harvest mouse operation energy to light up LEDs connected in antiparallel when the computer mouse operates a game. The TEGC has advantages of being flexible, light weight, durable, cost effective, and portable by folding or rolling into a small part. This work presents a significant progress toward the structure design of triboelectric generator for its practical applications.     

Effects of low electric current (LEC) treatment on pure bacterial cultures

AIMS: This research focused on the effects of low electric current (LEC) on the cell viability and metabolic activity of Escherichia coli and Bacillus cereus. METHODS AND RESULTS: Different LEC intensities at fixed amperage were applied, employing either graphite or copper electrode pairs, and the effects were determined by conventional cultural methods and bioindicators. On E. coli, the LEC with graphite electrodes at 5 and 10 mA led to no significant variation, but at 20 and 40 mA there was increasing inhibition of both the enzymatic activities and growth, and a reduction in ATP content. On B. cereus, similar experiments at the lower amperages did not have any inhibitor effects, however, the 40 mA current stimulated growth, ATP content and some enzymatic activities. The LEC treatment using copper electrodes caused, already at 5 mA, inhibition of bacterial growth and metabolic and enzymatic activities in both E. coli and B. cereus. CONCLUSIONS: On the basis of the obtained results using different amperages and electrodes, we can conclude that E. coli seem to be more sensitive compared with B. cereus. SIGNIFICANCE AND IMPACT OF THE STUDY: The study increases the knowledge on LEC treatment effects on the pure bacterial cultures.     

Asymmetric Rate Behavior of Si Anodes for Lithium‐Ion Batteries: Ultrafast De‐Lithiation versus Sluggish Lithiation at High Current Densities

The combined effect of lithium‐ion diffusion, potential‐concentration gradient, and stress plays a critical role in the rate capability and cycle life of Si‐based anodes of lithium‐ion batteries. In this work, Si nanofilm anodes are shown to exhibit an asymmetric rate performance: around 72% of the total available capacity can be delivered during de‐lithiation under a high current density of 420 A g^‐1 (100C where C is the charge‐rate) in 22 s; in striking contrast, only 1% capacity can be delivered during lithiation. A mathematical model of single‐ion diffusion is established to elucidate the asymmetric rate performance, which can be mainly attributed to the potential‐concentration profile associated with the active material and the ohmic voltage shift under high currents; the difference in chemical diffusion coefficients during lithiation and de‐lithiation also plays a role. This clarifies that the charge and discharge rates of lithium‐ion‐battery electrodes should be evaluated separately due to the asymmetric effect in the electrochemical system.