Charge recombination in a poly(para-phenylene vinylene)-fullerene derivative composite film studied by transient, nonresonant, hole-burning spectroscopy

Transient, nonresonant, hole-burning spectroscopy was used to study the charge recombination process in poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1-4-phenylene vinylene] (MDMO-PPV):methanofullerene (PCBM) composite films. The position and intensity of the spectral hole in the absorption band of MDMO-PPV were monitored as a function of time in the 10 ns-10 ms time range. A time-dependent red shift was obsd. The intensity of the spectral hole decayed with time according to a power law (~t-a). The exponent a ~ 0.5 is nearly independent of the excitation fluence in the range 0.05-2 mJ/cm2. The depth of the spectral hole depends sublinearly on the excitation fluence (I) and can be described by (~G-b) with b.apprx.0.5. The time-dependent red shift and the power-law type time decay can be reproduced by numerical simulations. The Monte Carlo method is used to simulate the hopping dynamics of the photoinduced charges in a lattice of energetically disordered sites before they eventually recombine at the MDMO-PPV:PCBM interface. Charge sepn. is assisted by disorder and, in the 10 ns-10 ms time range, the recombination rate is limited by the detrapping of the cationic charge carriers in MDMO-PP

Trapping of electrons in metal oxide-polymer memory diodes in the initial stage of electroforming

Metal oxide-polymer diodes require electroforming before they act as nonvolatile resistive switching memory diodes. Here we investigate the early stages of the electroforming process in Al/Al2O3 /polyspirofluorene /Ba/Al diodes using quasistatic capacitance-voltage measurements. In the initial stage, electrons are injected into the polymer and then deeply trapped near the polyspirofluorene-Al2O3 interface. For bias voltages below 6 V, the number of trapped electrons is found to be CoxideV/q with Coxide as the geometrical capacitance of the oxide layer. This implies a density of traps for the electrons at the polymer-metal oxide interface larger than 31017 m−2

An assessment of heterosexuals\u27 perceived risk for HIV infection

Microalgae are a sustainable source of lipids. A commonly used strategy for lipid accumulation in microalgae is a two-step batch cultivation, with a growth phase followed by a nitrogen starvation phase. A problem with this process is the decrease in photosynthetic efficiency during the nitrogen starvation phase, which leads to low lipid productivities. In this research, a new process strategy was studied with the aim to improve lipid productivity of the microalgae Nannochloropsis gaditana. The nitrogen concentrations were chosen to assure consumption of most part of the nitrogen during the night. An improvement of the photosystem II maximum quantum yield and an increase in the dry weight and TAG concentration was achieved from day 7 of nitrogen starvation onwards when the culture was fed with nitrogen each night compared to a culture without nitrogen addition. Consequently, the time-average TAG yield on light was also higher after 7 days of nitrogen starvation. However, since the maximal time-averaged triacylglycerol (TAG) yield on light was reached after 3 days of nitrogen starvation, the improved photosynthetic activity did not lead to an increase of the maximal time-averaged TAG yield on light. The culture with nitrogen addition had a higher protein concentration (1.1 compared to 0.7 g L−1), showing that the added nitrogen was mainly used for protein production. A higher chlorophyll a content (2.0 compared to 0.8 μg mg−1) showed improved photosystem and that a small part of nitrogen was used for chlorophyll a. Small nightly nitrogen additions during batch cultivation of nitrogen starved N. gaditana did result in improvement in photosystem II maximal quantum yield, biomass concentration, TAG production and a higher time-averaged maximal TAG yield on light, after 7 days of nitrogen starvation.</p

Lithium fluoride injection layers can form quasi-Ohmic contacts for both holes and electrons

Thin LiF interlayers are typically used in organic light-emitting diodes to enhance the electron injection. Here, we show that the effective work function of a contact with a LiF interlayer can be either raised or lowered depending on the history of the applied bias. Formation of quasi-Ohmic contacts for both electrons and holes is demonstrated by electroluminescence from symmetric LiF/polymer/LiF diodes in both bias polarities. The origin of the dynamic switching is charging of electrically induced Frenkel defects. The current density-electroluminescence-voltage characteristics can qualitatively be explained. The interpretation is corroborated by unipolar memristive switching and by bias dependent reflection measurements.</p

Energy transfer in hybrid quantum dot light-emitting diodes

Energy transfer in a host-guest system consisting of a blue-emitting poly(2,7-spirofluorene) (PSF) donor and red-emitting CdSe/ZnS core shell quantum dots (QDs) as acceptor is investigated in solid films, using time-resolved optical spectroscopy, and in electroluminescent diodes. In the QD:PSF composite films, the Förster radius for energy transfer is found to be 4–6 nm. In electroluminescent devices lacking an electron transport layer, the electroluminescence (EL) spectrum of the QD:PSF polymer composite is similar to the photoluminescence (PL), giving evidence for energy transfer from PSF to the QDs. The addition of an electron transport layer between the emitting layer and the cathode results in a significant change in the EL spectrum and a considerable improved device performance, providing almost pure monochromatic emission at 630 nm with a luminance efficiency of 0.32 cd/A. The change in spectrum signifies that the electron transport layer changes the dominant pathway for QD emission from energy transfer from the polymer host to direct electron-hole recombination on the QDs

Relation between the electroforming voltage in alkali halide-polymer diodes and the bandgap of the alkali halide

Electroforming of indium-tin-oxide/alkali halide/poly(spirofluorene)/Ba/Al diodes has been investigated by bias dependent reflectivity measurements. The threshold voltages for electrocoloration and electroforming are independent of layer thickness and correlate with the bandgap of the alkali halide. We argue that the origin is voltage induced defect formation. Frenkel defect pairs are formed by electron-hole recombination in the alkali halide. This self-accelerating process mitigates injection barriers. The dynamic junction formation is compared to that of a light emitting electrochemical cell. A critical defect density for electroforming is 10(25)/m(3). The electroformed alkali halide layer can be considered as a highly doped semiconductor with metallic transport characteristics. (C) 2014 Author(s)