Decrease In Quantum Efficiency Research Articles

Photocurrent Spectra and Fast Kinetic Studies of P3HT/PCBM Mixed with a Dye for Photoconversion in the Near-IR Region

Photoconversion properties are demonstrated for a device based on a small dye molecule, absorbing light in the near-IR region, mixed with two organic charge transport materials and together forming a dye-sensitized organic bulk heterojunction. The organic dye molecule, phthalocyanine (1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine), mixed with a blend of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C-61 (PCBM), shows a photoconversion spectrum extended more than 150 nm toward longer wavelengths, as compared to a device without such dye sensitization. In the dye-sensitized region of the photoconversion spectrum the maximum infernal quantum efficiency was estimated to 40%. With higher dye concentrations the internal quantum efficiency decreases. Transient laser spectroscopy measurements show that after excitation of the dye there is an electron transfer from the dye to PCBM and a subsequent hole transfer from the dye to P3HT, which results in a long-lived (P3HT(+)/dye/PCBM-) charge-separated state.

Improved Durability of Eu Chelate Encapsulated by Silica Glass Film Prepared by Sol-Gel Method

We demonstrated an improved durability of Eu(HFA)3(TPPO)2 [tris(hexafluoroacetylacetonato)- europium(III) 1,2-phenylenebis(diphenylphosphine oxide)] encapsulated by a sol-gel derived silica glass film. The film was constructed using phenyltrimethoxysilane and diethoxy- dimethylsilane as a starting solution. Unlike cases in which conventional tetramethoxysilane is used, no decrease in internal quantum efficiency was observed at room temperature. We further improved the thermal and long-term stability of Eu(HFA)3(TPPO)2 inside the transparent glass film at elevated annealing temperatures when compared to the Eu(HFA)3(TPPO)2 powder itself: The photoluminescence quantum efficiency of the transparent film was 60% at 160℃ and 32% at 20℃. Properly prepared glass networks protect the Eu chelate from free oxygen and water in the transparent glass film, which is promising for future practical applications.

Electron sources utilizing thin CsBr coatings

We present experimental results obtained in solid Cu targets coated with ∼18 nm thick CsBr films operating in a reflection mode. The results indicate a factor of 50X increase in quantum efficiency relative to uncoated Cu samples. The CsBr/Cu samples are very robust allowing brief exposures to air without a substantial decrease in quantum efficiency and lifetime. The results are very encouraging from the point of view of reliable high efficiency electron sources for lithography or other applications including free electron lasers.

Novel iridium complexes as high-efficiency yellow and red phosphorescent light emitters for organic light-emitting diodes

A series of novel biscyclometallated iridium complexes based on spirobifluorene ligands and acetyl acetonate (acac) ancillary ligands have been synthesized and characterized. Their electrochemical properties were investigated by cyclic voltammetry (CV). HOMO, LUMO, and energy band gaps of all the complexes were calculated by the combination of UV–vis absorption spectra and CV results. TGA and DSC results indicated their excellent thermal stability and amorphous structure. All the iridium complexes were fabricated into organic light-emitting devices with the device configuration of ITO/PEDOT:PSS (50 nm)/PVK (50 wt %):PBD (40 wt %):Ir complex (10 wt %) (45 nm)/TPBI (40 nm)/LiF (0.5 nm)/Ca (20 nm)/Ag (150 nm). Yellow to red light emission has been achieved from the iridium complexes guest materials. Complex C1 (yellow light emission) achieved an efficiency of 36.4 cd/A (10.1%) at 198 cd/m 2 and complex C4 (red light emission) reached external quantum efficiency of 4.6%. The slight decrease of external quantum efficiency at high current density revealed that the triplet–triplet (T 1–T 1) annihilation was effectively suppressed by the new developed complexes.

Temperature dependence and physical properties of Ga(NAsP)/GaP semiconductor lasers

We report on the properties of GaNAsP/GaP lasers which offer a potential route to producing lasers monolithically on silicon. Lasing has been observed over a wide temperature range with pulsed threshold current density of 2.5 kA/cm2 at 80 K (λ=890 nm). Temperature dependence measurements show that the radiative component of the threshold is relatively temperature stable while the overall threshold current is temperature sensitive. A sublinear variation of spontaneous emission versus current coupled with a decrease in external quantum efficiency with increasing temperature and an increase in threshold current with hydrostatic pressure implies that a carrier leakage path is the dominant carrier recombination mechanism.

Changes in photosynthetic capacity and antioxidant enzymatic systems in micropropagated Zingiber officinale plantlets during their acclimation

Ginger (Zingiber officinale Rosc.) plantlets were propagated in vitro and acclimated under different photosynthetic photon flux densities (60 and 250 µmol m-2 s-1 = LI and HI, respectively). Increases in chlorophyll (Chl) content and Chl a/b ratio were found under both irradiances. In vitro plantlets (day 0) exhibited a low photosynthesis, but chloroplasts from in vitro leaves contained well developed grana and osmiophillic globules. Photoinhibition in leaves formed in vitro was characterized by decrease of photochemical efficiency and quantum efficiency of photosystem 2 photochemistry in HI treatment during acclimation. The new leaves formed during acclimation in both treatments showed a higher photosynthetic capacity than the leaves formed in vitro. Also activities of antioxidant enzymes of micropropagated ginger plantlets changed during acclimation.

Photoassembly of the Manganese Cluster in Mutants Perturbed in the High Affinity Mn-Binding Site of the H2O-Oxidation Complex of Photosystem II

The light-driven, oxidative assembly of Mn2+ ions into the H2O-oxidation complex (WOC) of the photosystem II (PSII) reaction center is termed photoactivation and culminates in the formation of the oxygen-evolving (Mn4-Ca) center of the WOC. Initial binding and photooxidation of Mn2+ to the apoprotein is critically dependent upon aspartate 170 of the D1 protein (D1-D170) of the high affinity Mn site [Nixon and Diner (1992) Biochemistry 31, 942-948]. Three O2-evolving mutant strains of Synechocystis, D1-D170E, D1-D170H, and D1-D170V, were studied in terms of the kinetics of photoactivation under both continuous and flashing light. Photoactivation using single turnover flashes revealed D1-D170H and D1-D170V, but not D1-D170E, were prone to form substantial amounts ( approximately 40-50%) of inactive centers ascribed to photoligation of aberrant nonfunctional Mn based upon the reversibility of the inactivation and similarity to previous in vitro results [Chen, C., Kazimir, J., and Cheniae, G. M. (1995) Biochemistry 34, 13511-13526]. On the other hand, D1-D170E lowers the quantum efficiency of photoactivation compared to the wild-type by the largest amount (80% decrease) versus D1-D170H and D1-D170V, which do not produce measurable decreases in quantum efficiency. The low quantum efficiency of photoactivation in D1-D170E is due to the destabilization of photoactivation intermediates. Numerical analysis indicates that the PSII centers in D1-D170E are heterogeneous with respect to photoactivation kinetics and that the majority of centers are characterized by intermediates that decay approximately 10-fold more rapidly than the wild-type control. Additionally, the kinetics of O2 release during the S3-S0 transition was markedly retarded in D1-D170E, in contrast to D1-D170H and D1-D170V, which did not exhibit a discernible slow-down compared to the wild-type.

Detector comparison for sulfur and chlorine detection with laser induced breakdown spectroscopy in the near-infrared-region

Laser-induced breakdown spectroscopy has been employed for the investigation of the sulfur and chlorine content of building materials. Both, chloride and sulfate ions are major damaging species affecting the stability and lifetime of a structure. Chlorine and sulfur are mostly detected in the VUV and the NIR. In case of building materials the main elements like calcium or iron have many strong spectral lines over the whole spectral range, so that trace elements can only be detected in spectral windows unaffected from these lines. With regard to a preferably simply, robust against dust and vibrations and portable setup only the NIR spectral features are used for civil engineering applications. Most detectors, mainly CCD cameras have rapidly decreasing quantum efficiency in the NIR. Also the quantum efficiency of the photocathode of CCD-Detectors with image intensifier is decreasing in the NIR. Different CCD-detectors were tested with respect to high quantum efficiency and high dynamic range, which is necessary for simultaneous detection of weak spectral lines from trace elements and intense spectral lines from main elements. The measurements are made on reference samples consisting of cement, hydrated cement, cement mortar and concrete with well-defined amounts of the trace elements. Experimental conditions are chosen for an optimum intensity of the trace element spectral lines. The detector systems are compared by limit of detections and the signal to noise ratio.

Effects of copper and cadmium on photosynthesis in cucumber cotyledons

The effects of 20 and 50 µM concentrations of Cu and Cd on photosynthesis in cucumber (Cucumis sativus L.) cotyledons were studied by the measurements of gas exchange characteristics, chlorophyll (Chl) fluorescence parameters, photosynthetic pigment contents, and two Calvin cycle enzymes activities: glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and 3-phosphoglyceric acid kinase (PGK). To minimize indirect metal action, seedlings were treated with metals in the stage of green, fully developed cotyledons. The metals reached the cotyledon tissue after 48 h of treatments, though symptoms of metal action were not visible at that time. The effect of metals on the light phase of the photosynthesis parameters such as potential efficiency of photosystem 2 (PS2; Fv/Fm), and photochemical and nonphotochemical quenching of Chl fluorescence (qP and qNP) was negligible. In contrast, a decrease of PS2 quantum efficiency (ΦPS2) was much more noticeable. Changes in the pigment contents were slight, as only 50 µM Cd decreased Chl a and b contents in small extent. On the contrary, metals in both concentrations drastically decreased (50 and more % of control) the net photosynthetic rate and the stomatal conductance, but not the internal CO2 concentration. The activities of both GAPDH and PGK were also decreased by metals, although the effect on PGK was more prominent, particularly on its potential activity (dithiothreitol in extraction and incubation media). Hence Cu and Cd affected the synthesis of enzyme proteins rather than they influenced their modifications. The effects of both metals on most of the measured photosynthesis parameters were similar, but the accumulation of Cd in the cotyledons was significantly higher than Cu accumulation. Thus Cu was more toxic for the photosynthesis of cucumber cotyledons than Cd.

Structure–property relationships in a new series of two-photon absorbing D–π–D divinyl-biphenyl derivatives

The study on the structure–property relationships of divinyl-biphenyl derivatives demonstrates the following results: Changing the charge transfer dipoles only by substituting the terminal donor group, the two-photon absorption cross-sections of these molecules range from 9.1 to 745 GM, while the emission peak remains in blue region and the quantum yield vary slightly. As the intra-molecular charge transfer dipole larger than a value the symmetrical π-system will rotate around the center single bond, and it will decrease their complanarity of π-conjugated plane, which limits the further enhancement of two-photon absorption cross-section, together with the quantum efficiency decrease.

Quantum efficiency and formation of the emission line in light-emitting diodes based on InGaN/GaN quantum well structures

The spectra of electroluminescence, photoluminescence, and photocurrent for the In0.2Ga0.8N/GaN quantum-well structures are studied to clarify the causes for the reduction in quantum efficiency with increasing forward current. It is established that the quantum efficiency decreases as the emitting photon energy approaches the mobility edge in the In0.2Ga0.8N layer. The mobility edge determined from the photocurrent spectra is E me = 2.89 eV. At the photon energies hv > 2.69 eV, the charge carriers can tunnel to nonradiative recombination centers with a certain probability, and therefore, the quantum efficiency decreases. The tunnel injection into deep localized states provides the maximum electroluminescence efficiency. This effect is responsible for the origin of the characteristic maximum in the quantum efficiency of the emitting diodes at current densities much lower than the operating densities. Occupation of the deep localized states in the density-of-states “tails” in InGaN plays a crucial role in the formation of the emission line as well. It is shown that the increase in the quantum efficiency and the “red” shift of the photoluminescence spectra with the voltage correlate with the changes in the photocurrent and occur due to suppression of the separation of photogenerated carriers in the field of the space charge region and to their thermalization to deep local states.

Effect of host materials on eelectrophosphorescence properties of PtOEP‐doped organic light‐emitting diodes

We have studied the effect of host materials on the electrophosphorescence properties by comparing three different host materials such as tris(8‐hydroxyquinoline)‐aluminum (III) (Alq3), bis(8‐hydroxyquinoline)‐zinc (II) (Znq2), and 4,4'‐N,N'‐dicarbazole‐biphenyl (CBP) doped with a red‐emissive phosphorescent dye, 2,3,7,8,12,13,17,18‐octaethyl‐21H,23H‐porphyrin platinum (II) (PtOEP). The EL spectra show a strong red emission (peak at 650 nm) from the triplet excited state of PtOEP and a very weak emission from an electron transport layer of Alq3 and a hole transport layer of N,N'‐diphenyl‐N,N'‐bis(3‐methylphenyl)‐1,1‐biphenyl‐4,4'‐diamine (TPD). We find that the triplet exciton lifetime and the quantum efficiency decrease in the order of CBP, Alq3, and Znq2 host materials. The results are interpreted as a poor exciton confinement in Alq3, and Znq2 host compared with in CBP. Therefore, it is very important for the triplet‐exciton confinement in the emissive layer for obtaining a high efficiency.

Changes in photochemical and antioxidant enzyme activities in maize (Zea mays L.) leaves exposed to excess copper

Changes in photosynthetic and antioxidant activities in maize (Zea mays L.) leaves of cultivars 3223 and 31G98 exposed to excess copper (Cu) were investigated. Cu treatment reduced the shoot and root length of both cultivars. No significant difference of Cu accumulation in the roots of both cultivars was observed while the cultivar 3223 accumulated significantly higher Cu in leaves than 31G98. The observed decreases in effective quantum efficiency of PSII, ETR and qP indicate an over excitation of photochemical system in 3223 compared to 31G98. The leaf chlorophyll and carotenoid contents of both cultivars decreased with increasing Cu concentration. A far higher production of anthocyanins in 31G98 has been observed than that of 3223. At 1.5 mM Cu concentration, all antioxidant enzyme activities increased in leaves of the cultivar 31G98 while there were no significant changes in SOD and GR activities in 3223 compared to the control except increased APX and POD activities. The lower Cu accumulation in leaves and higher antioxidant enzyme activities in 31G98 suggested an enhanced tolerance capacity of this cultivar to protect the plant from oxidative damage.

The effects of ultraviolet radiation on photosynthetic performance, growth and sunscreen compounds in aeroterrestrial biofilm algae isolated from building facades

The effects of artificial ultraviolet radiation [UVR; 8 W m(-2) ultraviolet-A (UVA), 0.4 W m(-2) ultraviolet-B (UVB)] on photosynthetic performance, growth and the capability to synthesise mycosporine-like amino acids (MAAs) was investigated in the aeroterrestrial green algae Stichococcus sp. and Chlorella luteoviridis forming biofilms on building facades, and compared with the responses of two green algae, from soil (Myrmecia incisa) and brackish water (Desmodesmus subspicatus). All species exhibited decreasing quantum efficiency (Fv/Fm) after 1-3 days exposure to UVR. After 8-12 days treatment, however, all aeroterrestrial isolates exhibited full recovery under UVA and UVA/B. In contrast, D. subspicatus showed only 80% recovery after treatment with UVB. While Stichococcus sp. and C. luteoviridis exhibited a broad tolerance in growth under all radiation conditions tested, M. incisa showed a significant decrease in growth rate after exposure to UVA and UVA/B. Similarly D. subspicatus grew with a reduced rate under UVA, but UVA/B led to full inhibition. Using HPLC, an UV-absorbing MAA (324 nm-MAA) was identified in Stichococcus sp. and C. luteoviridis. While M. incisa contained a specific 322 nm-MAA, D. subspicatus lacked any trace of such compounds. UV-exposure experiments indicated that all MAA-containing species are capable of synthesizing and accumulating these compounds, thus supporting their function as an UV-sunscreen. All data well explain the conspicuous ecological success of aeroterrestrial green algae in biofilms on facades. Biosynthesis and accumulation of MAAs under UVR seem to result in a reduced UV-sensitivity of growth and photosynthesis, which consequently may enhance survival in the environmentally harsh habitat.

Analysis of dependence of electroluminescence efficiency of AlInGaN LED heterostructures on pumping

AbstractThe work is devoted to explanation of the decrease of external quantum efficiency (QE) with increase of pumping density typically observed for AlInGaN heterostrucures. It is shown as a result of numerical modeling that while the increase of QE at low pumping density is due to the competition between radiative and non‐radiative recombination, the decrease of QE at large pumping density is caused by decrease of injection efficiency for the holes into the active area. A modified LED heterostructure is suggested, for which the effect of QE decreasing is not expected. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Salinity stress and hydrogen peroxide regulation of antioxidant defense system in Ulva fasciata

The regulation of antioxidant defense system in macroalgae exposed to salinity stress was examined in Ulva fasciata Delile. As compared to the 30‰ control, a long-term (4 days) exposure to hyposaline (5, 15‰) and hypersaline (60, 90, 120, 150‰) conditions inhibited growth rate and TTC reduction ability. A decrease in maximum quantum efficiency (F v/F m ratio) and the maintenance of superoxide dismutase activity under salinity stress indicate the potential generation of reactive oxygen species in chloroplasts. An exposure to 15, 60, and 90‰ decreased seawater H2O2 contents but increased thallus H2O2 contents that are positively correlated with TBARS and peroxide contents. Alleviation of oxidative damage and H2O2 accumulation at 15 and 90‰ by a H2O2 scavenger, dimethylthiourea, suggests that oxidative damage occurring under moderate hyposaline and hypersaline conditions is ascribed to accumulated H2O2. Increased glutathione reductase activity and glutathione content and decreased ascorbate content are responsible for accumulated H2O2 at 15, 60, and 90‰, while ascorbate peroxidase activity increased only at salinity ≥ 90‰. Catalase and peroxidase activities also increased at 60 and 90‰ for H2O2 removal, but only catalase showed activity increase at 15‰. For the regeneration of ascorbate, the activities of both dehydroascorbate reductase and monodehydroascorbate reductase were increased at 5 and 15‰ while only monodehydroascorbate reductase activity increased at 60 and 90‰. It is hypothesized that the availability of antioxidants and the activities of antioxidant enzymes are increased in U. fasciata to cope with the oxidative stress occurring in hyposaline and hypersaline conditions.

Effet du déficit hydrique au niveau de la phase photochimique du PSII chez deux variétés d'olivier

The effect of the water deficit, on two olive tree varieties ‘ Chetoui’ and ‘ Chemlali’ at the level of photosystem II photochemistry (PSII) was studied through the following parameters: leaf water potential ( Ψ Hb ), quantum yield of PSII ( ΦPSII), maximum quantum yield of PSII ( Φ max PSII), electron transfer rate ( J T ) and photochemical quenching (qP). The results obtained show a reduction in the leaf water potential and a decrease in quantum efficiency of PSII. Besides, electron transfer rate and photochemical quenching showed an increase in response to water deficit. These modifications present some differences according to the variety. These observations are discussed in relation to the strategies developed to grow drought-resistant olive trees in arid areas. To cite this article: M.F. Kasraoui et al., C. R. Biologies 329 (2006).

'Charge pump' effect and mechanisms of charge carriers localisation in oxidised nano-Si

A new effect of 'two-stroke charge pump' has been experimentally observed in nano-Si, which is based on the Auger process ejecting charge carriers from a silicon nanocrystalline core into enveloping SiOx layer. The effect is important for understanding the nature of such phenomena as dimensional decrease of quantum efficiency of emission, space separation of charge carriers and peculiarities of their localisation. A new mechanism of charge carrier localisation by 'topological' traps in undulating quantum wires has been considered. The developed models are supported by the results of comprehensive spectral investigations of photoluminescence, thermoluminescence, and tunnelling luminescence.

Cadmium stimulates the accumulation of salicylic acid and its putative precursors in maize (Zea mays) plants

The effect of 10, 25 and 50 µM Cd(NO3)2 on the salicylic acid (SA) metabolism was investigated in young maize seedlings (Zea mays L., hybrid Norma). Cadmium (Cd) was translocated into the leaves and induced oxidative damage, as indicated by the reduced chlorophyll content, the decreased quantum efficiency of photosystem II and the enhanced malondialdehyde (MDA) content, especially after 7 days. The activity of glutathione reductase (EC 1.6.4.2) increased from the fourth day and that of guaiacol peroxidase (EC 1.11.1.7) after 7 days of Cd stress compared with the control leaves. These effects of Cd exhibited a correlation with the concentration. Under these conditions, Cd did not affect the MDA content or the antioxidant enzyme activities in the roots. After 7 days, Cd increased the levels of free and bound forms of benzoic acid (BA), O‐coumaric acid (O‐hydroxy‐cinnamic) (O‐HCA) and SA in the leaves, but in the roots, only the 50 µM rate of Cd caused changes in the free O‐HCA acid and bound BA content.

Light-Triggered Action Potentials and Changes in Quantum Efficiency of Photosystem II in Anthoceros Cells

Capillary microelectrodes and pulse amplitude-modulated microfluorometry were used to study light-triggered changes in cell membrane potential, chlorophyll fluorescence, and photochemical yield of PSII in chloroplasts of a hornwort Anthoceros sp. The action potential was generated by illuminating the plant sample for a few seconds. It was accompanied by a reversible decrease in quantum efficiency of PSII and by nonphotochemical quenching of fluorescence that continued as long as 10 min after the light stimulus. The presence of ammonium ions (2 mM) enhanced the amplitude and prolonged the duration of dark changes of fluorescence parameters in accordance with the reported increase in duration and amplitude of the light-triggered action potential in the presence of NH 4 + . A rapid retardation of PSII activity within the first seconds of illumination was also evident from absorbance changes at 810 nm reflecting the redox conversions of chlorophyll P700. The PSII-dependent stage of reduction in the induction curves of P700 absorbance was strongly suppressed, and the amplitudes of signals induced by white and far-red light (717 nm) differed insignificantly. It is concluded that a short-term irradiation triggers the generation of ΔpH at the thylakoid membranes, which is accompanied by inhibition of the plasma membrane H+ pump and by reversible inactivation of PSII due to increased thermal dissipation of chlorophyll excitations.