Mercaptophosphonic acids as efficient linkers in quantum dot sensitized solar cells

This work was supported by the Agence Nationale de la Recherche, project QuePhélec (ANR-13-BS10-0011-01). MTS and IDWS are thankful to ERC for financial support for exciton diffusion project, grant number 321305. AKB and IDWS also acknowledge financial support from EPSRC programme grant: structured light, EP/J01771X.Control over the deposition of quantum dots (QDs) on nanostructured semiconductors is very important for the photovoltaic performance of QD sensitized solar cells. The best control is typically achieved using bifunctional molecular linkers, such as mercaptopropionic acid (MPA), to attach the QDs to metal oxides in a specific manner; however some materials, such as ZnO, are not compatible with these molecules due to their pH sensitivity. We have developed new linkers, mercaptophosphonic acids of different length, which allow efficient functionalization of ZnO nanowires and also mesoporous TiO2 without damaging their surface. Detailed XPS and contact angle studies of the mechanism of self-assembly of these acids show that their strong chelation of the oxide surface prevents protonic attack and etching. Using these linkers, we show that colloidal ternary quantum dots, CuInS2, can be conformally and homogeneously deposited on the functionalized metal oxides. Photophysical studies by means of time-resolved photoluminescence spectroscopy confirm efficient electron transfer from the QDs to the metal oxides with the rate and efficiency scaling with respect to the linker length and nature. The efficiency of the QD sensitized solar cells fabricated with such assemblies also strongly depends on the linkers used and follows the trends observed for the charge transfer.PostprintPeer reviewe

In situ formation and photo patterning of emissive quantum dots in small organic molecules

Nanostructured composites of inorganic and organic materials are attracting extensive interest for electronic and optoelectronic device applications. Here we report a novel method for the fabrication and patterning of metal selenide nanoparticles in organic semiconductor films that is compatible with solution processable large area device manufacturing. Our approach is based upon the controlled in situ decomposition of a cadmium selenide precursor complex in a film of the electron transporting material 1,3,5-tris(N-phenyl-benzimidazol-2-yl)-benzene (TPBI) by thermal and optical methods. In particular, we show that the photoluminescence quantum yield (PLQY) of the thermally converted CdSe quantum dots (QDs) in the TPBI film is up to 15%. We also show that laser illumination can form the QDs from the precursor. This is an important result as it enables direct laser patterning (DLP) of the QDs. DLP was performed on these nanocomposites using a picosecond laser. Confocal microscopy shows the formation of emissive QDs after laser irradiation. The optical and structural properties of the QDs were also analysed by means of UV-Vis, PL spectroscopy and transmission electron microscopy (TEM). The results show that the QDs are well distributed across the film and their emission can be tuned over a wide range by varying the temperature or irradiated laser power on the blend films. Our findings provide a route to the low cost patterning of hybrid electroluminescent devices

Solubilised bright blue-emitting iridium complexes for solution processed OLEDs

EZ-C acknowledges the University of St Andrews for financial support. IDWS and AKB acknowledge support from EPSRC (EP/J01771X). The authors would like to thank the Engineering and Physical Sciences Research Council for financial support for Adam Henwood: EPSRC DTG Grants: EP/J500549/1; EP/K503162/1; EP/L505097/1.Combining a sterically bulky, electron-deficient 2-(2,4-difluorophenyl)-4-(2,4,6- trimethylphenyl)pyridine (dFMesppy) cyclometalating C^N ligand with an electron rich, highly rigidified 1,1’-(α,α’-o-xylylene)-2,2’-biimidazole (o-xylbiim) N^N ligand gives an iridium complex, [Ir(dFMesppy)2(o-Xylbiim)](PF6), that achieves extraordinarily bright blue emission (ΦPL = 90%; λmax = 459 nm in MeCN) for a cationic iridium complex. This complex is compared with two reference complexes bearing 4,4’-di-tert-butyl-2,2’- bipyridine, and solution-processed organic light emitting diodes (OLEDs) have been fabricated from these materials.Publisher PDFPeer reviewe

pH-Induced transformation of ligated Au 25 to brighter Au 23 nanoclusters

Thiolate-protected gold nanoclusters have recently attracted considerable attention due to their size-dependent luminescence characterized by a long lifetime and large Stokes shift. However, the optimization of nanocluster properties such as the luminescence quantum yield is still a challenge. We report here the transformation of Au25Capt18 (Capt labels captopril) nanoclusters occurring at low pH and yielding a product with a much increased luminescence quantum yield which we have identified as Au23Capt17. We applied a simple method of treatment with HCl to accomplish this transformation and we characterized the absorption and emission of the newly created ligated nanoclusters as well as their morphology. Based on DFT calculations we show which Au nanocluster size transformations can lead to highly luminescent species such as Au23Capt17

Energy Efficiency and Throughput for TCP Traffic in Multi-Hop Wireless Networks

Ahwrrrcr — We study the performance metrics associated with TCPregulated traffic in multi-hop, wireless networks that use a cnmmnn physical channel (e.g., IEEE 802.11). In contrast to earlier analyses, we focus simultaneously on two key operating metrics- the energy efficiency and the session throughput. Using analysis and simulations, we show how these metrics are strongly influenced hy the radio transmission range of individual nodes. Due to tradeoffs hetween the individual packet transmission energy and the likelihood of retransmissinns, the total energy consumption is a cnnvex function of the numher of hops (and hence, of the transmission range). On the other hand, the TCP session throughput decreases supralinearly with a decrease in the transmission range. In certain scenarios, the overall network capacity can then he a cnncave function of the transmission range. Based on our analysis of the performance of an individual TCP session, we finally study how parameters such as the node density and the radio transmission range affect the overall network capacity under different operating conditions. Our analysis shows that capacity metrics at the TCP layer hehave quite differently than corresponding idealized link-layer metrics. 1

Tuning the exciton diffusion coefficient of polyfluorene based semiconducting polymers

The authors acknowledge financial support from FP7 project “Laser Induced Synthesis of Polymeric Nanocomposite Materials and Development of Micro‐patterned Hybrid Light Emitting Diodes (LED) and Transistors (LET)” – LAMP project (G.A. 247928). M.T.S., A.R., and I.D.W.S. acknowledge support from the European Research Council (EXCITON grant 321305). I.D.W.S. acknowledges a Royal Society Wolfson Research Merit Award. The authors are grateful to EPSRC for an equipment grant (EP/L017008/1).Exciton diffusion plays an important role in functional materials used in organic optoelectronic devices, such as solar cells, organic light emitting diodes, and lasers. Here we explore how exciton diffusion can be controlled in highly fluorescent blue‐emitting polyfluorene materials by changing the length and type of side chains. We find that the exciton diffusion coefficient (D) decreases from 1.2 × 10−3 cm2 s−1 to 0.2×10−3 cm2 s−1 when the side chain length is increased from 8 to 12 carbon atoms. Other changes to the side chains led to enhancement of D up to 1.6 × 10−3 cm2 s−1. Our results show that small adjustments to the molecular structure can be helpful for the future development of high‐brightness organic light emitting devices.PostprintPostprintPeer reviewe

An Organic Vortex Laser

Optical vortex beams are at the heart of a number of novel research directions, both as carriers of information and for the investigation of optical activity and chiral molecules. Optical vortex beams are beams of light with a helical wavefront and associated orbital angular momentum. They are typically generated using bulk optics methods or by a passive element such as a forked grating or a metasurface to imprint the required phase distribution onto an incident beam. Since many applications benefit from further miniaturization, a more integrated yet scalable method is highly desirable. Here, we demonstrate the generation of an azimuthally polarized vortex beam directly by an organic semiconductor laser that meets these requirements. The organic vortex laser uses a spiral grating as a feedback element that gives control over phase, handedness, and degree of helicity of the emitted beam. We demonstrate vortex beams up to an azimuthal index <i>l</i> = 3 that can be readily multiplexed into an array configuration

Spectroscopic and lasing characterization of electroluminescent organic materials

Some electroluminescent molecules are investigated by optical spectroscopy. Their lasing ability is studied in liquid solutions using a low�Q laser oscillator arrangement. In neat films wave-guided amplification of spontaneous emission is investigated. Some studies concerning the distributed feedback laser action of neat films on corrugated gratings are made. The investigated materials are the triphenylamine molecule TPA, the triphenylamine starbust molecule m-MTDAB, the triphenylamine dimer TPB, the napthyl-diphenylamine dimer beta-NPD, the oligomer dicarbazovinylene-MEH-benzene 2CzV-MEH-B, and the phenylene-bipyridine polymer PPBpy. The molecule triphenylamine (TPA) in tetrahydrofuran (THF) and the starburst triphenylamine oligomer 1,3,5-tris(3-methylphenylphenylamino)benzene (m-MTDAB) in THF and as neat film are characterized. The fluorescence decay is studied by picosecond laser excitation and time-resolved signal detection. Reverse saturable absorption is observed in picosecond laser nonlinear transmission measurements (laser duration 35 ps, laser wavelength 347.15 nm). The lasing ability of the compounds is tested by picosecond laser transverse pumped amplification or attenuation of spontaneous emission. The excited-state absorption cross-section spectra of the samples in the fluorescence spectral regions are extracted from the amplification/attenuation of spontaneous emission. Weak amplification of spontaneous emission is observed for m-MTDAB in THF around the fluorescence maximum (370 to 405 nm). For TPA in THF and m-MTDAB neat film attenuation of spontaneous emission occurs over the whole fluorescence region (excited-state absorption stronger than stimulated emission). The molecules N,N,N�,N�-tetraphenylbenzidine (TPB, triphenylamine dimer TAD) and N,N�-bis(2-naphtalenyl)-N,N�-bis(phenylbenzidine) (beta-NPB, naphtyl-diphenylamine dimer beta-NPD) dissolved in tetrahydrofuran (THF) and as neat films are studied. The excited-state absorption at the pump laser wavelength is determined by saturable absorption measurement. Low-Q laser oscillation of TPB in THF is achieved by transverse pumping the solution in a cell. The excited-state absorption of TPB in THF at the laser wavelength is extracted from the laser threshold. In TPB neat films wave-guided travelling-wave lasing is achieved. No laser action is obtained for &#61538;-NPB because of very small S1-S0 stimulated emission strength, and the presence of the excited-state absorption in the fluorescence wavelength region. The oligomeric molecule 1,4-bis(9-ethyl-3carbazovinylene)-2-methoxy-5-(2�-ethyl-hexyloxy)-benzene (abbreviated 2CzV-MEH-B) dissolved in tetrahydrofuran (THF) and as neat film is investigated upon its optical spectroscopic properties and its lasing properties. Again the excited-state absorption at the pump-laser wavelength is determined by saturable absorption measurements. Laser oscillation of the dye in THF in a rectangular cell is achieved by transverse pumping. From the emission behaviour around laser threshold the excited-state absorption cross-section spectrum in the laser active spectral region of the material is extracted. The wave-guided travelling-wave lasing behaviour of the emitter as neat film is studied by analysis of the amplification of the transverse pumped spontaneous emission. Surface emitting distributed-feedback lasing is achieved with a neat film on corrugated second-order periodic gratings. The luminescent polymer, poly[2,2�-bipyridine-5,5�-diyl-(2,5-dihexyl-1,4-phenylene)] (abbreviated PPBpy) dissolved in tetrachloroethane (TCE) and as neat film is characterized optically by absorption, emission spectroscopy and saturable absorption measurements. Low-Q laser oscillation of the polymer in TCE is achieved and used to determine the effective stimulated emission cross-section at the lasing wavelength. Travelling-wave lasing of a neat film of the polymer is achieved. Surface-emitting distributed-feedback lasing is obtained with a neat film on corrugated second-order periodic gratings. The investigated electroluminescent molecules are applied in organic light emitting devices (OLEDs). The investigated polymer is a potential candidate for polymer light emitting devices. For their application in light emitting devices a high quantum yield of electroluminescence is sufficient. But for the application of the electroluminescent materials in laser devices it is indispensible that the stimulated emission cross-section is higher than the excited state absorption cross-section. Good OLED performance of an electroluminescent material is a precondition for good organic lasers, but in each case the effective stimulated emission cross-section (stimulated emission cross-section�excited state absorption cross-section) has to be determined to get information about its lasing ability as is done in this work. The oligomeric emitter 2CzV-MEH-B, the triphenylamine dimer TPB, and the phenylene-bipyridine polymer PPBpy turned out to be good organic active media for laser action in the violet and blue spectral region

Wearable organic optoelectronic sensors for medicine

This work was supported by Engineering and Physical Sciences Research Council Programme Grant “Challenging the limits of photonics: Structured light” (grant number EP/J01771X/1) and MRC.Organic optoelectronic devices that can be used to make compact, wearable sensors for medicine and sports are shown and two examples are given: a tissue-oxygenation sensor and a muscle-contraction sensor. It is demonstrated how the muscle-contraction sensor on a volunteer's arm can successfully control the robot arm.Publisher PDFPeer reviewe

Effect of Market Crisis of Financial Efficiency on Underpricing: an VAR Overview of Indian IPOs

This paper attempts to design for and tests empirical models, which integrate theoretical and firm’s economic capabilities. Which are interacting to explain financial efficiency, i.e. several financial &amp; liquidity ratios at the level of underpricing after the Indian stock market crunch? The study is founded on IPO that listed at Bombay stock exchange given that April-2008 to Dec-2011. VAR (vector autoregressive analysis) is used to distinguish the relationship between all independent variables with the dependent variable, i.e. level of underpricing. The results of VAR display that variable of DEBTQ, APATM, ROCE and RONW have a consequential association at the level of Market adjusted return ratio (MARRO), AND VAR indicates 46% R2 and 30% adjusted R2. Nevertheless, ROCE and RONW have a significant difference in the level of traditional underpricing, and VAR indicates 49% R2 and 32% adjusted R2. However, in the mutually of a case i.e. LOGMAARO &amp; LOGUNDER verified the assumptions of Akaike information criterion and Schwarz's criterion. We executed unit root testing to verify stationary of time series and autocorrelation function. Durbin Watson’s value subsisted 1.58. Which indicate, there is no heteroskedasticity problem exist for the model? Key words: IPOs, Post market crisis, ownership structure, share holding pattern, BSE, Underpricing, firm specific factors, market related variables