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

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 -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

Multi-criteria decision making approach for the selection of software effort estimation model

Software development with minimum effort has become a challenging task for the software de-velopers. Software effort may be defined as the prediction process of the effort required to de-velop any software. Many software effort estimation models have been developed in the past, but it is observed that none of them can be applied successfully in all kinds of projects in differ-ent environments that raise the problem of the software effort estimation model selection. To se-lect the suitable software effort estimation model, many conflicting selection criteria must be con-sidered in the decision process. The present study emphasizes on the development of a fuzzy multi-criteria decision making approach by integrating Fuzzy Set Theory and Weighted Distance Based Approximation. To show the consistency of the proposed approach, methodology valida-tion is also performed by making comparison with existing methodologies and to check the criti-cality of the selection criterion, sensitivity analysis is also performed

Tuning the emission of cationic iridium (III) complexes towards the red through methoxy substitution of the cyclometalating ligand

EZ-C acknowledges CFI (Canadian Foundation for Innovation), NSERC (the Natural Sciences and Engineering Research Council of Canada), FQRNT (Le Fonds québécois de la recherche sur la nature et les technologies) and the University of St Andrews for financial support. C. R-C. would like to thank the MINECO for the financial support of this research in the framework of project CTQ2010-17481, the Junta de Andalucía (CICyE) for special financial support (P08-FQM-4011 and P10-FQM-6703) and the MECD (Spanish Ministry of Education, Culture, and Sport) for an FPU grant. This work has been supported by the Spanish Ministry of Economy and Competitiveness (MINECO) (MAT2014-55200), the Generalitat Valenciana (Prometeo/2012/053). IDWS and AKB acknowledge financial support from the EPSRC of the UK (grant: EP/J01771).The synthesis, characterization and evaluation in solid-state devices of a series of 8 cationic iridium complexes bearing different numbers of methoxy groups on the cyclometallating ligands are reported. The optoelectronic characterization showed a dramatic red shift in the absorption and the emission and a reduction of the electrochemical gap of the complexes when a methoxy group is introduced para to the Ir-C bond. The addition of a second or third methoxy group does not lead to a significant further red shift in these spectra. Emission maxima over the series ranged from 595 to 730 nm. All complexes possessing a motif with a methoxy group at the 3-position of the cyclometalating ligands show very short emission lifetimes and poor photoluminescence quantum yields whereas complexes having a methoxy group at the 4-position are slightly blue shifted compared to the unsubstituted parent complexes, resulting from the inductively electron withdrawing nature of this directing group on the Ir-C bond. Light-emitting electrochemical cells were fabricated and evaluated. These deep red emitters generally show poor performance with electroluminescence mirroring photoluminescence. DFT calculations accurately model the observed photophysical and electrochemical behavior of the complexes and point to an emission from a mixed charge transfer state.Publisher PDFPeer reviewe