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We have studied the optical properties of different thienyl-S,S-dioxide oligothiophenes under strong excitation. No stimulated emission is observed in neat films due to singlet-singlet bimolecular annihilation. The bimolecular recombination constant is quantified from a rate equation model in the limit of constant annihilation rate. We demonstrate that tunable stimulated emission in the range 490–660 nm, due to optical gain with an estimated cross section of the order of σg≈10-17 cm2, can be obtained by blending the active molecule with small quantities of inert polycarbonate poly(bisphenol-A-carbonate) (PC). The presence of amplified spontaneous emission (ASE) for a PC:active molecule relative concentration as small as 1:500 suggests that the polycarbonate role is to reduce the intermolecular diffusion rate rather than to isolate the active molecules. Moreover, for higher PC content, a continuous decrease of the bimolecular quenching role is observed. These results demonstrate that the absence of stimulated emission in neat films is not necessarily due to intrinsic molecular properties, as strong ASE can be obtained by slightly modifying the molecule interaction during the deposition process. This approach could allow a considerable extension of the number of molecules showing stimulated emission for organic laser applications. [ABSTRACT FROM AUTHOR]

We investigated the temperature dependence of the poly(9,9-dioctylfluorene) β phase photoluminescence (PL) spectra in spin coated thin films from tetrahydrofuran solutions. As the temperature increases from 18 to 300 K a continuous blueshift of the 0-0 PL peak of about 25 meV and an increase of the peak full width at half maximum (FWHM) of about 49 meV are observed. We show that the PL spectra temperature dependence is not due to a temperature dependent average conjugation length, as often assumed, but instead it can be quantitatively explained in the frame of a thermal quasiequilibrium model for excitons in an inhomogeneously broadened excited states distribution. We demonstrate that the emission blueshift and broadening are mainly due to the increase of the excitons’ temperature with the sample one. This effect is partially compensated by an increasing efficiency of the exciton energy migration. The interplay between these two processes quantitatively explains the observed temperature dependence of the PL peak energy and of its FWHM. On the contrary we show that the PL spectra are almost independent of the absorption blueshift with temperature. [ABSTRACT FROM AUTHOR]

We demonstrate a static fabrication approach to make free-standing ordered arrays of fluorescent nanofibres through control of the transverse electrospinning field. The alignment and the density of the nanofibre arrays are optimised by careful design of both the source and collector electrode geometries which can control the transverse electric field over the full path of the jet. In doing so, we fabricate suspended fluorescent nanofibres with an aspect ratio of 104, and with a substantially increased density and order parameter (by a factor of ~10 compared to random deposition). Electrostatic modelling suggests that the field distribution of the component is the main contribution to the ordering between the plates. This method offers increased efficiency for the creation of ordered fibres collected over a small area and the characterisation of their photoluminescent properties. [ABSTRACT FROM AUTHOR]

Simultaneous photoluminescence (PL) and external quantum efficiency (EQE) confocal mapping is used to investigate the correlation between the local PL and the EQE in a regioregular poly(3-exylthiophene):poly(9,9-dioctylfluorene-co-benzothiadiazole) inverted bulk heterojunction solar cell. We show that the charge generation and charge collection are strongly non-uniform on a length scale up to 100 µm. Our results evidence that organic solar cells optimization requires not only the control of the submicrometric active materials arrangement but also the control of the large scale device uniformity. [ABSTRACT FROM AUTHOR]

The authors investigated the energy transfer between the chromophors of a white emitting polyspirobifluorene copolymer containing a blue emitting backbone, a green, and a red emitting dye. They demonstrate that the green dye is mainly excited by blue→green Förster resonant energy transfer (FRET), with a Förster radius R0bg=35 Å. The red dye is instead excited by direct blue→red FRET (R0br=21 Å) and by sequential blue→green→red FRET (R0gr=31 Å). Finally, the authors show that the FRET processes, fundamental to have white emission, are of intermolecular nature, while intramolecular energy migration and transfer are negligible. [ABSTRACT FROM AUTHOR]

We have studied the optical properties of a thienyl-S, S-dioxide oligothiophene under strong excitation. No stimulated emission is observed in neat films due to singlet-singlet bimolecular annihilation, with a bimolecular constant of A=(4.5±0.5)×10-11 cm3 s-1. On the contrary we show that strong stimulated emission is observed if the active molecule is blended to an inert polymer [poly(bisphenol A carbonate] (PC) even for a PC content as low as one PC molecule every 500 active molecules. We suggest that small PC quantities modify the film superamolecular packing, thus reducing the bimolecular recombination efficiency. These results are expected to be quite general and could allow a considerable extension of the number of molecules showing stimulated emission for organic laser applications. [ABSTRACT FROM AUTHOR]

We demonstrate that 20 nm thick indium tin oxide (ITO) layers deposited by pulsed laser deposition present sheet resistance as low as 130 Ω/□ and very uniform morphology, with an average roughness of about 0.4 nm, and peak-to-valley roughness as low as 8.2 nm. This good uniformity allowed us to realize a single layer polyfluorene active waveguide with both top and bottom ITO electrodes showing clear amplified spontaneous emission and electrode induced losses as low as 3.0 cm-1. We investigated the effects of hole injection in the ASE intensity concluding that complete gain suppression due to polaron absorption would take place for current density of about 360 mA cm-2. [ABSTRACT FROM AUTHOR]

We studied the microscopic dependence of poly(9,9-dioctylfluorene) photoluminescence (PL) on the deposition conditions. We show that in films spin coated from chloroform phase separation of β and glassy phases is present, with micrometric β phase clusters covering about 6% of the sample surface. The exposure to toluene vapors leads to the disappearance of the β phase clusters, but increases the β phase content in the films due to swelling induced polyfluorene chain planarization. The deposition from toluene solution leads to nonuniform PL intensity, dominated by the β phase emission, attributed to an interplay between aggregation during the solvent evaporation and solvent swelling induced chain planarization. [ABSTRACT FROM AUTHOR]

We report on random lasing in substituted quinquethienyl S,S-dioxide neat films. Despite the absence of highly efficient scatterers in the film, a fine structure with laser-like peaks as narrow as 5 Å is observed in the emission spectra. The far-field emission pattern is studied through angle-resolved emission measurements, demonstrating that random lasing emission is directional, with a 8° divergence but different individual emission patterns. The origin of the scattering centers providing the feedback for lasing has been analyzed through atomic force microscopy measurements of the film surface. We demonstrate that the random lasing is induced by sequential scattering from 50 nm diameter holes in the film with an average distance of 500 nm, while thickness fluctuations are not relevant. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A correction to the article "Interplay between stimulated emission and singlet singlet annihilation in oligothiophene dioxide thin films" which was published online on December 4, 2008 is presented.

We investigated the temperature dependence of the poly(9,9-dioctylfluorene) beta phase photoluminescence (PL) spectra in spin coated thin films from tetrahydrofuran solutions. As the temperature increases from 18 to 300 K a continuous blueshift of the 0-0 PL peak of about 25 meV and an increase of the peak full width at half maximum (FWHM) of about 49 meV are observed. We show that the PL spectra temperature dependence is not due to a temperature dependent average conjugation length, as often assumed, but instead it can be quantitatively explained in the frame of a thermal quasiequilibrium model for excitons in an inhomogeneously broadened excited states distribution. We demonstrate that the emission blueshift and broadening are mainly due to the increase of the excitons' temperature with the sample one. This effect is partially compensated by an increasing efficiency of the exciton energy migration. The interplay between these two processes quantitatively explains the observed temperature dependence of the PL peak energy and of its FWHM. On the contrary we show that the PL spectra are almost independent of the absorption blueshift with temperature.