M. Anni

White light emission from blends of blue-emitting organic molecules: A general route to the white organic light-emitting diode?

We show that all possible binary combinations of molecules from four different families of organics—a diamine derivative, N,N′-bis(3methylphenyl)-N,N′-diphenylbenzidine, an oxidiazole derivative, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, a substituted thiophene dioxide, 2,5-bis(trimethylsilyl thiophene)-1,1-dioxide, and poly(9-vinylcarbazole)—produce white or near-white emission. We suggest that this is due to exciplex formation, and that this is likely to be a general phenomenon for blends of blue-emitting aromatic organics. This implies that films of spin-coated blends of blue-emitting organics represent a general, simple, and cheap route to white-emitting organic light-emitting diodes.

Förster energy transfer from blue-emitting polymers to colloidal CdSe∕ZnS core shell quantum dots

We have studied the interactions in the solid phase between a blue-light-emitting organic conjugated polymer {poly[(9,9-dihexylfluorenyl-2,7-diyl)-alt-co-(9,ethyl-3,6-carbazole)]} and colloidal CdSe∕ZnS core shell quantum dots. We demonstrate by time resolved photoluminescence measurements that efficient Forster energy transfer takes place from the polymer acting as the donor and the CdSe∕ZnS dots, acting as the acceptors. A Forster radius of about 80±15A is extracted from the analysis of the relaxation dynamics based on the solution of the donor-acceptor coupled rate equations. This value is in good agreement with the value extracted from the steady state spectra and indicates that efficient energy tranfer from a polymeric host to a nanocrystal guest can occur. Our results are relevant to the application of hybrid organic/inorganic systems to light-emitting devices.

Multicolor oligothiophene-based light-emitting diodes

We demonstrate wide tunability, from green to near infrared, of the electroluminescence emission of substituted oligothiophene compounds. The compounds are characterized by high chemical stability, electron affinities up to 3.1 eV and photoluminescence efficiencies up to 70%. These characteristics make these materials excellent candidates for application in light-emitting diodes. We obtain low turn-on voltage devices with electroluminescence efficiency up to 0.2%, more than one order of magnitude larger than the values reported for unsubstituted oligothiophene compounds.

Amplified spontaneous emission and efficient tunable laser emission from a substituted thiophene-based oligomer

We investigated gain and lasing in spin-coated films of a soluble substituted oligothiophene. With increasing excitation power, the photoluminescence spectra show a clear line narrowing due to amplified spontaneous emission. We measure a low threshold (20 μJ cm−2) for line narrowing and a large gain cross section (6×10−16 cm2), indicating that this molecule is a promising active material for organic solid-state lasers. As a demonstrator, we realize a transverse electromagnetic (TEM00) single-mode laser with tunable emission from the yellow to the red (a range of 37 nm), with a pump threshold as low as 18 μJ cm−2 and efficiency of 1.9%. These results are among the best so far reported for organic lasers.

Color engineering by modified oligothiophene blends

Fully tunable light emission is demonstrated with combinations of binary blends of modified oligothiophenes of high efficiency, covering the entire spectrum of colors according to the standards of the Commission International de l’Eclairage. The emission spectrum of each blend is determined by the Forster transfer when the energy separation between the highest occupied molecular orbital–lowest unoccupied molecular orbital gap of the constituent molecules is smaller than 0.56 eV. For larger energy separation, the blend emission is just given by the superposition of the emission spectra of the constituent molecules.

Single-mode tunable organic laser based on an electroluminescent oligothiophene

Single-mode tunable laser emission is obtained from a stable cavity based on a prototype compound of a class of functionalized electroluminescent oligothiophenes. Laser emission is demonstrated in the red spectral region with a tunability wavelength range of 30 nm, centered at 607 nm. The laser exhibits a well-defined pump threshold and good emission characteristics. The results suggest the use of the family of functionalized oligothiophenes as optical amplifiers covering the whole visible spectral region.

Amplified spontaneous emission from a soluble thiophene-based oligomer

Optical gain is demonstrated, through pump-probe measurement, in a spin-coated film of a quinquethiophene-S,S-dioxide. The temporal evolution of the gain as a function of the pump intensity shows a progressive shortening of the lifetime as the pump intensity increases. The photoluminescence spectra show line narrowing for excitation intensity higher than 960 μJ cm−2. From these measurements, we conclude that oligothiophene-S,S-dioxides are valuable candidates for thin-film organic light amplifier and lasers.

The Role of Intrinsic and Surface States on the Emission Properties of Colloidal CdSe and CdSe/ZnS Quantum Dots

Time Resolved Photoluminescence (TRPL) measurements on the picosecond time scale (temporal resolution of 17 ps) on colloidal CdSe and CdSe/ZnS Quantum Dots (QDs) were performed. Transient PL spectra reveal three emission peaks with different lifetimes (60 ps, 460 ps and 9–10 ns, from the bluest to the reddest peak). By considering the characteristic decay times and by comparing the energetic separations among the states with those theoretically expected, we attribute the two higher energy peaks to ± 1Uand ± 1L bright states of the fine structure picture of spherical CdSe QDs, and the third one to surface states emission. We show that the contribution of surface emission to the PL results to be different for the two samples studied (67% in the CdSe QDs and 32% in CdSe/ZnS QDs), confirming the decisive role of the ZnS shell in the improvement of the surface passivation.

Low electrode induced optical losses in organic active single layer polyfluorene waveguides with two indium tin oxide electrodes deposited by pulsed laser deposition

We demonstrate that 20nm thick indium tin oxide (ITO) layers deposited by pulsed laser deposition present sheet resistance as low as 130Ω∕◻ and very uniform morphology, with an average roughness of about 0.4nm, and peak-to-valley roughness as low as 8.2nm. 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 3.0cm−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 360mAcm−2.

Far-field emission and feedback origin of random lasing in oligothiophene dioxide neat films

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 A 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.