Laura Favaretto

High-efficiency oligothiopene-based light-emitting diodes

We report investigations of the photoluminescence (PL) and electroluminescence (EL) of a thiophene oligomer for which we have devised a variety of substitutions aimed at enhancing the solid-state efficiency. We find that the absolute PL quantum efficiency in the solid state is up 37% for both powders or spin-coated thin films of the compound. The material thus becomes competitive for applications in organic light-emitting diodes (LEDs). EL efficiencies up to 1.2 cd/A are demonstrated in LEDs prepared with indium–tin–oxide and Ca–Al electrodes.

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.

Oligomer-based organic distributed feedback lasers by room-temperature nanoimprint lithography

Room-temperature nanoimprint lithography in air is used in order to pattern a nonthermoplastic, low-molar-mass thiophene-based pentamer with excellent gain properties. No degradation of the luminescence efficiency of the active medium was observed after patterning. In this way, we fabricated single-mode emission distributed feedback lasers having a threshold excitation fluence of 140 μJ/cm2. The lasing line is peaked at 637 nm and exhibits a linewidth of less than 0.7 nm and a well-behaved input-output characteristic in the whole range of pump fluences. These results demonstrate room-temperature nanoimprint lithography as powerful and straightforward fabrication technique for oligomer-based nanostructured optoelectronic devices.

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.

Thienopyrrolyl dione end-capped oligothiophene ambipolar semiconductors for thin film- and light emitting transistors

The design, synthesis and structure-property investigation of a new thienopyrrolyl dione substituted oligothiophene material showing reduced band gap energy, low lying LUMO energy level and ambipolar semiconducting behaviour is described.

Oligothiophene-S,S-dioxides as a class of electron-acceptor materials for organic photovoltaics

Oligothiophene-S,S-dioxides are proposed as electron acceptors materials in organic blended photovoltaic devices. Photoinduced charge transfer is demonstrated in blends between a regioregular poly(3-hexylthiophene) and the oligomers, via photoluminescence spectroscopy. The enhanced photovoltaic performance exhibited by the blended cells, with respect to that of pristine devices in which the polymer is the active layer, represents further evidence for exciton dissociation. An increase of the power conversion efficiency up to sixty-fold is achieved by blending the polymer with the oligothiophene-S,S-dioxides.

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.

Surface and bulk phenomena in conjugated polymers devices

Abstract We report a number of investigations into a variety of surface and bulk phenomena which are particularly relevant to the design of polymer optoelectronic devices, and in particular to light-emitting diodes (LEDs). First, we consider the surface properties of the most common anode, a thin layer of indium tin oxide (ITO), as modified by chemical and physical treatments. We show how the treatments affect the electrode workfunction, surface energy, morphology, sheet resistance and therefore, the LEDs efficiency and lifetime. We will also consider the effect of a conducting polymer hole-transport layer, such as poly(styrene sulfonate) doped poly(3,4-ethylene dioxythiophene) (PEDOT), incorporated between the ITO and the active layer. Secondly, we turn our attention to the optimisation of the photoluminescence (PL) efficiency, and report examples of how the polymer microstructure and possibly the engineering of disorder in a class of poly( p -phenylene vinylene)s and poly- or oligo-thiophenes can determine a significant increase of the PL efficiencies in the solid state.

Branched thiophene-based oligomers as electron acceptors for organic photovoltaics

Two branched V-shaped benzo[b]thiophene-based structures were investigated as novel electron-acceptor materials in blended photovoltaic devices. Photoinduced charge transfer was demonstrated in blends between a regioregular poly(3-hexylthiophene) and the oligothiophenes, via photoluminescence spectroscopy. The enhanced photovoltaic performance exhibited by the blended cells, with respect to that of pristine devices in which the polymer was the active layer, represented further evidence for exciton dissociation. Remarkable improvement of the performance of the blended devices upon mild thermal annealing was achieved. Furthermore, the effect of the molecular structure of the branched benzo[b]thiophenes on the blend photoresponse is discussed.

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.