Giuseppe Gigli

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.

Increase of charge carriers density and reduction of Hall mobilities in oxygen-plasma treated indium–tin–oxide anodes

We report investigations of the electronic transport properties carried out by means of the Hall technique for indium–tin–oxide thin films on glass after a variety of surface treatments. We find that oxygen-plasma treatments induce a significant increase in the carrier concentration, and a less significant decrease of mobilities with respect to “as-received” or aquaregia treated substrates. We consider that this is indicative of an increased concentration of defects, as a result of the plasma exposure.

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.

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.

High photo and electroluminescence efficiency oligothiophenes

Abstract We report photo and electroluminescence efficiency of a thiophene oligomer functionalized to enhance its solid-state efficiency. We find that the absolute PL quantum yield is up to 37% for spin-coated thin films of the compound. The material has been used as active material in organic light-emitting diodes (LEDs). EL efficiencies up to 0.9 cd/A are demonstrated in LEDs prepared with a blend indium tin oxide and Ca–Al electrodes.

Direct assessment of tunable Schottky barriers by internal photoemission spectroscopy

Al/GaAs(001) junctions in which the Schottky barrier was tuned through fabrication of a pseudomorphic Si interface layer were characterized by internal photoemission spectroscopy. Well-defined photoabsorption onsets corresponding to Schottky barrier heights ranging from 0.3 to 1.1 eV were observed in different devices. Our results point to the possible exploitation of tunable Schottky barriers in metal/semiconductor and metal/semiconductor/metal photon detectors.

Shaping X-rays by diffractive coded nano-optics

In this paper we report results obtained in the fabrication and use of novel coded diffractive nano-optics that, beyond focusing, can perform new optical functions. In particular, the intensity of light in the space beyond the optical elements can be redistributed with almost complete freedom. These novel X-ray optical elements have been tested and found to perform multi-focusing in single or multiple focal planes and beam shaping of a generic monochromatic beam into a desired continuous geometrical pattern. Already available extreme ultraviolet and X-ray sources are suitable as ideal sources for such diffractive optical elements. Their new optical functions have been tested in differential interference contrast microscopy and we suggest their use also in maskless lithography and chemical vapour deposition induced by extreme ultraviolet and X-ray radiation.

An organic solution to the Kelvin myth? White light with true color temperature via exciplex formation

Abstract Many white light sources are described as having a “color temperature”, a practice that has been termed the “Kelvin myth”, since, in general, only incandescent light-emitters have a spectral distribution which closely approximates that of a black-body curve. We show that the white light emitted, via exciplex formation, from blends of two blue-emitting organic materials, has the same spectral form as black-body light, as perceived by the human eye. The color temperature is tunable, depending on the relative concentrations of the two materials. This suggests that organics could provide a white light source with a tunable, true, color temperature.

Organic microcavities based on thermally evaporated TeOx-LiF dielectric mirrors

Abstract We report on the realization of high-quality organic microcavities consisting of distributed Bragg reflectors (DBRs) based on lithium fluoride (LiF) and tellurium dioxide (TeO x ) deposited by thermal evaporation. The materials are transparent in the range from 350 nm to 5 μm and have an evaporation temperature of about 1000 K . The large difference in the refractive index (about 0.9 in the visible and near infrared range) allows one to obtain a reflectivity higher than 99% over a spectral region about 200 nm wide with a small number of periods. The mirror deposition technique is suitable for the fabrication of organic quantum microcavities in a single deposition process. Three fully evaporated organic λ cavities with Phyrrometene 580 as active material are described. The cavities show Q -value up to 300, good uniformity and reproducibility.

Highly efficient photometrics tailoring by means of optimized bell- shaped lens arrays

An innovative optical panel provided with an hexagonal array of refractive lenses having a properly optimized doublecurvature profile has been simulated by ray-tracing and fabricated by injection-moulding. Such lenses are constituted by a concave profile (having negative curvature radius) on their bottom and a convex profile (having positive curvature radius) on their top. We demonstrate that, if compared to refractive elements with conventional geometry, bell-shaped microlenses allow to collect incident rays within a wider angular range (so reducing the number of rays lost by TIR ) and to properly re-direct them. When installed on fluorescent tubes-based professional lighting systems, such refractive elements allow to reduce undesirable glare as prescribed by EN12464-1 Interior Lighting Design Standards and to finely control photometric outputs of luminaries. Besides, bell-shaped microlenses-based films were also simulated to be applied onto a bottom-emitting lambertian OLED. We demonstrate that, by properly tailoring both concave and convex profile shape, it is possible to increase the outcoupling efficiency, as well as the luminous flux emitted by the exit surface, by a factor up to 1.95 and, at the same time, to produce far-field photometric outputs characterized by uniform isocandela distribution maps with an aperture angle up to 60°.