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Today, the field of electroluminescence of organic semiconductors is dominated by two kinds of materials: phosphorescent and fluorescent small molecules, in particular for applications where high emission power is needed, like lighting. Although phosphorescent compounds seem to be the most promising in terms of external quantum efficiency and low power consumption, fluorescent compounds show high stability and the possibility to be deposited avoiding codeposition with a host. This is particularly true for oligothiophenes which show high stability and the possibility to tune the emission wavelength in a very wide range, from green-bluish to near infrared without the need of coevaporation. The possibility to functionalize these compounds in a very flexible way and finely tailor their properties, make this class of molecules strongly competitive with respect to standard ones (also phosphorescent), although much research must still be carried out to further improve the stability and the efficiency of devices based on these materials. We are currently pushing up this research field trying to mix the best technology for OLEDs (p-i-n technology) with the best thiophene oligomeric materials with the aim to generate new kinds of electroluminescent devices for different pourposes: from display to lighting and automotive.

The development of alternative deposition techniques of conjugated polymeric compounds is an important step towards the fabrication of low cost multi-layered organic light emitting diodes suitable for display or lighting applications. In this work we show a white light -emitting polymeric hetero-structured diode consisting of three-stacked blue, red and green light emitting polymers. In order to circumvent the issue of selecting orthogonal solvents in solution -deposition approaches, we combine spin-coating with the Matrix Assisted Pulsed Laser Evaporation technique, resulting in the realization of a polymeric multilayered stack. By controlling the carriers and the energy transfer across the three light emitting layers interfaces, as well as the interplay between the deposition parameters, a pure white colour emission with Commission Internationale de l'Éclairage coordinates (X=0.327, Y=0.374) and a Color Rendering Index of 70 have been achieved. Our study represents the first proof of a light emitting diode made by multilayer polymeric thin films emitting white light.

Three different families of chemically engineered rod-shaped CdSe/CdS colloidal nanocrystals have been embedded into a poly(methyl methacrylate) matrix to realize a set of color-tunable photoluminescent filters for RGB light emitting devices, which demonstrate excellent optical transparency (in the range of wavelengths not corresponding to nanocrystal absorption), efficient photoluminescence and good thermal- and photo-stability. Accurate morphological and optical characterization of nanocomposite foils is provided as a function of nanorod size and content, and their color conversion properties are investigated in combination with a blue-emitting LED source. This approach combines the tunable optical features of inorganic quantum-confined light emitters with the facile processability of the polymeric host and offers a highly versatile design tool, which can be exploited in a wide spectrum of lighting and photonic devices. The preparation procedure reported here is even compatible with the implementation of an engineered array of microlens on the front-end surface of the nanocomposite foil and thus makes possible a tailored control of the color-converted photometric pattern.

Extremely low voltage white organic light-emitting devices (WOLEDs) with fluorescent emitters are realized exploiting p-i-n structure. White light is obtained by two complementary colors system, in which the yellow and the blue emitting components are based on 5,6,11, 12-tetraphenylnaphthacene (rubrene) and 4,4-bis-2, 2-diphenylvinyl-1, 1-spirobiphenyl (Spiro-DPVBi), respectively. The effects on the device performances of various electron blocking layers and hosts for rubrene are discussed. The best device shows a luminance of 1000 cd/m(2) at bias of as low as 2.9 V, and 10 000 cd/m(2) at 4.7 V with a maximum power efficiency of 8.7 lm/W. The Commission Internationale de l'Eclairage chromaticity coordinates change from (0.36, 0.45) at 1000 cd/m(2) to (0.33, 0.42) at 10 000 cd/m(2) showing high color stability. (C) 2008 American Institute of Physics.

Very low voltage organic light-emitting diodes using a fluorescent linear S,S-dioxide oligothiophene as emitting layer has been realized using a p-i-n structure. The device reaches a remarkable luminance of 10 000 cd/m(2) at only 9 V, which is two orders of magnitude higher than the simple bilayer structure already reported for this active material. Due to the doping of the transport layers, a maximum power efficiency of 2.1 lm/W was reached against 0.2 lm/W of the corresponding undoped device. As a consequence of this higher power efficiency, the reduced self-heating of the p-i-n device structure, compared to the undoped devices, determines the best operating condition to check the intrinsic stability of the emitting layer. Aging measurements reveal indeed a very high stability, with extrapolated device lifetimes at about 10(8) and 2200 h at starting luminances of 100 and 3200 cd/m(2), respectively.

We demonstrate efficient red phosphorescent organic light emitting diodes with a bipolar emission structure (D-EML) formed by two different layers doped with a red phosphorescent dye. Due to its self-balancing character, the recombination zone is shifted far from the emission/carrier-blocking-layer interfaces. This prevents the accumulation of carriers at the interfaces and reduces the triplet-triplet annihilation, resulting in an improved efficiency of the D-EML device compared with the standard single-EML architecture. However, a current efficiency of 8.4 cd A(-1) at 10 mA cm(-2) is achieved in the D-EML device compared with 3.7 cd A(-1) in the single-EML device.