In optoelectronics, inkjet printing (IJP) technology is being developed as an alternative to the traditional techniques for organic materials deposition. In this work, we report the fabrication of organic light-emitting diodes (OLEDs) on the flexible substrate by studying the effect of a surface chemical treatment on the inkjet printed polymer film morphology. The employed piranha treatment increases the substrate surface energy and improves the wettability, thus inducing a decrease in the IJ printed drop thickness. The IJ printed polymer (poly(9,9-dihexyl-9H-fluorene-2,7-diyl)) is the hole-transporting layer (HTL) of a hybrid structure in which the other layers are deposited by vacuum thermal evaporation. Furthermore, in order to determine the effect of the IJ deposition method on the manufactured OLED performances, we compare them to those of devices fabricated using standard technologies. With this aim, OLEDs with the same structure are fabricated by replacing the IJ printed polymer with a spin-coated film employing the same polymer solution. The electrical and optical properties of the electroluminescent devices are investigated and discussed. Despite the lack of thickness uniformity in IJ printed film, which is an intrinsic, technological limit, OLEDs with IJ printed HTL show electro-optical characteristics that are similar to the ones of OLEDs with spin-coated HTL.

The electroluminescence (EL) of isotactic and syndiotactic poly(N-pentenyl-carbazole) (PPK), achieved by coordination polymerization, is studied in order to investigate the interrelation between thepolymer tacticity and theirphysical-chemical properties. The use of these polymers in organic light-emitting diode (OLED) fabrication is also explored. Thermal and x-ray diffraction analyses of PPKs show that the isotactic stereoisomer is semicrystalline, whereas the syndiotactic one is amorphous. Optical analysis of both stereoisomers, carried out on film samples, reveals the presence of two different excimers:'sandwich-like' and 'partially overlapping'. Nevertheless, the emission intensity ratio between 'sandwich-like' and 'partially overlapping' excimers is higher in the isotactic than in the syndiotactic stereoisomer. Using the synthesized polymers as OLED emitting layers, the influence of the polymer tacticity on theEL properties of the device is highlighted. In detail, while blue OLEDs are obtained by using the syndiotactic stereoisomer, OLEDs witha multilayer structure fabricated with the isotactic stereoisomer emit white light. The contribution of three different emissions (fluorescence, phosphorescence and electromer emissions) with comparable intensities to the detected white light is discussed.

In this paper, we present a correlation study between the working temperature of OLEDs (Organic Light Emitting Diodes) and the electroluminescence and driving voltage changes. The aim is to investigate the relationship between the operating temperature and the aging mechanisms. We have found that performances degradation of devices is strictly related to the glass transition temperature (Tg) of organic layers, and that electrical failure is reached only for temperatures higher than Tg.

Highly sensitive alternate current (ac) impedance measurements with variable temperature have been performed to investigate the optical and electrical failure mechanisms during the glass transition phenomena in the archetypal ITO/TPD/Alq3/Al organic light emitting diode (OLED) structure. Since the device degradation is mainly related to the lower glass transition temperature (Tg) of the N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), this study is focused on the frequency response of thin TPD films approaching the glassy region. The related experimental data are discussed in the framework of the universal dielectric response model. By ac measurements, TPD glass transition temperature is located and temperature regions with different OLED behaviors are evidenced. The relation between the behaviors of TPD frequency response and of the OLED electro-optical response, while the temperature approaches the glass transition region, is discussed.

In this work, experimental measurements on polystyrene-ZnO nanocomposite scattering films and on organic light-emitting device with and without the scattering layers are presented. The results are also compared with Henyey-Greenstein radiative-transfer model to narrow down the parameters that can be important in the identification of more suitable scattering layers. As a result, an increase of efficiency of about 30% has been obtained that it can be translated in 60% of outcoupled light in respect to the total generated amount.