G. Nenna

Optical properties of polystyrene-ZnO nanocomposite scattering layer to improve light extraction in organic light-emitting diode

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 radiativetransfer 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.

Monitoring of concentrated radiation beam for photovoltaic and thermal solar energy conversion applications

Methods for evaluating the light intensity distribution on receivers of concentrated solar radiation systems are described. They are based on the use of Lambertian diffusers in place of the illuminated receiver and on the acquisition of the scattered light, in reflection or transmission mode, by a CCD camera. The spatial distribution of intensity radiation is then numerically derived from the recorded images via a proprietary code. The details of the method are presented and a short survey of the main applications of the method in the photovoltaic and thermal solar energy conversion field is proposed. Methods for investigating the Lambertian character of commercial diffusers are also discussed.

Insights into thermal degradation of organic light emitting diodes induced by glass transition through impedance spectroscopy

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.

Nanostructured PEDOT:PSS film with two-dimensional photonic quasi crystals for efficient white OLED devices

A polymeric PEDOT:PSS film nanostructured with Photonic Quasi Crystals that opens the path towards more efficient white OLEDs is presented. For the first time three different quasi crystal families were fabricated (octagonal, dodecagonal and Thue-Morse) onto a conductive polymeric film combining high-resolution electron beam lithography (EBL) and plasma etching techniques to improve light extraction and to control spectral tunability. The efficiency gain obtained in light extraction holds great promise for the use of quasi crystals as functional components in polymeric based White Organic Light Emitting Diode (WOLED) devices.

Novel organic LED structures based on a highly conductive polymeric photonic crystal electrode

In this work we demonstrate the possibility to realize a novel unconventional ITO-free organic light emitting diode (OLED) utilizing a photonic polymeric electrode. Combining electron beam lithography and a plasma etching process to partially structure the highly conductive poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) it is possible to realize an embedded photonic crystal (PC) structure. The realized PC-anode drastically reduces the light trapped in the OLED, demonstrating the possibility to eliminate further process stages and making it easier to use this technology even on rollable and flexible substrates.

Graded-size microlens array by the pyro-electrohydrodynamic continuous printing method.

In the present work, the pyro-electrohydrodynamic technique was used for the realization of tunable-size microlens arrays. Poly(methyl methacrylate) dissolved in different solvent mixtures was used as the polymeric material for the realization of the microstructures. By controlling the experimental parameters and in particular, the volume of the drop reservoir, graded-size square arrays of tens of microlenses with focal length in the range 1.5-3 mm were produced. Moreover, the optical quality and geometrical features were investigated by profilometric and interferometric analysis.

Effect of PEDOT:PSS ratio on the electrical and optical properties of OLEDs

We report on the employ of several kinds of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) dispersions as hole injection layer to increase the stability and the charge injection in organic light emitting diodes (OLEDs). The PEDOT:PSS dispersions are different for PEDOT:PSS ratios and their properties were characterized by contact angle measurement, UV-Vis-IR transmittance and I-V characteristics. Multi layer ITO-PEDOT:PSS-PF6-Alq3- Al OLEDs have been manufactured and the electrical and optical properties have been extensively investigated and discussed in function of PEDOT:PSS ratio. We have found that a major quantity of PEDOT induces a decrement of ITO hole barrier and so an increment of OLED current and luminance. Performed transport analysis have pointed out that PEDOT:PSS carrier density ND has a notable role in electrical transport at high bias.

Particle size dependence of resonant-tunneling effect induced by CdS nanoparticles in a poly(N-vinylcarbazole) polymer matrix

CdS nanoparticles of different sizes were synthesised in poly(N-vinylcarbazole) and studied in device structures glass/indium tin oxide (ITO)/PVK:CdS/Al. Electrical bistability and negative differential resistance (NDR) effects were observed in the current-voltage characteristics. In addition, the devices showed a considerable enhancement of the current magnitude. A dependence of the current conduction on the nanoparticle size and size distribution in the polymer was studied through electrical impedance measurements. The study revealed the importance of the charge effects of the nanoparticles resulting in a bistable behavior. A resonant tunneling current model was proposed to explain the NDR and its relation with the nanoparticle size and size distribution.

Effect of electrodes properties on OLED performances

The effects induced by different electrical contacts, both for the anode and for the cathode, have been analyzed in Organic Light Emitting Diodes (OLEDs). The properties of anode electrode, Indium Tin Oxide (ITO), have been varied through different surface treatments allowing roughness control, carbon impurity removal, spikes decrement. These induce changes of ITO surface chemical-physical characteristics as roughness, surface energy and surface polarity. OLEDs manufactured employing treated ITO have showed an improvement of 25 times in luminance. Thermionic injection model has been used to estimate decrement in effective hole barrier at ITO-organic layer. It is shown that this effect is correlated to ITO surface energy. The second step of process optimization has concerned the cathode electrode investigation. In order to perform this task, Al, Ca/Al, Ag, Mg/Ag have been used to realize different ITO/PEDOT:PSS/PF6/Alq3/Metal OLEDs. Measurements of electrical and optical behaviour have been performed. A thermionic injection model, with and without Schottky barrier decrement, has been used to calculate the change of the cathode electrical barrier.

Double-cavity radiometer for high-flux density solar radiation measurements

A radiometric method has been developed, suitable for both total power and flux density profile measurement of concentrated solar radiation. The high-flux density radiation is collected by a first optical cavity, integrated, and driven to a second optical cavity, where, attenuated, it is measured by a conventional radiometer operating under a stationary irradiation regime. The attenuation factor is regulated by properly selecting the aperture areas in the two cavities. The radiometer has been calibrated by a pulsed solar simulator at concentration levels of hundreds of suns. An optical model and a ray-tracing study have also been developed and validated, by which the potentialities of the radiometer have been largely explored.