Paolo Tassini

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.

Electrooptical Analysis of Effects Induced by Floating Metallic Interlayers in Organic LEDs

The aim of this paper is to investigate the electrical and optical property modifications that can be related to the presence of a nanometric metallic layer at the interface between two organic emissive materials in a stacked structure. For purposes of comparison, reference devices have also been analyzed to emphasize the increase of electrical switching and hysteresis behaviors in current-voltage plots and spectral variations in electroluminescence. In this paper, we have tried to summarize the electrical effects of the floating nanometric thin metallic layer by extracting a small number of parameters which can represent the current state of the device.

Electrical stress effects on organic thin-film transistor with solution-processed TIPS-pentacene and poly-4-vinylphenol dielectric

Organic thin-film transistors (OTFTs) with bottom-gate top-contacts architecture have been fabricated on glass substrates using solution-processed TIPS-pentacene (triisopropylsilyl pentacene) as semiconductor and cross-linked PVP (poly(4-vinylphenol)) as insulator, with Gold contacts evaporated through shadow masks. Applying constant electrical stress for short time on both gate and drain, an increase of drain current respect just finished devices have been observed in output and transfer characteristics.

FLEXIBLE SUBSTRATES COMPARISON FOR PLED TECHNOLOGY

Flexible substrate displays are critical to organic electronics, e‐paper’s and e‐ink’s development. Many different types of materials are under investigation, including glass, polymer films and metallic foils. In this work we report a comparison study of polymer films as flexible substrates for polymer light emitting diodes (PLEDs) technology. The selected polymer substrates are two thermoplastic semi‐crystalline polymers (PET and PEN) and a high Tg material that cannot be melt processed (PAR). Firstly, the chosen films were characterized in morphology and optical properties with the aim to confirm their suitability for optoelectronic applications. Transmittance was analysed by UV‐Vis spectrophotometry and roughness by a surface profilometer. Finally, the surface energy of substrates (untreated and after UV‐ozone treatment) was estimated by contact angle measurements in order to evaluate their wettability for active materials deposition.Flexible substrate displays are critical to organic electronics, e‐paper’s and e‐ink’s development. Many different types of materials are under investigation, including glass, polymer films and metallic foils. In this work we report a comparison study of polymer films as flexible substrates for polymer light emitting diodes (PLEDs) technology. The selected polymer substrates are two thermoplastic semi‐crystalline polymers (PET and PEN) and a high Tg material that cannot be melt processed (PAR). Firstly, the chosen films were characterized in morphology and optical properties with the aim to confirm their suitability for optoelectronic applications. Transmittance was analysed by UV‐Vis spectrophotometry and roughness by a surface profilometer. Finally, the surface energy of substrates (untreated and after UV‐ozone treatment) was estimated by contact angle measurements in order to evaluate their wettability for active materials deposition.

2D/3D switchable displays through PDLC reverse mode parallax barrier

ABSTRACT In this paper, we present a new type of parallax barrier, for switchable 2D/3D display vision, obtained by using a Reverse Mode Polymer Dispersed Liquid Crystal display (RV-PDLC). The parallax barrier was prepared by sandwiching the RV-PDLC film between two ITO conductive glass supports. Strips of ITO were removed from one of the supports, in order to alternate conductive to not conductive strips. In this way the RV-PDLC films could be electrically switched in a parallax barrier able to turn a 2D image in a 3D one. The RV-PDLC barrier was obtained in tree steps: (1) a nematic diacrylate monomer (NDM) was dissolved in a nematic liquid crystal (NLC) in the presence of a small quantity of radical polymerisation initiator; (2) the mixture was homeotropically aligned between the glass conductive supports, previously treated with an aligning chemical; (3) the liquid crystal monomer was polymerised by UV curing. A 2D/3D switchable device was finally obtained by coupling the parallax barrier with a 2D tablet display. GRAPHICAL ABSTRACT

Electron Beam Curing Technology for Very High-Throughput Manufacturing of Flexible Alternating Current Powder Electroluminescent Devices

Thick-film alternating current powder-based electroluminescent (ACPEL) succeeds on the market as mature technology for large-area light sources. An additional boost for its development may come from the radiation curing technology. Since it is totally compatible with high-speed roll-to-roll processing, radiation curing can offer multiple advantages to further lower costs and make easier the fabrication process of ACPEL devices. In this paper, the application of the electron beam (EB) curing technology to produce flexible ACPEL devices was explored for the first time. In particular, devices with emitting layer made by EB irradiation were successfully fabricated on poly(ethylene terephthalate) (PET) substrate. Device properties were evaluated and compared with those obtained using the conventional ultraviolet curing process. Smaller driving voltages and higher luminous output were observed for the EB treated samples as a consequence of a more cross-linked polymeric binder of the emitting layer generated. In addition, possible effects of EB overdose were also investigated; experiments revealed that excessively high doses can induce the degradation of both polymeric binder and emitting particles. Therefore, the feasibility of using the EB curing was proven to fabricate ACPEL devices, launching it as the next future technology for more sustainable, very fast, and one-step manufacturing of powder-based alternating current EL devices.

Organic LED safe-operating area investigated through high applied field dependences

In this paper, the electrical and optical properties of organic light emitting diodes (OLEDs) are analyzed, deriving the devices limits at high applied electric fields. In particular, for different device thicknesses, it is possible to conclude that the optical breakdown can be attributed to a surface phenomenon, that the temperature of electrical failure is significantly different from the optical one, that the two phenomena of failure are markedly similar at different thicknesses and that there is a maximum field beyond which the thin cathode may be destroyed due to phenomena not agreeable to thermal events.