Mario Petrosino

Switching Device Based on a Thin Film of an Azo-Containing Polymer for Application in Memory Cells

Results obtained on a conductive multilevel device realized with a 100 nm azo-polymer film are reported. The capability to switch between three different conductance states makes the device versatile both for Write Once Read Many and Write Many Read Many memory cell application. The analysis of the - curves indicates that the hysteretic conductance change with the external voltage can be explained in terms of different hopping distances existing between the nearest neighbors intervening in the two conformational states of the molecular structure.

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.

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.

Admittance spectroscopy of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) based organic light emitting diodes

Admittance spectroscopy of Indium Tin Oxide (ITO)–poly(3,4-ethylenedioxythiophene) (PEDOT)–polyfluorene–Al organic light emitting diodes is analyzed by varying PEDOT dispersion. Loss–frequency diagrams show one or two peaks. The results have been interpreted by using the Nicollian and Brews admittance model. Single level and multi level interface state distributions are found for each kind of PEDOT. Surface state density is about 1011 eV−1cm−2 for all distributions while the interface time constant varies between 471 μs and 220 ns. Good agreement is found between experimental data and analytical model. The combined effect of PEDOT conductivity and the inferred interface state density is analyzed and discussed.

Photosensing Properties of Pentacene OFETs Based on a Novel PMMA Copolymer Gate Dielectric

In the present work, bottom-gate top-contact organic field effect transistors (OFETs) were fabricated by evaporating a pentacene semiconductor film on top of a new insulating poly(methyl methacrylate) (PMMA) copolymer containing methacrylate units. The PMMA copolymer was synthesized in order to combine the well-known insulating properties of PMMA with the possibility to be efficiently photocured enabling photopatterning-based organic circuitry integration processes. The properties of the pentacene layer deposited on ITO/PMMA copolymer stack were studied through morphological and structural analyses. Device photoresponses and photoexcitated transients were investigated and compared to reference devices based on standard PMMA gate dielectric.

The effect of ITO surface energy on OLED electrical properties

Two simple empirical models are proposed in order to relate the ITO surface chemical physical properties to the OLED electrical behaviour. Two different organic active layers have been deposited over the ITO substrate: evaporated small organic molecule or spun polymer. In the first case it has emerged that the device current depends on the ITO surface energy through an exponential law, while in the second it is the anode potential barrier that is influenced by the surface energy decreasing. These models allow to tune the device electrical characteristics changing the materials technological features.

Analysis of pinning effect at the Alq3/metal interface in organic light emitting diodes

In order to study the improvement of electrons injection in organic light emitting diodes (OLEDs), we have realized ITO/PEDOT/PF6/Alq3/cathode OLEDs with four different metallic electrodes: Ca, Mg, Al, and Ag. The cathode barrier increment/decrement has been estimated between the four kinds of devices by using the thermionic injection theory. These results have allowed to see a very low dependence of the cathode barrier on the metal work function, explained in terms of pinning at the cathode interface. The authors propose that this pinning effect is due to the presence of a distribution of states at the Alq3/metal interface; the application of the Cowley-Sze model has returned a density of states of about 6 × 1014 eV−1 cm−2, high value satisfying the observed independence of the cathode barrier on the employed metallic electrodes.

A study on the gate voltage dependence of the activation energy in Meyer‐Neldel rule for charge mobility in pentacene OTFTs

A temperature analysis of the OTFT having pentacene as channel semiconductor and PMMA as gate dielectric has been performed. The experimental results have been studied by extracting the field‐effect mobility of the TFTs and relating it to the sample’s temperature. We have found the mobility to follow the Meyer‐Neldel rule. This behavior can be considered imputable to the channel carrier hopping. The gate voltage effect on the thermal activation energy for the mobility and the asymptotic parameter has been also taken into account.

Organic memory device with large conductance switching based on oxadiazole—containing polyether thin films

Organic memory devices were realized using oxadiazole-containing polyether as active organic layer. A thin film containing this molecule was sandwiched between n-type silicon substrate (cathode) and aluminum (anode) electrodes. The memory cells showed an on-off ratio of 4.9 orders of magnitude at 1.3 V and a threshold voltage of about 2.9 V. The analysis of transport was realized considering Space Charge Limited Conduction (SCLS) and Pai-Gill hopping models. Thanks to the Si-technology integrability and the simplicity of the realization technique, the proposed device is more attractive for low-cost applications.

Admittance electrical model of polyfluorene based organic light emitting diodes

In order to obtain an AC electrical model of polyfluorene-PEDOT poly(3,4-ethylenedioxythiophene) based organic light emitting diodes, different devices have been realized using three PEDOT dispersions. Static current-voltage characteristics have revealed exponential like transport in all the samples and the bias has shown no dependence on admittance spectra. An electrical model is proposed and the estimated capacitance has been analyzed in terms of polyfluorene bulk intra-gap state distribution obtaining a state density of about 1016 eV−1 cm−3. The hypothesis of a space charge involving completely the polyfluorene layer has been discussed.