L. Zuppiroli

High mobility n‐type charge carriers in large single crystals of anatase (TiO2)

Resistivity, thermopower, and Hall‐effect measurements on large single crystals of the anatase form of TiO2 all indicate high mobility n‐type carriers that are produced by thermal excitation from a density of ∼1018 cm−3 putatively present shallow donor states. The decrease of the mobility with increasing temperature is consistent with the scattering of carriers by the optical phonons of TiO2.

Internal electric field and charge distribution in multilayer organic light-emitting diodes

The internal electric field in multilayer organic light-emitting diodes (OLEDs) is investigated using a combination of experimental measurement and numerical device modeling. This approach results in a detailed understanding of the functioning of a multilayer OLED. The method is applied to a standard device structure that has received broad attention in literature. From the experimental part, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. Device simulations fully clarify the situation, giving an insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we find that there is a leakage of unrecombined holes towards the cathode at low voltages. We also find a strong variation of the electric field in the tris(8-hydroxyquinolinato)aluminum layer.

Numerical model for organic light-emitting diodes

An extensive numerical model recently developed for the multilayer organic light-emitting diode is described and applied to a set of real devices. The model contains a detailed description of electrical contacts including dipolar layer formation, thermionic and tunneling injection, space charge effects, field dependent mobilities and recombination processes. The model is applied to simulate several single layer devices and the family of bilayer devices made in our group. It provides insight into the energy level shifts, internal electric fields and charge distribution (and consequently recombination) throughout the device. Finally, the analysis is extended to the optimization of bilayer device.

Investigation of the charge transport through disordered organic molecular heterojunctions

We present a three-dimensional multiparticle Monte Carlo (3DMPMC) simulation of hopping transport in disordered organic molecular media. We used this approach in order to study the charge transport across an energetically disordered organic molecular heterojunction which is known to strongly influence the characteristics of the multilayer devices based on thin organic films. The role of the energetic disorder and its spatial correlations, which govern the transport in the bulk, are examined here for the bilayer homopolar system where the heterojunction represents the bottleneck for the transport. We study the effects of disorder on both sides of the heterojunction, including the effects of the spatial correlation within each material and among the layers. The 3DMPMC approach allowed us to correctly tackle the effects of the Coulomb interaction among carriers in the region where the charge accumulation in the device is particularly important and the Coulomb interaction most pronounced. The Coulomb interact...

Swelling and microcracking of boron carbide subjected to fast neutron irradiations

The neutron absorbing ceramic boron carbide was irradiated in the Phenix fast breeder reactor between 600 and 1500u2009°C. Selected pellets were observed at different magnifications using optical ceramography, scanning electron microscopy, and transmission electron microscopy. The bulk swelling was measured as a function of the burn‐up and compared to a recent model based on microstructural observations. Intergranular microcracking was identified as the main source of troubles in the use of boron carbide at high fluence levels and studied in relation with our recent models.

Polarization effects in the channel of an organic field-effect transistor

We present the results of our calculation of the effects of dynamical coupling of a charge carrier to the electronic polarization and the field-induced lattice displacements at the gate interface of an organic field-effect transistor (OFET). We find that these interactions reduce the effective bandwidth of the charge carrier in the quasi-two-dimensional channel of a pentacene transistor by a factor of 2 from its bulk value when the gate is a high-permittivity dielectric such as (Ta2O5), while this reduction essentially vanishes using a polymer gate insulator. These results point to carrier effective bandwidth as a possible trigger of the dielectric effects on the mobility reported recently in OFETs.

The dielectric response of boron carbide due to hopping conduction

We present a quantitative analysis of ac electrical conductivity and dielectric constant measurements that were performed on boron carbide samples. This material is crystalline but disordered on the atomic scale because of the existence of a large stoichiometry range from B9C to B4C. The polaronic character of the hopping process is well recognized in this disordered solid. Moreover, the boron carbides of this study are also ceramic materials with grain sizes ranging from 1 to 100 μm, depending on the sample. After having considered most of the models which account for the dielectric response, we have applied Dyre’s model which assumes a uniform distribution of activation energies between two extreme values and uses an effective‐medium‐type calculation. It permits an elegant interpolation between dc and ac results and a reasonable estimate of the density of localized states (∼1018/cm3u2009K). The microstructure has little or no influence on the dielectric response in the explored frequency and temperature ran...

Fluorescent nanopigments: Quantitative assessment of their quantum yield

In the last few years, an intense research effort has focused on the synthesis of fluorescent nanopigments for functional inks, light harvesting, tagging, tracing, (bio)labeling, imaging, and lighting applications. Moreover, combined with dielectric matrices, these fluorescent nanoparticles may open the way to the realization of novel optophotonic devices. In particular, due to the large variety of available organic fluorescent dyes, their encapsulation into either an inorganic or an organic host is a very promising approach to synthesize a large palette of new fluorescent nanopigments. However, since the dye encapsulation may affect the fluorescence efficiency, measuring the quantum yield of fluorescent nanopigments is of paramount importance for the development of any connected application. In this article, we present a diffuse reflectance (DR) technique that enables the quantitative assessment of the quantum yield of fluorescent nanoparticles such as zeolite L nanocrystals and poly(methyl methacrylate) nanospheres both loaded with fluorescent perylene molecules. Our method is validated by measuring a well known fluorescence standard and by comparing the results obtained for a model zeolite nanopigment with those provided by an alternative DR technique. Reliable and reproducible quantum yield values are obtained for both low- and high-efficiency fluorescent nanoparticles. Our technique can thus enable systematic and quantitative studies that may yield an important insight in the mechanisms affecting the fluorescence efficiency of a large variety of nanopigments.

Optical and electrical properties of evaporated 2,5‐bis‐methylthio‐7,7’,8,8’‐tetracyanoquinodimethane

2,u20095u2009‐bis‐methylthiou2009‐u20097u2009,u20097’,u20098,u20098’u2009‐u2009tetracyanoquinodimethane (BMT‐TCNQ), a strong π‐electron acceptor, has been newly synthesized and its optical and electrical properties have been investigated in evaporated films. A large bathochromic shift of the visible absorption band (474–590 nm) has been observed in going from solution to the solid state. This is attributed to enhanced electron delocalization in the solid state through close intermolecular S‐N contacts (3.2 A) between the S atom of the ‐SCH3 group of one BMT‐TCNQ molecule and the N atom of the ‐C≡N group of the neighboring molecule. Evaporated BMT‐TCNQ exhibits a high electrical conductivity of 2×10−5 Su2009cm−1 at room temperature as well as a high spin concentration of 2×1017 spins/cm3. The origin of the charge carriers is presumably ascribed to electron‐donating impurities which cause the ESR signals, whereas the close intermolecular S‐N contacts considerably facilitate the carrier hopping throughout the crystal.