Judikaël Le Rouzo

Optical-electrical simulation of organic solar cells: excitonic modeling parameter influence on electrical characteristics

This paper deals with Organic Solar Cells (OSCs) simulation using finite element method. Optical modeling is performed via Finite Difference Time Domain method whereas the continuity and Poisson’s equations are solved to obtain electrical characteristics of the OSC. In this work, simulation results point out the influence of physical parameters such as the exciton diffusion coefficient or the exciton lifetime on OSC performances. The comparison of modeling results and experimental measurement allows the exciton recombination, dissociation rate and lifetime to be determinated.

Metamaterial filters at optical-infrared frequencies.

We propose two distinctive designs of metamaterials demonstrating filtering functions in the visible and near infrared region. Since the emissivity is related to the absorption of a material, these filters would then offer a high emissivity in the visible and near infrared, and a low one beyond those wavelengths. Usually, such a system find their applications in the thermo-photovoltaics field as it can find as well a particular interest in optoelectronics, especially for optical detection. Numerical analysis has been performed on common metamaterial designs: a perforated metallic plate and a metallic cross grating. Through all these structures, we have demonstrated the various physical phenomena contributing to a reduction in the reflectivity in the optical and near infrared region. By showing realistic geometric parameters, the structures were not only designed to demonstrate an optical filtering function but were also meant to be feasible on large surfaces by lithographic methods such as micro contact printing or nano-imprint lithography.

Optical properties of dielectric thin films including quantum dots

Depending on the minimum size of their micro/nanostructure, thin films can exhibit very different behaviors and optical properties. From optical waveguides down to artificial anisotropy, through diffractive optics and photonic crystals, the application changes when decreasing the minimum feature size. Rigorous electromagnetic theory can be used to model most of the components, but, when the size is a few nanometers, quantum theory also has to be used. The materials, including quantum structures, are of particular interest for many applications, in particular for solar cells because of their luminescent and electronic properties. We show that the properties of electrons in periodic and nonperiodic multiple quantum well structures can be easily modeled with a formalism similar to that used for multilayer waveguides. The effects of different parameters, in particular the coupling between wells and well thickness dispersion, on possible discrete energy levels or the energy band of electrons and on electron wave functions are given. When such quantum confinement appears, the spectral absorption and extinction coefficient dispersion with wavelength are modified. The dispersion of the real part of the refractive index can be deduced from the Kramers-Kronig relations. Associated with homogenization theory, this approach gives a new model of the refractive index for thin films including quantum dots. The bandgap of ZnO quantum dots in solution obtained from the absorption spectrum is in good agreement with our calculation.

Flat-top and patterned-topped cone gratings for visible and mid-infrared antireflective properties

Achieving a broadband antireflection property from material surfaces is one of the highest priorities for those who want to improve the efficiency of solar cells or the sensitivity of photo-detectors. To lower the reflectance of a surface, we are concerned with the study of the optical response of flat-top and patterned-topped cone shaped silicon gratings, based on previous work exploring pyramid gratings. Through rigorous numerical methods such as Finite Different Time Domain, we first designed several flat-top structures that theoretically demonstrate an antireflective character within the middle infrared region. From the opto-geometrical parameters such as period, depth and shape of the pattern determined by numerical analysis, these structures have been fabricated using controlled slope plasma etching processes. In order to extend the antireflective properties up to the visible wavelengths, patterned-topped cones have been fabricated as well. Afterwards, optical characterizations of several samples were carried out. Thus, the performances of the flat-top and patterned-topped cones have been compared in the visible and mid infrared range.

Fast and reliable approach to calculate energy levels in semiconductor nanostructures

Abstract. We propose a method under the effective mass approximation with an original formulation that applies to quantum wells, circular quantum wires, and spherical quantum dots of arbitrary materials with sizes as small as 1 nm. Hundreds of structures are resolved on the second scale on a laptop, allowing for optimization procedures. We demonstrate its capability by confronting bandgap calculations with exhaustive literature data for CdS, CdSe, PbS, and PbSe nanoparticles. Our approach includes a correction of the mass to address the nonparabolicity of the band structure. The correction gives an accuracy comparable to more demanding calculation methods, such as eight-band k·p, tight-binding, or even semiempirical pseudopotential methods. The effect of the correction is shown on the intrasubband optical properties of InGaAs/AlGaAs coupled quantum wells.

Optical properties of antireflective flat or rough patterned topped silicon cones gratings

Abstract Achieving a broadband antireflection property from material surfaces is one of the highest priorities in photosensing applications. To demonstrate a low reflectance, we are concerned with the study of the optical response of flat-, patterned- and rough-top silicon cone gratings. Based on previous work exploring pyramid gratings, we first designed several flat top structures that theoretically demonstrate an antireflective character within the middle infrared region. In order to extend the antireflective properties up to the visible wavelengths, patterned- and rough-topped cones have been fabricated as well. Optical characterizations of several samples were carried out in the visible and mid infrared range, in order to determine the effect of structuration on the performances of the different cones gratings.

Inverted cones grating for flexible metafilter at optical and infrared frequencies

By combining the antireflective properties from gradual changes in the effective refractive index and cavity coupling from cone gratings and the efficient optical behavior of a tungsten film, a flexible filter showing very broad antireflective properties from the visible to short wavelength infrared region and, simultaneously, a mirror-like behavior in the mid-infrared wavelength region and long-infrared wavelength region has been conceived. Nanoimprint technology has permitted the replication of inverted cone patterns on a large scale on a flexible polymer, afterwards coated with a thin tungsten film. This optical metafilter is of great interest in the stealth domain where optical signature reduction from the optical to short wavelength infrared region is an important matter. As it also acts as selective thermal emitter offering a good solar-absorption/infrared-emissivity ratio, interests are found as well for solar heating applications.

Van Hove singularities in intersubband transitions in multiquantum well photodetectors

Photocurrent spectra of quantum well infrared photodetector (QWIP) devices have been studied over more than three orders of magnitude, revealing features which have been largely overlooked before. Electric field assisted tunneling and, more surprisingly, Van Hove singularities at the miniband edges are shown to play an important role in the low and high energy parts of the QWIP photocurrent spectra, respectively. The photoresponse of QWIPs away from their peak responsivity is found to be non-negligible (>1% in the 3–5μm for a 8–12μm detector), which has to be taken into consideration when optimizing multispectral devices.

Model of self assembled monolayer based molecular diodes made of ferrocenyl-alkanethiols

There has been significant work investigating the use of self assembled monolayers (SAMs) made of ferrocenyl terminated alkanethiols for realizing molecular diodes, leading to remarkably large forward-to-reverse current rectification ratios. In this study, we use a multiband barrier tunneling model to examine the electrical properties of SAM-based molecular diodes made of HSC 9 Fc, HSC 11 Fc, and HSC i FcC 13Ai (0 i 13). Using our simple physical model, we reproduce the experimental data of charge transport across various ferrocenyl substituted alkanethiols performed by Nijhuis, Reus, and Whitesides [J. Am. Chem. Soc. 132, 18386–184016 (2010)] and Yuan et al. [Nat. Commun. 6, 6324 (2015)]. Especially, the model allows predicting the rectification direction in HSC i FcC 13Ai (0 i 13) based molecular diodes depending on the position of the ferrocenyl (Fc) moiety within the molecules. We show that the asymmetry of the barrier length at both sides of the Highest Occupied Molecular Orbital of the ferrocenyl moiety strongly contributes to the rectifying properties of ferrocenyl-alkanethiol based molecular junctions. Furthermore, our results reveal that bound and quasi-bound states play an important role in the charge transport.

Low-dimensional optics

Abstract. Thanks to progress in material science and nanotechnologies, surfaces and thin films can now be structured at different scales. Photonics components take advantage of this possibility to fulfill still more and more complex functions. They are composed of organic and inorganic materials, dielectrics, semiconductors, and metallic materials, or a mixture of them. Multiscale and chiral structures can be used to control both spectral and spatial distributions of light together with its polarization state. The optical mode density in the near field and in the far field can then be designed in particular by combining more or less resonant structures for the optical waves, associating diffraction, interferences, and anisotropic structures like Fabry–Perot, waveguide, plasmons, and photonic crystals. Artificially nanostructured materials, often called metamaterials, exhibit new properties. Different phenomena, including optical topological insulator and structures for vortex waves transporting angular momentum of photons, are discussed and illustrated. With the development of nanometer size structures, another step is taken toward allowing control of the intimate interaction of optical waves with materials to tune their basic electronic properties and permittivity. Both optical and electronic properties are also strongly dependent on coupling effects and need a global approach.