The extraordinary progresses in the design and realization of structures in inorganic or organic thin films, whether or not including nanoparticles, make it possible to develop devices with very specific properties. Mastering the links between the macroscopic optical properties and the optogeometrical parameters of these heterogeneous layers is thus a crucial issue. We propose to present the tools used to characterize and to model thin film layers, from an optical point of view, highlighting the interest of coupling both experimental and simulation studies for improving our knowledge on the optical response of the structure. Different examples of studies are presented on copper indium gallium selenide, perovskite, P3HT:ZnO, PC70BM, organic layer containing metallic nanoparticles, and colored solar cells.
The extraordinary progresses in the design and realization of structures in inorganic or organic thin films, whether or not including nanoparticles, make it possible to develop devices with very specific properties. Mastering the links between the macroscopic optical properties and the opto-geometrical parameters of these heterogeneous layers is thus a crucial issue. We propose to present the tools used to characterize and to model thin film layers, from an optical point of view, highlighting the interest of coupling both experimental and simulation studies for improving our knowledge on the optical response of the structure. Different examples of studies are presented on CIGS, Perovskite, P3HT:ZnO, PC70BM, organic layer containing metallic nanoparticles and colored solar cells.
The photo conversion efficiencies of the 1st and 2nd generat ion photovoltaic solar cells are limited by the physical phenomena involved during the photo-conversion processes. An upper limit around 30% has been predicted for a monojunction silicon solar cell. In this work, we study 3rd generation solar cells named rectenna which could direct ly convert visible and infrared light into DC current. The rectenna technology is at odds with the actual photovoltaic technologies, since it is not based on the use of semi-conducting materials.<p> </p> We study a rectenna architecture consist ing of plasmonic nano-antennas associated with rectifying self assembled molecular diodes. We first opt imized the geometry of plasmonic nano-antennas using an FDTD method. The optimal antennas are then realized using a nano-imprint process and associated with self assembled molecular diodes in 11- ferrocenyl-undecanethiol. Finally, The I(V) characterist ics in darkness of the rectennas has been carried out using an STM. The molecular diodes exhibit averaged rect ification ratios of 5.
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 have decided to study the optical response of a top-flat cone shaped silicon grating, based on previous work exploring pyramid gratings.<p> </p>Through rigorous numerical methods, such as Finite Different Time Domain or Rigorous Coupled-Wave Analysis, we then designed several structures theoretically demonstrating 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. Afterwards, optical characterizations of several samples were carried out. The reflectance of the grating in the near and middle infrared domains has been measured by Fourier Transform Infrared spectrometry and a comparison with numerical analysis has been made.<p> </p>As expected, those structures offer a fair antireflective character in the region of interest. Further numerical investigations led to the fact that patterning the top of the cone could enlarge the antireflective domain to the visible region. Thus, as with the simple cone grating, a comparison of the numerical analysis with the experimental measurements is made. Finally, diffracted orders are studied and compared between both structures. Those orders are critical and must be limited as one wants to avoid crosstalk phenomena in imaging systems.
Cu(In<sub>1-x</sub>,Ga<sub>x</sub>)S<sub>2</sub> was studied using photoreflectance spectroscopy. In this study, efforts are devoted to optimizing PR set-up for measuring CIGS grown by electrodeposition: issues such as photoluminescence perturbation, high roughness and scattering are addressed. Dual frequency photoreflectance, where both probe and pump beams are modulated, is proposed here to over come the poor signal to noise ratio. Considering the low electric field regime, material parameters are extracted by employing the third derivative functional form of dielectric functions to fit data. The reliability of the technique is finally tested by measuring PR spectra on a specific 15 x 15 cm<sup>2</sup> wafer and explanations of PR line-shape evolution on this wafer are discussed.
We theoretically and experimentally study the structuration of organic solar cells in the shape of photonic crystal slabs.
Using a Finite Difference Time Domain (FDTD) method, we investigate the double structuration of the PEDOT:PSS
layer and the metallic electrode. By taking advantage of the optical properties of photonic crystals slabs, we show the
possibility to couple Bloch modes with very low group velocities in the active layer of the cells. Such Bloch modes, also
called slow Bloch modes (SBMs), allow increasing the lifetime of photons within the active layer. We show that an
absorption gain ranging between 4% and 11% is possible according to the band gap of the organic material. Finally, we
present experimental demonstration performed using nanoimprint to directly pattern the standard organic semiconductor
P3HT :PCBM blend in thin film form in the shape of a photonic crystal able to couple SBMs.
Optical surface structuration is of primary interest for applications such as photovoltaics or photodetectors.
Over last years, periodical patterns allowing antireflective effects with efficient properties have been designed
and fabricated. Some specific issues such as diffraction of undesired high energy orders are a direct consequence
of the periodical nature of this kind of pattern.
Random rough surfaces allow the antireflective effect without these undesired diffraction effects. By tuning
their statistics, random rough surfaces offer new degrees of freedom for antireflection but also for controlling
the scattering (polarization, spatial distribution). The two main parameters of such surfaces are the height
probability density function and the autocorrelation function. The height probability density function carries
information about height of the structures. The autocorrelation function is a representation of the lateral
distribution of the surface.
Our photofabrication method uses a speckle pattern recorded on a photoresist. By controlling the exposure
parameters, such as the number of exposure and the beam intensity distribution, one is able to control the
statistics of the speckle, and so of the photofabricated surfaces. Using a chromatic confocal sensor, height
mapping of these surfaces are performed. From these mappings, the height probability density and the
correlation function are calculated.
The experimental statistics are compared with the predicted theoretical ones showing a good agreement.
Results are presented showing a significant modification of the statistics of the photofabricated surfaces.
Cyclopropanofullerenes have been synthesized in the aim of being used as acceptors in blends based on regioregular poly (3,5-Hexylthiophene) RR-P3HT for photovoltaic (PV) plastic cells. These molecules used with RR-P3HT in bulk heterojunction (BHJ) configuration provided interesting characteristics: 1.5% solar conversion efficiency, 9.29 mA/cm2 current density, 0.51 V open circuit voltage, and 0.34 fill factor. The IPCE spectrum for P3HT: cyclopropanofullerene cells shows a peak around 430 nm with 71% external quantum efficiency. This result justifies the increased current density.
Experimental and numerical results concerning the influence of silver nanoparticles on the optical absorption of organic
devices are presented. The metallic nanoparticles (NPs) are placed inside an interpenetrated poly(3-hexylthiophene):
[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) layer using a physical vapor deposition technique. An
absorption enhancement by comparison to devices without NPs is shown. An increase of the absorption by annealing is
also observed. Moreover, calculations are performed via a numerical analysis based on a Finite Difference Time Domain
(FDTD) method. We demonstrate that the light absorption can mainly occur inside the active layer instead of inside the
Depending on the minimum size of their micro/nano structure, 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 of a few nanometers,
quantum theory has also to be used. These materials including quantum structures are of particular interest for other
applications, in particular for solar cells, because of their luminescent and electronic properties.
We show that the properties of electrons in multiple quantum wells can be easily modeled with a formalism similar to
that used for multilayer waveguides. The effects of different parameters, in particular coupling between wells and well
thickness dispersion, on possible discrete energy levels or energy band of electrons and on electron wave functions is
given. When such quantum confinement appears the spectral absorption and the extinction coefficient dispersion with
wavelength is modified. The dispersion of the real part of the refractive index can then be deduced from the Kramers-
Krönig relations. Associated with homogenization theory this approach gives a new model of refractive index for thin
films including quantum dots. Absorption spectra of samples composed of ZnO quantum dots in PMMA layers are in
preparation are given.
Broadband antireflection properties of material surfaces are of primary interest for a wide variety of applications: to
enhance the efficiency of photovoltaic cells, to increase the sensitivity of photodetectors, to improve the performance of
light emitting diodes, etc...
In the past, broadband antireflection multilayer coatings were widely used and recently very low refractive index
materials in thin film form have been fabricated by several groups. The research work presented in this paper aims at
modeling and fabricating bi-periodic micro-structured silicon surfaces exhibiting broadband antireflection properties in
the infrared range. These structures of pyramidal shape, which typical dimensions are smaller than the wavelength, are
not in the Effective Medium Theory (EMT) validity domain. The optimization of the optical properties of such patterned
surfaces needs a fully Finite Difference Time Domain (FDTD) rigorous description of light propagation phenomena. The
influence of various opto-geometrical parameters such as period, depth, shape of the pattern is examined. The
antireflective properties of such bi-periodic patterned surfaces is then discussed using the photonic crystal theory and
photonic band diagrams description. The structure is considered as a two dimensional periodic structure with a nonuniform
third dimension. Correlations between the density of Bloch modes, flatness of dispersion curves and the surface
reflectance are presented. The last part of this paper is devoted to the presentation of the fabrication and the
characterization of the structures. Low cost and large surface processing techniques are proposed using wet anisotropic
etching through a silica mask obtained by photolithography or nanoimprinting.
Our work deals with the improvement of "light harvesting" in organic photovoltaic cells by using photonic nanostructures. We have theoretically studied a periodically nanostructured poly
(3-hexylthiophene)(P3HT)/6,6-phenyl C61-butyric acid methyl ester (PCBM) thin film in order to increase its absorption in the near infrared spectral range. We have used a software, based on the FDTD (Finite-Difference Time-Domain) method, to calculate the absorption of light in organics solar cells. We have also considered the nanostructured photoactive layer of solar cells as a photonic crystal
and we have computed band diagrams to study the dispersion curves of this structure. We have first studied a blend (bulk heterojunction) with the same proportions of P3HT and PCBM. This material
provide at this time the best results in terms of photovoltaic efficiency. Nevertheless, in order to improve the
transport of charges to the electrodes, a model with P3HT and PCBM independently nanostructured (ordered
heterostructure) was also used. Moreover, this periodic nanostructuration allows "slow Bloch modes" to be coupled
inside the device with a low group velocity of electromagnetic waves. Thus, the interaction duration between light and
organics materials is improved.
The P3HT/PCBM photonic crystal parameters have been adjusted to maximize the density of Bloch modes and to obtain
flat dispersion curves. We have found that the light matter interaction was strongly enhanced which resulted in a 35.6%
increase of absorption in the 600 nm to 700 nm spectral range. In order to realize nanostructured organic solar cells, we
are also developing an experimental prototype, based on a patented process, which allows to nanostructure several kinds of polymers.
The concept of refractive index has to be revisited for micro or nano structured materials. The refractive index is
generally linked to the group phase velocity of the light wave traveling through a media. With such a definition it must
be associated to the effective index in waveguides or in photonic crystals. This last point leads to the concept of
metamaterials. The fundamental problem of the optical properties of nanostructured materials is tackled. With the
progresses of nanotechnologies it becomes now possible to control the nanostructure of materials. Refractive index
engineering, artificial anisotropy and antireflection structures are now possible. This paves the way to new applications
concerning photonic crystals, integrated optics, micro sensors, solar cells... Some examples of applications are given.
The Four Quadrant Phase Mask is a key component for the design of advanced coronagraphs that may be used to search exo-planets. The validity of this concept has been demonstrated in the visible and need now to be demonstrated in the mid infrared. For this purpose, two components are manufactured for wavelengths 4.75 and 16.25 μm. This manufacturing requires the deposition of ZnSe layers using Ion Assisted Deposition, followed by a lift off process.
We have designed and fabricated a silicon grating which shows antireflection properties in the [4μm ; 6 μm] spectral region. It is shown both theoretically and experimentally that, even if the refractive index and the grating period are in the extend that a simple homogenization theory can not be used, a substantial broadband antireflection effect can be obtained. The grating was made using a wet anisotropic etching technique. The reflectance was calculated with a modal method and compared successfully with the experimental results. It is shown that the grating reduces the silicon substrate
reflectance in the whole [4 μm ; 6 μm] spectral domain by a factor greater than 10.
Optical and electro-optical properties of pulse laser deposited PLZT thin films are measured using the m-lines technique. Sample preparation is described. Correlation between the deposition process and optical and electro-optical properties is presented.