J. Le Rouzo

Photonic crystals for improving light absorption in organic solar cells

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.

Photonic crystals for light trapping within organic solar cells

We propose a methodology allowing the design of the active layer of organic solar cells in the shape of a photonic crystal. An optimised photonic crystal allows trapping the light in a layer at specific wavelengths thanks to a coupling of a low group velocity mode called slow Bloch mode. This method is used to design two structures which allow to improve absorption of light in organic solar cells for wavelengths close to the band gap of an active layer composed of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C61-butiryc acid methyl ester (PCBM). Nevertheless, while the first structure does not allow an efficient charges harvesting by the electrodes, the second structure can be beneficial for both the optical and the electrical properties of the cell thanks to the structuring of the Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer.

Optical behavior of silver nanoparticles embedded in polymer thin film layers

The study of metal nanoparticles (NPs) is challenging for the control of the light matter interaction phenomena. In this context, our work is focused on optical characterization and modeling of polymer thin films layers with inclusions of previously chemically synthesized NPs. Through the presence of metallic NPs in polymer thin films, the optical properties are assumed to become tunable. Thin film layers with inclusions of differently shaped and sized silver NPs, such as nanospheres and nanoprisms, are optically characterized to get the scattering, the reflection and the absorption of the layers. One step and two step seed based methods of silver ions reduction are used for the chemical synthesis of nanospheres and nanoprisms. The plasmonic resonance peaks of these colloidal solutions range from 360 to 1300 nm. A poly vinyl pyrrolidone (PVP) polymer matrix is chosen for its light non-absorbing and NP-stabilizing properties. Knowledge on the shape and size of the NPs embedded in the spin coated layers is obtained by transmission electron microscopy (TEM) imaging. The optical properties include spectrophotometry and spectroscopic ellipsometry (SE) measurements to get the reflectance, the transmittance, the absorptance and the optical indices n and k of the heterogeneous layers. A redshift in absorption is measured between deposited nanospheres and other shaped NPs. FDTD simulations allow calculation of far and near field properties. The visualization of the NP interactions and the electric field enhancement, on and around the NPs, are studied to improve the understanding of the far field properties.

Toward a nanoimprinted nanoantenna to perform optical rectification through molecular diodes

This work presents investigations about the realization and modelization of rectenna solar cells. Rectennas are antennas coupled with a rectifier to convert the alternative current originating from the antenna into direct current that can be harvested and stored. By reducing the size of the antennas to the nanoscale, interactions with visible and near-infrared light become possible. If techniques such as nanoimprint lithography make possible the fabrication of sufficiently small plasmonic structures to act as optical antennas, the concept of rectenna still faces several challenges. One of the most critical point is to achieve rectification at optical frequencies. To address this matter, we propose to use molecular diodes (ferrocenyl-alkanethiol) that can be self-assembled on metallic surfaces such as gold or silver. In this paper, we present a basic rectenna theory as well as finite-difference time-domain (FDTD) optical simulations of plasmonic structures and experimental results of both nanoimprint fabrication of samples and characterizations by electron microscopy, Raman spectroscopy, and cyclic voltammetry techniques.

Characterization and modeling tools for light management in heterogeneous thin film layers

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.

Low dimensional optics

Thanks to progresses in material science and nanotechnologies, surfaces and thin films can now be structured at different scales. Photonics components take benefit of this possibility to fulfill still more and more complex functions. They are composed as well of organic as inorganic materials, dielectric, semiconductor, and metallic materials, or a mixture of them. Multiscale and chiral structures can be used to control both spectral, spatial distribution 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, photonic crystals ... Artificially nanostructured materials often called metamaterials exhibit new properties. Different phenomena recently considered, including optical topological insulator and structures for vortex waves transporting angular momentum of photons, will be also discussed and illustrated. With the development of nanometer size structures another step is overtaken allowing the 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 needing a global approach.

Innovative approaches in thin-film photovoltaic cells

Abstract: This chapter discusses the use of new approaches in thin film photovoltaic solar cells. The chapter first reviews devices which use nanowires and quantum dots in inorganic thin film solar cells. The second part is devoted to organic solar cells, explaining their working principles and strategies for light trapping and efficiency enhancement. The last part describes new chalcopyrite materials deposited by state-of-the-art technologies at the industrial level such as spray, electrodeposition, or doctor blading. Such technologies allow low-cost deposition of advanced materials on large surfaces for harvesting sun energy.