Philippe Torchio

Slow Bloch modes for enhancing the absorption of light in thin films for photovoltaic cells

This paper deals with the improvement of “light harvesting” in photovoltaic cells by using photonic nanostructures. We theoretically study a poly-3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) thin film periodically nanostructured in order to increase its absorption. The periodic nanostructuration allows “slow Bloch modes” (group velocity close to zero) to be coupled inside the material. The P3HT/PCBM photonic crystal parameters are adjusted to maximize the density of Bloch modes and obtain flat dispersion curves. The light-matter interaction is thus strongly enhanced, which results in a 35.6% increase of absorption in the 600–700nm spectral range.

High-reflectivity HfO 2 /SiO 2 ultraviolet mirrors

High-reflectivity dense multilayer coatings were produced for the ultraviolet spectral region. Thin-film single layers and UV mirrors were deposited by ion plating and plasma ion-assisted deposition high- energetic technologies. Optical characterizations of HfO2 and SiO2 single layers are made. The optical constants obtained for these two materials are presented. HfO2 and SiO2 mirrors with a reflectance of ∼99% near 250 nm are reported.

Effect of the thickness of the MoO3 layers on optical properties of MoO3/Ag/MoO3 multilayer structures

The electrical and optical properties of MoO3/Ag/MoO3 multilayer structures have been studied using the Ag deposition rate and layer thicknesses as parameters. When the silver film is deposited at 0.20 nm/s rate, the silver layer thickness necessary to achieve the percolation threshold of the resistivity ρ towards conductive structures is 10 nm. Below 10 nm, the films are semiconductor and above the films are conductors. In the present work, the variation of the thicknesses of top and bottom MoO3 layers is shown to strongly modify the optical properties of the multilayer structures. By using a Ag thickness of 10 nm, we demonstrate an increasing of the transmittance of the MoO3/Ag/MoO3 structures by optimizing the MoO3 layers thicknesses. When the MoO3 bottom layer is 20 nm thick, and the MoO3 top layer is 35 nm, the maximum transmission is 86% at the wavelength of 465 nm, while the averaged transmission in the visible range (350 nm-800 nm) is 70%. The best measured conductivity, σ = 1.1 × 105 (Ω cm)-1, corresponds also to this MoO3 (20 nm)/Ag (10 nm)/MoO3 (35 nm) structure. A good qualitative agreement between the theoretical calculations of the variation of the optical transmittance and reflectance of the MoO3/Ag/MoO3 structures is also highlighted.

Simple layer-by-layer photonic crystal for the control of thermal emission

We present a theoretical and experimental study of a simple layer-by-layer photonic crystal structure designed for the control of the thermal emission in the infrared wavelength domain. We show that a relatively simple structure made of alternated ZnSe homogenous layers and gold microstructured grids can act as a thermal source itself giving us the unique opportunity to tailor its emission spectra. Comparisons between computed and measured transmission and emissivity spectra illustrate the relevance of our approach.

Optical modeling of organic solar cells based on CuPc and C60.

We have investigated the influence of the poly(3,4-ethylenedioxythiophene)-blend-poly(styrene-sulfonate) (PEDOT:PSS) layer on the short-circuit current density (J(sc)) of single planar heterojunction organic solar cells based on a copper phthalocyanine (CuPc)-buckminsterfullerene (C(60)) active layer. Complete optical and electrical modeling of the cell has been performed taking into account optical interferences and exciton diffusion. Comparison of experimental and simulated external quantum efficiency has allowed us to estimate the exciton diffusion length to be 37 nm for the CuPc and 19 nm for the C(60). The dependence of short-circuit current densities versus the thickness of the PEDOT:PSS layer is analyzed and compared with experimental data. It is found that the variation in short-circuit current densities could be explained by optical interferences.

Long range nanostructuring of silicon surfaces by photonic nanojets from microsphere Langmuir films

Large arrays of sub-micrometre blind holes and with a filling ratio up to 60% on areas of millimetre square are realized on silicon. The structuration ensues from combining both Langmuir–Blodgett deposition technique and ultraviolet nanosecond laser-assisted photonic nanojet ablation through C18 functionalized silica microspheres. Different laser fluence ranges and numbers of laser shots are studied to understand the tradeoff between size, quality of the craters and surface morphology after laser irradiation. In particular, tuning the irradiation fluence yields selectivity of the characteristic lateral dimension of the imprinted craters on the substrate and laser operation in multishot mode allows obtaining high quality and regularity of the surface morphology of the resulting millimetre square arrays of holes. This simple, fast, long-range and low-cost near-field nanolithography technique is of interest for fabricating devices with new functionalities and finds applications in many fields in nanoscience and nanoengineering.

Sand-castle biperiodic pattern for spectral and angular broadening of antireflective properties

This Letter deals with the antireflective properties of top-patterned pyramids, looking like sand castles, bi-periodically repeated on a silicon surface. It is demonstrated numerically that such an original pattern allows a dramatic spectral and angular broadening of the antireflective efficiency. Design examples are given for wavelengths ranging from 0.5 microm to 5 microm and incidence angles of 30 degrees and 45 degrees. Applications of such antireflective surfaces on photodetectors and solar cells are soon expected.

Radio frequency sputtered Al:ZnO-Ag transparent conductor: A plasmonic nanostructure with enhanced optical and electrical properties

Optimization of metal-based transparent conductors (MTCs) made of silver and aluminium-doped zinc oxide (AZO) prepared by radio-frequency (r.f.) sputtering has been carried out through tuning of metal film properties. The influence of morphology and related plasmonic features of AZO/Ag/AZO MTCs on their optical and electrical performance is demonstrated and it is shown that the nominal thickness of the silver layer itself is not the most crucial value determining the MTC performance. The MTC performance has been optimized by a search of deposition conditions ensuring fractal-type metal layer formation up to a certain coalescence state that enables full gaining from silver optical properties, including its plasmonic features. For 150 W- and 200 W-deposited silver, MTCs with maximum transmittance as high as 83.6% have been obtained. These coatings have a figure of merit as good as 0.01 Ω−1 and a remarkably wide spectral transparency region: transmittance higher than 70% down to 1200 nm for 200W-samples. Modelling of the MTC coatings is proposed additionally, based on variable angle spectroscopic ellipsometric measurements, which takes into account the variation of the optical properties of silver when deposited in various conditions and embedded in a semiconductor stack.

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.

Optical modeling of the ultimate efficiency of pentacene : N, N' -ditridecylperylene -3, 4, 9,10-tetracarboxylic diimide-blend solar cells

Molecular blends of pentacene: N, N′-ditridecylperylene-3, 4, 9, 10-tetracarboxylic diimide (PTCDI-C13H27) permit to cover the visible part of the solar spectrum with an absorption onset at 730nm. Although charge mobilities of pentacene and PTCDI are rather large, the efficiency of pentacene:PTCDI-C13H27 blended devices is still lower than that of poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester devices. Comparisons between experimental results and optical modeling indicate that more than 30% of the photocurrent is lost in the blend while the short circuit current should reach 13mAcm−2 in the absence of recombination processes.