Sylvain Vedraine

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.

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: Influence of light trapping by photonic crystal and ZnO spacer on electrical characteristics

This paper deals with organic solar cells (OSC) simulation using finite element method. Optical modeling is performed via finite difference time domain method whilst the continuity and Poisson’s equations are solved to obtain electrical characteristics of the OSC. In this work, simulation results point out the OSC structure influence on its performances, either by the interface grating or by the ZnO optical spacer introduced between the active layer (P3HT:PCBM layer) and the metallic electrode. The comparison of modeling results and experimental measurement allows us to confirm and forecast the enhancement of the photovoltaic properties such as the power conversion efficiency.

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.

Optical role of the thin metal layer in a TiOx/Ag/TiOx transparent and conductive electrode for organic solar cells

One possible alternative to ITO, the most commonly used transparent and conductive electrode (TCE) for Organic Solar Cells (OSCs) and other optoelectronic components, is to use an oxide|metal|oxide multilayer. Glass|cathode|ZnO (20 nm)|P3HT:PCBM (250 nm)|PEDOT:PSS (50 nm)|Ag (150 nm) inverted OSC structures are realized, where the cathode can be a TiOx|Ag|TiOx or ITO reference TCE. The sizing of the TiOx|Ag|TiOx (TAT) TCE structure is numerically realized by optimization of the normalized squared electric field inside the active P3HT:PCBM layer. The optimized TAT design in the whole design is different from the one involving optimization of transparency at the output of the trilayer structure in air. A photovoltaic efficiency of 2.7% is obtained for OSC with the TiOx (22 nm)|Ag (15 nm)|TiOx (19 nm) structure and can be compared to the 3.14% of efficiency obtained with the ITO reference. The short-circuit current density is identified as the crucial photoelectrical parameter. The morphology of the silver layer in TAT can give rise to an exaltation of the electromagnetic field, leading to an enhanced and undesirable absorption inside the metal layer. This exaltation is dependent of the thickness of the metal layer and induces changes in current density proportional to the normalized squared electric field inside this layer. The lost in short-circuit current density is estimated between 0.3 and 0.6 mA cm−2, and is comparable to a thickness variation of 20 nm for both TiOx layers or 2.5 nm of the silver layer. We define an exaltation coefficient of the bare electrode, which can be considered as a factor of merit to qualify the quality of the optical role of the silver layer and thereby of the trilayer electrode.

Numerical simulation of HTM-free and WO x based perovskite cells: Effects of interface conditions

Hole transport material (HTM) free and WOx based perovskite solar cells are theoretically investigated by using driftdiffusion and small signal models. The influence of interface states and leakage current is studied, and the current-voltage (J-V) and capacitance-voltage (C-V) characteristics are reproduced in reasonable agreement with experimental data, including build in potential (Vbi) variation, open circuit voltage (VOC) loss and hysteresis effects.

Surface Plasmons for Performance Enhancement of R.F. Sputtered Silver-AZO Transparent Electrodes

Dependence between optical, electrical and morphological properties of Ag-AZO coatings is studied. The main factor determining their performance as transparent electrodes is an optimal coalescence state of Ag film yielding proper plasmonic properties

Surface plasmon effect on metallic nanoparticles integrated in organic solar cells

Experimental results concerning the influence of plasmon effect from silver nanoparticles on the organic photovoltaic device performance are presented. The metallic nanoparticles (NPs) are placed on top of ITO layer using a physical vapor deposition technique. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) followed by an interpenetrated poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend layer are then spin-coated. The aluminum electrode is finally evaporated on. Photovoltaic properties compared to devices without NPs are shown. A spectrophotometric characterization is carried on. Moreover, a ToF-SIMS measurement is performed in order to obtain the depth chemical profiles of solar cell containing such NPs. Silver NPs diffusion inside other layers of the cell is investigated.

State of the Art of Photonic Structures for Solar Cells

A state of the art on photonic structures such as geometrical configurations, thin-films, gratings, photonic crystals and plasmons in solar cells is presented. Recent results obtained by several research groups are described and discussed.