Wilfried Vervisch

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.

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-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.

Influence on electrical characteristics of the design of 4H-SiC ultraviolet photodetectors: Theoretical analysis and simulations

This paper deals with the simulation of the reverse current density of 4H-SiC ultraviolet- (UV) photodetector devices based on p-i-n diodes. Simulations using the finite-element method presented in this paper lead to an understanding of the photodetector current density levels in dark field, as well as under UV exposure. The theoretical study coming from these simulations allows us to propose some UV-photodetector performance enhancements. A way to improve the UV-photodetector performance is to increase the current density at reverse bias. This study demonstrates the improvement in electrical characteristics achievable by either optical or electrical means. Optical simulations prove that an increase in photon harvesting is possible by using a specific patterned surface, a surface grating which behaves as a photonic crystal. In addition to the absorption enhancement achieved caused by this kind of surface, we point out wavelength selectivity. Nevertheless, the electrical simulation confirms that the electr...

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.

4H-SiC P+N UV Photodiodes : A Comparison between Beam and Plasma Doping Processes

This paper presents a study of 4H-SiC UV photodetectors based on p+n thin junctions. Two kinds of p+ layers have been implemented, aiming at studying the influence of the junction elaborated by the ion implantation process (and the subsequent annealing) on the device characteristics. Aluminum and Boron dopants have been introduced by beam line and by plasma ion implantation, respectively. Dark currents are lower with Al-implanted diodes (2 pA/cm2 @ - 5 V). Accordingly to simulation results concerning the influence of the junction thickness and doping, plasma B-implanted diodes give rise to the best sensitivity values (1.5x10-1 A/W @ 330 nm).

Optical and Electrical Simulation of 4H-SiC UV Photodetector by Finite Element Method

This paper focuses on UV-photodetector simulation. The calculus method description and the physical equations which occur in this model are presented as well as the UV-photodetector structure (p+n--n+ diode). Based on the Finite Element Method the electrical part solves the continuity and Poisson equation, and the optical part solves by Maxwell’s equation, FDTD [1]. Simulation works point out the influence of the p+-type layer on the electrical characteristics such as the current densities versus reverse bias. Indeed, simulation results show the current density increase with the decrease doping concentration or the p+-type layer thickness.

Simulations of Interactions between Fast Neutrons and 4H-SiC Detectors

Among particle detectors, particle detectors based on the wide gap semiconductor materials are many used in the nuclear area. For the reliable uses in hard and severe environment, the 4H-SiC is mainly used to the realization of nuclear detector components. This is a part of the topic of the I_SMART European project which proposes to study the nuclear detection of the thermal and fast neutron and gamma rays. In this paper, we deal with the Monte Carlo simulation results of interactions between particles and 4H-SiC detector. In particular, simulation works present the results between fast neutron and 4H-SiC sensor with a comparison between the simulation and experimental results. This article tries to point out the effect of the space charge region depletion, in particular the electric field on the signal response strength.

Optical and Electrical Simulations of Solar Cell Based on Silicon and Silicon Carbide

Among the different ways to improve the performances of light harvesting inside solar cells, multilayer configuration can be used. The bandgap of each single layer will contribute to absorption in a well defined wave-lengths range, enhancing the overall performances. Here, we investigate such performances in the case of solar cells made by two layers: a silicon one, and a SiC-based layer, and show the increasing of electrical working by means of computer simulations. These simulations are based on Finite Difference Time Domain (FDTD) for the optical calculations, on one side, and the Finite Element Method (FEM) for the electrical ones, on the other. The main goal is to show the enhancement of the electrical performances in heterostructure solar cells. In this paper, we investigate the influence of each different structure on the optical and electrical response. Our results show the influence of the device structures, in particular, the enhancement of the UV-ligth absorption inside the solar cell. Moreover, the difference structures allow us to show an improvement in the harvesting charge carrier by the heterojunction.

Full Simulation Study of UV Photodetectors Based on Pn Junctions in Silicon Carbide

This paper deals with optical and electrical simulations of 4H-SiC UV-Photodetectors based on pn junctions. The simulations are performed under the UV light, with wavelengths varying between 200 nm and 300 nm. Under reverse bias, the simulation results point out the influence of surface patterns on the current density. The studied structures of the patterns consist in a semicircle with or without a flat surface. The patterned surfaces are parametrized according to the semicircle radius R and the flat surface length L. We show that the optical absorption strongly depends on these parameters, giving a maximum value whatever the wavelength with R = 100 nm and L = 0 nm (no flat surface). However, to optimise the carrier harvest, it is important for the space charge region to be situated in a zone where the optical generation is high. This study shows that the photodetector current density increases within three orders of magnitude (from 9x10-14 A.cm-2 to 3x10-10 A.cm-2), by using the specific surface pattern given above.