J.J. Simon

Enhanced antireflecting properties of micro-structured top-flat pyramids.

This paper aims at modeling bi-periodic micro-structured silicon surfaces exhibiting broadband antireflection properties in the infrared range using Rigorous Coupled-Wave Analysis (RCWA). These structures of pyramidal shape, which typical dimensions are smaller than the wavelength, are not in the Effective Medium Theory (EMT) validity domain. 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 are then discussed using the photonic crystal theory and photonic band diagrams description. Correlations between the density of Bloch modes, their localizations with respect to the incident medium light line and the surface reflectance are presented.

Spherically shaped micro-structured antireflective surfaces

An antireflecting micro-structured interface, working in the resonance domain, and made from a bi-periodic array of semi-spherical hollowing-out in a silicon substrate is presented. Its parameters such as sphere radius and position of sphere centers from the surface are optimized numerically. A simple and robust process is described allowing such kind of antireflective surfaces to be fabricated for the infrared range. Spectral and angular reflectance measurement demonstrates the efficiency of the antireflective micro-structured interface which can easily be adapted for the visible range and for photovoltaic applications by a simple homothetic modification of the micro-structure typical dimensions.

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.

Overcoming the V oc limitation of CZTSe solar cells

The Voc deficit issue is considered to be the most significant limitation of high-performance kesterite devices [1]. Secondary phases, defects, band tailing and interface recombination at the kesterite/CdS heterojunction are possible reasons for the low Voc. Raman spectroscopy and PL imaging are applied here for the detection of these phenomena. Their impact on Voc and the efficiency of CZTSe devices is investigated. Process steps are optimized so as to limit their negative influence. Control of secondary phase removal and optimization of the CZTSe/CdS interface lead to significant Voc enhancement. CZTSe devices with 466 mV open circuit voltage and 8.2% power conversion efficiency are achieved. An even higher Voc of 490 mV is obtained for a 4.5% CZTSe/In2S3 device.

1D and 2D numerical simulations of Cu2ZnSnSe4 solar cells

1D and 2D numerical simulations can be employed to perform optimizations of thin film solar cells and analysis of the physical mechanisms affecting the performances. In this work we discuss optical optimizations of a CZTSe solar cell predicted by a 1D model and a 2D model implementing grain boundaries as p-type phases with low band gap. The results of the simulations point out the significant impact that these GBs can have on the Voc loss and possible correlations with other experimental results obtained by K-AFM and C-AFM.

Design and fabrication of infrared antireflecting bi-periodic micro-structured surfaces

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.

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.

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.