Etienne Moulin

Plasmonic reflection grating back contacts for microcrystalline silicon solar cells

We report on the fabrication and optical simulation of a plasmonic light-trapping concept for microcrystalline silicon solar cells, consisting of silver nanostructures arranged in square lattice at the ZnO:Al/Ag back contact of the solar cell. Those solar cells deposited on this plasmonic reflection grating back contact showed an enhanced spectral response in the wavelengths range from 500 nm to 1000 nm, when comparing to flat solar cells. For a particular period, even an enhancement of the short circuit current density in comparison to the conventional random texture light-trapping concept is obtained. Full three-dimensional electromagnetic simulations are used to explain the working principle of the plasmonic light-trapping concept.

Design of nanostructured plasmonic back contacts for thin-film silicon solar cells

We report on a plasmonic light-trapping concept based on localized surface plasmon polariton induced light scattering at nanostructured Ag back contacts of thin-film silicon solar cells. The electromagnetic interaction between incident light and localized surface plasmon polariton resonances in nanostructured Ag back contacts was simulated with a three-dimensional numerical solver of Maxwells equations. Geometrical parameters as well as the embedding material of single and periodic nanostructures on Ag layers were varied. The design of the nanostructures was analyzed regarding their ability to scatter incident light at low optical losses into large angles in the silicon absorber layers of the thin-film silicon solar cells.

Photoresponse enhancement in the near infrared wavelength range of ultrathin amorphous silicon photosensitive devices by integration of silver nanoparticles

We have investigated the contribution of localized surface plasmon polaritons (LSPPs) in silver nanoparticles with radii smaller than 20 nm to the photocurrent of ultrathin photosensitive devices based on amorphous silicon. An increased light absorption and an enhanced photocurrent are found for wavelengths between 600 nm and 1150 nm in presence of nanoparticles. As amorphous silicon absorbs light efficiently only at wavelengths up to 750 nm, the increased photocurrent in the near infrared range is explained in terms of LSPP-induced photoemission of electrons within and in close vicinity of the nanoparticles.

Optical simulations of microcrystalline silicon solar cells applying plasmonic reflection grating back contacts

Light trapping is a key issue for high efficiency thin-film silicon solar cells. The authors present three-dimensional electromagnetic simulations of an n-i-p substrate-type micro- crystalline silicon solar cell applying a plasmonic reflection grating back contact as a novel light- trapping structure. The plasmonic reflection grating back contact consists of half-ellipsoidal silver nanostructures arranged in square lattice at the back contact of thin-film silicon solar cells. Experimental results of prototypes of microcrystalline silicon thin-film solar cells showed significantly enhanced short-circuit current densities in comparison to flat solar cells and, for an optimized period of the plasmonic reflection grating back contact, even a small enhancement of the short-circuit current density in comparison to the reference cells applying the conventional random texture light-trapping structure. The authors demonstrate a very good agreement between the simulated and experimental spectral response data when taking experimental variations into account. This agreement forms an excellent basis for future simulation based optimizations of the light-trapping by plasmonic reflection grating back contacts. Furthermore, from the simulated three-dimensional electromagnetic field distributions detailed absorption profiles were calculated allowing a spatially resolved evaluation of parasitic losses inside the solar cell.

2-D Periodic and Random-on-Periodic Front Textures for Tandem Thin-Film Silicon Solar Cells

We evaluate the performance of thin-film silicon micromorph tandem solar cells deposited on transparent superstrates with embossed micrometer-scale 2-D gratings. Once coated with a thin conductive layer of hydrogenated indium oxide, the textured superstrates can be used as 2-D periodic single-texture front electrodes. Combining these almost loss-free front electrodes with a highly transparent, random self-textured zinc oxide layer (with a thickness ≤ 1 μm) deposited by low-pressure chemical vapor deposition (LPCVD), we obtain double-texture transparent front electrodes. The potential of both single- and double-texture front electrodes is estimated by varying the illumination spectrum of the solar simulator, thereby assessing the maximum efficiency of the tandem cells under optimal current-matching conditions. Our results demonstrate the complementary roles of the 2-D gratings and the LPCVD-ZnO layers in double textures: Cell efficiencies as high as with our state-of-the-art 2.3-μm-thick LPCVD-ZnO front electrode are obtained with significantly reduced ZnO layer thicknesses. Additionally, we show that equivalent efficiencies are also within reach with 2-D periodic single textures if the proper cell configuration is applied.

Core–shell CdTe–TiO2 nanostructured solar cell

A prototype core–shell nanostructured solar cell with a p–n CdTe–TiO2 radial junction is constructed by pulse potential electrodeposition of CdTe into a TiO2 nanotube array. 1 : 1 stoichiometry of CdTe is achieved with a high filling rate. Measurements are done to characterize the photovoltaic properties of the as-fabricated solar cell with CdTe nanowire array as the absorber layer.

Plasmon-induced photoexcitation of “hot” electrons and “hot” holes in amorphous silicon photosensitive devices containing silver nanoparticles

We report on a plasmon-induced photocurrent in photosensitive devices based on hydrogenated amorphous silicon (a-Si:H) containing silver nanoparticles (NPs). The photocurrent is measured in a spectral region corresponding to optical transitions below the band gap of a-Si:H. Photoexcitation of “hot” electrons in the NPs or in defect states present in the vicinity of the NPs, resulting from plasmon decay in the NPs, is often cited as being responsible for this effect. In this study, we demonstrate that plasmon induced photogeneration of “hot” holes is also able to contribute to a photocurrent. A bifacial symmetrical transparent device was prepared in order to compare the internal quantum efficiency of both processes, the first based on the photogeneration of “hot” electrons and the second based on the photogeneration of “hot” holes.

Light trapping in thin-film solar cells measured by Raman spectroscopy

In this study, Raman spectroscopy is used as a tool to determine the light-trapping capability of textured ZnO front electrodes implemented in microcrystalline silicon (μc-Si:H) solar cells. Microcrystalline silicon films deposited on superstrates of various roughnesses are characterized by Raman micro-spectroscopy at excitation wavelengths of 442 nm, 514 nm, 633 nm, and 785 nm, respectively. The way to measure quantitatively and with a high level of reproducibility the Raman intensity is described in details. By varying the superstrate texture and with it the light trapping in the μc-Si:H absorber layer, we find significant differences in the absolute Raman intensity measured in the near infrared wavelength region (where light trapping is relevant). A good agreement between the absolute Raman intensity and the external quantum efficiency of the μc-Si:H solar cells is obtained, demonstrating the validity of the introduced method. Applications to thin-film solar cells, in general, and other optoelectronic devices are discussed.

Design of periodic nano- and macro-scale textures for high-performance thin-film multi-junction solar cells

Surface textures in thin-film silicon multi-junction solar cells play an important role in gaining the photocurrent of the devices. In this paper, a design of the textures is carried out for the case of amorphous silicon/micro-crystalline silicon (a-Si:H/mu c-Si:H) solar cells, employing advanced modelling to determine the textures for defect-free silicon layer growth and to increase the photocurrent. A model of non-conformal layer growth and a hybrid optical modelling approach are used to perform realistic 3D simulations of the structures. The hybrid optical modelling includes rigorous modelling based on the finite element method and geometrical optics models. This enables us to examine the surface texture scaling from nano- to macro-sized (several tens or hundreds of micrometers) texturisation features. First, selected random and periodic nanotextures are examined with respect to critical positions of defect-region formation in Si layers. We show that despite careful selection of a well-suited semi-ellipsoidal periodic texture for defect-free layer growth, defective regions in Si layers of a-Si: H/mu c-Si: H cell cannot be avoided if the lateral and vertical dimensions of the nano features are optimised only for high gain in photocurrent. Macro features are favourable for defect-free layer growth, but do not render the photocurrent gains as achieved with light-scattering properties of the optimised nanotextures. Simulation results show that from the optical point of view the semi-ellipsoidal periodic nanotextures with lateral features smaller than 0.4 mu m and vertical peak-to-peak heights around or above 0.3 mu m are required to achieve a gain in short-circuit current of the top cell with respect to the state-of-the-art random texture (>16% increase), whereas lateral dimensions around 0.8 mu m and heights around 0.6 mu m lead to a > 6% gain in short-circuit current of the bottom cell.

Optical simulations and prototyping of microcrystalline silicon solar cells with integrated plasmonic reflection grating back contacts

Light-trapping is a key issue for high efficiency thin-film silicon solar cells. In this work, the interaction of incident light with microcrystalline silicon solar cells applying a plasmonic reflection grating back contact is studied with threedimensional electromagnetic simulations and via the measured spectral response of prototypes. The investigated plasmonic reflection grating back contact consists of half-ellipsoidal silver nanostructures arranged in square lattice at the back contact of a n-i-p substrate type microcrystalline silicon solar cell. Experimental results of prototypes of these solar cells show significantly enhanced short circuit current densities in comparison to flat cells and even a small enhancement of the short circuit current density in comparison to the conventional random texture light-trapping concept of thin-film silicon solar cell. A very good agreement was found for the simulated and measured spectral response of the solar cell. From the simulated three-dimensional electromagnetic field distributions detailed absorption profiles were calculated allowing a spatially resolved evaluation of parasitic losses inside the n-i-p type microcrystalline silicon solar cell.