The record in photovoltaic conversion efficiency is detained by multi-junction solar cells based on III-V semiconductors. However, the wide adoption of these devices is hindered by their high production cost, to a large extent due to the expensive III-V substrates. As an alternative, a hybrid geometry has been proposed [LaPierre JAP 2011], which combines a 2D Si bottom cell with a III-V nanowire top cell in a tandem device. This approach, which may reach theoretical efficiencies of approx. 34%, requires smaller amounts of expensive III-V materials compared to conventional III-V tandem cells and benefits from the nanowire light trapping effects.
In this work, we report the fabrication and nanoscale characterization of two types of nanostructures for solar cells: radial GaAlAs and axial GaAsP p-n junction nanowires. Nanowires are grown by gallium-assisted molecular beam epitaxy using Be and Si as doping sources. The composition (probed by EDX and cathodoluminescence) was adjusted to tune the bandgap toward the optimal value for a III-V-on-Si tandem cell (approx. 1.7 eV). Local I-V characteristics and electron beam induced current (EBIC) microscopy under different biases are used to probe the electrical properties and the generation pattern of individual nanowires. For radial junction nanowires, EBIC mappings revealed a homogeneous collection of carriers on the entire nanowire length. For axial junction nanowires, the doping concentrations and the minority carrier diffusion lengths were extracted from the EBIC generation profiles. The effect of an epitaxial GaP passivating shell on the optical and generation properties was assessed.
Increasing efforts on the photovoltaics research have recently been devoted to material savings, leading to the emergence of new designs based on nanotextured and nanowire-based solar cells. The use of small absorber volumes, light-trapping nanostructures and unconventional carrier collection schemes (radial nanowire junctions, point contacts in planar structures,…) increases the impact of surfaces recombination and induces homogeneity in the photogenerated carrier concentrations. The investigation of their impacts on the device performances need to be addressed using full 3D coupled opto-electrical modeling.
In this context, we have developed a new tool for full 3D opto-electrical simulation using the most advanced optical and electrical simulation techniques. We will present an overview of its simulation capabilities and the key issues that have been solved to make it fully operational and reliable. We will provide various examples of opto-electronic simulation of (i) nanostructured solar cells with localized contacts and (ii) nanowire solar cells. We will also show how opto-electronic simulation can be used to simulate light- and electron-beam induced current (LBIC/EBIC) experiments, targeting quantitative analysis of the passivation properties of surfaces.
Recent studies led in the ﬁeld of infrared spectroscopy focused on the use of nanoantennas to enhance electromagnetic ﬁeld on the bonds of molecules, in order to improve detection. We propose to take beneﬁt from dipolar optical resonances in dielectric free-standing nanorod arrays as an innovative component to achieve spectroscopy in the mid-infrared wavelength range. The particularity of this component is not only to allow electromagnetic ﬁeld enhancement, but also its ability to shift spectrally the resonance according to incidence angle. Spectroscopy is thus possible on a wide wavelength range for a given geometry of the nanorods. We present here numerical studies of the impact of the size of nanorods on the reﬂection spectra. We use the permittivity of a test molecule determined by transmission spectra. In addition, we introduce the fabrication process, transmission and reﬂection measurements of nanorods.
In order to develop photovoltaic devices with increased efficiency using less rare semiconductor materials, the
concentrating approach is applied on Cu(In,Ga)Se2 thin film devices. For this purpose, Cu(In,Ga)Se2 microcells with a
mesa design are fabricated. The influence of the edge recombination signal is analyzed. It is found that with an
appropriate etching procedure, devices as small as 50x50 μm do not experience edge recombination efficiency
limitations. Under concentration, significant Voc gains are seen, leading to an absolute efficiency increase of two points
We present a compact real-time multispectral camera operating in the mid-infrared wavelength range. Multispectral images of a scene with two differently spectrally signed objects and of a burning solid propellant will be shown. Ability of real-time acquisition will thus be demonstrated and spectra of objects will be retrieved thanks to inversion algorithm applied on multispectral images.
Conventional light trapping techniques are inefficient at the sub-wavelength scale. This is the main limitation for the thickness reduction of thin-film solar cells below 500nm. We propose a novel architecture for broadband light absorption in ultra-thin active layers based on plasmonic nano-cavities and multi-resonant mechanism. Strong light enhancement will be shown numerically for photovoltaic materials such as CIGSe and GaAs. First experiments on ultrathin nano-patterned CIGSe solar cells will be presented.
In order to develop photovoltaic devices with increased efficiency using less rare semiconductor materials, the concentrating approach was applied on Cu(In,Ga)Se2 thin film devices. Microscale solar cells down to a few micrometers wide were fabricated. They show, at around x475, an efficiency of 21.3%, thanks to concentrated
illumination (532 nm laser), compared to 16% efficiency under non-concentrated illumination. Due to the miniaturization, ultrahigh fluxes can be studied (< ×1000), without damaging the device. We analyse the high concentration regime of these micro-devices. Under ultrahigh light fluxes the collection efficiency decreases on certain devices. We attribute this effect to the screening of the electric field at the junction under high illumination. Numerical simulations of p-n junctions under intense fluxes corroborate this hypothesis. We built a homemade finite element method program, solving Poisson and continuity equations without resorting to the minority carrier approximation. We study the electric field at a p-n junction as a function of illumination intensity, and highlight the screening phenomena. Cu(In,Ga)Se2 thin films prove to be appropriate for a use under concentration, leading to significant gains in terms of efficiency and material usage. On these particular devices, ultrahigh illuminations can be used and the electric regime studied.
We study experimentally and theoretically band-pass filters based on guided-mode resonances in free-standing metal-dielectric structures with subwavelength gratings. A variety of filters are obtained: polarizing filters with lD gratings, and unpolarized or selective polarization filters with 2D gratings, which are shown to behave as crossed-lD structures. In either case, a high transmission (up to ≈ 79 %) is demonstrated, which represents an eight-fold enhancement compared to the geometrical transmission of the grating. We also show that the angular
sensitivity strongly depends on the rotation axis of the sample. This behavior is explained with a detailed description of the guided-mode transmission mechanism.
We present the experimental study of a new design of band-pass filter based on guided-mode resonances in a
free-standing metal-dielectric structure with subwavelength gratings. Component consists of a subwavelength
gold grating with narrow slits deposited on a silicon nitride membrane. High optical transmission is measured
with up to 78% transmission at resonance. Experimental angularly resolved spectra are presented: they reveal
the role of the diffracted orders and of the waveguide eigenmode in the resonance. Spectra have a typical profile of
Fano resonances: we show that this profile is due to interferences between a direct transmission channel through
the 0th order, and an indirect transmission channel which results from the excitation by the ±1 diffracted orders
of a waveguide eigenmode.
This study addresses the potential of different approaches to improve the generated current density in ultrathin
Cu(In,Ga)Se2 (CIGSe) based solar cells down to 0.1 μm. Advanced photon management, involving both absorption
enhancement and reflection reduction in the absorber, is studied. In this contribution, the three main approaches used
- The reduction of the CIGSe thickness by chemical etching which combines thickness reduction and smoothing effect
on the absorber.
- Optical management by front contact engineering and by the replacement of the back contact by the "lift-off" of CIGSe
layer from the Mo layer and the deposition of a new reflective back contact.
- Application of plasmonic structures to CIGSe solar cells enabling light confinement at the subwavelength scale.
Cu(In,Ga)Se2 microcells are photovoltaic devices of increased efficiency and low semiconductor consumption. They
show an increase in efficiency due to concentrated illumination up to more than ×100, which is a breakthrough as thin
films were previously limited to low concentration applications (about 10 suns). New measurements, made under
concentrated natural solar illumination are presented, which confirm the conclusions of laser experiments. We also
extend our approach to an other direction, that of using thin Cu(In,Ga)Se2 layers. This reduces further the volume of the
solar cells and gives an insight in the effect of thickness as a key parameter controlling the performances of thin film
microcells. On thinner microcells, optimum efficiencies are reached at illumination intensities over ×400. Due to their
favorable architecture, microcells present efficient resistive and thermal management, leading to gains in efficiency and
Application of up-conversion in photovoltaic is limited by the up-conversion efficiency of materials. We propose to use a
realistic plasmonic structure proposed in the literature to exceed this limitation. Erbium doped yttrium fluoride thin layer
has been elaborated by ALD at 250°C to be used in such a structure as active up-converter material. Up-conversion
properties of samples have been characterized using a confocal microscope. Samples, having a thickness below 100 nm,
deposited onto a gold mirror, exhibit an up-conversion visible with naked eyes. The measurement of the enhancement
factor of structures associated with the plasmonic resonnator has been performed by luminescence cartography. From
these data, total enhancement factor up to 35 has been achieved by the use of the plasmonic structures, as compared to a
layer deposited on a bare glass substrate.
We will present a brief overview of the interest in subwavelength gratings for spectral filtering in the mid-infrared wavelength range. Guided-mode, plasmonic and dipolar resonances will be considered. We will particularly focus on components fabricated in our laboratories, achieving band-pass or cut-band filtering. Optical characterization will be shown, revealing resonances with high quality factors. Multispectral camera has been realized by integrating our components into a cooled infrared focal plane array.
We provide the first experimental evidence of sharp resonant extinction in free-standing arrays of non-resonant
dielectric nanorods. Nearly perfect optical extinction is shown for transparent material. High-resolution optical
measurements (absolute transmission and reflection) of one dimensional gratings with very low fill factors have
been obtained. The results can be fully explained by coherent multiple scattering in arrays of non-resonant
subwavelength nanorods and are in good agreement with an analytical model.
Plasmonic has demonstrated the ability to enhance performances of photodetectors at a
resonant wavelength. Absorption in a photodetector can reach 100% using nanophotonic
plasmonic array. Plasmonic devices are confining light at the interface metal/dielectric, as a
consequence, detection volume is smaller (100 to 1000 times) than in usual photodetectors
leading to a decrease in dark current of infrared photodetectors and therefore a higher working
temperature. The second consequence of a short detection volume is a higher collection
efficiency of photocarriers as the transit time is smaller than the lifetime.
Sub-wavelength gratings allow to code complex transmittance functions that introduce both amplitude and
phase variations in the propagation of a given wavefront. These micro-structures are a promising technique
to miniaturize optical functions such as light polarizing, light confinement, spectral filtering... Realizations in
the visible and the infrared domain have been fulfilled: for example micro-lenses, anti-reflection coatings or
sinusoidal-transmittance can easily be coded. This technique is all the more advantageous in the mid-wavelength
infrared (MWIR) or long-wavelength infrared (LWIR) spectral range since there are only a few materials available
in this spectral range. However the characterization of these structures is problematical, since it involves phase
and amplitude measurements. It is even more complicated in the far infrared domain (8 - 14 μm), as will be
detailed. Besides, the finite size of the gratings introduces phase steps, which is well-known to be a problematic
issue. We describe here a dedicated bench to characterize sub-wavelength gratings in the LWIR spectral range.
The core of the bench is a quadri-wave lateral shearing interferometer based on a diffraction grating, which allows
a complete two-dimensional characterization of both phase and amplitude in a single measurement. We present
here theoretical and experimental results of a characterization of such a sub-wavelength grating.
Lateral shearing interferometers (LSIs) are efficient tools for optical analysis. They allow classical optical wave-front
aberrations measurements as well as the precise evaluation of abrupt steps. The basic element of an LSI
is the transmittance grating, which diffracts a number of orders (two in the case of a mono-dimensional LSI,
ideally three or four non coplanar orders in the case of bi-dimensional LSI). This brings the need for specifically
designed transmittance gratings. For instance, a mono-dimensional LSI needs a sinusoidal-shaped transmittance,
since its Fourier transform carries exactly 2 orders. Such transmittances are however either impossible or at least
extremely costly to design using classical macroscopic techniques, mainly because the usual thin film deposition
techniques require several technological steps, in order to get the desired light filtering effect.
Given these constraints, we made use of sub-wavelength structures in order to build a new class of LSI. They
are made of sub-wavelength lamellar metallic gratings specifically designed for the mid-infrared, and allow the
precise coding of the desired transmission shape all over the LSI grating.
Bolometers cooled to very low temperature are currently the most sensitive detectors for low spectral resolution
detection of millimetre and sub-millimetre wavelengths. The best performances of the state-of-the-art bolometers allow
to reach sensitivities below the photon noise of the Cosmic Microwave Background for example. Since 2003, a french
R&D effort called DCMB ("Developpement Concerte de Matrices de Bolometres") has been organised between different
laboratories to develop large bolometers arrays for astrophysics observations. Funded by CNES and CNRS, it is intended
to get a coherent set of competences and equipments to develop very cold bolometers arrays by microfabrication. Two
parallel developments have been made in this collaboration based on the NbSi alloy either semi-conductive or
superconducting depending on the proportion of Nb. Multiplexing schemes have been developed and demonstrated for
these two options. I will present the latest developments made in the DCMB collaboration and future prospects.
Subwavelength dielectric and metallic gratings embedded in vacuum can act as highly-resonant spectral filters. We review the theoretical principles for the design of symmetric dielectric and metal gratings to conceive efficient optical filters in the mid and far infrared range, and we show how both resonance width and resonance wavelength can be tuned. We describe an original process for the fabrication of free-standing SiC gratings, and we present the first samples obtained with 10 &mgr;m period. Experimental angularly resolved transmission spectra show evidences of their filtering properties.
Subwavelength metallic structures are used to design gratings with a great variety of transmittance levels. Such gratings can answer growing needs for complex transmittance devices, particularly useful for wave-front analysis applications. Having in mind the conception of a perfectly sinusoidal transmittance for the mid-infrared, we have decided to test the ability of subwavelength lamellar gratings to code the transmittance with several levels. In order to calibrate gratings transmission, as a function of the fill factor, we have designed, realized and measured samples made of six 2mm x 2mm gratings, with transmittance ranging from 10% to 95%. Experimental results for TM- and TE-polarized light are reported and analysed.
New concepts for efficient light absorption in nanoscale metal-semiconductor-metal photodetectors are analyzed from both theoretical and experimental point of view. They are based on sub-wavelength metallic gratings which allows light confinement in tiny volumes (< 100 nm) close to electrodes (< 100 nm). Two photodetector structures are proposed: (i) a resonant-cavity-enhanced subwavelength metal-semiconductor-metal photodetector, and (ii) a nanoscale metal-semiconductor grating photodetector. External quantum efficiency as high as 9 % has been obtained in 40 x 100 nm2 cross-section GaAs wires, limited by fabrication technology. These results show promising features for highly efficient and ultrafast photodetectors.