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Conventional photovoltaic devices are currently made from relatively thick semiconductor layers, ~150 µm for silicon and 2–4 µm for Cu(In,Ga)(S,Se)2, CdTe or III–V direct bandgap semiconductors. Ultrathin solar cells using 10 times thinner absorbers could lead to considerable savings in material and processing time. Theoretical models suggest that light trapping can compensate for the reduced single-pass absorption, but optical and electrical losses have greatly limited the performances of previous attempts. Here, we propose a strategy based on multi-resonant absorption in planar active layers, and we report a 205-nm-thick GaAs solar cell with a certified efficiency of 19.9%. It uses a nanostructured silver back mirror fabricated by soft nanoimprint lithography. Broadband light trapping is achieved with multiple overlapping resonances induced by the grating and identified as Fabry–Perot and guided-mode resonances. A comprehensive optical and electrical analysis of the complete solar cell architecture provides a pathway for further improvements and shows that 25% efficiency is a realistic short-term target. Ultrathin solar cells having thicknesses below 1 µm can still reach efficiencies comparable to their thicker counterparts, but require less material to manufacture. By exploiting light-trapping nanostructures, Chen and colleagues achieve GaAs solar cells with 20% efficiency at just 205 nm thicknesses.

International audience; Optical nanoantennas are studied to manipulate light and enhance light matter interactions. Here, we experimentally demonstrate the optical Helmholtz resonance in a metallic slit-box structure, which is predicted to be harmonic and to enhance the electric field intensity by several orders of magnitude. It is fabricated thanks to a two step electron beam lithography process, between which the box was filled with benzocyclobutene (BCB). Up to 80% of the light is absorbed at a λ = 2.84 μm wavelength under a beam focused by a Cassegrain objective (NA = 0.4), even if the dimensions of this resonator are deeply subwavelength for both the slit (width λ/55 and height λ/77) and the box (width λ/7 and height λ/37). As expected from the inductance nature of the box, the optical properties of the BCB filling the box have no influence on the resonance behavior.

Coupled waveguide arrays lend themselves very well to photonic engineering through tailoring of the propagation and coupling constants [1]. Especially interesting is the manipulation of collective Floquet Bloch waves (extended beams see Figure 1 top, a and d) that propagate over several waveguides through evanescent coupling and their manipulation for classical and quantum data processing. Phase-front shaping of these waves is crucial for compact imaging and photonic circuitry in the classical and quantum regime up to multiqbit architectures where phase-shifters are instrumental [2]. Segmentation of a waveguide enables a change of its propagation constant [3] and thus segmentation patterns can shape the phase front of a beam. Image reconstruction has indeed been demonstrated using π shifts [4]. We demonstrate here complete phase-front shaping in waveguide arrays by means of more general segmented patterns for a novel imaging approach. We implement three emblematic functions related to three phase profiles in III-V waveguide arrays: beam steering (linear wedge-profile), focusing and defocusing (quadratic lens-like profile) and diffraction (cosine profile). We evaluate the operation of those patterns by measuring maps of the output intensity profile as a function of the position of injection (x injection =0 corresponds to the center of the patterns). The experimental maps clearly exhibit the desired effects (see Figure 1 for steering and focusing). The excellent agreement between the simulation and the experiment validates the concept of extensive phase front shaping via segmentation patterns, the fabrication and our capacity to design and fabricate more complex functions.

Most plasmonic super absorbers use the excitation of surface plasmons to achieve perfect absorption in metal nanostructures. We present a design for an ultrathin metal–semiconductor–metal super absorber where light is mainly absorbed within a flat 25 nm thick GaAs semiconductor layer. The top metal layer is patterned as a two-dimensional grid with a subwavelength period and is covered with a thin embedding dielectric layer. We show numerically multiresonant absorption with low polarization and angular dependence. We experimentally demonstrate optical absorption above 80% over the 450–830 nm spectral range using a gold nanogrid. This work opens perspectives toward ultrathin active plasmonic optoelectronic devices and, in particular, highly efficient ultrathin solar cells.

We review and propose light trapping strategies in ultra-thin solar cell. We demonstrate JSC=24.8 mA/cm2 in 205 nm-thick GaAs solar cell using a nanostructured TiO2/Ag back mirror fabricated by Nanoimprint lithography.

International audience; Ultrathin c-Si solar cells have the potential to drastically reduce costs by saving raw material while maintaining good efficiencies thanks to the excellent quality of monocrystalline silicon. However, efficient light trapping strategies must be implemented to achieve high short-circuit currents. We report on the fabrication of both planar and patterned ultrathin c-Si solar cells on glass using low temperature (T < 275 °C), low-cost, and scalable techniques. Epitaxial c-Si layers are grown by PECVD at 160 °C and transferred on a glass substrate by anodic bonding and mechanical cleavage. A silver back mirror is combined with a front texturation based on an inverted nanopyramid array fabricated by nanoimprint lithography and wet etching. We demonstrate a short-circuit current density of 25.3 mA/cm2 for an equivalent thickness of only 2.75 μm. External quantum efficiency (EQE) measurements are in very good agreement with FDTD simulations. We infer an optical path enhancement of 10 in the long wavelength range. A simple propagation model reveals that the low photon escape probability of 25% is the key factor in the light trapping mechanism. The main limitations of our current technology and the potential efficiencies achievable with contact optimization are discussed.

We propose a new guide mode resonant photodiode demonstrating better sensitivity for the infrared imaging. This comprises a backside dielectric sub-wavelength periodic structure and an absorbing region as thin as 90 nm. In this study, electro-optical characterizations of individual InGaAs detector featuring focal-plane array-compatible geometries are fully explained by electro-magnetic simulations. In particular, we observe near perfect collection of the photo-carriers and external quantum efficiency of up to 71% around 1,55 µm.

We investigate a full-dielectric guided mode resonant photodiode. It has been designed to enhance the absorption by excitation of several resonances in the SWIR domain. The device consists of an InP/InGaAs/InP P-i-N heterojunction containing an active layer as thin as 90 nm on top of a subwavelength lamellar grating and a gold mirror. We successfully compared the electro-optical characterizations of individual pixels with electro-magnetic simulations. In particular, we observe near perfect collection of the photo-carriers and external quantum efficiency (EQE) of up to 71% around 1.55 μm. Moreover, compared with InGaAs resonator state-of-the-art detector, we show a broader spectral response in the 1.2-1.7 μm range, thus paving the way for SWIR low dark current imaging.

In this paper we report the fabrication and characterization of single wall carbon nanotube film based devices. Carbon nanotubes (CNT) film is deposited on a silica substrate and usual clean room processes have been used to make Pt-electrodes on top of it and to pattern the CNT film. The obtained devices have been used to measure the evolution of the temperature coefficient of resistance (TCR) as a function of temperature. At room temperature the TCR is –0.2%K-1 and becomes even more important as the temperature goes down. Moreover the optical response in atmospheric band II and band III have been investigated at room temperature showing a variation of the device resistance up to 0.2% for an irradiance of ∼100 mW/cm2 in the 3–5 µm range. Such results are very promising for application of CNT films in bolometers for thermal infrared detection (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; We investigated the dark current components of thin planar InGaAs photodiodes grown by metalorganic vapor-phase epitaxy for optical nano-resonators. Owing to their high electric field enhancement, nano-resonators make it possible to substantially reduce the thickness of the active region to about 100 nm all the while maintaining high quantum efficiency. In the present study, to cover a broad spectral band, we combined several resonance peaks induced by guided-mode resonances in a given spectral range. This type of geometry allowed us to introduce InAlAs at the edge of a thin InGaAs active region in order to drastically reduce both the diffusion current and the generation/recombination current. We found that, in such devices, tunneling dark current components increase as the thickness of the active layer is reduced and dominate the reverse dark current. By optimizing the epitaxial stack, while keeping its total thickness constant (the optical properties of the nano-resonator remained unchanged), we showed that we are already able to achieve a specific detectivity of up to 1×10^13 cm√( Hz)W^−1 for λ=1.55 μm.

International audience; Degenerate two-photon absorption (TPA) is investigated in a 186 nm thick gallium arsenide (GaAs) p-in diode embedded in a resonant metallic nanostructure. The full device consists in the GaAs layer, a gold subwavelength grating on the illuminated side, and a gold mirror on the opposite side. For TM-polarized light, the structure exhibits a resonance close to 1.47 μm, with a confined electric field in the intrinsic region, far from the metallic interfaces. A 109 times increase in photocurrent compared to a non-resonant device is obtained experimentally, while numerical simulations suggest that both gain in TPA-photocurrent and angular dependence can be further improved. For optimized grating parameters, a maximum gain of 241 is demonstrated numerically and over incidence angle range of (−30°; +30°). This structure paves the way towards low-noise infrared detection, using non-degenerate TPA, involving two photons of vastly different energies in the same process of absorption in a large bandgap semiconductor material.

JUN 14-19, 2015; International audience; We report on the fabrication and characterization of both flat and patterned c-Si solar cells on glass. We use epitaxial layers grown by plasma enhanced chemical vapor deposition at low temperature (T

We study light management in a 430 nm-thick GaAs p-i-n single junction solar cell with 10 InGaAs/GaAsP multiple quantum wells (MQWs). The epitaxial layer transfer on a gold mirror improves light absorption and increases the quantum efficiency in the MQW region by a factor of 4 through the excitation of Fabry-Perot resonances. We show a good agreement with optical simulation and achieve around 9% efficiency. This promising result can be further improved by anti-reflection layers and additional light-trapping solutions.

Light trapping enhancement is a major research field in photovoltaics. Scarce and expensive resources for semiconductor material drive the research on light management in thin absorber layer. This paper reviews some of the known techniques, from back reflector to nanophotonic technologies such as nanowires or plasmonic-enhanced photovoltaic devices. Light trapping enhancement can reach ~100 fold and experimental demonstrations of device exceeding the ray optics limits have been reported.

International audience; We describe a method of controlled evaporation on a textured substrate for self-assembling and shaping gold-nanorod-based materials. Tridimensional wall features are formed over areas as large as several square millimeters. Furthermore, analyses by small-angle X-ray scattering and scanning electron microscopy techniques demonstrate that colloids are locally ordered as a smectic B phase. Such crystallization is in fact possible because we could finely adjust the nanoparticle charge, knowledge that additionally enables tuning the lattice parameters. In the future, the type of ordered self-assemblies of gold nanorods we have prepared could be used for amplifying optical signals.

We investigate a resonant photodiode transferred onto a gold mirror. This comprises a backside dielectric subwavelength periodic structure and an absorbing region as thin as 90 nm. Electro-optical characterizations of individual pixels featuring focal-plane array-compatible geometries are fully explained by electro-magnetic simulations. In particular, we observe a near perfect collection of the photo-carriers and external quantum efficiency of up to 71% around 1.55 μm.

The "material" and "device" linewidth enhancement factor of 5 quantum-dot layer single mode lasers emitting at 1.3 /spl mu/m are investigated using two methods for the first time, demonstrating a record value of 0.67 for this wavelength.

We study light management in a 430 nm-thick GaAs p-i-n single junction solar cell with 10 pairs of InGaAs/GaAsP multiple quantum wells (MQWs). The epitaxial layer transfer on a gold mirror improves light absorption and increases the external quantum efficiency below GaAs bandgap by a factor of four through the excitation of Fabry-Perot resonances. We show a good agreement with optical simulation and achieve around 10% conversion efficiency. We demonstrate numerically that this promising result can be further improved by anti-reflection layers. This study paves the way to very thin MQWs solar cells.

International audience; 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 1D gratings, and unpolarized or selective filters with 2D gratings, which are shown to behave as two crossed-1D 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.