Powering AI at the edge: A robust, memristor-based binarized neural network with near-memory computing and miniaturized solar cell.

Memristor-based neural networks provide an exceptional energy-efficient platform for artificial intelligence (AI), presenting the possibility of self-powered operation when paired with energy harvesters. However, most memristor-based networks rely on analog in-memory computing, necessitating a stable and precise power supply, which is incompatible with the inherently unstable and unreliable energy harvesters. In this work, we fabricated a robust binarized neural network comprising 32,768 memristors, powered by a miniature wide-bandgap solar cell optimized for edge applications. Our circuit employs a resilient digital near-memory computing approach, featuring complementarily programmed memristors and logic-in-sense-amplifier. This design eliminates the need for compensation or calibration, operating effectively under diverse conditions. Under high illumination, the circuit achieves inference performance comparable to that of a lab bench power supply. In low illumination scenarios, it remains functional with slightly reduced accuracy, seamlessly transitioning to an approximate computing mode. Through image classification neural network simulations, we demonstrate that misclassified images under low illumination are primarily difficult-to-classify cases. Our approach lays the groundwork for self-powered AI and the creation of intelligent sensors for various applications in health, safety, and environment monitoring.

Humidity-Induced Degradation Processes of Halide Perovskites Unveiled by Correlative Analytical Electron Microscopy.

Improving the stability of lead halide perovskite solar cells (PSCs) for industrialization is currently a major challenge. It is shown that moisture induces changes in global PSC performance, altering the nature of the absorber through phase transition or segregation. Understanding how the material evolves in a wet environment is crucial for optimizing device performance and stability. Here, the chemical and structural evolution of state-of-the-art hybrid perovskite thin-film Cs0.05 (MA0.15 FA0.85 )0.95 Pb(I0.84 Br0.16 )3 (CsMAFA) is investigated after aging under controlled humidity with analytical characterization techniques. The analysis is performed at different scales through Photoluminescence, X-ray Diffraction Spectroscopy, Cathodoluminescence, Selected Area Electron Diffraction, and Energy Dispersive X-ray Spectroscopy. From the analysis of the degradation products from the perovskite layer and by the correlation of their optical and chemical properties at a microscopic level, different phases such as lead-iodide (PbI2 ), inorganic mixed halide CsPb(I0.9 Br0.1 )3 and lead-rich CsPb2 (I0.74 Br0.26 )5 perovskite are evidenced. These phases demonstrate a high degree of crystallinity that induces unique geometrical shapes and drastically affects the optoelectronic properties of the thin film. By identifying the precise nature of these specific species, the multi-scale approach provides insights into the degradation mechanisms of hybrid perovskite materials, which can be used to improve PSC stability.

GaAs/GaInP nanowire solar cell on Si with state-of-the-art Voc and quasi-Fermi level splitting.

With their unique structural, optical and electrical properties, III-V nanowires (NWs) are an extremely attractive option for the direct growth of III-Vs on Si for tandem solar cell applications. Here, we introduce a core-shell GaAs/GaInP NW solar cell grown by molecular beam epitaxy on a patterned Si substrate, and we present an in-depth investigation of its optoelectronic properties and limitations. We report a power conversion efficiency of almost 3.7%, and a state-of-the-art open-circuit voltage (VOC) for a NW array solar cell on Si of 0.65 V. We also present the first quantification of the quasi-Fermi level splitting in NW array solar cells using hyperspectral photoluminescence measurements. A value of 0.84 eV is obtained at 1 sun (1.01 eV at 81 suns), which is significantly higher than qVOC. It indicates NWs with a better intrinsic optoelectronic quality than what could be expected from TEM images or deduced from electrical measurements. Optical and electronic simulations provide insights into the main absorption and electrical losses, and guidelines to design and fabricate higher-efficiency devices. It suggests that improvements at the n-type contact (GaInP/ITO) are key to unlocking the potential of next generation NW solar cells.

Industrially Compatible Fabrication Process of Perovskite-Based Mini-Modules Coupling Sequential Slot-Die Coating and Chemical Bath Deposition.

To upscale the emerging perovskite photovoltaic technology to larger-size modules, industrially relevant deposition techniques need to be developed. In this work, the deposition of tin oxide used as an electron extraction layer is established using chemical bath deposition (CBD), a low-cost and solution-based fabrication process. Applying this simple low-temperature deposition method, highly homogeneous SnO2 films are obtained in a reproducible manner. Moreover, the perovskite layer is prepared by sequentially slot-die coating on top of the n-type contact. The symbiosis of these two industrially relevant deposition techniques allows for the growth of high-quality dense perovskite layers with large grains. The uniformity of the perovskite film is further confirmed by scanning electron microscopy (SEM)/scanning transmission electron microscopy (STEM) analysis coupled with energy dispersive X-ray spectroscopy (EDX) and cathodoluminescence measurements allowing us to probe the elemental composition at the nanoscale. Perovskite solar cells fabricated from CBD SnO2 and slot-die-coated perovskite show power conversion efficiencies up to 19.2%. Furthermore, mini-modules with an aperture area of 40 cm2 demonstrate efficiencies of 17% (18.1% on active area).

Cathodoluminescence mapping of electron concentration in MBE-grown GaAs:Te nanowires.

Cathodoluminescence mapping is used as a contactless method to probe the electron concentration gradient of Te-doped GaAs nanowires. The room temperature and low temperature (10 K) cathodoluminescence analysis method previously developed for GaAs:Si is first validated on five GaAs:Te thin film samples, before extending it to the two GaAs:Te NW samples. We evidence an electron concentration gradient ranging from below 1 × 1018cm-3to 3.3 ×1018cm-3along the axis of a GaAs:Te nanowire grown at 640 °C, and a homogeneous electron concentration of around 6-8 × 1017cm-3along the axis of a GaAs:Te nanowire grown at 620 °C. The differences in the electron concentration levels and gradients between the two nanowires is attributed to different Te incorporation efficiencies by vapor-solid and vapor-liquid-solid processes.

Investigation of the effect of the doping order in GaN nanowire p-n junctions grown by molecular-beam epitaxy.

We analyse the electrical and optical properties of single GaN nanowire p-n junctions grown by plasma-assisted molecular-beam epitaxy using magnesium and silicon as doping sources. Different junction architectures having either a n-base or a p-base structure are compared using optical and electrical analyses. Electron-beam induced current (EBIC) microscopy of the nanowires shows that in the case of a n-base p-n junction the parasitic radial growth enhanced by the magnesium (Mg) doping leads to a mixed axial-radial behaviour with strong wire-to-wire fluctuations of the junction position and shape. By reverting the doping order p-base p-n junctions with a purely axial well-defined structure and a low wire-to-wire dispersion are achieved. The good optical quality of the top n nanowire segment grown on a p-doped stem is preserved. A hole concentration in the p-doped segment exceeding 1018 cm-3 was extracted from EBIC mapping and photoluminescence analyses. This high concentration is reached without degrading the nanowire morphology.

Backside light management of 4-terminal bifacial perovskite/silicon tandem PV modules evaluated under realistic conditions.

Perovskite/silicon tandem modules have recently attracted growing interest as a potential candidate for new generations of solar modules. Combined with a bifacial configuration it can lead to considerable energy yield improvement in comparison to conventional monofacial tandem solar modules. Optical modeling is crucial to analyze the optical losses of perovskite/silicon solar modules and achieve efficient light management. In this article we study the optical properties of four-terminal bifacial tandem modules, using metal-halide perovskite top solar cell and a conventional industrial crystalline silicon PERC bottom solar cell. We propose a method to analyze bifacial gains, improve back side light management and challenge it under realistic spectral conditions at several locations with various albedos. We show that both optimized designs for the back side show comparable advantages at all locations. These results are a good sign for the standardization of bifacial four-terminal perovskite/silicon modules.

Stable and high yield growth of GaP and In0.2Ga0.8As nanowire arrays using In as a catalyst.

We report the first investigation of indium (In) as the vapor-liquid-solid catalyst of GaP and InGaAs nanowires by molecular beam epitaxy. A strong asymmetry in the Ga distribution between the liquid and solid phases allows one to obtain pure GaP and In0.2Ga0.8As nanowires while the liquid catalyst remains nearly pure In. This uncommon In catalyst presents several advantages. First, the nanowire morphology can be tuned by changing the In flux alone, independently of the Ga and group V fluxes. Second, the nanowire crystal structure always remains cubic during steady state growth and catalyst crystallization, despite the low contact angle of the liquid droplet measured after growth (95°). Third, the vertical yield of In-catalyzed GaP and (InGa)As nanowire arrays on patterned silicon substrates increases dramatically. Combining straight sidewalls, controllable morphologies and a high vertical yield, In-catalysts provide an alternative to the standard Au or Ga alloys for the bottom-up growth of large scale homogeneous arrays of (InGa)As or GaP nanowires.

Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications.

Axial p-n and p-i-n junctions in GaAs0.7P0.3 nanowires are demonstrated and analyzed using electron beam induced current microscopy. Organized self-catalyzed nanowire arrays are grown by molecular beam epitaxy on nanopatterned Si substrates. The nanowires are doped using Be and Si impurities to obtain p- and n-type conductivity, respectively. A method to determine the doping type by analyzing the induced current in the vicinity of a Schottky contact is proposed. It is demonstrated that for the applied growth conditions using Ga as a catalyst, Si doping induces an n-type conductivity contrary to the GaAs self-catalyzed nanowire case, where Si was reported to yield a p-type doping. Active axial nanowire p-n junctions having a homogeneous composition along the axis are synthesized and the carrier concentration and minority carrier diffusion lengths are measured. To the best of our knowledge, this is the first report of axial p-n junctions in self-catalyzed GaAsP nanowires.

Nanoplasmonics-enhanced label-free imaging of endothelial cell monolayer integrity.

Surface plasmon resonance imaging (SPRI) is a powerful label-free imaging modality for the analysis of morphological dynamics in cell monolayers. However, classical plasmonic imaging systems have relatively poor spatial resolution along one axis due to the plasmon mode attenuation distance (tens of µm, typically), which significantly limits their ability to resolve subcellular structures. We address this limitation by adding an array of nanostructures onto the metal sensing surface (25 nm thick, 200 nm width, 400 nm period grating) to couple localized plasmons with propagating plasmons, thereby reducing attenuation length and commensurately increasing spatial imaging resolution, without significant loss of sensitivity or image contrast. In this work, experimental results obtained with both conventional unstructured and nanostructured gold film SPRI sensor chips show a clear gain in spatial resolution achieved with surface nanostructuring. The work demonstrates the ability of the nanostructured SPRI chips to resolve fine morphological detail (intercellular gaps) in experiments monitoring changes in endothelial cell monolayer integrity following the activation of the cell surface protease-activated receptor 1 (PAR1) by thrombin. In particular, the nanostructured chips reveal the persistence of small intercellular gaps (<5 µm2) well after apparent recovery of cell monolayer integrity as determined by conventional unstructured surface based SPRI. This new high spatial resolution plasmonic imaging technique uses low-cost and reusable patterned substrates and is likely to find applications in cell biology and pharmacology by allowing label-free quantification of minute cell morphological activities associated with receptor dependent intracellular signaling activity.

Evidence and control of unintentional As-rich shells in GaAs1-x P x nanowires.

We report on the detailed composition of ternary GaAsP nanowires (NWs) grown using self-catalyzed vapor-liquid-solid (VLS) growth by molecular beam epitaxy. We evidence the formation of an unintentional shell, which enlarges by vapor-solid growth concurrently to the main VLS-grown core. The NW core and unintentional shell have typically different chemical compositions if no effort is made to adjust the growth conditions. The compositions can be made equal by changing the substrate temperature and the P/As flux ratio in the vapor phase. In all cases, we still observe the existence of a P-rich interface between the GaAsP NW core and the unintentional shell, even if favorable growth conditions are used.

Investigation of GaN nanowires containing AlN/GaN multiple quantum discs by EBIC and CL techniques.

In this work, nanoscale electrical and optical properties of n-GaN nanowires (NWs) containing GaN/AlN multiple quantum discs (MQDs) grown by molecular beam epitaxy are investigated by means of single wire I(V) measurements, electron beam induced current microscopy (EBIC) and cathodoluminescence (CL) analysis. A strong impact of non-intentional AlN and GaN shells on the electrical resistance of individual NWs is put in evidence. The EBIC mappings reveal the presence of two regions with internal electric fields oriented in opposite directions: one in the MQDs region and the other in the adjacent bottom GaN segment. These fields are found to co-exist under zero bias, while under an external bias either one or the other dominates the current collection. In this way EBIC maps allow us to locate the current generation within the wire under different bias conditions and to give the first direct evidence of carrier collection from AlN/GaN MQDs. The NWs have been further investigated by photoluminescence and CL analyses at low temperature. CL mappings show that the near band edge emission of GaN from the bottom part of the NW is blue-shifted due to the presence of the radial shell. In addition, it is observed that CL intensity drops in the central part of the NWs. Comparing the CL and EBIC maps, this decrease of the luminescence intensity is attributed to an efficient charge splitting effect due to the electric fields in the MQDs region and in the GaN base.

Material challenges for solar cells in the twenty-first century: directions in emerging technologies.

Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan-French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.

Optoelectrical modeling of solar cells based on c-Si/a-Si:H nanowire array: focus on the electrical transport in between the nanowires.

By coupling optical and electrical modeling, we have investigated the photovoltaic performances of p-i-n radial nanowires array based on crystalline p-type silicon (c-Si) core/hydrogenated amorphous silicon (a-Si:H) shell. By varying either the doping concentration of the c-Si core, or back contact work function we can separate and highlight the contribution to the cell's performance of the nanowires themselves (the radial cell) from the interspace between the nanowires (the planar cell). We show that the build-in potential (V bi) in the radial and planar cells strongly depends on the doping of c-Si core and the work function of the back contact respectively. Consequently, the solar cell's performance is degraded if either the doping concentration of the c-Si core, or/and the work function of the back contact is too low. By inserting a thin (p) a-Si:H layer between both core/absorber and back contact/absorber, the performance of the solar cell can be improved by partly fixing the V bi at both interfaces due to strong electrostatic screening effect. Depositing such a buffer layer playing the role of an electrostatic screen for charge carriers is a suggested way of enhancing the performance of solar cells based on radial p-i-n or n-i-p nanowire array.

Determination of n-Type Doping Level in Single GaAs Nanowires by Cathodoluminescence.

We present an effective method of determining the doping level in n-type III-V semiconductors at the nanoscale. Low-temperature and room-temperature cathodoluminescence (CL) measurements are carried out on single Si-doped GaAs nanowires. The spectral shift to higher energy (Burstein-Moss shift) and the broadening of luminescence spectra are signatures of increased electron densities. They are compared to the CL spectra of calibrated Si-doped GaAs layers, whose doping levels are determined by Hall measurements. We apply the generalized Planck's law to fit the whole spectra, taking into account the electron occupation in the conduction band, the bandgap narrowing, and band tails. The electron Fermi levels are used to determine the free electron concentrations, and we infer nanowire doping of 6 × 1017 to 1 × 1018 cm-3. These results show that cathodoluminescence provides a robust way to probe carrier concentrations in semiconductors with the possibility of mapping spatial inhomogeneities at the nanoscale.

Ultrathin Epitaxial Silicon Solar Cells with Inverted Nanopyramid Arrays for Efficient Light Trapping.

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/cm(2) 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.

Infrared spectroscopy of molecules with nanorod arrays: a numerical study.

Nanorod arrays with diameters much smaller than the wavelength exhibit sharp resonances with strong electric-field enhancement and angular dependence. They are investigated for enhanced infrared spectroscopy of molecular bonds. The molecule 3-cyanopropyldimethylchlorosilane (CS) is taken as a reference, and its complex permittivity is determined experimentally in the 3-5 µm wavelength range. When grafted on silicon nitride nanorods, we show numerically that its weak absorption bands due to chemical bond vibrations can be enhanced by several orders of magnitude compared with unstructured thin film. We propose a figure of merit (FoM) to assess the performance of this spectroscopic scheme, and we study the impact of the nanorod cross section on the FoM.

Influence of a Boron Precursor on the Growth and Optoelectronic Properties of Electrodeposited Zinc Oxide Thin Film.

Highly transparent and conductive materials are required for many industrial applications. One of the interesting features of ZnO is the possibility to dope it using different elements, hence improving its conductivity. Results concerning the zinc oxide thin films electrodeposited in a zinc perchlorate medium containing a boron precursor are presented in this study. The addition of boron to the electrolyte leads to significant effects on the morphology and crystalline structure as well as an evolution of the optical properties of the material. Varying the concentration of boric acid from 0 to 15 mM strongly improves the compactness of the deposit and increases the band gap from 3.33 to 3.45 eV. Investigations were also conducted to estimate and determine the influence of boric acid on the electrical properties of the ZnO layers. As a result, no doping effect effect by boron was demonstrated. However, the role of boric acid on the material quality has also been proven and discussed. Boric acid strongly contributes to the growth of high quality electrodeposited zinc oxide. The high doping level of the film can be attributed to the perchlorate ions introduced in the bath. Finally, a ZnO layer electrodeposited in a boron rich electrolyte was tested as front contact of a Cu(In, Ga)(S, Se)2 based solar cell. An efficiency of 12.5% was measured with a quite high fill factor (>70%) which confirms the high conductivity of the ZnO thin film.

Ultrasensitive Characterization of Mechanical Oscillations and Plasmon Energy Shift in Gold Nanorods.

Mechanical vibrational resonances in metal nanoparticles are intensively studied because they provide insight into nanoscale elasticity and for their potential application to ultrasensitive mass detection. In this paper, we use broadband femtosecond pump-probe spectroscopy to study the longitudinal acoustic phonons of arrays of gold nanorods with different aspect ratios, fabricated by electron beam lithography with very high size uniformity. We follow in real time the impulsively excited extensional oscillations of the nanorods by measuring the transient shift of the localized surface plasmon band. Broadband and high-sensitivity detection of the time-dependent extinction spectra enables one to develop a model that quantitatively describes the periodic variation of the plasmon extinction coefficient starting from the steady-state spectrum with only one additional free parameter. This model allows us to retrieve the time-dependent elongation of the nanorods with an ultrahigh sensitivity and to measure oscillation amplitudes of just a few picometers and plasmon energy shifts on the order of 10(-2) meV.

Electrodeposition of ZnO window layer for an all-atmospheric fabrication process of chalcogenide solar cell.

This paper presents the low cost electrodeposition of a transparent and conductive chlorine doped ZnO layer with performances comparable to that produced by standard vacuum processes. First, an in-depth study of the defect physics by ab-initio calculation shows that chlorine is one of the best candidates to dope the ZnO. This result is experimentally confirmed by a complete optical analysis of the ZnO layer deposited in a chloride rich solution. We demonstrate that high doping levels (>10(20) cm(-3)) and mobilities (up to 20 cm(2) V(-1) s(-1)) can be reached by insertion of chlorine in the lattice. The process developed in this study has been applied on a CdS/Cu(In,Ga)(Se,S)2 p-n junction produced in a pilot line by a non vacuum process, to be tested as solar cell front contact deposition method. As a result efficiency of 14.3% has been reached opening the way of atmospheric production of Cu(In,Ga)(Se,S)2 solar cell.