RbF post-deposition treatments provide an effective way to increase the performances of Cu(In,Ga)Se2 (CIGSe) solar cells, but the role of heavy alkali elements at the microscopic scale is still unclear. Here, we investigate the impact of an RbF post-deposition treatment under S atmosphere. An increase of the conversion efficiency from 12.4% to 16.3% is enabled by a better collection of charge carriers and a 100 mV increase in Voc. Using high-resolution hyperspectral cathodoluminescence mapping, we show that the RbF(S) treatment passivates low-energy defects attributed to copper vacancies, effectively resulting in an efficient reduction of non-radiative recombination at grain boundaries and in grain interiors.

We address the question of the optimal broadband absorption of waves in an open dissipative system and its key implications for solar cells. We develop a general framework for absorption induced by multiple overlapping resonances, based on quasinormal modes and radiative and nonradiative decay rates. We demonstrate that the upper bounds on broadband absorption in a slab of thickness d take the simple form A=1−exp⁡(−Fαd), where α is the absorption coefficient and F is the path enhancement factor. We apply these results to sunlight absorption in photovoltaics and answer the long-standing debate on the best light-trapping strategy in solar cells. For angle-independent absorption enhanced by scattering, we derive the isotropic upper bound F=4n^{2} (where n is the refractive index), extending the well-known Lambertian (or Yablonovitch) limit beyond the ray-optics and weak-absorption regimes. For angle-restricted illumination, we show that F can be further increased up to 8πn^{2}/sqrt[3] using multiresonant absorption induced by periodical patterning. These results have a general scope in the field of wave physics and open new opportunities to maximize absorption, detection, and attenuation of electromagnetic or mechanical waves in ultrathin devices.

Re-using the substrate is identified as a method for reducing the cost of high efficiency III-V solar cells. The approach investigated here consists in inserting a graphene layer onto a (001)GaAs substrate prior to the epitaxial growth of GaAs. To obtain a monocrystalline GaAs grown layer, the graphene layer is patterned, followed by a two-step epitaxial growth, here performed by molecular beam epitaxy (MBE). The first step is a selective area growth of GaAs in graphene openings, followed by a lateral overgrowth, under a modulated Ga flux. The second step, after reaching coalescence, consists in a regular growth under continuous Ga supply. It is observed that the pattern orientations relative to the crystallographic direction of the GaAs substrate below the graphene have an influence on GaAs morphology and quality. The best result was obtained for patterns oriented along [ 110]+22,5 degrees with a graphene coverage of 50%, with a significantly reduced roughness down to 3,3 nm.

Our multifunctional bonding layer for III-V//Si tandem solar cells uses an innovative architecture boasting high transparency, conductivity, and Si surface roughness accommodation, eliminating costly polishing requirements. An optical loss analysis demonstrated the feasibility of current-matching.

Nanowires (NWs) offer unique possibilities to control semiconductor heterostructures and polytypes at the nanometer scale. The crystal structure of GaAs can be switched from bulk cubic zinc blende (ZB) to the hexagonal wurtzite (WZ) phase, but the properties and doping of WZ GaAs are still poorly known. Here, we grow high-quality GaAs NWs containing large segments of pure ZB and WZ phases using self-catalyzed, vapor-liquid-solid molecular beam epitaxy. Undoped, Be-doped and Si-doped WZ GaAs are investigated by high-resolution cathodoluminescence (CL) at low temperature (10 K). The luminescence originating from the WZ region is unambiguously distinguished by its strong anisotropy, evidenced by polarimetry. In undoped GaAs, the WZ CL peak is found ∼1 meV higher than the free exciton energy in ZB. The recombination dynamics is probed by time-resolved CL and features a lifetime of 0.6 ns for exciton recombination and 1.65 ns for the free-electron-to-acceptor transition. From Be-doped NWs, we infer an ionization energy of ∼30 meV for the Be acceptor in GaAs WZ. The CL spectra broaden and redshift with increasing Be concentration due to the bandgap narrowing, following a trend similar to GaAs ZB. Si-doped WZ GaAs exhibits a low-energy CL peak (1.47 eV) attributed to the donor-acceptor pair recombination involving Si impurities. The degree of polarization of WZ luminescence decreases with increasing doping levels for both p-type and n-type. These results shed light on the properties and doping of WZ GaAs and show that time-resolved CL and CL polarimetry constitutes a powerful tool to characterize the crystal phase, local defect, transport and recombination mechanism at the nanoscale.

This work aims at reducing the cost of III-V solar cell by substrate recycling, investigating growth on graphene covered substrates. It was shown in the literature that the graphene layer allowed the growth of monocrystalline layers with the same orientation as the substrate, while still allowing the exfoliation thanks to the mechanically weak graphene plane. Two possible underlying physical mechanisms could play a role in obtaining a crystallographic alignment of the epi-layer: a remote interaction through the graphene, or a nucleation at graphene holes followed by a lateral growth. With our developed process, we have not observed remote interactions. We have therefore investigated the possibility of optimizing growth on patterned graphene. We show that a crucial parameter in obtaining a good quality III-V is the graphene opening orientation relative to the substrate. Stripes oriented along <100> directions on (001) substrates provided the smoothest nanostructures prior coalescence. Surprisingly, afterwards, higher index directions, e.g. <120>-4 degrees, resulted in the smoothest surfaces, with RMS roughness down to 3.3 nm, and the highest luminescence emission. First exfoliation tests have been carried out, showing a dependance on the peel-off direction relative to the stripes orientation, which will be further studied in coming works.

Nanowires (NWs) offer unique possibilities to control semiconductor heterostructures and polytypes at the nanometer scale. The crystal structure of GaAs can be switched from bulk cubic zinc blende (ZB) to the hexagonal wurtzite (WZ) phase, but the properties and doping of WZ GaAs are still poorly known. Here, we grow high-quality GaAs NWs containing large segments of pure ZB and WZ phases using self-catalyzed, vapor-liquid-solid molecular beam epitaxy. Undoped, Be-doped and Si-doped WZ GaAs are investigated by high-resolution cathodoluminescence (CL) at low temperature (10 K). The luminescence originating from the WZ region is unambiguously distinguished by its strong anisotropy, evidenced by polarimetry. In undoped GaAs, the WZ CL peak is found ∼1 meV higher than the free exciton energy in ZB. The recombination dynamics is probed by time-resolved CL and features a lifetime of 0.6 ns for exciton recombination and 1.65 ns for the free-electron-to-acceptor transition. From Be-doped NWs, we infer an ionization energy of ∼30 meV for the Be acceptor in GaAs WZ. The CL spectra broaden and redshift with increasing Be concentration due to the bandgap narrowing, following a trend similar to GaAs ZB. Si-doped WZ GaAs exhibits a low-energy CL peak (1.47 eV) attributed to the donor-acceptor pair recombination involving Si impurities. The degree of polarization of WZ luminescence decreases with increasing doping levels for both p-type and n-type. These results shed light on the properties and doping of WZ GaAs and show that time-resolved CL and CL polarimetry constitutes a powerful tool to characterize the crystal phase, local defect, transport and recombination mechanism at the nanoscale.

This work reports on the room temperature sputtering growth of gallium nitride thin films, exhibiting band edge luminescence without thermal annealing or post-deposition processing. In particular, we investigate and correlate their luminescence spectra with their structural properties (amorphous or polycrystalline phases), the presence of defects, and the grain features by combining cathodoluminescence and HR-TEM characterizations. The working pressure and Ar/N2 flow ratio are found to have a profound effect on both the structural and optical properties of the films. Notably, the only film with band edge luminescence is grown at the highest investigated pressure (13.3 Pa) and 40% N2 concentration and exhibits single-grained crystallites of small sizes (∼5 nm). Moreover, this polycrystalline film has an almost equal fraction of amorphous and crystalline phases. This suggests that the presence of smooth and nanometer-sized crystals in an amorphous matrix (inducing grain surface passivation) could be a synergetic combination to achieve luminescent films grown at room temperature.

We present a low-cost colloidal lithography process for creating correlated disorder nanostructures to enhance light trapping in ultra-thin silicon solar cells. This approach optimizes light-matter interactions and reduces manufacturing costs.

We have developed a general framework for multi-resonant absorption: we provide light-trapping upper bounds in solar cells, resolving a tension between numerical results and theoretical limits. We also discuss the implications for optimal light-trapping strategies.

Nowadays, Si is the main technology deployed on terrestrial applications. However, it is limited to applications in flexible applications, such as for electric vehicles, and space applications. Furthermore, the efficiency limitation on single junctions below 30% is fundamental in these applications as they are naturally limited in area. Thinner Si cells (

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.

Large surface transfer is a long-standing challenge for applications that require stacking 2D and 3D materials with only a limited number of combinations and techniques currently found in the literature. We report a systematic study of CVD graphene transfer to GaAs surfaces by mechanical exfoliation from Ge(110) substrates using highly stressed Ni layers. A uniform contact at the 2D-3D interface, achieved by aircushion pressing, enables a high yield of the transfer process to GaAs and other substrates exceeding 95% of the initial surface over cm-scales. Raman spectroscopy verifies that the crystalline quality of the transferred graphene is similar to that of the as-grown graphene, with its initially high compressive strain partially relaxed and no unintentional doping. After a two-step selective etching of the Ni layer in H2SO4: Sodium-n nitrobenzene sulfonate: thiourea (Transene etchant TFG) and HCl, we studied the surface chemistry of the resulting graphene/GaAs surfaces by X-ray photoelectron spectroscopy (XPS). Undoped GaAs remains unoxidized during the transfer process with an increase of the As-As related peaks due to preferential Ga dissolution during the acid-based deoxidation and etching processes, whereas p-type GaAs in contact with graphene showed corrosion damage attributed to a galvanic process with graphene acting as the cathode. This work provides new insights on the potential and processing constraints of dry-transferred graphene/GaAs heterostructures.

Solar photovoltaics (PV) is expected to play a crucial role in achieving carbon neutrality by 2050. The past 15 years have witnessed remarkable progress in both research and industry. This review provides an overview of current technologies, including dominant silicon PV, emerging perovskite materials, premium III-V semiconductors, and alternative thin-film technologies. We also introduce the main research avenues, with a particular focus on the emerging field of tandem solar cells and the role of photonics.

We present a universal model of broadband absorption in a slab of semiconductor. The theoretical framework, based on the description of multiple overlapping resonances in the frequency domain, has a very broad domain of validity. We derive simple analytical formulas for reference light-trapping models and for absorption upper bounds. Two light-trapping strategies are compared: multi-resonant absorption achieved with a sub-wavelength periodical pattern, and isotropic scattering obtained with random texturing. We provide an answer to the long-debated question of the best strategy for light-trapping in solar cells, and guidelines for the design of ultrathin solar cells. They apply to both silicon and thin-film solar cells. The new upper bounds on absorption presented in this work could be used to revisit the maximum efficiency of single-junction silicon solar cells.

The introduction of selenium in CdSeTe/CdTe solar cells has led to improved device performances attributed to the passivation of bulk defects. In this work, high-resolution cathodoluminescence experiments are performed on a series of CdSeTe/CdTe thin films with different Se concentrations to quantify the mechanisms and the passivation role of Se. We demonstrate a universal dependence between the Se concentration and the radiative efficiency and a ten-fold enhancement of the luminescence between CdTe and CdSe0.4Te0.6. Raw luminescence maps are converted into maps of the Se concentration, revealing its graded profile within the stack. We demonstrate the diffusion of Se along CdTe grain boundaries induced by the cadmium chloride annealing treatment and determine the diffusion coefficients, which are more than eight times higher at grain boundaries than in grain interiors. These results provide microscopic insights into the distribution of Se and its impact on the passivation of CdSeTe/CdTe solar cells.

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 Cs 0.05 (MA 0.15 FA 0.85 ) 0.95 Pb(I 0.84 Br 0.16 ) 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 (PbI 2 ), inorganic mixed halide CsPb(I 0.9 Br 0.1 ) 3 and lead‐rich CsPb 2 (I 0.74 Br 0.26 ) 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.

Perovskite solar cells are one of the most actively studied next-generation solar cells. This is mainly because high power conversion efficiencies can be achieved even with simple solution-based fabrication processes. In addition, wide bandgap perovskite solar cells can be used as top sub-cells in multi-junction solar cells due to their easily tunable bandgap properties. On the other hand, colloidal quantum dots (CQDs), whose band gap depends on the quantum dot size, are one of the few options that are compatible with solution processes and can be employed as lower sub-cells. Here, we show the potential of both types for the construction of multi-junction solar cells. To this end, we constructed a wide bandgap perovskite solar cell that is ideal for monolithic 2-junction perovskite/GaAs solar cells. We also developed a colloidal quantum dot solar cell with infrared absorbing PbS CQDs and constructed a spectral splitting multi-junction solar cell as a proof of concept.

We address the question of the optimal broadband absorption of waves in an open, dissipative system. We develop a general framework for absorption induced by multiple overlapping resonances, based on quasi-normal modes and radiative and non-radiative decay rates. Upper bounds on broadband absorption in a slab of thickness d take the simple form: A= 1-exp(-F α d), where α is the absorption coefficient and F the path enhancement factor. We apply these results to sunlight absorption in photovoltaics and answer the long-standing debate on the best light-trapping strategy in solar cells. For angle-independent absorption, we derive the isotropic scattering upper bound F = 4 n^2 (n the refractive index), extending the well-know Yablonovitch limit beyond the ray optics and weak absorption regimes. For angle-restricted illumination, we show that F can be further increased up to 8 π n^2 / √(3) using multi-resonant absorption induced by periodical patterning. These results have a general scope in the field of wave physics and open new opportunities to maximize absorption, detection, and attenuation of electromagnetic or mechanical waves in ultrathin devices.

In this work we employ the transfer matrix method for the analysis of optical materials properties to simulate and optimize monolithic tandem solar cell devices based on CuIn 1− x Ga x Se 2 , CI(G)S, and perovskite (PVK) absorbers. By finding models that fit well the experimental data of the CI(G)S solar cell, the semitransparent perovskite solar cell (PSC) and the PVK/CI(G)S monolithic tandem solar cell, we were able to perform a detailed optical loss analysis that allowed us to determine sources of parasitic absorption. We found better substitute materials for the transport layers to increase the power conversion efficiency and, in case of semitransparent PSCs, sub-bandgap transmittance. Our results set guidelines for the monolithic PVK/CI(G)S tandem solar cells development, predicting an achievable efficiency of 30%.