III–V solar cells are mainly grown on GaAs or Ge substrate, which significantly contributes to the final cost and affects the sustainable use of these rare materials. A so‐called PEELER process is developed, in which a porosification technique is used to create a weak layer between a Ge substrate and the epitaxial layers. This method enables the separation of the grown layers, allowing for the subsequent reuse of germanium and a reduction in the environmental and economic cost of optoelectronic devices. Technology validation using the device performance is important to assess the technology interest. For this purpose, the performance of 22 nondetached single‐junction GaAs photovoltaic cells grown and manufactured on porosified 100 mm Ge wafer without antireflection coating is fabricated and compared. All the cells exhibit comparable performance to state‐of‐the‐art GaAs solar cells (grown or Ge or GaAs) with high efficiency (21.8% ± 0.78%) and thereby demonstrate the viability of growing high‐performance optoelectronic devices on detachable Ge films.

New massive markets for space multi-junction solar cells are being discussed globally. For such an explosive increase in demand to materialize, a more sustainable and affordable Ge substrate technology is required. To this end, lithography-based Ge-on-Nothing and electrochemical process-based porous Ge wafers were developed. Both approaches yield uniform and smooth monocrystalline Ge-on-Ge engineered substrates after annealing, of which the top layer is weakly attached to the mother substrate. High-quality space solar cells were grown on them, followed by successful foil detachment and surface reconditioning. These results clearly demonstrate the feasibility of the reusable Ge substrate concept.

Mesoporous germanium (MP-Ge) emerges as a very appealing material for many applications such as anode material for Lithium-Ion batteries due to it high specific area and large void spaces or, in optoelectronics as sacrificial layer for III-V materials growth and detachment, allowing notably several uses of a single Ge substrate. These porous nanostructures are distinguished by a large specific surface area and are prone to degradation with time due to exposure to the environment. To understand and be able to reduce this effect, we studied the chemical and morphological evolution of porous germanium layers under various ambient storage conditions for 3 months to identify the main parameters responsible for material degradation. This study demonstrates that the ambient air environment leads to the growth of native oxide, leading to major morphology changes. Scanning electrons microscope (SEM) showed the formation of clusters and the enlargement of the pores after 90 days. These structural modifications are caused by the oxidation of Ge, and more specifically by the creation of GeO2 matrices due to the synergy of dioxygen (O2) and humidity (H2O(g)). The energy brought by light can exacerbate these phenomena and thus accelerate the degradation rate of the pore morphology. Based on these experimental results, we propose efficient solutions to limit the GeO2 proportions and the clusters' appearance, by storing them under a dry neutral atmosphere (Ar) or by adding a hydrogen halide pre-treatment (10s 1% HBr solution).

Germanium (Ge) is increasingly used as a substrate for high-performance optoelectronic, photovoltaic, and electronic devices. These devices are usually grown on thick and rigid Ge substrates manufactured by classical wafering techniques. Nanomembranes (NMs) provide an alternative to this approach while offering wafer-scale lateral dimensions, weight reduction, limitation of waste, and cost effectiveness. Herein, we introduce the Porous germanium Efficient Epitaxial LayEr Release (PEELER) process, which consists of the fabrication of wafer-scale detachable monocrystalline Ge NMs on porous Ge (PGe) and substrate reuse. We demonstrate monocrystalline Ge NMs with surface roughness below 1 nm on top of nanoengineered void layer enabling layer detachment. Furthermore, these Ge NMs exhibit compatibility with the growth of III-V materials. High-resolution transmission electron microscopy (HRTEM) characterization shows Ge NMs crystallinity and high-resolution X-ray diffraction (HRXRD) reciprocal space mapping endorses high-quality GaAs layers. Finally, we demonstrate the chemical reconditioning process of the Ge substrate, allowing its reuse, to produce multiple free-standing NMs from a single parent wafer. The PEELER process significantly reduces the consumption of Ge during the fabrication process which paves the way for a new generation of low-cost flexible optoelectronics devices.

III-V photovoltaic cells typically use germanium (Ge) as a substrate for the epitaxial growth, however, this material contributes significantly to the overall price of the multijunction solar cells. In order to reduce the environmental and economic cost of the solar cells, we have developed a porosification technique using bipolar electrochemical etching (BBE) to create a weak layer between the Ge substrate and the epitaxial layers. This approach allows the easy separation of the grown layers and the subsequent reuse of germanium. As evidence of the potential of this method, we have compared the performances of non-detached single-junction III-V solar cells grown and fabricated (without anti-reflection coating-ARC) on porosified Ge, and on bulk Ge as a reference. All the final cells show mirror-like monocrystalline III-V layers with comparable characteristics notably concerning the Voc (VocGePorous=0.862V vs VocGeBulk=0.882V) and good efficiencies (Eff.GePorous=13.03% vs Eff.GeBulk=15.96%) in comparison with current literature values on similar substrates (Vocliterature= 0.75V and FFliterature=61%). These promising results open the path toward thin III-V solar cells and multiple Ge substrates reuse.

Epitaxial thin film detachment and substrate reuse is one of the promising approaches to reduce the weight and the cost of triple junction (3J) solar cells on Ge substrate for both terrestrial and space PV. This approach is based on epitaxial growth of high-quality solar cell materials on porosified Ge substrate. The mesoporous layer created by electrochemical etching undergoes thermal induced reconstruction leading to the formation of voided weak layer suitable for epilayers detachment. This approach is low-cost, scalable to large surfaces and allows the substrate reuse for several epitaxial cycles upon appropriate reconditioning. Accordingly, the success of the reconditioning step is conditional to both reliability and cost-effectiveness. In this context, we report the first successful proof-of-concept of Ge substrate reuse for epitaxy after the epilayer detachment. We demonstrate that chemical etching with HF-based mixture allows to recondition the detached substrate providing a low surface roughness of 1.3 nm without any CMP step. The reconditioned substrate was then porosified giving rise to homogenous porous layer suitable for epitaxial regrowth. A second growth cycle has been successively performed on the reconditioned and reporosified substrate. The epitaxial Ge layer from the second cycle is found to have high crystalline quality and low surface roughness as revealed by X-ray diffraction and atomic force microscopy investigations. Our results demonstrate a CMP-free reliable Ge substrate reconditioning process for epitaxy, which paves the way to the substrate multi-reuse for triple junction solar cell cost-reduction.

Germanium (Ge) substrates are usually used for epitaxial growth of III- V materials but represent a significant part of the cell total cost. The lift-off technique using porosification by bipolar chemical etching is a promising approach to detach the active layers from the Ge substrate and allows Ge substrate reuse. However, this solution raises challenges concerning the delamination and possible structural deterioration of the mem-brane during the micro-fabrication of the solar cell. In this paper, we successfully apply the main micro-fabrication steps on a Ge membrane grown on porous Ge. The front side process using UV lithography and Au-Ni deposition for contacts has been successfully performed without membrane degradation. Those contacted membranes were also successfully transferred on a host substrate. X rays measurements (XRD) were also performed before and after detachment and shows no damage on the crystalline quality of Ge membrane.

Currently, III-V multijunction solar cells holds the highest efficiency. However, they are expensive to produce and this prevents their large-scale use. For single GaAs solar cells, most of the total cost of the cell comes from the substrate. For III-V cells on Ge substrate, the thickness of the substrate is usually between 150 μm and 180 μm, whereas only 1 μm would be sufficient to maintain the performance of the cell. Moreover, 95% of the weight of the cell comes from the substrate which is a problem for the space domain. It is therefore necessary to find an economical and reliable approach for layer separation and reuse of Ge substrate. The approach proposed in this work shows the epitaxial growth of high-quality monocrystalline Germanium template on a 4-inch porous Ge substrate. The porous layer reconstruction leaves voided interface with nanometer scale pillars allowing defect free detachment of the epitaxial layer. Furthermore, the Ge epilayer demonstrates very low surface roughness, comparable to that of the epi-ready bulk Ge substrate with a preserved miscut angle suitable for III-V materials and solar cells heteroepitaxy. Accordingly, high crystal quality GaAs epilayer has been successfully demonstrated on the designed Ge template. Our finding paves the way to a reliable and cost-effective approach towards III-V/Ge multijunction solar cells detachment and substrate reuse that may constitute a technological breakthrough for both space and terrestrial PV applications.