The development of “green” energy sources has attracted much intention over the last decade and is currently of a crucial importance for our societies. Because of the greenhouse effect the policies for the development of carbon free energies have become central. Nevertheless, the terawatt-scale energy demand will remain. The electrical energy supply will therefore have to be an energy mix. Photovoltaics will be the backbone of such a renewable energy system. To fulfill the challenging price targets and to develop new markets (e.g. building integration) the design and production of solar cells working at highest efficiencies is essential. Crystalline Si dominates the PV market (85%) and will be essential for still a very long time. However, as maximum efficiency is limited to 29.4 %, strategies must be developed to maintain its dominant role in the PV market. One way to reduce losses is to add a top cell with a higher band gap above a conventional silicon cell to create a tandem cell with potentially more than 40 % efficiency. Different ways of developing new top absorbers have been investigated but require the use of indium and gallium. Significant volatility in the price and supply of such matter over the past years has led to considerable concern given their critical roles and their use in a wide range of large scale electronic devices including solar cells. Moreover, III-V tandem cells require the use of epitaxial growth that remain both expensive and limiting for large scale realization and will therefore negatively influence the production costs even at high production capacity. Finaly, the materials used are toxic and their acceptance in society is therefore limited. It is so important to study and develop new indium-free Earth abundant and non-toxic materials with optimized properties for the realization of innovative solar cell demonstrating affordable cost for mass production. The OPERA project aims at developing a new kind of low-cost, indium/gallium-free, non-toxic nitride absorber and to realize first test nitride cell by using easy-to-use, up-sizeable and cost affordable production technique (sputtering). This is an ambitious goal that would be a major step forward in the field of solar energy. Moreover, each intermediate step of the project would bring new knowledge more especially about single top nitride solar cell or related materials fundamental properties. Indeed, the family of Zn-IV-N2 alloys is promising as it could span the solar spectrum and could then replace the InGaN alloys as absorbers. Nonetheless, data about ZnSnN2 alloys remain scarce. The interest of such alloy for PV has increased the last two years, but numerous efforts remain to be done to better understand its fundamental properties. The OPERA project gathers four laboratories with different specialties and complementary skills: the Jean Lamour Institute - CNRS - France dedicated to material science. The Institute of Electronics Microelectronics and Nanotechnology – CNRS – France with skills in material sciences and device technologies. Institut National de l’Energie Solaire – Commissariat à l’Energie Atomique et aux Energies Alternatives: a well-known French institute for solar technologies and transfer to industry. The Group of electrical engineering of Paris (GeePs), formerly LGEP, is one of the main PV laboratories in France, a founding member of the CNRS PV Photovoltaic Federation (FedPV), and an active partner of IPVF (Institut Photovoltaïque Francilien).

Thanks to their nanoscale dimensions and large surface-to-volume ratio, the sub-100 nm wide semiconducting nanowires (NWs) undergo a broad range of new physical phenomena, which are non-existing or non-expressed at micrometric scale. Surface charge effects (SCE) are one of these phenomena strongly expressed at nanoscale dimensions and deeply affecting the NW properties, and thus the device performances integrating them, such as piezoelectric (PZ) generators. Despite their crucial influence, SCE are too often neglected in the design of theses NW-based devices, since they are unknown to a large extent and/or non-mastered. The SCENIC project aims at addressing the fundamental questions regarding the impact of these surface charges in order to control their effects on the properties of sub-100 nm wide GaN and ZnO NWs, two typical NW systems with the same wurtzite structure, in which SCE are highly pronounced. These surface charges being modulated by the material morphology and properties constituting the NWs (nature, dimensions, doping…), and dependent upon the NW functionalization (with inorganic material according to a core-shell configuration, chemical adsorbates…), we propose to investigate different coupling between the active NWs and the environment surrounding them. Based on various nanoscale characterization tools using atomic force microscopy (AFM) in different modes, predictive numerical simulations using finite-element-method including many physical phenomena (i.e. mechanics, electro-mechanical effects, semiconductor physics), and a direct invaluable comparison between the two closed GaN and ZnO NW systems grown by vapor phase techniques (i.e. MBE and MOCVD, respectively) and exhibiting a high crystalline quality, the project will address these crucial nanoscale phenomena in details. By establishing the SCE mechanisms expressed into NWs, the SCENIC project aims at offering SCE nano-engineering solutions for optimizing the surface band bending (surface Fermi level pinning) in link with the targeted PZ application. Then, these advantageous GaN and ZnO NW nano-architectures will be validated through the fabrication and testing of PZ NW-based devices, in which SCE play a major role on their performances. The SCENIC project is a fundamental project, at a cutting-edge domain of research involving nanomaterials (NWs), physics at nanometer scale (SCE), modelling, and nano- and micro-technology (device validation), with applicative target in the field of piezoelectric generators. The project is based on a complementary partnership between Center for Nanosciences and Nanotechnologies (C2N), Laboratoire des Matériaux et du Génie Physique (LMGP), Group of electrical engineering of Paris (GeePs) and Institut de Microélectronique Electromagnétisme et Photonique and LAboratoire d'Hyperfréquences et de Caractérisation, (IMEP-LaHC). The consortium presents a strong expertise in nanomaterial growth, physical, electrical, chemical characterizations at nanoscale dimensions with specifically advanced and ultimate AFM-based characterization equipments, simulations and device processing and testing, then gathering all the competencies required to address the ambitious objectives of the present project.

The OXIGEN project aims to develop a new crystalline silicon (c-Si) photovoltaic (PV) cell generation, and to obtain = 23% efficiency on large area devices. The studies will focus on the fabrication of ultra-thin junctions and functionalized oxides to reach transparent and passivated contacts using industrial processes. Two technologies will be highlighted in this project, the first one being Plasma Immersion Ion Implantation (PIII) which is ideal to obtain ultra-thin junctions. The second one, based on fast Atomic Layer Deposition (ALD), is developed by the French company Encapsulix and will be used for the fabrication of innovative electrodes allowing both surface passivation and charge carrier collection. This collaboration in the field of functionalized oxides for c-Si PV cells will be great to share high level scientific knowledge and research tools. The project will be coordinated by CEA-LITEN (LHMJ) because most of the process integration will be done at INES facilities. The scientific expertise of four academic labs (INL, LMGP, IMEP LAHC, GEEPS-IPVF) on the thin films/interface/device fabrication, simulation and characterizations will be necessary for all technological improvements of OXYGEN cells structures. All technological and scientific improvements will be done in collaboration with a start-up (ENCAPSULIX), which will offer specific skills in industrial process development.

Patterning of silicon surfaces is essential for the design of opto and microelectronic devices. Creating microstructures like trenches, macropores or rods are essentials for the development of several component families in many research fields and commercial applications. Silicon etching is carried out either chemically or by using reactive-ion etching, usually involving one or more lithography steps. This is accompanied by some important constraints. The fabrication of complex structures involves a large number of technological steps that represent eventually a significant cost in the final device cost (and lithography/dry etching equipment also requires considerable investments). Moreover, one is often restricted in the design of perfectly suited surface structures because of limitations due for instance to the influence of the crystallography or to insufficiently high aspect ratios. In addition, in some industries (e.g. photovoltaic), lithography is incompatible with the cost issues and technical constraints of production lines, which strongly limit the surface structures that can be realized and therefore their effectiveness. Recently, a new method to etch silicon has emerged. It is based on the contact at the nanoscale of silicon with a noble metal in the presence of HF and an oxidizing agent (e.g. H2O2, anodic current). The metal acts as a catalyst allowing for a localized dissolution with a resolution of a few nm only. More recently, a Japanese group has shown that it is also possible with this method to use micrometer-sized electrodes made of Pt to etch large structures in silicon. Based on this principle, the PATTERN project aims at the development of a new silicon etching process using metal electrodes that perform "nano-imprints" by direct contact with silicon in a single electrochemical step. The goal is to be able to replicate a large variety of forms such as pores and trenches of high aspect ratio, inverted pyramids, micro-pillars, etc., cumulating high accuracy (~ 10 nm), multi-level etching profile and large surface area processing capabilities (> cm2). The biggest technological issue is the necessity to provide a nanostructured interface with a triple contact Silicon/Electrolyte/Metal. The innovative aspect is the development and use of volumetric nanoporous metal electrodes that can ensure this triple contact whatever the considered dimensions are, and be used several times in industrial processes. PATTERN is intended to provide the basis for this new contact etching process, including a demonstrator in the field of surface treatments for solar cells. PATTERN is divided into three tasks according to a multidisciplinary approach combining microelectronics, optics, powder metallurgy, electrochemistry of semiconductors and modeling of electronic interfaces: T1- Development of nanoporous metal electrodes with defined patterns (IEMN). Production of molds with basic and advanced periodic structures; Elaboration of nanoporous metal imprints by sintering metal powders in the molds. T2 - Contact etching of silicon (ICMPE). Study of the etching process and the transfer of the imprint electrodes surface pattern to the silicon substrate, with different oxidizing agents and metals; Optimization of the operating conditions. T3- 2D modeling of the Silicon/Electrolyte/Metal electronic interface (LGEP). Localization of the anodic currents around metal dots ~10x10 nm2 in size, as a function of the metal work function, the metal polarization, the inter-dot spacing. To conduct the project, three research teams specialized in microelectronics (IEMN), silicon (electro)chemical etching (ICMPE), and modeling of semiconductor electronic properties (LGEP) gathered together. They are acknowledged of a high level expertize in microelectronics, optoelectronics and photovoltaics applications, with several international patents taken in these fields.

While silicon-based solar cell technologies dominate the photovoltaic (PV) market today, their performance is limited. Indeed, the world record efficiency for Si-based PVs has been static at 25% for several years now. III-V multijunction PVs, on the other hand, have recently attained efficiencies > 40% and new record performances emerge regularly. Although tandem PV geometries have been developed combining crystalline and amorphous silicon, it has not been possible so far to form devices with efficiencies to rival III-V multijunctions. NOVAGAINS aims to benefit from combining the maturity of the Si technology with the potential efficiency gains associated with IIIV PV through the development of a novel tandem PV involving the integration of an InGaN based junction on a monocrystalline Si junction by means of a compliant ZnO interfacial template layer which doubles as a tunnel junction. Although the (In)GaN alloy has been used extensively in LEDs, its’ use in solar cell technology has drawn relatively little attention. Nevertheless, the InGaN materials system offers a huge potential to develop superior efficiency PV devices. The primary advantage of InGaN is the direct-band gap, which can be tuned to cover a range from 0.7 eV to 3.4 eV. As such, this is the only system which encompasses as much of the solar spectrum. Indeed, the fact that InGaN can provide such tunability of the bandgap means that PV conversion efficiencies greater than 50% can be anticipated. Unfortunately, it is very difficult to grow GaN based films of high materials quality directly on Si because they do not have a good crystallographic match. ZnO can be grown more readily on such substrates, however, because of its’ more compliant nature. Indeed, well-crystallized and highly-oriented ZnO can even be grown directly on the native amorphous SiO2 layer. Since ZnO shares the same wurtzite structure as GaN and there is less than 2% lattice mismatch it has been demonstrated that it is then possible to grow InGaN/GaN epitaxially on ZnO/Si using the specialized know-how offered by the consortium. Modeling indicates that when optimized, stacked InGaN and Si cells coupled by tunneling through a ZnO interlayer offer the perspective of tandem cells with overall solar conversion efficiencies in excess of 30%.