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%.

Today, single junction silicon technology dominates the photovoltaic (PV) market, with more than 90% of market share. However, the power conversion efficiency of silicon solar cells is now close to the theoretical limit. Indeed, the record has been pushed to 26.7 %, which is close to the silicon single junction theoretical limit of approximately 29% when the unavoidable Auger recombination is taken into account. To increase solar cell efficiency above 30% while keeping the abundant, cheap and stable silicon material as a basis, one solution is to couple silicon with another semiconductor having a larger bandgap in a tandem cell configuration. Currently, silicon based tandem technology follows two paths: the monolithic two terminals tandem (2TT) where the top and the bottom sub-cells are electrically and optically connected, and the four terminals tandem (4TT) where the two sub-cells are electrically independent. However, the 2TT architecture needs to manage photocurrent matching and to optimize the tunnel junction charges transport mechanisms between the top and the bottom sub-cells, while the 4TT device has to deal with issues related to the buried contacts shadowing and access and losses induced by the adhesive interconnection. The THESIS proposal aims at developing an original 3 terminals tandem solar cell (3TT). The approach is threefold: - To propose a new solar cell technology with 3 terminals. This allows us to suppress the constraint of photo-current matching for the two cells constituting the tandem cell. Furthermore, a 3-terminal tandem cell does not need a tunneling junction. - To facilitate the access to the different contacts of the top and bottom cells without the need for etching and without having to align buried contact grids, - To combine the advantages of reliable and mastered silicon technology with those of emerging technologies, allowing the creation of a heterojunction stack with the silicon. This new 3-terminals tandem cell technology we have patented is made possible in an innovative and simple way by using a silicon PV cell with interdigitated back contacts (IBC) on the rear face as a bottom sub-cell and depositing a larger bandgap semiconductor on top of the c-Si surface with a selective band offset barrier (BOB) at the interface in order to form a front heterojunction stack (FHS) realizing a top heterojunction sub-cell. This barrier is chosen so that the heterojunction allows a separation of the operation of the two cells. In the THESIS project, we propose to focus on the emerging perovskites as the absorber of the p-type FHS. The interface between the perovskite and silicon will be actively studied in the project and will need deep investigations to improve the interface quality and device operation. We plan to use also p/i a-Si:H stack as the FHS, forming a (p) a-Si:H/ (i) a-Si:H/ (n) c-Si vertical front subcell. Of course, we do not expect the best photovoltaic performances with this subcell due to the limited transport properties of a-Si:H. However, the growth of device quality a-Si:H for the top subcell, and the c-Si IBC technology are already well mastered in the consortium, so this will allow us to fabricate a proof of concept device for this innovative 3TT architecture. This will be a breakthrough in the PV world, since the 3TT architecture has never been demonstrated so far.

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.

The huge increase in today`s information and data communication services imposes higher data rate transmission networks and processing, requiring high frequency (HF) operations. Tunneling devices (TDs) offer potentially superior performance than thermionic-based devices as they do not suffer from limitations due to thermal activation and can lead to negative differential resistance, a unique and differentiating feature for the development of HF sources and detectors. Unfortunately, as TDs are very sensitive to the chemical and electronic structure at the tunneling interface, their performance are hampered by a number of issues due to the covalent bonding at the interface when fabricated using Si, Ge or III-V semiconductors. It is then mandatory to explore emerging materials to improve the tunneling interface quality. The absence of surface dangling bonds in two-dimensional transition metal dichalcogenides (TMDs) alleviates the interface covalent bonding issue and allows the formation of van der Waals heterostructures rendering strain-free integration possible. In recent years several reports have confirmed the added value of TMDs for tunneling devices with band-to-band tunneling as the governing conduction mechanism. However, to date, TMD-based tunnel devices have been elaborated from exfoliated or transferred layers which raises severe issues regarding the interface integrity and process reliability. The Tunne2D project aims at assessing the capabilities of TMDs for tunneling devices. To this end, different tunnel diodes will be fabricated at wafer scale and fully characterized at low and high frequency. The objectives are: i) the growth of TMD heterostructures using improved chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) scalable growth techniques and their thorough characterization; ii) the design of tunnel diodes based on the simulation of material and transport properties; iii) the reliable fabrication of TMD-based tunnel diodes; iv) the comparison between tunneling devices fabricated with either CVD or MBE to get insights in the pros/cons of the two approaches; v) the benchmarking of the resulting TMD-based diodes versus conventional semiconductor-based ones. The success of TUNNE2D will not only pave the way for more complex tunneling devices, like Tunnel-FET for low power applications, but the resulting high-quality heterostructures will offer new opportunities for the development of optoelectronic devices like infrared photodetectors and photocatalytic cells for H2 production. The project will focus on the Se-based TMD family offering the variety of materials, namely metallic, p- and n-type semiconducting layers with different electron affinities requested to fabricate these diodes. Our approach relies on the strong interaction between material issues, modeling and simulation and electrical investigation of the devices. It gathers 4 academic partners (IEMN, CINTRA, C2N, CP2M) with complementary skills and equipment in material elaboration and characterization, device processing and simulation. IEMN has launched a MBE system fully dedicated to TMD growth and is expert in the development of 2D-based devices with advanced fabrication techniques. Besides standard MBE, an original approach using single source precursors will be explored, in strong collaboration with CP2M, expert in the precursor design for gas-phase deposition techniques. CINTRA, very active in the field of TMD CVD growth, has demonstrated the versatility of the CVD technique for more than 30 materials and the interest of metalorganic precursors for large scale MoS2 continuous films. This approach will be extended to selenide ones. C2N has a long experience in tunneling device simulations and has recently focused on TMDs. The success of the consortium will rely on a long-term partnership between IEMN and CINTRA, a long running experience in joint projects of IEMN and C2N and the great habit of CP2M to work with material physicists.