The SMASH-IBC2 project aims at reducing the cost of PV electricity by developing innovative high efficiency solar cells and modules. Two different technologies currently dominate the PV market. The first one is based on crystalline silicon (c-Si) devices which historically lead the market due to its proven reliability and efficiency. Thin film technologies also show a great potential in terms of efficiency and cost reduction. In this project both technologies will be merged to obtain high efficiency solar cells on thin c-Si wafers with simplified processes. This may be achieved through an innovative solar cell design called IBC Si-HJ (Interdigitated Back Contact Silicon Hetero-Junction). IBC Si-HJ cells have a high efficiency potential (=25%) achievable on thin wafers (=100µm) with a low temperature fabrication process (=200°C). Moreover a simplified and aesthetic module interconnection (coplanar) can be developed with these structures. To obtain a cost effective structure we will study different processes from the thin film technology and try to transfer them for c-Si solar cells fabrication. We will focus on one hand on thin layers and contact formation (chemical and physical vapor deposition, electrodeposition, epitaxy). On the other hand, simplified cell fabrication steps (laser contacting, ablation and scribing) will be developed to achieve a low cost and industrial process. The main goal of the project is to validate a cost-effective method for fabricating high efficiency PV modules, using 24% efficient c-Si solar cells, based on thin (100 µm) and large area (150 cm2) silicon wafers. The metallisation of these devices will be ITO- (Indium Tin Oxide) and Ag-free to reduce the cell cost. This achievement will be based on well identified scientifical and technological issues linked with different tasks in the project. Thanks to the previous projects on the same topic (QC-Passi, SHARCC, TopShot), an important knowledge has been developed by the different partners. A precise and realistic roadmap has therefore been determined, as well as associated milestones.

The main challenge targeted by the ULTiMeD project is to gain an atomic level control over an innovative fabrication of sulfide-based lamellar materials and ultimately 2D transition metal dichalcogenides (TMD), a recognized class of emerging materials of large potential in microelectronics. The originality is based on an intermediate metal-thiolate (polymer) film deposited by a combination of Atomic Layer Deposition and Molecular Layer Deposition (ALD/MLD) followed by a thermal treatment (annealing) en route to the crystalline final phase (metal = Mo, Sn, Ti, Ga). The consortium possesses a first proof-of-concept for MoS2 [CAD2017-1] (and WS2 [CAD2017-2]). The targeted materials are MoS2, mainly as a case study, then SnS2, TiS2 and for comparison GaSx (the consortium has an ongoing collaboration on GaSx). We will explore the possibility of obtaining by this method 2D hybrid heterostructures with TMDs and graphitic-materials inside which could allow new electrodes or electrical contacts, in particular of p+ type, a solution that could avoid noble or expensive metals. Also, it is known that H2S precursors, which is the common sulphur source in thin film technology, is not environmentally friendly. Alternatively, the ULTiMeD project proposes another process that is to substitute ethanedithiol for H2S. We identify that one of the likely advantages of this molecule (ethanedithiol) is to limit/protect against adventitious hydrolysis (and hence formation of oxysulfides rather than sulfides), a known pitfall with H2S. To date, the consortium definitely need a better understanding of the surface chemical phenomena taking place during growth and annealing at the substrate surface and in the layer. Therefore, one of the major effort of the ULTiMeD project is to gather a complementary suite of theoretical and experimental tools to determine the chemical reaction route (first principle calculations) and perform in situ chemical and structural studies during growth and annealing. We will perform ALD/MLD ab initio modelling by using the Density Functional Theory. We will use advanced structural and chemical characterization techniques in our laboratories (in situ Raman spectroscopy, high resolution X-ray fluorescence, X-ray Photoelectron Spectroscopy, DRIFT, …) and a dedicated reactor (MOON) to study in situ ALD/MLD process at the early stage and the film evolution during the thermal annealing (at “home-lab” with a Residual Gaz Analyser and by ellipsometry). Besides, MOON will allow to use in situ synchrotron radiation-based techniques, as for instance x-ray fluorescence spectroscopy, x-ray absorption and x-ray diffraction, at the synchrotron SOLEIL (St Aubin) [BOI2016] to obtain a comprehensive picture of the incipient growth. Ab initio calculations will provide a very powerful tool for the interpretation of structural and spectroscopic data. Standard electrical characterization and KFM measurements will be also performed. We expect that the ULTiMeD project lead to improved control over the 2-step ALD/MLD process followed by mild annealing en route to the crystalline final phase, and ultimately over the ability to tailor the properties of large area 2D sulfide materials atomically. Our original methodology opens up the possibility to grow new emerging sulfide based materials including 2D dichalcogenides (SnS2, TiS2) which would find application after thorough investigation of their phase segregation.

Reducing the absorber thickness is one of the main issues for most photovoltaic technologies, because of the material cost or scarcity, and potential efficiency improvements with higher carrier concentration. Hence, highly efficient energy conversion in ultra-small volumes of semiconductors is a major challenge for next-generation solar cells. However, it requires the development of novel light-trapping strategies, semiconductor growth techniques and nanofabrication processes scalable to large surface areas. Following these guidelines, an ideal solar cell would be made of semiconductor nanostructures (nanocell photovoltaic devices) and can be drawn as follows: it is composed of very small volume absorber (typical dimensions 100 nm), with high light absorption and carrier collection efficiencies, and low parasitic losses. In principle, this strategy should result in significant efficiency improvement due to less bulk recombination and higher carrier concentration (higher Voc), and a strong semiconductor material saving. Moreover, this ideal solar cell would be fabricated by bottom-up processes on reusable substrates and transferred on host substrates, in order to guaranty large scale and low cost fabrication. In this project, we aim at developing new architectures, technologies and methods for next-generation solar cells with ultra-low absorber volumes. The goals of this project are twofold: (1) to pave the way towards fully optimized (optically, electronically, technologically) nanocell photovoltaics devices made of semiconductor nanostructures fabricated by bottom-up approaches (localized growth, in particular), (2) to have short-term impact on thin-film photovoltaics by developing new technological processes (nanopatterning, heterostructure epitaxial growth, and transfer processes with scalable and potentially low cost fabrication techniques) and tools (characterization and modeling). As an intermediate step towards the final design, we will fabricate a GaAs nanowire solar cell on inactive silicon substrate, before the development of the final goal of the project: a GaAs nanowire-based solar cell with fully optimized light trapping, transferred on a metallic back contact-mirror on glass. Nanocell solar cells will be based on the localized growth of III-V nanostructures (GaAs) on Si substrates, with the aim of reaching small volume solar cells, at the frontier of the possible volume reduction. The optimization of the nanophotonic absorption enhancement with an efficient material system will lead to concepts transferable to other polycristalline thin film technologies. Importantly, we propose to use small aspect ratios nanowires, leading to less transport issues and less interface recombination than in hitherto proposed nanowire solar cells. High efficiencies can only be achieved if this approach is coupled to advanced nanophotonics to avoid a strong photocurrent loss penalty. The selective growth of well-organized nanowire arrays on Si substrates, that will be transferred on host substrates, and combined with optimized nanophotonic/plasmonic designs and efficient passivation, are the main issues that will be addressed in this project. The high-level expertise gathered by the partners of this consortium covers the wide scope of activities required, and will enable to tackle these issues. Each of these achievements will put our results at the highest level as compared to current state-of-the-art, and will pave the way towards next-generation solar cells. We target 20% conversion efficiency.

INDEED proposes an original and innovative way to fabricate efficient transparent electrodes with varying haze factor values as well as their integration into solar cells, with the goals of using low-cost and abundant materials coupled with low cost deposition methods. The involved multidisciplinary consortium can tackle the tasks with complementary approaches dealing with the fabrication, characterization of physical properties of these thin Transparent Conductive Oxide (TCO) layers, modelling of the carrier transport and optical properties as well their integration into two types of promising solar cells: thin Si films (LPICM) and CIGS (IRDEP). A very high value of the haze (40-50%), coupled with excellent transparency and low sheet resistance, appears indeed very promising for enhancing solar cell performances. The presence of Lotus Synthesis Company within the consortium is also a clear asset in terms of precursor development and up-scaling of the diffusing Fluor-doped Tin Oxide (FTO). Transparent electrodes are one of key components of the information technology (displays) and energy applications (photovoltaics, architecture and window glasses). The associated markets related to TCO have grown exponentially over the past decade due to the proliferation of large LCDs, thin film solar cells, OLEDS, or touch screens. Therefore, an intensive research has been devoted to this type of materials. Indium Tin Oxide is the main used TCO but it is of rare supply and indium-free solutions are therefore requested. TCO play a crucial role in solar cell technologies which is to collect photo-generated charges in the absorber, thus extracting current from the active region of the cell and allowing the generated carriers to be collected. At the same time, they must be transparent to incident light in the required spectral range to allow light into the cell to generate electron-hole pairs. This double function is usually accomplished by TCOs. A key property is the haziness of TCOs, which is the ratio between diffused and total transmitted light intensities. A large haze factor is required for solar cell applications since it can increase the optical path length and then the photon absorption by the photo-active layer. This allows the use of thinner absorbing layers, which may lower recombination losses and therefore enhance the solar cell efficiency. The project INDEED is based on a very recent way proven to grow efficiently TCO (especially FTO) with varying haze factors on the one hand and on the expertise, competence and complementarity between the different teams of the consortium on the other hand. While INDEED will be focused on rigid substrates (glass), the flexibility can be reached since diffusive FTO can also be fabricated on polymeric substrates which exhibit improved mechanical resistance when compared with non-diffuse FTO layers. INDEED proposes innovative solutions in order to fabricate efficient transparent electrodes with varying haze factor values (up to very large values: 40-50%) as well as their integration into two types of solar cells, with the objectives of developing low cost methods and abundant materials for enhancing solar cell efficiency. The results of electrical and optical characterization will be used to quantify, via optical modelling, the impact of the material properties on the power conversion efficiency. These electrodes will be tested in single junction PIN and tandem PIN/PIN Si devices fabricated at LPICM as well as CIGS solar cells at IRDEP. The choice of the two types of solar cells appears pertinent since the high efficiency and low cost are possible for both silicon thin film devices which have reached record efficiencies of 16.3% (for a triple junction) and CIGS solar cells which reach 21.7%. They exhibit a high potential development in the PV market and the French research community is foreseen to play an important role in their development.

The control of surface chemistry or interface is one of the key steps in the control of nanotechnology and photonics industry for high added values components. This fact is generally observed for the III-V semiconductors (III-Vs) where it is still difficult to achieve their exceptional performances predicted by physics. The EPINAL program is based on a passivation involving a total rate covering of the surface of InP by an ultra thin film of polyphosphazene. This is an original and novel passivation route that the Consortium wants to extend to other III-V semiconductors. This process involves a controlled chemistry of nanoscale "P-N" bonds which is achieved by electrochemistry or electroless procedure in liquid ammonia ( 55 °C, atmospheric pressure). EPINAL is a multidisciplinary project combining 4 academic research groups (ILV, C2N, ICMPE and IRDEP). This highly contrasted but complementary multidisciplinary approach (chemistry, electrochemistry, physics and optics) is an original and innovative way to define the better performances of passivation films on III-Vs semiconductors. EPINAL aim is the understanding of fundamental modifications observed at the interface in order to reveal the best passivation as possible. The preliminary results obtained by the consortium have been encouraging on InP surface by electrochemistry. The film formation has been indeed obtained in a fully controlled way by electrochemistry (thickness, homogeneity and spatial distribution). The chemical stability of the film on different solvents and its thermal stability have been revealed. The functionalisation of the film by inorganic fragment complexation has been observed and the possibility to extend this passivation to pattern formation has been shown using SiO2 masks. This passivation will also explore other III-Vs involving new “V-N” bonds such as “Sb-N” and “Ga-N” that are present in many ternary and quaternary alloys. The success of this extended passivation to other III-Vs semiconductors would allow many applications in the field of microelectronics and optoelectronic devices. Examples include passivation on the cleaved or engraved surfaces of SC waveguides to improve their resistance in high optical power densities. High-reflectivity treatments of mirror faces to increase laser performance are also essential. Passivation and isolation of laser ribbons for deep wave peak waveguide technology are other examples.