The aim of the project is to provide low-cost and high efficiency tandem cells grown on crystalline silicon (c-Si) substrates, with merging both the monocristalline Si approach with the high-efficiency monocristalline multijunction approach based on III-V materials. These CPV cells will be used under natural lighting and under low light concentrators (100 suns) developed by IRDEP-CNRS, and benchmarked under medium concentration by Heliotrop sas. The PV cells efficiency is one of the most important parameters for the final cost of electricity, since it impacts directly the ratio between produced energy and production cost. With 22% efficiency modules based on c-Si, the technology seems to reach its limits. To increase further the efficiency of c-Si cells and modules, going to multijunction devices (association of two different absorbing layers in the same cell) seems to be the obvious choice. While many projects tend to focus on all silicon technology, best high bandgap cells are yet based on III-V compounds. This project proposes to demonstrate the proof-of-concept for a monolithic integration of high efficiency multijunction CPV device on a low cost monocristalline silicon substrate upon which a III-V lattice-matched material will be grown using molecular beam Epitaxy (MBE). This Lattice-Matched heterostructure with its very low structural defect densities (Dislocations, AntiPhase Domains, point defects) will be capable of sustaining III-V high performing PV devices onto silicon with long life-time. This novel route overcomes the problems of high cost substrates (as compared to Ge or III-V substrates used currently for this kind of CPV), the killer structural defect formation and reliability issues of lattice mismatched systems (metamorphic approach) and the low reliability and low lifetime of hybrid techniques (such as wafer bonding). The integration of photovoltaic functions onto a single silicon substrate will also achieve a reduction in the use of III-V based semiconducting materials in high-efficiency multijunction CPVs. The two main scientific and technologic objectives of the project are : 1) The achievement of GaAsPN (1.7 eV) single cell on Si (with a 15% efficiency under low concentration, i.e. 100 sun). 2) The demonstration of a high efficiency and low cost multi-junction solar cell: GaAsPN pn cell at 1.7 eV on Si pn cell at 1.1 eV (25% efficiency under low concentration, i.e. 100 sun, as a first step towards very high-efficiencies >30%) Lattice-matched layers and slightly tilted substrates are used to overcome the two main difficulties faced by the growth of III-V materials on silicon substrates: misfit dislocations and antiphase lattice defects, in order to obtain defect-free III-V materials and to get large minority carrier diffusion lengths for the PV applications. The PV devices will consist in high efficiency tandem cells III-V/Si double-pn-junctions separated with a Buried Tunnel Junction. The final structure will include a first bottom Si pn (1.1eV low gap) grown on the Si substrate, then a thin GaP layers is grown by MBE to prevent structural defects formation, a top cell GaAsPN pn (1.7eV large gap) junction is then grown on top of it. The project relies on a high quality consortium which brings together six french partners, and an associated European partner, with high, established competence and complementary methodology and expertise in their fields and leading appropriate workpackages: FOTON (growth of III-V materials), INL (Si-based PV technology), CEMES-CNRS (structural characterizations), IRDEP-CNRS (research in PV development), EDF R&D (a European leader in the Energy sector), HELIOTROP (French manufacturer of high concentration photovoltaic modules (HCPV)) and AALTO (a Finnish associated academic partner specialised in point defects analysis). The partners are active in European research consortia and in networks of excellence and they drive many projects on the national and international level.

The objective of this project is to make important advances in quantum Monte Carlo (QMC) for computational chemistry. Our long-term motivation is to explore whether or not QMC could become a reference method of the field in the years to come. As known, the present approaches of computational chemistry are the Density Functional Theory approaches and various types of post-Hartree-Fock methods (Coupled Cluster, Configuration Interaction, perturbative approaches, etc.). However, despite remarkable results a number of limitations still restrict their use either in terms of predictability (e.g, choice of the exchange-correlation potential) and/or computational cost as a function of the system size. Quantum Monte Carlo methods have been developed as a promising alternative to these standard approaches. In computational physics they are extensively employed and considered as mature methods. However, it is not the case in computational chemistry, although a number of interesting results on real systems have been obtained. In this project we propose to make important progress on what we consider to be the most important practical and theoretical bottlenecks responsible for the present limited use of QMC in chemistry. From a theoretical point of view, we thus propose to work on: (i) The absence of a simple, general, and systematic strategy for constructing accurate enough trial wave functions for general molecular systems. QMC requiring such an input, present algorithms rest too much on expertise and lengthy optimizations, thus avoiding the practical use of QMC by the «ordinary» (computational) chemist not expert in QMC. (ii) The absence of a stable and accurate algorithm for computing forces and allowing large-scale geometry optimizations. This work will be the continuation of a thesis work presently underway. In particular, linear-scaling techniques (O(N) approach) to treat large systems will be developed and implemented. Besides these methodological aspects, the following important practical aspects will also be pursued: (iii.) To provide a general-public, general-purpose, and easy-to-use version of our code QMC=Chem including the two previous aspects. Such a version will be delivered at the project end. Interfaced with the standard codes of quantum chemistry (GAUSSIAN, MOLPRO, etc.) it will allow to treat easily and accurately large electronic systems using our multi-scale strategy with minimal and automated trial wave function re-optimization, linear-scaling O(N), and efficient force computations. (iv.) To deliver a parallelized version of the code with maximal efficiency. Monte Carlo methods are known to be intrinsically suited to parallelism and, more generally, to High Performance Computing. Common believe is that this practical but fundamental feature will certainly be a key feature for the success of such methods. Finally, to promote QMC within the theoretical chemistry community we propose to demonstrate its high potential via selected QMC «critical» applications for problems of high scientific interest and particularly difficult to study with standard methods. We also propose to organize an international workshop at the end of the project to foster the diffusion of our results. The core of the project will be realized at LCPQ (Toulouse). The financing of a PhD thesis is asked for the implementation of our general multi-scale strategy. Critical QMC applications will be done in collaboration with the Professor A. Ramírez-Solís from Mexico. Regarding the development of large-scale QMC geometry optimizations using our newly developed forces and multi-scale wave functions, and the implementation of O(N) linear-scaling techniques a 36-month post doctoral financing is demanded. This post-doctoral work will be done on a shared basis: Half-time at LCPQ (QMC, localized molecular orbitals) and half-time at the SRMC laboratory at Nancy under the supervision of Dr. A. Monari.