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Design numérique de métamatériaux pour des applications photovoltaïques

Abstract : The purpose of the thesis was to simulate the absorption spectrum of meta-materials for photovoltaic applications. By meta-material, we mean an assembly of nanometric size objects at mesoscopic distance. The underlying idea is that by adjusting the size of the nano-object and the geometric arrangement, one could tune the absorption edge. To calculate these quantities, I used state-of-the art formalism, namely ab-initio methods.The first step of the work has been dedicated to the calculation of the absorption of an isolated object (slab of silicon, graphene, hBN). In the framework of periodic codes, one uses a supercell with vacuum to isolate the object, and a method has been developed previously in the Theoretical spectroscopy group at LSI, to provide results independent of vacuum. It is called “Selectd-G” method, and was successfully applied to silicon surfaces. For an isolated slab, a modified expression of the reciprocal space Coulomb potential, called “slab potential”, must be used. To validate the use of the slab potential on the microscopic dielectric matrix, I have simulated Electron Energy Loss spectra for slabs of few graphene layers, and successfully reproduced available experimental data. This has also offered the possibility to study the plasmon dispersion of a single graphene layer, and discuss the nature of electronic excitations in the system (intraband transitions or 2D-plasmon).The second step has been dedicated to the study of the absorption spectrum of an array of interacting slabs. Since it has been evidenced that the supercell formalism acts as an effective medium theory with vacuum, with the spurious effect of having spectra dependent on the size of the supercell, I have reversed the procedure to extract the spectrum of the interacting slab, "cured" from the vacuum problem. First, the feasibility has been demonstrated on slabs of hBN, as their semi-conducting characteristics with a the large gap prevent numerical instabilities. Then, it has allowed us to understand the reason why the absorption of the interacting slab of silicon appears at lower energy than its bulk counterpart: it is due to the presence of surface states in the gap of the bulk band structure. Nevertheless, the difference with the isolated slab must be further investigated.The third part has been dedicated to the study of materials currently used or candidates for photovoltaic applications: InP and InSe. I have first studied the band structures of bulk InP and InSe. To correct for the underestimation of the band gap in the local density approximation (LDA), I have used GW corrections and the Heyd-Scuseria-Ernzerhof (HSE) exchange-correlation functional. The absorption spectrum for bulk InP has been calculated by means of the solution of the Bethe-Salpeter equation to correctly account for the excitonic effects. As expected, the experimental macroscopic function is well reproduced. Since the calculation is numerically demanding, I have also compared the results with the much lighter calculation using TDDFT where I used the long range kernel to mimic the excitonic effects. For bulk InSe, I have calculated the HSE corrections for the eigenvalues and obtained a good agreement with the experimental band gap. The spectrum obtained within TDDFT, with the long range kernel, gives satisfying results. We have started the calculations for slabs of these two materials. Thick slabs of InP and InSe have been considered and a 2x2 reconstruction have been performed for the InP slab to recover the semi-conducting surface. The LDA band structures and absorption spectra have been calculated. Then, such large systems being out of range of HSE corrections calculations, the study has been focused on much thiner slabs in the case of InSe.
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Submitted on : Friday, March 8, 2019 - 3:54:06 PM
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  • HAL Id : tel-02062171, version 1


Ilia Iagupov. Design numérique de métamatériaux pour des applications photovoltaïques. Materials Science [cond-mat.mtrl-sci]. Université Paris Saclay (COmUE), 2018. English. ⟨NNT : 2018SACLX117⟩. ⟨tel-02062171⟩



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