Atomic structure, electronic states and relaxation dynamics in photovoltaic materials and interfaces from photoemission-related spectroscopies

Abstract : The efficiency of the photovoltaic process depends on the electronic band structure of the active material and the charge carrier dynamics. In this thesis, we have studied how these issues are related to the atomic structure in materials for two different technologies of solar cells, namely silicon heterostructure solar cells, and hybrid organic-inorganic perovskite solar cells. In silicon heterostructure solar cells, we have analyzed the impact of defects on the electronic properties of amorphous silicon heterostructures (a-Si:H/a-SIC:H/c-Si) by core level and valence band spectroscopies. In particular, we have quantified the number of dangling bonds inside a-Si:H layer upon irradiation, we have identified the electronic states associated to them, and we have understood the transitions previously observed by photoluminescence. In perovskite solar cells, we have correlated the atomic structure, the electronic structure and the electronic dynamics for two- and three-dimensional hybrid organic-inorganic perovskites. We have used with this goal a whole panel of complementary techniques: X-ray diffraction, angle-resolved photoemission spectroscopy, inverse photoemission spectroscopy, and time-resolved two-photon photoemission. In the two-dimensional perovskite (C₆H₅C₂H₄NH₃)₂PbI₄, the valence and conduction bands have been determined experimentally and compared to spectral function simulations. In the three-dimensional perovskite CH₃NH₃PbI₃, we have again determined the band structure and simulated it. Very broad spectral features have been experimentally observed, which relax the optical transition conditions impacting in the solarcell efficiencies. In both experiments and calculations, we observe that the spectral weight follows a cubic periodicity while the system is structurally in the tetragonal phase. This apparent contradiction is explained by the band broadness, which hides the band folding of the tetragonal distortion. As for the relaxation dynamics, we have observed that the photoexcited carriers thermalize in a subpicosecond time scale through the coupling to organic cation vibrations. At longer timescales (10~100 picoseconds), the electron diffusion controls the dynamics. This dynamics is affected by the annealing-induced defects, which localize the photoexcited electrons for more than 300 picoseconds.
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Min-I Lee. Atomic structure, electronic states and relaxation dynamics in photovoltaic materials and interfaces from photoemission-related spectroscopies. Materials Science [cond-mat.mtrl-sci]. Université Paris-Saclay, 2018. English. ⟨NNT : 2018SACLS220⟩. ⟨tel-02179632⟩

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