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@PHDTHESIS{Czaja:878036,
      author       = {Czaja, Philippe},
      title        = {{A}b initio perspective on hydrogenated amorphous silicon
                      for thin-film and heterojunction photovoltaics},
      volume       = {494},
      school       = {Universität Mainz},
      type         = {Dissertation},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2020-02592},
      isbn         = {978-3-95806-474-4},
      series       = {Schriften des Forschungszentrums Jülich Reihe Energie $\&$
                      Umwelt / Energy $\&$ Environment},
      pages        = {107 S.},
      year         = {2020},
      note         = {Dissertation, Universität Mainz, 2019},
      abstract     = {Hydrogenated amorphous silicon (a-Si:H) has applications in
                      photovoltaics as an absorber material in thin-film solar
                      cells and as a passivation material in
                      silicon-heterojunction cells, where it forms an interface
                      with the crystalline silicon (c-Si) absorber. The physical
                      processes occurring at this interface have crucial impact on
                      the characteristics of the entire photovoltaic device. The
                      key to improving the solar cell performance lies therefore
                      in the optimization of the interface, in particular with
                      respect to its transport and recombination properties. This
                      requires a profound understanding of the microscopic
                      structure of a-Si:H and a-Si:H/c-Si interfaces, and of its
                      effect on the macroscopic properties relevant for
                      photovoltaics, such as absorption, optical and mobility gap,
                      band offsets, and local density of gap states. In this
                      thesis we present an ab initio study that seeks to provide
                      insight into the atomic and electronic structure of bulk
                      a-Si:H and a-Si:H/c-Si interfaces, extract the relevant
                      electronic and optical properties, and explore the
                      computational limitations that have to be overcome in order
                      to arrive at a predictive ab initio simulation of the
                      silicon hetero junction. In the first step bulk a-Si:H is
                      investigated, for which we use atomic configurations of
                      a-Si:Hwith 72 and 576 atoms, respectively. These were
                      generated with ab initio molecular dynamics, where the
                      larger structures are defect free, closely matching the
                      experimental situation and enabling the comparison of the
                      electronic and optical properties with experimental results.
                      Density functional theory calculations are applied to both
                      configurations in order to obtain the electronic wave
                      functions. These are analyzed and characterized with respect
                      to their localization and their contribution to the density
                      of states, and are used for calculating abinitio absorption
                      spectra of a-Si:H. The results show that both the size and
                      the defect structure of the configurations affect the
                      electronic and optical properties and in particular the
                      values of the optical and mobility gap. These values can be
                      improved by calculating quasi particle(QP) corrections to
                      the single-particle spectra using the G$_{0}$W$_{0}$ method.
                      Thereby we find that the QP corrections can be described by
                      a set of scissors shift parameters, which can also be used
                      in calculations of larger structures. The analysis of
                      individual contributions to the absorption by evaluating the
                      optical matrix elements indicates that strong localization
                      enhances the optical coupling, but has little effect on the
                      average transition probability, for which we find a
                      dependence E$^{2}$+ const on the photon energy E,
                      irrespective of the nature of the initial or final state. In
                      the second step the previously analyzed defect-free a-Si:H
                      structure is combined with c-Si to form a realistic
                      a-Si:H/c-Si interface structure, which undergoes a
                      high-temperature annealing in order to obtain a very low
                      defect density. Throughout the annealing, we monitor the
                      evolution of the structural and electronic properties. The
                      analysis of the bonds by means of the electron localization
                      function reveals that dangling bonds move toward the free
                      a-Si:H surface, leaving the interface region itself
                      completely defect free. The hydrogen follows thi smovement,
                      which indicates that in the case under consideration,
                      hydrogen passivation does not play a significant role at the
                      interface. A configuration with a satisfactory low density
                      of defect states is reached after annealing at 700 K. A
                      detailed characterization of the electronic statesin this
                      configuration in terms of their energy, localization, and
                      location reveals that, despite the absence of dangling bonds
                      near the interface, localized interface states still exist,
                      lying mostly below the conduction band edge from where they
                      seem to move deeper into the gap throughout the annealing.
                      The quantitative description of electronic localization also
                      allows for the determination of the a-Si:H mobility gap,
                      which, together with the c-Si band gap, yields band offsets
                      that are in qualitative agreement with experimental
                      observations. We find, however, that the error in
                      determining the band edges is too large for an accurate
                      calculation of the band offsets, and can be decreased only
                      by using larger configurations.},
      cin          = {IEK-5},
      cid          = {I:(DE-Juel1)IEK-5-20101013},
      pnm          = {899 - ohne Topic (POF3-899)},
      pid          = {G:(DE-HGF)POF3-899},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      urn          = {urn:nbn:de:0001-2020081215},
      url          = {https://juser.fz-juelich.de/record/878036},
}