TY  - THES
AU  - Czaja, Philippe
TI  - Ab initio perspective on hydrogenated amorphous silicon for thin-film and heterojunction photovoltaics
VL  - 494
PB  - Universität Mainz
VL  - Dissertation
CY  - Jülich
M1  - FZJ-2020-02592
SN  - 978-3-95806-474-4
T2  - Schriften des Forschungszentrums Jülich Reihe Energie & Umwelt / Energy & Environment
SP  - 107 S.
PY  - 2020
N1  - Dissertation, Universität Mainz, 2019
AB  - 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.
LB  - PUB:(DE-HGF)3 ; PUB:(DE-HGF)11
UR  - https://juser.fz-juelich.de/record/878036
ER  -