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@PHDTHESIS{Lehnen:202687,
      author       = {Lehnen, Stephan},
      title        = {{I}nvestigation of light propagation in thin-film silicon
                      solar cells by dual-probe scanning near-field optical
                      microscopy},
      volume       = {270},
      school       = {RWTH Aachen},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2015-04871},
      isbn         = {978-3-95806-066-1},
      series       = {Schriften des Forschungszentrums Jülich Reihe Energie $\&$
                      Umwelt / Energy $\&$ Environment},
      pages        = {120 S.},
      year         = {2015},
      note         = {RWTH Aachen, Diss., 2015},
      abstract     = {In this work, the light propagation in microcrystalline
                      silicon thin film solar cells is investigated. For this
                      purpose, a dual-probe scanning near-field optical microscope
                      (SNOM) wasdeveloped and set up from scratch. The microscope
                      is equiped with two separated probes for local illumination
                      and detection on a subwavelength scale. Applying newly
                      developed modes of measurement, exclusively available at
                      dual probe SNOMs, the microscope allows for measuring the
                      propagation of light in thin layers with high precision.
                      Within the framework of this thesis, the different physical
                      challenges of dual probe scanning near-field optical
                      microscopy are outlined and the technological solutions are
                      described. The reliability of the setup was thoroughly
                      tested at measurements of light propagation in flat and
                      textured microcrystalline silicon (μc-Si:H) thin film solar
                      cells in nip-configuration. The measured raw data are
                      analysed by multiple methods. It is observed, that the
                      lateral intensity decay of light is strongly influenced by
                      local surface features. Therefore, an advanced dual-probe
                      scan mode is introduced which compensates for the
                      non-constant coupling efficiencies, caused by local surface
                      features. In dual-probe operation, only a small share of the
                      photons emitted by the illumination probe finally reaches
                      the detection probe. Hence, the different loss mechanisms,
                      which are accounted for the strong attenuation of the
                      propagating light, are theoretically investigated by means
                      of a ray-tracing approach. Ray-tracing allows to examine the
                      loss mechanisms separately. The simulation reveals, that for
                      a wavelength of 750nm the intensity decay within the first 5
                      μm is dominated by the radial distribution of light inside
                      the layer. At longer distance, absorption is the major
                      mechanism for a decrease in light intensity. The intensity
                      decay due to a transmittance at the front interface strongly
                      attenuates the propagation of light with small angles of
                      incidence. Although ray-tracing neglects the wave nature of
                      light, the approach is capable to reproduce the lateral
                      intensity decay of light propagating in a 1 μm thick
                      μc-Si:H layer. The ray-tracing approach is supplemented
                      with finite-difference time-domain (FDTD) simulations which
                      provide the field distribution on a sub-wavelength scale. By
                      including the illumination probe to the simulated layer
                      stack, an improved compliance of simulation and experiment
                      is achieved. Based on the FDTD simulation, the light
                      distribution insideand above the layer stack is investigated
                      and compared to a half-space model which represents a system
                      without light-trapping properties. It is demonstrated, that
                      at least for undamaged, perfect probes, a direct light
                      transfer between illumination and detection probe, bypassing
                      the absorber layer, is negligible. Furthermore, the FDTD
                      simulations provide the angular distribution of the light
                      emitted by a SNOM probe, placed at subwavelength distance
                      above the surface. Thereby, the simulations complement the
                      experimentally determined angular resolved far-field
                      emission characteristic of a probe in air. It is shown that
                      a large share of $64\%$ of the light intensity emitted by a
                      SNOM probe, placed at subwavelength distance above a
                      μc-Si:H absorber layer, is coupled into angles which
                      exceeds the angle of total reflection. Finally, the
                      intensity decay of the propagating light, determined by FDTD
                      simulations, revealed good accordance with the measured
                      data. A modulation of the intensity decay, which originates
                      from interference induced by multiple reflections, is
                      observed in the measurement as well as in FDTD simulations.
                      The good compliance between simulation and experiment
                      indicates that macroscopic absorption coefficients are
                      suitable for the description of light propagation even on
                      microscopic scales.},
      cin          = {IEK-5},
      cid          = {I:(DE-Juel1)IEK-5-20101013},
      pnm          = {121 - Solar cells of the next generation (POF3-121)},
      pid          = {G:(DE-HGF)POF3-121},
      typ          = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
      url          = {https://juser.fz-juelich.de/record/202687},
}