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@PHDTHESIS{Mller:202690,
      author       = {Müller, Thomas Christian Mathias},
      title        = {{L}ight {A}bsorption and {R}adiative {R}ecombination in
                      {T}hin-{F}ilm {S}olar {C}ells},
      volume       = {272},
      school       = {RWTH Aachen},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2015-04874},
      isbn         = {978-3-95806-068-5},
      series       = {Schriften des Forschungszentrums Jülich Reihe Energie $\&$
                      Umwelt / Energy $\&$ Environment},
      pages        = {ii, 146 S.},
      year         = {2015},
      note         = {RWTH Aachen, Diss., 2015},
      abstract     = {Solar cells and light emitting diodes are generally the
                      same kind of device. Whereas solar cells are optimized for
                      light absorption, light emitting diodes are optimized for
                      light emission, i.e. radiative recombination. Both processes
                      are present in each of these devices. The
                      electroluminescence depends on the transport of injected
                      charge carriers and radiative recombination, whereas the
                      external quantum efficiency originates from light absorption
                      and the extraction of photo generated charge carriers.
                      According to Donolato and Rau, the external quantum
                      efficiency and the luminescence are connected by the
                      reciprocity relation. However, the reciprocity relation only
                      holds under certain circumstances. Whereas these
                      circumstances are given in defect-free solar cells made from
                      crystalline silicon for instance, the situation can be
                      different in thin-film devices. The physics in thin-film
                      devices can be affected by localized inter-band defect
                      states, which also affect the reciprocity relation. These
                      states are found in thin-film chalcopyrite Cu(In,Ga)Se$_{2}$
                      np heterojunction, hydrogenated microcrystalline silicon
                      pin, and hydrogenated amorphous silicon pin devices as
                      investigated in this thesis. This thesis is structured
                      within this sequence, studying systems with increasing
                      concentrations of defect states in their band gap to
                      investigate these circumstances, where the reciprocity
                      relation still holds. The requirements of the reciprocity
                      relation are investigated with temperature and charge
                      carrier injection dependent experiments as well as
                      comprehensive simulations. Since the electroluminescence is
                      affected by series resistance, in most cases it is
                      complemented with photoluminescence, where charge carrier
                      transport is negligible. An expanded Fourier transform
                      infrared spectroscopy setup is used to perform the
                      luminescence experiments, and the external quantum
                      efficiency is measured with a constant photocurrent method
                      setup. The simulations use a commercial one dimensional
                      numerical device simulator, which solves the optics and the
                      semiconductor equations. A self-developed program, which
                      uses full band diagrams of the onedimensional device
                      simulator as input parameters, yields the luminescence and
                      external quantum efficiency calculations. The luminescence
                      in Cu(In,Ga)Se$_{2}$ originates from transitions between the
                      bands and localized band-tail states. Since the band-tail
                      densities of states are rather steep, the almost unshifted
                      luminescencespectra are compatible to the external quantum
                      efficiency in terms of the reciprocity relation. Around room
                      temperature, the radiative ideality factor, which is
                      determined from luminescence/voltage characteristics by
                      fitting a common diode law, is found to be close to unity.
                      However, temperature dependent experiments and simulations
                      show that the reciprocity relation only holds if the thermal
                      energy is higher than the characteristic energy of the
                      band-tail densities of states. Transitions between band-tail
                      states yield the luminescence in hydrogenated
                      microcrystalline silicon. However, the reciprocity relation
                      between the luminescence and the external quantum efficiency
                      only holds in a small spectral range, since the band-tails
                      are broad, i.e. their characteristic energies are higher
                      than the thermal energy, even at room temperature. This
                      yields blue shifted luminescence spectra with increasing
                      charge carrier injection and a temperature dependent
                      radiative ideality factor, which exceeds unity and increases
                      with decreasing temperature. However, detailed simulations
                      show that the reciprocity relation holds if the device under
                      luminescence conditions is close to thermal equilibrium.
                      These conditions yield very low luminescence intensities,
                      which are not accessible within experiments. The
                      luminescence in hydrogenated amorphous silicon originates
                      from transitions between localized band-tails, which are
                      even broader than in hydrogenated microcrystalline silicon.
                      Additionally, around room temperature and/or at low
                      injection, transitions between band and neutral amphoteric
                      mid-gap defect states contribute to the luminescence beside
                      the tail-to-tail luminescence. The band-to-defect
                      luminescence spectra do not shift under different charge
                      carrier injection levels, since the occupation of mid-gap
                      defect states is almost constant, even if the charge carrier
                      injection is moderate. This yields a radiative ideality
                      factor of around two. However, the presence of additionally
                      Stokes-shifted tail-to-tail luminescence, where the
                      luminescence looses photon energy to the lattice, completely
                      hampers the validity of the reciprocity relation within
                      experiments. Only simulations can show that the reciprocity
                      relation holds if the device is almost in thermal
                      equilibrium at extreme low charge carrier injection.},
      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/202690},
}