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@PHDTHESIS{Merdzhanova:49633,
      author       = {Merdzhanova, Tsvetelina},
      title        = {{M}icrocrystalline silicon films and solar cells
                      investigated by photoluminescence spectroscopy},
      volume       = {41},
      school       = {Academy of Science, Sofia},
      type         = {Dr. (FH)},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {PreJuSER-49633},
      isbn         = {3-89336-401-3},
      series       = {Schriften des Forschungszentrums Jülich. Reihe
                      Energietechnik / Energy Technology},
      pages        = {X, 137 S.},
      year         = {2005},
      note         = {Record converted from VDB: 12.11.2012; Academy of Science,
                      Sofia, Diss., 2004},
      abstract     = {A systematic investigation on photoluminescence (PL)
                      properties of microcrystalline silicon ($\mu$c-Si :H) films
                      with structural composition changing from highly crystalline
                      to predominantly amorphous is presented. The samples were
                      prepared by PECVD and HWCVD with different silane
                      concentration in hydrogen (SC). By using photoluminescence
                      in combination with Raman spectroscopy the relationship
                      between electronic properties and the microstructure of the
                      material is studied. The PL spectra of gc-Si :H reveal a
                      rather broad ($\thicksim$0.13eV) featureless band at about 1
                      eV (`$\mu$c'-Si-band) . In mixed phase material of
                      crystalline and amorphous regions, a band at about 1.3eV
                      with halfwidth of about 0.3eV is found in addition to
                      `$\mu$c'-Si-band, which is attributed to the amorphous phase
                      (`a'-Si-band). Similarly to amorphous silicon, the
                      `$\mu$c'-Si-band is assigned to recombination between
                      electrons and holes in band tail states. An additional PL
                      band centred at about 0.7eV with halfwidth slightly broader
                      than the `$\mu$c'-Si-band is observed only for films
                      prepared at high substrate temperature and it is
                      preliminarily assigned to defect-related transitions as in
                      polycrystalline silicon. With decreasing crystalline volume
                      fraction, the `$\mu$c'-Si-band shifts continuously to higher
                      energies for all $\mu$c-Si :H films but the linewidth of the
                      PL spectra is almost unaffected. This is valid for all
                      deposition conditions investigated. The results are
                      interpreted, assuming decease of the density of band tail
                      states with decreasing crystalline volume fraction. The
                      reason for the band tails and their reduction is not clear
                      but strain might play a critical role and hydrogen or
                      hydrogenated amorphous silicon might be effective for strain
                      reduction. By applying the `carrier thermalization model'
                      developed for a-Si:H the slope, E$_{o}$, of the conduction
                      band tail states are derived from temperature dependence of
                      the PL intensity quenching (E$_{o}$ $\approx$ 0.035eV) and
                      from the shift of the PL peak energy (E $\approx$ 0.022eV).
                      The reason for this discrepancy is not clear yet, but the
                      simple model assuming that only one type of carriers are
                      involved in the process of thermal excitation might be too
                      simple for ~$\mu$c-Si :H. By using a new technique, namely
                      voltage modulated PL on solar cells, information on the
                      carrier distributions is obtained. It relates the splitting
                      of the quasi-Fermi-levels of electrons and holes and the
                      respective excess carrier distributions via open circuit
                      voltage V$_{oc}$ and the PL energy. This relationship is
                      studied as a function of SC, temperature and optical
                      generation rate go. An increase of the PL energy and
                      V$_{oc}$ is found for (i) increasing SC, (ii) increasing go
                      and (iii) decreasing temperature. It is suggested that the
                      reason in all cases is the shift of the distribution of
                      electrons and holes to higher energies. We propose that with
                      increasing SC, the density of band tail states is reduced
                      and the carrier distributions shift to higher energies as a
                      result of an increasing generation rate and increasing
                      carrier lifetime with decreasing temperature, respectively.
                      The shift of quasi-Fermi levels to higher energies is always
                      accompanied by a weak shift of carrier distributions in the
                      band tails. It is concluded that the maximum achievable V_
                      and the PL peak energy are determined by the band tail
                      states. At low temperature, a strongly reduced carrier
                      extraction is observed, which indicates reduced a drift or a
                      diffusion length. Multiple trapping processes in the band
                      tail states are suggested as the reason for this. A simple
                      model is proposed to simulate PL spectra and V$_{oc}$ in
                      $\mu$c-Si:H solar cells as a function of temperature, based
                      on carrier distributions in quasi-equilibrium conditions. In
                      the model is assumed symmetric density of states
                      distributions for electrons and holes in the conduction and
                      the valence band tail states. The best agreement between the
                      model calculations and experimental results for two solar
                      cells with different structural properties was obtained by
                      using a E$_{o}$ $\approx$ 0.03eV for the slope of both
                      exponential band tail states, which fits reasonably well
                      with E$_{o}$ $\approx$ 0.035eV from the temperature
                      dependence of the luminescence intensity.},
      cin          = {IPV},
      ddc          = {620},
      cid          = {I:(DE-Juel1)VDB46},
      pnm          = {Photovoltaik},
      pid          = {G:(DE-Juel1)FUEK247},
      typ          = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
      url          = {https://juser.fz-juelich.de/record/49633},
}