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@PHDTHESIS{Turan:283046,
      author       = {Turan, Bugra},
      title        = {{L}aser processing for the integrated series connection of
                      thin-film silicon solar cells},
      volume       = {306},
      school       = {RWTH Aachen},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2016-01728},
      isbn         = {978-3-95806-119-4},
      series       = {Schriften des Forschungszentrums Jülich Reihe Energie $\&$
                      Umwelt / Energy $\&$ Environment},
      pages        = {xii, 188 S.},
      year         = {2016},
      note         = {RWTH Aachen, Diss., 2016},
      abstract     = {The integrated series connection of solar cells is an
                      essential aspect for thin-film photovoltaic technology. With
                      a series connection a high output voltage of the module is
                      achieved while the output current is kept low. Thus, Ohmic
                      losses in the contact materials are kept low as well. In
                      thin-film silicon solar technology the steps to create the
                      interconnection are commonly done by laser ablation
                      integrated in-between the depositions of the solar cell
                      layer materials. In three steps laser scribing is used to
                      selectively remove layers locally in the form of lines
                      across the module substrate. In a first step the
                      front-contact is removed for electrical insulation and cell
                      stripe definition. Afterwards, the absorber is removed
                      locally exposing the front-contact beneath. Finally, the
                      interconnection is formed when the back-contact is removed
                      locally as well. The area that is needed for the
                      interconnection of two neighboring cells is no longer active
                      for current generation. Depending on the technology $5-10\%$
                      of active area is lost. The reduction of this area holds an
                      attractive potential for an increase of the module
                      efficiency. The topic of this thesis is the investigation of
                      the lower geometrical limits for the dead area reduction for
                      substrate side laser processing of thin-film silicon solar
                      cells. It is well-known that the interconnection and the
                      laser processes can have an impact on the performance of the
                      solar module. Therefore, the characterization of the impact
                      on the performance is of special importance when laser
                      processes are used that are capable of generating a reduced
                      interconnection width. P1: for the front-contact insulation
                      process it was found out that the scribe quality strongly
                      depends on the used laser wavelength. Ablation mechanisms
                      that are driven by material phase changes (scribing with
                      532nm or 1064nm) can lead to smoother scribe edges compared
                      to mechanisms dominated by stress-induced removal (355nm)
                      where non-uniform rip-off at the edges occurs. However, in
                      certain processing regimes, strong ablation debris
                      redeposition in direct vicinity of the P1 scribe is observed
                      when small beam spot radii (<10µm) are used. Such
                      redeposition has a severe impact on the solar cell
                      performance in this region. With proper wet-chemical
                      cleaning the amount of redeposited debris on the
                      front-contact and the negative impact on the solar module
                      can be minimized. Parasitic shunting of two neighboring cell
                      stripes by deposition of absorber material into the P1
                      scribe increases when the scribe width is reduced.
                      Measurements show that the overall magnitude of the shunt is
                      in a value range that impact on the solar module is
                      negligible for commonly used cell topologies. P2: the width
                      reduction approach was extended for the absorber removal
                      process (P2). To ensure the selectivity of silicon removal
                      without damaging of the front-contact beneath, only 532nm
                      was used for scribing. For this wavelength ablation is
                      strongly assisted by mechanical stresses generated by
                      hydrogen diffusion from the absorber layer and/or thermal
                      expansion of the absorber layer. Mechanical constraints
                      limiting the lower scribe width are found that depend on the
                      absorber thickness and the laser beam spot size. Such
                      behavior can be explained directly from linear elastic
                      fracture mechanics where removal of the layer is determined
                      by the relation between delamination at the interface and
                      fracture of the absorber along the circumference of the
                      spot. It can be concluded that for substrate side
                      laser-induced ablation thin scribe lines are only possible
                      for thin layers. The parasitic series resistance formed by
                      P2 also increases as the scribe width is decreased. However,
                      for processing of amorphous silicon absorbers, with a beam
                      radius 10µm, the minimal achievable resistance value is
                      strongly increased. In fact, much more than what would be
                      expected just by the geometrical contact area reduction.
                      This is most likely owed to changes of the specific contact
                      resistance due to increased debris redeposition within the
                      P2 scribe prior to back-contact deposition. In contrast,
                      such effects are not observed for processing of tandem
                      absorber where debris redeposition is less pronounced. Here,
                      low series resistances, with only minor impact on the module
                      performance, are achieved for all investigated beam spot
                      sizes. P3: the back-contact insulation process (P3) is
                      similar to P2 since the back-contact is removed indirectly
                      by removal of the absorber beneath. Shunting between front-
                      and back-contact can occur at the direct P3 scribe edges.
                      These shunts are possibly formed due to heat generated by
                      sub-threshold energy intake of excess energy from the
                      shoulders of Gaussian distribution of the laser. The
                      mechanical constraints on the minimal achievable scribe
                      widths are even stronger than what was observed for the
                      optimization of the P2 process. This is owed to the
                      additional overall thickness of the layer-stack due to the
                      back-contact. Furthermore, for tandem solar cell processing
                      the scribe edges are strongly distorted by delaminated
                      material while clean edges are obtained for a-Si:H solar
                      cells. The parasitic shunting by P3 scribing increases by
                      many orders of magnitude when a processing beam radius of
                      10µm is used. However, just like it was observed from P2,
                      an overall weaker deterioration is obtained for scribing of
                      tandem solar cells than for amorphous silicon cells. It is
                      possible that material modifications are more localized in
                      the a-Si:H top-cell. Together with the higher thickness of
                      the tandem cells (300nm vs. 1.4µm) the impact on the whole
                      device is not as pronounced.},
      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/283046},
}