Work Package 4-T4.3 - July 2018

Objectives and Status

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The front side electrodes of III-V solar cells comprise ofmetal grid lines on a patterned CAP. The task is to develop industrially scalable low cost processes for the front contact grid deposition suitable for tandem solar cells.

Task: 

Reducing the total series resistance of electrodes (contact resistance and bulk-resistance):

  • Metal inks were applied as contacts on the surface of GaAs by inkjet printing and spin coating.
  • Two sintering methods (laser and reducing atmosphere) for metal NP inks were developed. With respect to environmental impact; laser sintering is so far a more promising process.
  • Ohmic contact behavior is observed in first results. Further activities to achieve an optimized contact resistance are ongoing.

 

 Task:

Less than 2% of the front side area is shaded by electrodes.

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  • Fine interdigitated structures of metal inks were deposited and sintered on the surface of n-GaAs. Further activities aim to optimize thickness and thin film homogeneity in order to achieve the required contact resistance.
  • The quality of printed structures was investigated using different methods such  as XPS/UPS, AFM, SEM, parametric analyzer and profilometer.
  • Selective etching of GaAs versus Au was successfully conducted, further investigations of selective etching with other electrode metals will be continued.

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Development of bonding processes as an interconnection between top and bottom cells with high transparency and low contact resistance.

  • TCO thin films were spray-coated on highly doped Si and III-V epi layers and then annealed.
  • The quality of the applied TCO bonding films was investigated with electrical, mechanical (Maszara blade) and optical microscope characterization. Low resistance ohmic contact can be achieved for Si substrates. A high transmission is realized for the demanded wavelengths in general.
  • With respect to mechanical stability, a high bond strength is observed. Further investigations will focus on improving the contact resistance on III-V materials as well as on bond homogeneity.