Back Contacted Bottom Cells with Three Terminals: Maximizing Power Extraction from Current-Mismatched Tandem Cells

Adele Tamboli, Emily Warren, Pauls Stradins, Michael Rienaecker, Henning Schulte-Huxel, Raphael Niepelt, Rolf Brendel, Robby Peibst, Manuel Schnabel

Research output: Contribution to journalArticlepeer-review

32 Scopus Citations


Multi-junction cells can significantly improve the energy yield of photovoltaic systems over a single-junction cell. The internal interconnection scheme of the subcells is an important aspect in determining the resulting levelized cost of electricity. For a dual-junction cell, two approaches are commonly discussed: series-connected tandem cells with two terminals or independently working subcells in a four-terminal (4T) tandem device. In this paper, we explore the working principle and the operation modes of a third, rarely discussed option: a three-terminal (3T) tandem cell using a back-contacted bottom cell with 3Ts. We use current–voltage measurements of illuminated 3T interdigitated back contact cells and confirm that the front and rear base contacts are at similar quasi-Fermi level positions, which enables the bottom cell to either efficiently collect surplus carriers, in the case of a current-limiting or carrier injecting top cell, or inject majority carriers, in the case of a current-limiting bottom cell. As a result, no current matching is needed. The power output of an idealized 3T bottom cell without resistive effects is independent of the current density applied from the top cell. These characteristics of the 3T bottom cells enable a 3T tandem to operate as efficiently as a 4T tandem, while being compatible with monolithic design and not requiring intermediate grids. We propose a simple equivalent circuit model including additional resistive effects, which describes a real 3T bottom cell and achieves excellent agreement to the experiment. We deduce design guidelines for a 3T bottom cell in different operation regimes.

Original languageAmerican English
Pages (from-to)410-423
Number of pages14
JournalProgress in Photovoltaics: Research and Applications
Issue number5
StatePublished - 2019

Bibliographical note

Publisher Copyright:
© 2019 John Wiley & Sons, Ltd.

NREL Publication Number

  • NREL/JA-5900-71667


  • 3T
  • bipolar junction transistor tandem
  • bottom cell
  • current-mismatch
  • interdigitated back contact cell (IBC)
  • tandem solar cell
  • three terminal


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