2, ISFH, Hamelin, , Germany
Many different designs of tandem cells based on high efficiency III-V top cells and Si bottom cells have been proposed, and there is ongoing to debate as to whether the sub-cells should be wired in series (to create a tandem device with two terminals) or operated independently (four terminal). In this talk we will discuss the design and operating principles of three-terminal (3T) tandem cells fabricated by combining a III-V (GaInP or GaAs) top-cell with a 3T Si bottom cell that has a conductive front contact in addition to two interdigitated back contacts (IBCs). We have developed a two dimensional TCAD device physics model to study the behavior of Si and full tandem solar cells operated in a 3T configuration.
Our simulations show that this type of 3T tandems has the potential to provide a robust operating mechanism to efficiently capture the solar spectrum without the need to current match sub-cells (as in monolithic 2-terminal tandem) or fabricate complicated metal grids/interconnects between cells (as in 4T stacked tandem). Moreover, the module integration is relatively straightforward provided the top cell’s operating voltage is about 2x that of the bottom cell, as in the case of GaInP/Si tandem. Under AM1.5G illumination, we predict that 3T GaInP/Si tandems can operate at efficiencies >32%. We will discuss how adding a third terminal to a Si device complicates analysis of the current-voltage behavior of a solar cell and provide a framework for analyzing power production from a 3T device. Fabrication and testing of 3T tandems using a transparent conductive adhesive (TCA) approach will also be discussed.