Three Terminal Tandem Solar Cells*

Tandem and multi-junction solar cells provide a path to improved efficiencies and energy yields over single-junction solar cells. Mechanically-stacked, multi-terminal, multi-junction cells have proven to be the most efficient 1-sun devices at the laboratory scale. This approach enables the use of subcells grown by different techniques and does not require current or lattice-matching. For example, III-V or perovskite cells can be processed separately and then mechanically integrated with a Si bottom cell. However, this type of tandem device enables multiple approaches to interconnect the subcells, and each interconnection scheme has different implications for ease of fabrication, efficiency, and module assembly.

Three terminal tandem (3TT) solar cells can overcome some of the limitations of two terminal (current matched) and four terminal (independently operated) solar cell designs. This design is a compelling platform for tandem cell integration because 3TTs based on Si bottom cells with interdigitated back contacts enable the same robust performance of four terminal tandems but do not require lateral current extraction between the cells, which can become challenging when scaling devices to large areas. However, the coupled nature of 3TT devices adds a degree of complexity to the devices themselves and the ways that their performance can be measured and reported. While many different configurations of 3TT devices have been proposed, there is no standard taxonomy to discuss the device structure or loading topology. I will discuss a proposed taxonomy for classifying 3TT devices and explaining how to rigorously measure their performance. I will show how TCAD models agree with simple physical models and experimental results to explain the trends in the behavior of 3T tandems.