Three terminal tandem solar cells: from simulation to cells to strings

Multi-junction solar cells provide a path to improved
efficiencies and energy yields over single-junction
solar cells. There are multiple ways to interconnect
cells to create a tandem structure, and all have their
advantages and disadvantages. Three-terminal
tandem (3TT) solar cells are a promising platform for
tandem cell integration because they enables the
same robust performance of individually operated
subcells under varying illumination conditions but do
not require lateral current extraction between the
cells, which can become challenging when scaling
devices to large areas.

At NREL, we have focused work on understanding the performance of single III-
V/Si 3TT cells. We have developed a taxonomy to explain the operation and

measurement of 3TT devices and shown that 3TTs can achieve the same limiting
efficiencies as 4-terminal tandems, with a great deal of flexibility in material choice
and cell design. However, the circuitry for a 3TT string can become complex,
making circuit analysis more challenging. Using all III-V prototypes, we have
demonstrated the performance of strings of 3T devices and developed a
framework to understand the losses associated with making voltage-matched
strings of 3TTs devices.