SPLTRAK Abstract Submission
Drift-Diffusion Modelling of Four-Junction InGaP/InGaAs/SiGeSn/Ge Solar Cells
Laurier S. Baribeau, Robert F.H. Hunter, Christopher E. Valdivia, Karin Hinzer
SUNLAB, Centre for Research in Photonics, University of Ottawa, Ottawa, ON, Canada

The ternary alloy silicon germanium tin is a versatile candidate to extend the industry standard lattice matched InGaP/InGaAs/Ge multijunction solar cell to four junctions. Here, the SiGeSn composition space is discussed and its bandgap trend is visualized. Then, InGaP/InGaAs/SiGeSn/Ge solar cells are simulated using drift-diffusion modelling to ascertain SiGeSn quality limits, and the design challenges in the four-junction material system.
Power conversion efficiencies of 42.6% and 41.6% at 1000 suns AM1.5D are determined for designs implementing surface recombination velocities of 103 cm/s and 5×104 cm/s, respectively, at important interfaces in the device. These signify absolute efficiency gains of 1.3% and 0.4% with respect to like-modelled InGaP/InGaAs/Ge designs. The obtained power conversion efficiencies assume a Shockley-Read-Hall recombination lifetime of 1 µs in the SiGeSn material, however, lifetimes of 100 ns drop the efficiency by only ~1% (absolute).
The external quantum efficiency of the four-junction devices is near 90% across most of the solar spectrum. A plot of the fraction of incident light lost to various physical mechanisms in the solar cell is given and has been used to optimize the surface field layers to reduce minority charge carrier loss currents. Designs have been optimized for output power by thinning the top three subcells to ensure that the germanium does not limit the device’s 11.25 A/cm2 operating current at its maximum power point. This result indicates that current-matching limited by inefficient absorption and Auger recombination in the Ge subcell is one of the main design challenges of this material system and suggests avenues for possible improvements to the design, such as improved light trapping, refined bandgap engineering, and subcell segmentation.