III-V Multi-Junction Solar Cells on Si Substrates with a Voided Ge Interface Layer: A Modeling Study

Multi-junction solar cell efficiencies far exceed those attainable with silicon photovoltaics; however, the high cost of materials remains a barrier to their widespread use. Substantial cost reduction could be achieved by replacing the germanium substrate with a less expensive alternative: silicon. However, threading dislocations introduced by the lattice mismatch between silicon and other layers have a detrimental effect on performance. In this research, we seek to accommodate lattice mismatch by introducing a voided germanium interface layer on the silicon substrate to intercept dislocations and prevent them from reaching the active layers. We present simulation results exploring the effect of threading dislocations and substrate doping on device performance. For insufficient substrate doping, a potential barrier forms at the germanium/silicon interface, restricting current flow. This barrier becomes larger under concentration, necessitating higher doping ( 10
¹⁹
cm
^-3
) in the substrate layer. We show that for dislocation densities of 10
⁶
cm
^-2
, an efficiency of 28% is theoretically achievable under 100 sun concentration when the voided germanium interface and the silicon substrate are highly doped. For very low defect densities of 10
⁴
cm
^-2
, an efficiency of 36% could be achieved.