High Doping Performance of Sulfur and Zinc Dopants in Tunnel Diodes Using Hydride Vapor Phase Epitaxy

We evaluate the performances of sulfur (S) and zinc (Zn) as n- and p-type dopants in multijunction solar cells to develop tunnel diodes (TDs) by hydride vapor phase epitaxy (HVPE). HVPE has emerged as a low-cost alternative for the fabrication of III-V solar cells. In p-GaAs, all the Zn atoms can be activated, achieving free-hole concentrations greater than 10
¹⁹
cm
^-3
. On the contrary, n-GaAs becomes saturated in free electrons at the concentration of 5 × 10
¹⁸
cm
^-3
due to the formation of the VGa-SAs complexes. Nevertheless, GaAs TDs with excessive S incorporation at a density of 4 × 10
¹⁹
cm
^-3
yield a peak in the tunneling-current density of 5 A/cm
²
. Previously, we achieved conversion efficiencies of 20.8% for GaAs cells and 12.1% for InGaP cells using a triple-chamber HVPE. By integrating these components together in a single growth process, we develop an InGaP/GaAs dual-junction solar cell with a conversion efficiency of 21.8% and a minimal voltage loss in TDs under at least 1-sun operation.