Low bandgap GaAsNBi solar cells

The development of low bandgap GaAsNBi solar cells grown using MBE is reported. The devices include a pin heterostructure with GaAsNBi as the i-layer. The substrate rotation is stopped during epitaxy of the GaAsNBi layer (stationary growth), resulting in a continuous variation in the source fluxes over the wafer, enabling the investigation of a wide range of growth conditions in a single growth run. In particular, we investigate the photovoltaic properties as a function of the As/Ga flux ratio around the critical stoichiometric value. For slightly below stoichiometric As/Ga, the lattice mismatch and bandgap energy are minimized to 8 x 10-4 and 0.86 eV, respectively, while the crystal quality remains good. Increasing As/Ga over the stoichiometric ratio leads to a rapid increase in the tensile mismatch and bandgap energy, likely due to reduction in Bi incorporation. Despite a slightly higher open-circuit voltage offset at high As/Ga, this also leads to an clear increase in the external quantum efficiency and short-circuit current, suggesting significant differences in the carrier collection efficiencies. The results demonstrate the importance of the As/Ga flux ratio in controlling the solar cell performance, and the viability of GaAsNBi as a candidate for a low bandgap subjunction in multijunction solar cells.