Dilute nitride GaInNAs and GaInNAsSb solar cells by molecular beam epitaxy

Dilute nitride films with a roughly 1 eV band gap can be lattice-matched to gallium arsenide and germanium, and therefore could become a critical component in next-generation multijunction solar cells. To date most dilute nitride solar cells have been plagued with poor efficiency, due in large part to short diffusion lengths. This study focuses on two techniques aimed at improving the quality of dilute nitride films grown by molecular beam epitaxy: the utilization of biased deflection plates installed in front of the nitrogen plasma source, and the introduction of antimony during growth. Results from GaInNAs cells grown with and without deflection plates, and GaInNAsSb solar cells are reported. The use of biased deflection plates during GaInNAs growth improved every aspect of solar cell performance. For the GaInNAs devices grown with deflection plates, the dark current density, open-circuit voltage, and fill factor were the best of the devices studied. The GaInNAsSb cells had the highest quantum efficiency, almost 80% at maximum, mainly due to low background doping densities providing these devices with wide depletion widths. The GaInNAsSb materials also had quite narrow band gaps of 0.92 eV. Because of the high collection efficiency coupled with the narrow band gap, the sub-GaAs short-circuit current density produced by the GaInNAsSb cells is 14.8 mA∕cm2, which was the highest of the devices studied. This current is nearly 50% greater than the best dilute nitride solar cells in the literature, and is the first dilute nitride cell to produce enough current to current match the upper two subcells in a triple-junction device, composed of GaInP/InGaAs/GaInNAsSb.