Dilute nitride GaInNAs and GaInNAsSb solar cells, Gerald Soto, CRF 2010-1

Dilute nitride GaInNAs and GaInNAsSb solar cells

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.

The Solar Spectrum

•A single band gap cell will absorb photons with energies greater than or equal to the materials band gap Eg.

•Carriers with energy > Eg will quickly decay to Eg

•Idealized Efficiency = 25%

A GaInNAs 1eV Junction

GaInNAs was seen as a good material for this junction as it could be lattice matched to GaAs avoiding the loss mechanisms associated with strain.

• They grew a number of GaInNAs structures which achieved a photoresponse down to 1 eV

• The internal quantum efficiencies of the devices where low, typically bellow 0.2.

• The reason for the low internal quantum efficiency was due to diffusion lengths lower than the depletion widths

• The low diffusion lengths are attributed to a shallow electron trap and a deep recombination centre which so far seem unavoidable in GaInNAs.

• A tandem solar cell is limited by lowest current produced by any one junction so adding the junction could not improve the efficiency of the tandem cell.

• Later they grew p-i-n devices with intrinsic regions between 0.2 and 2.5μm

• The wider depletion region resulted in higher short circuit currents and quantum efficiencies than previous cells.

The National Renewable Energy Laboratory (NREL) in America suggested adding a junction to absorb photons with energies between 1 – 1.4eV would increase the idealized efficiency from 42% to 52%.

Researchers at the University of Tsukuba in Japan have also created bulk GaInNAs hetrojunction and homojunction cells.Their homojunction cell had an efficiency of 2.94%, They found that increasing the growth temperature from 480-520°C increased the hall mobility from 200 to 250 cm2/Vs.