Amorphous GaN1−xAsx alloys for multi-junction solar cells

We propose GaN_1-xAs_x as a new absorber material for single alloy multi-junction solar cells. Our recent results reported the first controlled growth of the material system across the full composition range and showed that the band gap can be tuned from 0.8eV to 3.4eV. Our low temperature molecular beam epitaxy (MBE) growth method results in amorphous films across much of the composition range (0.12 <; x <; 0.80), which eliminates the need for a lattice matched substrate. We present results for growth of homogeneous GaN_1-xAs_x films with strong optical absorption on Pyrex glass substrates with the potential for low cost multi-junction photovoltaics from one material system. Using photo-modulated reflectance spectroscopy and absorption spectroscopy, we have determined the band gaps for alloys with 0 <; x <; 0.88. We find that the band anticrossing model, developed for dilute highly mismatched alloys, can also explain the band gap dependence on composition across the full range. The band gap dependence on composition allows the determination of alloy compositions for single, double and triple junction solar cells with maximum theoretical conversion efficiency.