Thin film AlAs/GaAs on graphite solar cells

We have prepared cells of polycrystalline N-AlAs on p-GaAs on graphite, with the III-V semiconductor layers grown by chloride transport vapor phase deposition. From the photocurrent response versus wavelength we infer electron diffusion lengths of 1.8 and 1.0 µm in the polycrystal GaAs at hole concentrations p ∼ 4 and 8 × 10¹⁸cm^-3respectively. This indicates peak internal quantum efficiency exceeding 90%. External power efficiencies are limited by the large spreading resistance in the top AlAs layer due to poor majority carrier transfer across grain boundaries.