Fabrication of 0.9 eV bandgap a-Si/c-Si1−xGex heterojunction solar cells

We characterized hydrogenated amorphous Si / single-crystalline Si_1-xGe_x heterojunction solar cells grown on Si substrates in an effort to apply materials with a bandgap of 0.9-1.0 eV to the bottom cells of mechanically stacked tandem solar cells. Strain-relaxed Si_0.16Ge_0.84 films with a low dislocation density of 8.0 × 10⁴ cm^-2 were prepared using molecular-beam epitaxy by taking advantage of buffer layers with stepwise gradation of their composition. From the characterization of such films utilized as active layers in solar cells, their absorption edge were found to be extended to 1350 nm (~0.91 eV). The short-circuit current density, open-circuit voltage, fill factor, and efficiency of the Si_0.16Ge_0.84 heterojunction solar cell were 24.0 mA/cm², 163 mV, 0.491, and 2.0% respectively.