Fabrication of Si1−xGex heterojunction solar cells grown with stepwise graded buffer layers

We characterized strain-relaxed single crystal Si_1-xGe_x heterojunction solar cells grown by molecular beam epitaxy in an effort to obtain 0.9 eV bottom cells for mechanically stacked tandem solar cells. Stacked Si_1-xGe_x buffer layers, with each layer grown by varying the value of x, were employed to confine the dislocations within themselves. We achieved high-quality, strain-relaxed Si_1-xGe_x thin films with a low dislocation density (below 10⁵ cm^-2) with a maximum Ge content of x = 0.84. On characterization of such films, which act as solar cells, their absorption edge was found to have increased up to a maximum value of 1380 nm (~0.9 eV), in addition to them exhibiting higher external quantum efficiency in the entire wavelength range, upon increasing the Ge content up to x = 0.84. The short-circuit current density, open-circuit voltage, fill factor, and efficiency of the Si_1-xGe_x heterojunction solar cell (x = 0.84) were 20.43 mA/cm², 155 mV, 0.435, and 1.38% respectively.