Microcrystalline silicon–germanium solar cells for multi-junction structures

Microcrystalline silicon–germanium (μc-SiGe) solar cells were investigated for a unit cell of multi-junction structures. Despite having a thinner μc-SiGe layer of 0.5 μm, a conversion efficiency of 5.6% (the highest value ever reported for μc-SiGe solar cells) was achieved by using a μc-SiGe film with 20% Ge concentration. A short-circuit current (Isc) of 28 mA/cm2 was obtained in the case of 27% Ge concentration. This is higher than the Isc of the best μc-Si solar cells with 2 μm thickness. The collection efficiency spectrum shows more than 20% at 1000 nm, and sensitivity is observed in wavelengths up to 1200 nm. The results suggest that a reduction in the thickness of microcrystalline solar cells may achieve higher productivity, and the utilization of longer wavelength light allows us to obtain higher efficiency.