Short circuit current and efficiency improvement of SiGe solar cell in a GaAsP-SiGe dual junction solar cell on a Si substrate

SiGe materials on Si substrate are promising candidates to act as the bottom cell in a tandem structure, due to its high mobility, good process compatibility, adjustable lattice constant, and capability of absorbing light of wavelengths up to 1800nm. This work demonstrates the improvement of short circuit current and efficiency for the SiGe solar cell, operating in a GaAsP-SiGe two-terminal configuration tandem device. A numerical model is proposed to predict the short circuit current of the SiGe cell. Experimentally, the highest short circuit current density is increased from 12.9mA/cm2 to 19.4mA/cm2, with optimized fabrication process and device structure. The efficiency of the SiGe bottom cell is improved from 1.7% to 3.0% filtered by GaAsP. It is demonstrated that the cell has potential to achieve an efficiency of 4.6% at 20suns illumination. Different mole fractions of Ge in the structure are confirmed from the X-ray diffraction line scanning image.