Reducing optical and resistive losses in graded silicon-germanium buffer layers for silicon based tandem cells using step-cell design

Si solar cells with a SiGe graded buffer on top are fabricated as the initial step in GaAsP/Si tandem cell fabrication. Using this structure, the impact of the SiGe buffer layer on the Si solar cells is characterized. To mitigate the impact of the narrow-bandgap SiGe on the electrical and optical characteristics of the Si sub-cell, a portion of the underlying Si is exposed using a step-cell design. The step-cell design is demonstrated to increase the Jsc of the SiGe/Si stack from 5 to 20 mA/cm2. The layout of the top mesa is shown to have an impact on the device characteristics with the finger design giving better results than the rectangular mesa with respect to fill factor and series resistance. In addition, utilizing the step-cell design increases overall spectral response of the bottom cell, with significant improvements in the short wavelength range.