Material and device analysis of SiGe solar cell in a GaAsP–SiGe dual junction solar cell on Si substrate

Abstract

Low bandgap Si(1−x)Ge(x) solar cells are designed, fabricated, characterized and analyzed for the purpose of acting as the bottom cell in a GaAsP–SiGe tandem solar cell. The development of the SiGe cell under a GaAsP cell can lead to a 34% relative increase in efficiency over that of a silicon solar cell. This work focuses on making a SiGe bottom cell that can generate 21mA/cm2 of short circuit current (Jsc) at 1Sun from photons beyond 780nm and with a 450mV band gap–voltage offset (Woc) under 20Suns illumination. Numerical and analytical methods are introduced to demonstrate the SiGe solar cells׳ performance limits and to examine the trade-offs among I–V performance, cell structure and material composition. Material compositions are confirmed with energy-dispersive X-ray spectroscopy (EDS) and electrochemical capacitance voltage profiling (ECV). First principles analysis shows that a Si.15Ge.85 cell with a 5µm base can produce the expected current and an efficiency of 8.8% under 20Suns. By increasing base doping, we achieve a Woc of 435mV under 20Suns with a Si.18Ge.82 cell. The best 1Sun short circuit current measured from a Si.12Ge.88 cell at wavelengths beyond 780nm without anti-reflection coating, back surface reflector, or back texturing is 12.9mA/cm2. Adding a back surface field to the structure will lead to a higher Voc and lower Woc. Implementing light trapping with a higher base angle of pyramids can lead to the target Jsc of 21mA/cm2.