A Ge:Si solar cell under a silicon solar cell can lead to as much as a 5.5% absolute efficiency gain for a multi-junction solar module at 30× concentration. This work demonstrated short circuit current densities that were 93% of the model prediction and open circuit voltages that were 92% of the model predictions for 88% Ge content, Ge:Si solar cells below Si at 30 suns. Silicon solar cells can absorb few photons in the wavelength range above 1150 nm due to the effect of the absorption coefficient. One possible method to enhance the absorption of long wavelength photons is to apply a Ge solar cell below Si. However, this method is industrially impractical due to the high cost of Ge substrates. In this work, a low cost Ge:Si solar cell grown on silicon with strong long wavelength light sensitivity will be demonstrated. This work starts with an all epitaxial growth design, analyzes the performance limits, examines the trade-offs between solar cell performance, Ge composition and material quality and concludes with the pathways to higher efficiency. The high quality Ge:Si layers with Ge content above 85% were achieved on Si substrates using reduced pressure chemical vapor deposition (RPCVD) technology. Three high Ge content Ge:Si solar cells were designed, fabricated and analyzed. The encouraging results experimentally prove that low cost Ge:Si solar cells grown on Si can have high performance below Si. This has been achieved as a direct result of low dislocation density step graded Ge:Si buffers developed in this research. In this paper, the pathway to achieve low cost and high efficiency Ge:Si low band gap solar cells grown on silicon is described.
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