Design, fabrication and analysis of germanium: silicon solar cell in a multi-junction concentrator system

• Published in: Solar energy materials and solar cells Vol. 108; pp. 146 - 155
• Main Authors: Wang, Yi, Gerger, Andrew, Lochtefeld, Anthony, Wang, Lu, Kerestes, Chris, Opila, Robert, Barnett, Allen
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-01-2013; Elsevier
• Subjects: Applied sciences; Chemical vapor deposition; Design; Direct energy conversion and energy accumulation; Electrical engineering. Electrical power engineering; Electrical power engineering; Energy; Exact sciences and technology; Germanium; Germanium silicon; Low cost; Mathematical models; Multi-junction solar cell; Natural energy; Operation. Load control. Reliability; Photoelectric conversion; Photovoltaic cells; Photovoltaic conversion; Power networks and lines; Silicon; Silicon substrates; Solar cells; Solar cells. Photoelectrochemical cells; Solar energy; Wavelengths; Design; Germanium silicon; Chemical vapor deposition; Multi-junction solar cell; Performance evaluation; Costs; Ge-Si alloys; Power system economics; Dislocation density; Trade; Photovoltaic array; Power markets; Silicon; Gallium phosphide solar cells; Cost lowering; Silicon solar cells; High performance; Short circuit currents; Absorption coefficient; Buffer system; Forecasting; Open circuit voltage; Solar cell; Multijunction solar cells; Sensitivity; High efficiency; Germanium
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2012.08.016

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 1150nm 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.
► We designed three generation Ge:Si solar cells and predicated their performance below Si. ► We achieved Ge:Si solar cells on low cost Si substrates by RPCVD technology. ► We theoretically and experimentally proved that low cost Ge:Si on Si solar cells can have high efficiency below Si solar cell.