Material characterization and device optimization of silicon germanium solar cells grown on silicon substrates

Abstract

Successful growth of III-V solar cells on silicon (Si) substrates offers great promise by combining the high efficiency merits of III-V materials with the low cost and abundance of Si substrates. The aim of this thesis was to design and fabricate a highly efficient silicon germanium (SiGe) solar cell grown on a Si substrate. The performance of the SiGe on Si solar cell is evaluated based on the AM1.5G solar spectrum with wavelengths above 750 nm only. This is because the designed SiGe on Si solar cell will eventually work under a gallium-arsenide-phosphide (GaAsP) top cell in a GaAs0.84P0.16/SiGe on Si tandem device. This research first determined the optical properties of SiGe films and graded buffer layers. The optical constants of SiGe films grown on Si substrates with germanium (Ge) compositions of 77%, 82%, 85% and 88% from wavelengths of 400 nm to 1450 nm at room temperature were determined. Average absorption coefficients of SiGe graded buffer layers from 0% to 77%, 82%, 85% and 88% Ge at wavelengths beyond the band gap of Si were also determined. Following material characterization, the performance of SiGe on Si solar cells was predicted based on first principles. Then, a SiGe on Si solar cell was fabricated. Characterization and analysis were carried out on the first fabricated SiGe solar cell. Several possible pathways to improve the solar cell performance were proposed based on the analysis results. After this first demonstration, more SiGe on Si solar cells were fabricated to demonstrate different ideas for performance improvement. The short-circuit current density (Jsc) was improved by optimizing the thicknesses and doping densities of emitter and base, light trapping design, anti-reflection coating (ARC), Si substrate thinning and higher Ge compositions. The open-circuit voltage (Voc) was improved by adding a back surface field (BSF) layer, back surface passivation and hydrogen defect passivation. These improvements lead to the highest measured Jsc of 20.3 mA/cm2 and the highest measured Voc of 354 mV on different SiGe on Si solar cells. The best measured efficiency is 3.4% with a Voc of 348 mV, Jsc of 15.1 mA/cm2 and fill factor (FF) of 0.65. All the results reported in this thesis are based on the AM1.5G solar spectrum and only consider light with wavelengths above 750 nm unless stated otherwise. To the best of the author s knowledge, the 3.4% efficiency obtained here is the highest efficiency reported on a SiGe on Si solar cell when working under a GaAs0.84P0.16 top cell. With the techniques proposed from this work, a 20.6% efficient GaAsP/SiGe on Si tandem solar cell was fabricated.