| dc.description.abstract | III-V compound tandem solar cells that combine low and high bandgap materials tailored to the incident solar spectrum have very high conversion efficiencies (~44%). However most of the III-V tandem solar cells are grown on Ge or GaAs substrates, both are more expensive than Silicon substrate. In spite of unmatched performance of III-V solar cells, silicon not only have lower price in the market but also have mature technique. Therefore, germanium substrate replaced by silicon substrate is the main direction of research in the future.
In this study, we focused on the characteristics and influence of germanium thin film depositing on silicon solar cell. At first, we use PC1D simulation software to analyze and modulate thickness and concentration of each layer and find the current-matching value. From the simulation results, we found that when the thickness of gallium indium phosphide (GaInP) film is 1400nm, gallium arsenide (GaAs) film is 290nm and germanium (Ge) film is 30nm, we got the current-matching value is 12.8mA, conversion efficiency is 32.3%.
There are many methods to fabricate the silicon solar cell (Bottom cell), such as diffusion, ion implantation or chemical vapor deposition. We use method of ion implantation to fabricate silicon solar cell in our experiments. After ion implantation process, we use anneal to repair defects caused by ion implantation. Experimental results show that when we anneal at 900 °C for 2 minutes, the best full width at half maximum (FWHM) we can measured. The result of silicon solar cell conversion efficiency is 10.9%.
Finally, we discuss the optical and electrical properties of germanium thin films grown on silicon solar cell. The results showed that the thickness of the germanium thin film increases, the best values of FWHM we can obtain. In terms of electrical properties, as the thickness of germanium film increases, the defects and sheet resistance value increase. In terms of optical properties, as the thickness of germanium film increases, the transmittance at long wavelength decrease. After growing the germanium film at annealing 700 °C for 5 minutes, we can obtain the value of FWHM is 358.56 arcsec. When we grow the germanium film and then etching it on silicon solar cell, we measured the conversion efficiency decreased by 0.6 %. | en_US |