Analysis of the dark-current density in solar cells based on multicrystalline SiGe

We analyzed the dark-current density obtained from solar cells based on multicrystalline SiGe (mc-SiGe) using a modified two-diode model that includes two diodes with diode ideality factors of 1 and 2, shunt resistance, and several series resistances. The ratio of recombination area r, which corresponds to the domination of recombination current, is almost independent of the average Ge composition, while the shunt resistance R_sh shows little change up to 5% and a drastic reduction at an average Ge composition of 10%. These results indicate that the deterioration of solar cell properties with an average Ge composition of 10% is mainly due to its lower shunt resistance, which is caused by the generation of defects. It can also be said that the quality of mc-SiGe is preserved at an average Ge composition of up to 5%. This strongly supports the suitability to high-efficient solar cells of mc-SiGe that has the proper average Ge composition due to its enhanced absorption coefficient and the resultant increase in short-circuit current density.