$6.$ An ideal solar cell with a $100$ mW cm$-2$ incident solar intensity resulted in Isc$=12$ mA$/$cm$2,$ Voc $=1\mathrm{.}2$ V and fill factor $($ff$)=0\mathrm{.}76$ a$)$ Find the efficiency of this cell at one sun b$)$ Find I$0($reverse saturation current$)$