The theoretical calculations for the diode quality factornfor a metal‐insulator‐n‐semiconductor solar cell under illumination are presented here. These calculations show that, under illumination,nincreases with increasingdand decreases slightly ford>18 A˚ atJL=36 mA cm−2. Results of the calculations compare well with experimental results observed by H. C. Card [Solid‐State Electron.20, 971 (1977)].