Pie in the sky.
The graphene will degrade rather quickly with the electron exchange, and a thicker coat will reduce PV efficiency. Replacing the coating will entail more maintenance costs, both of which which reduces the already very poor ROI on PV panels. Besides, NO PV panel in existence stores solar energy for later conversion. NONE. The electricity is generated immediately upon light falling on the panel. No light, no power, reduced light, reduced power.
The real world physics always trumps the pie-in-the sky green dreams.