Polyolefin photovoltaic (PV) backsheets comprise a polypropylene layer stabilized with (A) at least one hindered amine with 2,2,6,6-tetraalkylpiperdine or 2,2,6,6 -tetraakylpiperazinone, either or both in combination with a triazine moiety, (B) a thioester, and, optionally, (C) at least one hindered hydroxybenzoate, and/or (D) an ortho hydroxyl triazine compound. These PV backsheets exhibit a low flame spread index of