The degradation of the polymer solar cells in the outdoor condition was studied. It was found that the devices with normal structure (ITO/PEDOT:PSS/PBDTTT-EFT:PC71BM/ZrOx/Al) show better stability than those with inverted device structure (ITO/ZnO/PBDTTT-EFT:PC71BM/MoO3/Ag). PBDTTT-EFT is poly[4, 8-bis(5-(2-ethylhexyl) thiophen-2-yl) benzo [1,2-b;4,5-b] dithiophene-2, 6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno [3,4-b] thiophene-) - 2-carboxylate-2-6-diyl)] and PC71BM is [6,6]-phenyl-C71-butyric acid methyl ester. The major cause of degradation was attributed to the ultraviolet (UV) part of the sunlight. When the UV filter with cut-off wavelength of 400 nm was used to protect the device, the decay rate was improved by 50 times and the protected device retains 95% of their initial power conversion efficiency after 13 days outside. In the meanwhile, the decay of device fabricated by toluene was found 8 times faster than device by chlorobenzene. Thin 20 nm of Ag capping layer can further improve the stability. Degradation under water immersion indicates that the major effect of Ag is to protect against the residual moisture inside the encapsulation.