Efficient organic photovoltaic devices show many interesting properties, but share a common drawback, namely their instability in atmosphere. We report on a shelf lifetime study of solar cells based on blends of two widely used polymeric semiconductors with 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61 (PCBM), encapsulated in a new flexible and transparent poly(ethylene naphthalate) (PEN)-based ultra-high barrier material. The barrier coating is entirely fabricated by plasma enhanced chemical vapor deposition (PECVD). The conjugated polymers used are poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV) and poly(3-hexyl)thiophene (P3HT). We have observed in this work that the encapsulation raises the shelf lifetime (50 % of the initial
efficiency) from a few hours into the range beyond 3,000 hours for MDMO-PPV based devices. Using the more stable P3HT, the lifetime could be increased to approximately 6,000 hours, or more than eight months.