In this paper, we elucidate the role of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the degradation of polymer:PCBM ((6,6)-phenyl C61-butyric acid methyl ester) solar cells. The study is done on unencapsulated cells exposed to ambient conditions in dark. The cell degradation results from reduced carrier extraction, and an investigation of the various interfaces within the cell allows us to correlate this to oxidation of the low work function metal cathode. We further show that this oxidation is caused by water vapor diffusion from the edges through the hygroscopic PEDOT:PSS layer. We demonstrate that only the hygroscopic nature of PEDOT:PSS, and not its acidity, has a detrimental impact. The oxidation of the cathode progresses in synchrony with the water ingress into the PEDOT:PSS layer from the edges of the device towards the central part, and results in a progressive constriction of the active area. When the PEDOT:PSS layer is replaced by an evaporated layer of MoO3, the device lifetime is improved considerably even with highly reactive metal cathodes. Finally, we provide a quantitative relationship between device lifetime and the level of humidity in the ambient, thus establishing a suitable accelerated shelf-life test for organic solar cells and their encapsulation.