The Photo-Stability of Polymer Solar Cells: Contact Photo-Degradation and the Benefits of Interfacial Layers

The organic/electrode interfaces in organic solar cells are systematically studied for their light, heat, and electrical stability in an inert atmosphere. Various extraction layers are examined for their effect on device stability, including poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and MoO3 for hole extraction layers, as well as LiF, Cs2CO3, and lithium acetylacetonate (Liacac) for electron extraction layers. The organic/metal interface is shown to be inherently photo-unstable, resulting in significant losses in device efficiency with irradiation. X-ray photoelectron spectroscopy measurements of the organic/aluminum interface suggest that the photo-induced changes are chemical in nature. In general, interfacial layers are shown to substantially reduce photo-degradation of the active layer/electrode interface. In spite of their photo-stability, several interfacial layers present at the active layer/cathode interface suffer from thermal degradation effects due to temperature increases under exposure to light. Electrical aging effects are proven to be negligible in comparison to other major modes of degradation.