Degradation X-ray photoelectron spectroscopy Inverted solar cells Time of flight secondary ion mass spectrometry a b s t r a c t Analysis of the degradation routes for poly[(4,4-bis(2-ethylhexyl)-cyclopenta-[2,1-b;3,4-b′]dithiophene)-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl] (PCPDTBT)-based solar cells under illumination and in the presence of air have been conducted using a combination of X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS) and solar cell device data. After ageing, XPS studies show that PCPDTBT appears as an oxygen-containing polymer, with data indicating that a break-up in the aromatic rings, formation of sulphates at the thiophene ring, chain scission in the polymer backbone and also loss of side chains. XPS studies have also been conducted on Phenyl-C71-butyric acid methyl ester (PC 71 BM) films and show a breakage of the fullerene cage, loss of molecular shape and oxidation of carbon atoms in the fullerene cage and side chains after ageing. XPS studies on active layers blends of PCPDTBT and PCBM also show significant changes in the vertical composition during ageing, with increased enrichment of PCPDTBT observed at the top surface and that the use of a processing additive (ODT) has a negative impact on the morphological stability. Based on these studies, it is shown that inverted structures are better suited than non-inverted devices for PCPDTBT:PCBM solar cells. An additional advantage of inverted devices is shown using TOF-SIMS; electrode degradation during ageing experiments leads to migration of indium and tin ions into the active layer in non-inverted devices, but is eliminated for inverted devices. &