We study the degradation of photovoltaic cells with poly(3-hexyl thiophene) (P3HT) and (6,6)-phenyl C(61)-butyric acid methyl ester (PCBM) blends under long-term continuous illumination as well as in shelf-life conditions, both in inert N(2) atmosphere. Degradation of the illuminated solar cells mainly occurs by a rapid decrease of the fill factor (FF) after 300 h, while short-circuit current and open-circuit voltage follow a linear decay after the initial burn-in. The sudden drop of the FF is correlated with an increase of the series resistance and proves irreversible upon annealing. Electrical measurements indicate that it stems from reduced charge extraction due to the photodegradation of the organic-electrode interfaces. Furthermore, as the external quantum efficiency (EQE) spectrum is evenly lowered over the entire wavelength range, we could exclude major changes in the blend morphology or significant changes to optical properties of the active layer. Introducing a thin C(60) layer leads to complete suppression of the FF decay over 1000 h, further proving that interface degradation dominates. Interestingly, similar improved lifetime over 1000 h was achieved by separate substitution of MoO(3) for PEDOT:PSS. (C) 2010 Elsevier B.V. All rights reserved.