We report on the longterm stability of 1.5-3% efficient inverted P3HT:PCBM solar cells sealed with glass plates, which have been glued onto the solar cells. We employed two different electron contact materials, titanium and chromium, and two different P3HT:PCBM layer thicknesses. By means of Auger electron spectroscopy we could show that the electron contact is partially oxidised during the processing of the inverted solar cells. It turns out that both, the electrode material and the thickness of the photoactive layer, have an influence on the longterm stability of such devices. After 1500 h of continuous illumination under a sulphur plasma lamp with a light intensity of ca. 1000 W/m(2) at 50 degrees C the most stable devices maintained 90% of the initial efficiency and the most efficient cells still had more than 2.5% power conversion efficiency. These best results are obtained with chromium as electron contact material. The light dose corresponds to approximately 1.5 years exposure to sunlight and is therefore a promising result. To our knowledge, this is the first systematic study on the longterm stability of organic solar cells using the inverted layer sequence. Compared to state of the art devices with usual layer sequence our results suggest that inverted organic solar cells can be at least as stable. (C) 2008 Elsevier B.V. All rights reserved.