In this paper, we discuss long-term stability measurements of tandem solar cells with mixed phthalocyanine: fullerene photoactive layers that exhibit an initial power conversion efficiency of about 4%. These devices are remarkably stable against exposure to halogen light as their power conversion efficiency decreases by less than 3% within more than 1400 In of permanent illumination at an intensity of approximately 185 mW/cm(2) at 50 degrees C. In addition, long-term stability measurements at an elevated temperature of 85 degrees C are performed. In comparison to the illumination experiment, the cells show a much faster degradation which is attributed to the low glass transition temperature of the hole transport layer. (c) 2007 Elsevier B.V. All rights reserved.