The advantages of high mechanical flexibility of organic polymers that are suitable for the roll-to-roll processing technique have recently assisted its fast progress towards fabricating large-area organic photovoltaic (OPV) devices on flexible substrates including plastics and metals. We fabricated the inverted OPV devices on various metals namely: stainless steel (SS), titanium coated steel (Ti-S), chromium coated aluminium (Cr-Al), and aluminium (Al). Around 3.1% power conversion efficiency (PCE) was achieved on the SS and Cr-Al substrates for the poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methylester (P3HT:PCBM) OPV devices while only 1.48% PCE of the OPV devices was obtained on the Ti-S substrate and no effective devices were presented on the Al substrate. Morphologies of the metal substrates and the active P3HT:PCBM layer were investigated by the scanning electron microscope (SEM) and the tapping mode atomic force microscope (AFM), separately. We revealed why different metal substrates can provide significantly different performance for the inverted OPV devices by the electrical impedance spectroscopy (EIS). (C) 2013 Elsevier B.V. All rights reserved.