Flexible electronic devices fabricated on plastic substrates require semiconductors, which can be deposited at low temperatures. While Indium-Gallium-Zinc-Oxide (IGZO) is a promising n-type oxide semiconductor, a p-type oxide semiconductor with similar performance is currently not available. Here, the room temperature deposition of nickel oxide (NiO) acting as a p-type oxide semiconductor on a flexible plastic foil is described. NiO exhibits a carrier density of +1.6 x 10(17) cm(-3) and a Hall mobility of 0.45 cm(2)/Vs. p-type NiO is combined with n-type IGZO to fabricate flexible pn diodes on a free-standing polyimide substrate. The diodes show an ideality factor of approximate to 3.2 and an on-off current-ratio of approximate to 10(4). The NiO/IGZO diodes stay fully operational when exposed to tensile or compressive mechanical strain of 0.25%, induced by bending to a radius of 10 mm. In addition, a 50 Hz AC signal was rectified using a flexible diode while flat and bent. (c) 2013 Elsevier Ltd. All rights reserved.