A photodiode with planar heterojunction was fabricated using copper (II) phthalocyanine (CuPc) organic semiconductor and zinc oxide (ZnO) inorganic nanoparticles (NPs, similar to 5 nm). The current-voltage (I-V) characteristics of ITO/ZnO NPs/CuPc/Ag device in dark and under illumination with a solar simulator were investigated in detail. The measurement results showed that the device exhibited good rectifying behavior in dark and under illumination. A rectification ratio (RR) of 15.44 at 1.95 V was achieved for the device under 100 mW/cm(2) illumination power. Also, the RR of the device as a function of light intensity was observed. The photoresponsive mechanism of the photodiode was illuminated in term of its energy band diagram. (C) 2012 Elsevier B.V. All rights reserved.