We report the use of solution processed zinc oxide (ZnO) nanoparticles as a buffer layer inserted between the top metal electrode and the photo-active layer in bulk-heterojunction (BHJ) organic solar cell (OSC) devices. The photovoltaic properties were compared for devices annealed before (Device A) or after (Device B) the deposition of the Al top electrode. The post-annealing treatment was shown to improve the power conversion efficiency up to 2.93% and the fill factor (FF) up to 63% under AM1.5 (100 mW/cm(2)) illumination. We performed the depth profile/interface analysis and elemental mapping using the time-of-flight secondary ion mass spectrometry (TOF-SIMS). Signals arising from Al-27, O-16, C-12, S-32, Zn-64, Si-28, Sn-120 and In-115 give an indication of successive deposition of Al, ZnO, P3HT:PCBM and PEDOT:PSS layers on ITO coated glass substrates. Furthermore, we discuss the surface imaging and visualize the chemical information on the surface of the devices. (C) 2014 Elsevier Inc. All rights reserved.