We present a simple post-processing treatment, which dramatically improves the electrical properties of inverted geometry bulk-heterojunction organic solar cells, by enhancing functionality of the zinc-oxide hole blocking layer. The results show that inherent problems associated with the ZnO layer (e.g. shunting and J-V inflection points) are of the same origin and can be efficiently treated and removed. (C) 2011 Elsevier BM. All rights reserved.