Indium-tin-oxide (ITO) surfaces were electrochemically treated with voltages from 0 to + 2.8 V in 0.1 M K(4)P(2)O(7) electrolyte. The initial growth mode of hole transport layer (HTL) was investigated by atomic force microscope (AFM) observation of thermally deposited 2 nm N,N-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) on the electrochemically treated ITO surfaces. The results showed that the morphology of NPB thin film was significantly influenced by the treating voltage via the change in surface energy. especially the polar component. The treatments with + 2.0 and + 2.4 V were found to be most effective for more uniform and denser nucleation of NPB. The influence of the electrochemical treatments on the nucleation and growth mode of HTL and therefore the device performance were discussed. (C) 2009 Elsevier B.V. All rights reserved.