By using electric-field-induced optical second-harmonic generation (EFISHG) measurement, we studied the effect of the use of bathocuproine (BCP) interface layer. The EFISHG measurements of indium-zinc-oxide (IZO)/C-60/Al diodes showed that the BCP layer inserted between C-60 and Al formed an electrostatic field vertical bar E-i vertical bar = 2.5 x 10(4) V/cm in the C-60 layer, pointing in a direction from the Al to the IZO. Accordingly, in the IZO/pentacene/C-60/BCP/Al organic solar cells (OSCs), holes (electrons) move to the IZO (Al) electrode, enhancing the short-circuit current. The EFISHG measurement is capable of directly probing internal fields in the layers used for OSCs, and is helpful for studying the contribution of the interface layer in OSCs. (C) 2013 Elsevier B.V. All rights reserved.