1, 8-Diiodooctane (DIO) has been known for its role of improving the polymer morphology and enhancing performance of polymer bulk heterojunction (BHJ) solar cell. In this work, the impedance spectroscopy was used to investigate the interface of poly(4,8-bis-alkyloxybenzo(1,2-b:4,5-b9dithiophene-2,6-diyl-alt- (alkyl thieno(3,4-b) thiophene-2-carboxylate)-2,6-diy1) (PBDTIT-C):PCOM in BHJ with DIO as additive. Based on our results, we were able to simulate the device into an equivalent circuit model, which allows us to conveniently analyze the organic/organic interfacial contact in the organic photovoltaic (OPV) device. Thus, we demonstrate that the impedance spectroscopy can an effective approach in characterizing the donor/acceptor interfaces, such that a direct correlation can be established between the morphology and the device performance of BHJ devices. (C)2014 Published by Elsevier B.V.