The EIS is a powerful, non-destructive and informative technique to study the electrical and electrochemical properties of the interfaces [33–36]. In order to acquire better insight into the interfacial charge transfer process within the DSSCs, EIS was carried out for the DSSC based on Siahkooti natural dye in the darkcondition [29]. The fitting results of EIS data are shown in Fig. 7. The high-frequency (>100 MHz) interception on the real axis can be attributed to series resistance (RS = 52 X). The two semicircles in the high and middle-frequency regions correspond to the resistance capacitance (RC) networks of the Pt/electrolyte and TiO2/dye/electrolyte interfaces, including the charge transfer resistance(RCT) and the constant phase element (CPE), respectively.