Nyquist plots of CdSe QDSSCs under dark condition and 1,000-W/m2illumination. (a) Nyquist plots of CdSe QDSSCs in dark; the equivalent circuit of the QDSSC with the representation of impedance at CE/electrolyte interface (subscript CE), QD-sensitized TiO2/electrolyte (subscript r) and series resistance (subscripts). The symbol R and CPE denote the resistance and constant phase element, respectively. (b) Details of plots (a) at high frequencies. (c) Nyquist plots of the same cells under 1,000-W/m2 illumination. (d) Details of plots (c) at high frequencies. The solid lines are the fitted curves.