Equivalent for small AC perturbation, which can explain the NC. R_c is the total series resistance generally caused by constant resistance of the cell; R_r is the recombination resistance including R_rac and R_rdc, and C_r is the recombination capacitance relate the electron transport process in film; C_μ is the chemical capacitance that stands for the change of electron density as a function of the Fermi level; R_ct is a charge-transfer resistance related to the traditional recombination of free carriers at electrolyte interface.