Understanding of the chopping frequency effect on IPCE measurements for dye-sensitized solar cells: From the viewpoint of electron transport and extinction spectrum

The incident monochromatic photon to electron conversion efficiency (IPCE) is an essential characterization method for the photoelectrical performance of solar cells. An IPCE measurement apparatus involving alternating current (ac) and direct current (dc) methods was set up. A chopping frequency effect on IPCE measurements was found for dye-sensitized solar cells (DSSCs), that is, with the increase in chopping frequency, the IPCE spectrum decreased significantly, and the different bands of the IPCE spectrum declined to different degrees. The chopping frequency effect was studied in detail by measuring the short-circuit current waveform, the extinction spectrum of the dye-coated TiO2 photoelectrode film and electrochemical impedance spectroscopy. The mechanism of the chopping frequency effect was investigated from the electron transport and extinction spectrum. The electron transport properties of the TiO2 photoelectrode film determined the slow response of DSSCs. From the extinction spectrum, the transport distance of electrons in the TiO2 film varied under the illumination of different monochromatic light. For DSSCs, the ac method was remarkably influenced by the trap states of electrons and the optical penetration depth, while the dc method was a steady-state measurement avoiding the impact of these two factors. Thus, the dc method is more suitable than the ac method for IPCE measurements of DSSCs.