Transient electrical response of dye-sensitized ZnO nanorod solar cells

A dye-sensitized solar cell was build with an electrode consisting of ZnO nanorods with a mean diameter of 80 nm instead of the commonly used nano colloidal TiO2 substrate. The electrical response of the cell under excitation with an alternating current (AC) voltage source was measured and analyzed in a wide frequency range (40 Hz-110 MHz). The results were compared to time-resolved measurements of the photocurrent generated by a nanosecond pulsed light source. Both measurements could be described by the same electrical equivalent circuit that consisted of passive circuit elements only. The slow response of the cell in the ms time range is mainly caused by the large space charge capacitance of the ZnO nanorod electrode. Because of the low impedance of the nanorod electrode, the photo response and the AC response is also influenced by the electrical properties of other interfaces in the cell, such as the counterelectrode. Thus the signal is more complex than the single RC response usually observed for planar photo anodes. SPICE simulations of the photocurrent response are given to illustrate the influence of the different interfaces on the overall signal. The differences observed in the electrical behavior of the Gratzel cell and the ZnO equivalent are discussed.