Photovoltaic Performance of Dye-Sensitized Solar Cells Containing ZnO Microrods

At an elevated temperature of 90 °C, a chemical bath deposition using an aqueous solution of Zn(NO₃)₂·6H₂O and (CH₂)₆N₄ resulted in the formation of both nanoflowers and microrods of ZnO on F-doped SnO₂ glass with a seed layer. The nanoflowers and microrods were sensitized with dyes for application to the photoelectrodes of dye-sensitized solar cells (DSSCs). By extending the growth time of ZnO, the formation of nanoflowers was reduced and the formation of microrods favored. As the growth time was increased from 4 to 6 and then to 8 h, the open circuit voltage (V_oc) values of the DSSCs were increased, whilst the short circuit current (J_sc) values varied only slightly. Changes in the dye-loading amount, dark current, and electrochemical impedance were monitored and they revealed that the increase in V_oc was found to be due to a retardation of the charge recombination between photoinjected electrons and I₃^- ions and resulted from a reduction in the surface area of ZnO microrods. A reduced surface area decreased the dye contents adsorbed on the ZnO microrods, and thereby decreased the light harvesting efficiency (LHE). An increase in the electron collection efficiency attributed to the suppressed charge recombination counteracted the decreased LHE, resulting in comparable J_sc values regardless of the growth time.