Role of dye-induced corrosion in determining the efficiency of ZnO-based DSSC: the case of ZnO nanoforest in N719

Abstract

ZnO-based DSSCs usually show less efficiency than TiO2-based cells despite ZnO being better transporter of electrons. More surprisingly, ZnO nanoforest, which provides seamless electron pathways and large surface area, shows a very modest efficiency of 2.6%. Usually dye complexation and dye aggregation are thought of as two reasons for such underperformance. In this work, we show (using electron microscopy) that significant corrosion of the photoanode occurs in dye solution and dye-induced corrosion is a major reason for low efficiency. It has been shown that the surface of the nanosized features of the photoanode got roughened within hours of dye loading. Such corrosion is much more severe for ZnO nanoforest because they contain more intricate structures in comparison with nanowires. In terms of dynamics, two surface processes occur simultaneously when the electrode is dipped into the ethanolic dye solution: adsorption and corrosion. Hence, a sweet spot is possible where the good amount of dye is adsorbed with little corrosion. It has been shown that the short-circuit current can be increased significantly by optimization of dye loading conditions. Such optimum conditions are found to be dependent on the intricacy of the mesoporous structure. A maximum photocurrent of 13 mA/cm2 and maximum efficiency of 2.9% has been achieved which is highest reported for the configuration used. Although it may appear that application of a passivation layer might appease the corrosion issue, our experiments show that such coatings reduce the photocurrent significantly.