Kinetics of Iodine-Free Redox Shuttles in Dye-Sensitized Solar Cells: Interfacial Recombination and Dye Regeneration (2015)

Dye-sensitized solar cells (DSCs) have gained widespread attentions owing to their low production cost, tunable optical response, and high light-to-electricity conversion. In DSCs, the performance of redox mediators with iodide/triiodide or iodine-free redox couples is vital to internal quantum efficiency. For a long time, iodide/triiodide based electrolytes are the most widely used mediators because of their desirable kinetics. Recently, exciting progress has been made with respect to iodine-free metallorganic and pure organic redox shuttles. Their tunable redox potential and diverse electron transfer behaviors enable the rational screening of electrolyte composition for enhancing the light-to-electricity conversion efficiency of DSCs toward the Shockley-Queisser limit. In this Account, we emphasize on current knowledge of two distinct but interrelated interfacial processes (electron recombination and dye regeneration), particularly for DSCs with iodine-free redox couples. We show that a deeper understanding of electron transfer kinetics of the alternative redox couples is fundamental to develop rational strategies for cell optimization. Compared with iodine electrolyte, iodine-free metallorganic redox couples such as iron, cobalt, and nickel complexes display much faster electron transfer kinetics in dye regeneration and interfacial recombination. Evidently, rapid regeneration enables the employment of more positive metal complex for attaining a higher photovoltage. However, severe recombination reactions have to be well controlled by using several effective surface treatments such as the addition of Brönsted bases and atomic layer deposition. Although these methods offer different pathways in surface passivation, a trade-off between charge injection efficiency and electron diffusion length is always observed. It follows that an appropriate LUMO level of sensitizer is essential to ensure efficient electron injection at the passivated TiO2 surface. Apart from fast recombination behavior, bulky metal complexes suffer from inefficient charge transport. Thus, the combination of thinner TiO2 film and sensitizers with high mole extinction coefficient has been employed for both enhancing diffusion-limited current and maintaining light-harvesting efficiency. Unlike metal complexes, most of organic sulfur redox couples in DSCs exhibit slow recombination kinetics. This allows the use of thicker TiO2 film to achieve an optimized light harvesting. However, the concomitant sluggish behavior of dye regeneration requires the use of sensitizers with more positive HOMO level, which is beneficial to efficient regeneration. Moreover, lower level of TiO2 band edge in DSCs based on organic sulfur mediators hinders the achievement of desirable photovoltage, spurring future explorations on this class of redox mediator. Based on the comparison of electron transfer behavior between iodine-free metallorganic complexes and pure organic redox couples, we aim to provide a comprehensive Account of the intriguing interfacial processes in iodine-free DSCs as the key scientific point is linked with the kinetics of interfacial reactions. This demonstrates the advantages as well as disadvantages of each class of iodine-free electrolyte and should shed light on to judicious selection of the energy levels for redox mediators, sensitizers, and the conduction band of TiO2 for DSCs. The knowledge of the reaction kinetics in DSCs should be also beneficial to the interface engineering on recent developed perovskite cells.