Transition metal complex redox shuttles for dye-sensitized solar cells

Abstract

An important link exists between the selected molecular structure of a sensitizer and the employed shuttle electrolyte to achieve high conversion efficiency in dye-sensitized solar cells (DSSCs). So far, the most commonly used redox mediator is iodide/triiodide (I⁻/I₃⁻), which has shown advantages such as desirable kinetic properties and high carrier collection efficiencies. However, it has several disadvantages including a low redox potential, corrosion toward metal materials and competitive blue light absorption. In this respect, the transition metal complex electrolytes represent valuable alternatives to replace traditional I⁻/I₃⁻ couples, and can overcome the drawbacks of the I⁻/I₃⁻ electrolyte. In recent years, the best efficiency of a DSSC was achieved with a porphyrin-sensitized solar cell with a transition metal complex-based redox electrolyte. In particular, in 2014, the Gratzel group published a record DSSC efficiency of 13% using a new porphyrin sensitizer with a Co-polypyridyl-based electrolyte. Here, we describe the engineering of the structure of a new transition metal complex electrolyte and the design of the molecular structure of a sensitizer at the same time. The main focus will be on the correlation between photophysical and electrochemical properties of the metal complex mediators and their DSSC performances. This review provides an in-depth investigation into exciting alternative electrolyte shuttles in DSSCs and the various advantages that they provide, including high conversion efficiency and non-corrosive properties.