Towards increased performance of iron(II)-based dye sensitized solar cells

Today, because of the current climate situation, renewable energy sources are of high importance. Over the last few decades, solar cells have become a vital technology due to their ability to convert sunlight into electrical energy. Most of the commercially available solar cells are made of crystalline silicon. Silicon-based solar cells are already well known in the market, but their costly and tedious fabrication motivates the examination of alternative systems. Since the pioneering work of O’Regan and Grätzel in 1991, dye-sensitized solar cells (DSCs) have become a promising substitute. The sandwich-type DSC structure is easy to manufacture, and a broad variety of sensitizers offers lower costs of materials. However, the use of metals such as ruthenium with low natural abundances still significantly increases the price and reduces the sustainability of DSCs.
In this work, the focus is set on n-type DSCs, which combine the use of Earth abundant iron(II) coordination complexes as dyes and the advantageous effects of using different additives in electrolytes. Tuning of electrolyte composition can also remarkably enhance the photoconversion efficiency (PCE) and, as shown for other dyes, has the potential to make iron-sensitizers a promising alternative to ruthenium-based compounds. It has been demonstrated that both the redox couple and the components of the electrolyte have a critical influence on the PCE, and this effect originates from its role as a charge transfer medium. The effects of lithium salts, ionic liquids with different counter-ions and solvents while retaining an I-/I3- redox shuttle are presented.
Sometimes small changes might lead to significant progress. The design of an alternative iron(II) based dye is proposed with a corresponding synthetic route. The synthesis towards the target complex is presented.
Moreover, a statistical study of electrochemical impedance (EIS) measurements was performed. EIS offers a possibility to study complex electronic systems and is commonly used for solar cells, but there is a general tendency in the literature to present impedance data only for one device. At the same time, the current density–voltage plots can illustrate that measurements may vary within one set of DSCs with identical components. The multiple DSCs impedance measurements are presented on the example of two dyes and provide the statistical analysis for their reproducibility between the cells.