Push–pull amino-substituted heteroleptic iron N-heterocyclic carbene photosensitizers in dye-sensitized solar cells: optimization and characteristics

In this work, four dyes—consisting of heteroleptic iron N-heterocyclic carbene (NHC) complexes—were investigated in dye-sensitized solar cell (DSC) applications to explore their behavior and interactions with the overall cell. Three of the four dyes contained amino-based electron donor moieties, while one was without this type of structure, and instead had only a hydrogen atom. Compared to earlier investigations, the characteristics of the DSC devices were improved by the use of a UV-curing glue to seal the cells, by lowering the concentration of sensitization baths, and by altering the composition of the TiO2 substrates. This resulted in devices with power conversion efficiencies of up to 1.3%. Optimizations previously made to the electrolyte for DSCs incorporating iron-NHC devices were however found to be detrimental for amino-substituted complexes, giving worse results for all measured characteristics of these dyes. There was furthermore a significant hysteresis effect for devices made from each dye, hitherto not reported for iron-NHC complexes as dyes, where the scan direction strongly influenced the open circuit voltage and thereby also the efficiency.