Record power conversion efficiencies for iron(ii)-NHC-sensitized DSSCs from rational molecular engineering and electrolyte optimization

Three Fe(ii) pyridylNHC-carboxylic heteroleptic complexes with (ARM7 and ARM11) or without spacers (ARM13) between the pyridine and the COOH anchoring group have been designed and characterized with the aim to increase the metal to surface charge separation and avoid undesired recombination processes in iron-sensitized DSCCs. The ARM13-sensitized DSSC scored the highest efficiency ever reported for an iron-sensitized solar cell (1.44%) providing that Mg2+ cations and NBu4I were present in the electrolyte, thus substantially boosting the photocurrent. The gain in efficiency derived from the use of MgI2-based electrolytes was rationalized by employing DFT calculations for the isolated dye sensitizers and dye/TiO2 interface models.