A deep insight into the role of basic additives in the charge injection of Fe(II)-based complexes

Since the pioneering contribution of Ferrere and Gregg, iron complexes have demonstrated the ability to undergo charge injection into a semiconductor substrate, despite the low Power Conversion Efficiency (PCE) of the resulting Iron-Sensitized Solar Cells (Fe(II)-SScs) [1]. Recently, we reported a panchromatic co-sensitization strategy based on Fe(II)-NHC complexes, achieving an exceptionally high photocurrent density of 9 mAcm-2 [2]. Nevertheless, the PCE remains limited by the low open-circuit photovoltage (VOC), which generally is typically improved by tuning the TiO2 Fermi Level position through the addition of an electron-rich base, such as tert-butyl pyridine (TBPy), to the electrolyte. However, despite the injection driving force on the order of 1 eV, which suggests no thermodynamic limitations, the presence of these alkaline additives proves detrimental in the system’s efficiency. To date, the reasons behind the photocurrent loss remains cryptic. Herein, we reported a deep investigation on the role of tert-butyl pyridine in Fe(II)-SSCs and its impact on photocurrent suppression, combining photoelectrochemical analysis with ultra-fast spectroscopy. Thermodynamics effects contribute to the performance degradation, as depicted by the Marcus parabola trend. Nevertheless, compared to well-established ruthenium complexes, it is not sufficient to outline the observed losses. References: [1] S. Ferrere, B. A. Gregg, JACS 1998, 120 (4), 843-844 [2] A. Reddy-Marri, E. Marchini, V. Diez Cabanes, R. Argazzi, M. Pastore, S. Caramori, P.C. Gros, Chem. Sci.2023,14, 4288-4301