Polypyridyl Iron Complex as a Hole Transporting Material for Formamidinium Lead Bromide Perovskite Solar Cells

An efficient hole-transporting material (HTM) is indispensable for high-performing perovskite solar cells (PSCs), which have recently emerged as a breakthrough photovoltaic technology. Here, we demonstrate the capacity of the transition metal complex (6,6'-bis(1,1-di(pyridin-2-yOethyl)-2,2'-bipyridine)-iron(II/III) trifluoromethanesulfonate ([Fe(bpyPY4)](OTf)(2+x)) to act as an additive-free, solution-processable HTM in PSCs based on the formamidinium lead bromide absorber. State-of-the-art physical methods have been employed to characterize [Fe(bpyPY4)](OTf)(2+x) and, in particular, to demonstrate its significantly higher conductivity compared to that of the conventional HTM Spiro-OMeTAD. A maximum power conversion efficiency of 2.2% was obtained for a device employing [Fe(bpyPY4)](OTf)(2+x), which is the first evidence of the applicability as a HTM in a PSC of a solid material in which conductivity is provided by a redox transformation of a transition metal.