Since their discovery in 2009, perovskite solar cells (PSCs) have attracted an enormous interest in the solar and renewable energy community and have been addressed as the next big thing in the solar field. Their ease of fabrication, the large availability and low cost of the materials together with some unique properties as the low exciton binding energy, large carrier diffusion length and tuneable band gap make them a good candidate for the next generation of solar devices and for tandem applications with silicon solar cells. Among the materials comprising PSCs, the hole transporting material (HTM) is the one that raises the most the final device price and the one that is most in need of an optimisation of its chemical and electronic properties. So far, most of the research in this field has focussed on organic materials. The “golden standard” HTM for perovskite solar cells, spiro-OMeTAD, allows the fabrication of high efficiency devices but presents many drawbacks. The difficulty of its synthesis and purification results in a very high final material cost and its use in solar devices requires mixing with additives and p-dopants to enhance its conductivity, which is very low in its pristine form.

Transition metal complexes are a class of materials that have been largely disregarded so far in solid-state solar applications, with the exception of their role of light harvesters in dye-sensitised solar cells. These complexes have found use in PSCs as p-dopants for organic HTMs but little accounts exist on their use as the HTM itself.Transition metal complexes possess properties that make them a good candidate for HTM applications in thin film solar cells. They present highly tuneable energy levels – which can be modified by tweaking the ligand structure or by changing the metal centre – and electrical conductivities much higher than organic compounds. The metal complexes used as HTMs reported in the literature – phthalocyanines and silver complexes with large aromatic ligands – present a very flat molecular structure that gives rise to high intermolecular π-π interactions and a crystalline morphology. Here, we present for the first time a polypyridyl iron complex with high denticity and different metal oxidation states. Its solid-state films are largely amorphous and present a good conductivity of 1.36×10⁻⁴ S/m. Devices fabricated with this metal complex and formamidinium lead bromide perovskite present a power conversion efficiency of 2.9%, close to the one of devices fabricated with spiro-OMeTAD (3.8%).