In Situ Formation of δ-FAPbI3 at the Perovskite/Carbon Interface for Enhanced Photovoltage of Printable Mesoscopic Perovskite Solar Cells

Formamidinium lead triiodide (FAPbI(3)) perovskite has attracted intensive research attention due to its ideal band gap and low defect density for photovoltaic applications. Particularly, the existence of non-photoactive delta-phase FAPbI(3) has been considered detrimental to regular-structured perovskite solar cells (PSCs). Here, in hole-conductor-free triple-mesoscopic PSCs, we observe that alpha-phase FAPbI(3) transforms to delta-phase FAPbI(3) at the perovskite/carbon interface and in situ forms an alpha/delta-phase junction when the as-fabricated cells are exposed to humid atmosphere. The alpha/delta-phase junction shows a favorable band alignment and significantly suppresses the charge recombination at the interface. By controlling the relative humidity of the atmosphere, we fabricate FAPbI(3)-based cells that deliver a champion efficiency of 17.11% with an enhanced open-circuit voltage (V-OC) of 1020 mV. This work demonstrates the potential of delta-phase FAPbI(3) for benefiting the device performance of PSCs and proposes the concept of constructing perovskite-based junctions at the interface between a perovskite-absorbing layer and charge-transporting layers for enhancing the V-OC of PSCs.