High Open-Circuit Voltage: Fabrication of Formamidinium Lead Bromide Perovskite Solar Cells Using Fluorene–Dithiophene Derivatives as Hole-Transporting Materials

Four different fluorene–dithiophene derivative-based hole-transporting materials (HTMs) (SO7–10) have been synthesized via a facile route and were successfully used in the fabrication of formamidinium lead bromide perovskite solar cells. Detailed characterization of the new compounds was carried out through 1H/13C NMR spectroscopy, mass spectrometry, ultraviolet–visible and photoluminescence spectroscopy, and cyclic voltammetry. Under AM1.5 G illumination, the mesoscopic CH(NH2)2PbBr3 perovskite solar cell employing SO7 as the HTM displayed an outstanding photovoltage (Voc) of 1.5 V with an efficiency (η) of 7.1%. The photovoltaic performance is on par with the device using the state-of-the-art Spiro-OMeTAD as HTM, which delivered a Voc of 1.47 V and a maximum η of 6.9%. A density functional theory approach with GW simulations including spin–orbit coupling and electrochemical measurements revealed deeper highest occupied molecular orbital levels for newly synthesized fluorene–dithiophene derivatives, which eventually makes them promising HTMs for perovskite solar cells, especially when high photovoltage is desired.