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 H-1/C-13 NMR spectroscopy, mass spectrometry, ultraviolet visible and photoluminescence spectroscopy, and cyclic voltammetry. Under AM1.5 G illumination, the mesoscopic CH(NH2)(2)PbBr3 perovskite solar cell employing SO7 as the HTM displayed an outstanding photovoltage (V-oc) of 1.5 V with an efficiency (eta) 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 V-oc, of 1.47 V and a maximum eta 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.