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 character-
ization of the new compounds was carried out through 1H/13C NMR
spectroscopy, mass spectrometry, ultraviolet-visible and photolumi-
nescence 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 (?) 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 ? 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.