A-D-A-type S,N-heteropentacene-based hole transport materials for dopant-free perovskite solar cells

This work reports the design and synthesis of acceptor-donor-acceptor (A-D-A) type low band gap hole transport materials (HTM) comprising S,N-heteropentacene central units for solid-state perovskite-based solar cells. The optical and electrochemical properties were tuned by the insertion of thiophene or ethylenedioxythiophene units in the molecular backbone. These HTMs showed strong absorption in the visible region and suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies with respect to the CH3NH3PbI3 perovskite. Mesoscopic solid state perovskite solar cells prepared by solution-processing using these HTMs generated power conversion efficiencies (PCE) of 10.3-11.4% without the use of any additive or dopant. The charge transfer behavior between photoexcited perovskite and the HTMs was further investigated by photo-induced absorption spectroscopy.