Naphtho[1,2-: B:4,3- b ′]dithiophene-based hole transporting materials for high-performance perovskite solar cells: Molecular engineering and opto-electronic properties

As a pump for extracting carriers from the absorber to the cathode, hole transporting materials (HTMs) play indispensable roles in promoting the power conversion efficiencies (PCE) of perovskite solar cells (PSCs). In this context, we present two simple HTMs using 2,2′-(1,2-phenylene) bisthiophene or naphtho[1,2-b:4,3-b′]dithiophene as a π-bridge to link two electron-donor units (4′,4′′-dimethoxytriphenylamine): PBT and NDT. Their frontier molecular orbital energies match well with those of perovskites as is consistent with the DFT calculation results. The PCE measured under mimetic solar illumination (AM 1.5, 100 mW cm^-2) increases from 13.6% (PBT) to 18.8% (NDT), which surpass that of the spiro-OMeTAD (18.1%) reference. The modified annulated core in NDT makes it perform better in photovoltaic conversion than PBT. The reason for this is discussed from aspects of hole mobility, charge collection probability (P_c) and recombination kinetics.