A 2,1,3-Benzooxadiazole Moiety in a D–A–D-type Hole-Transporting Material for Boosting the Photovoltage in Perovskite Solar Cells

A donor–acceptor–donor molecule with a 2,1,3-benzooxadiazole moiety, named BTPA-3, was successfully synthesized and employed as a hole-transporting material (HTM) for Cs0.05(MA0.17FA0.83)0.95Pb­(I0.83Br0.17)3- and CH3NH3PbBr3-based perovskite solar cells (PSCs). The best CH3NH3PbBr3-based PSC with BTPA-3 exhibited a superior efficiency (5.91%) compared to the cell with spiro-OMeTAD (5.61%), even an 80 mV higher open-circuit voltage (V oc) being recorded in the reverse scan (average V oc: 1.41 V for BTPA-3 and 1.33 V for spiro-OMeTAD). The higher V oc is attributed to larger recombination resistance and the resulting longer photovoltage lifetimes in the cells with BTPA-3. In the meantime, for the Cs0.05(MA0.17FA0.83)0.95Pb­(I0.83Br0.17)3-based PSC, BTPA-3 exhibited a much lower conversion efficiency (9.81%) compared with the cell with spiro-OMeTAD (13.2%) due to a small driving force for the hole injection. The morphologies and conductivities of the hole-transport layers were also investigated, and the results are discussed in relation to the performances of the PSCs. Finally, PSCs with BTPA-3 as an HTM based on both perovskite lead halide materials exhibited good long-term stabilities.