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 Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 - and CH 3 NH 3 PbBr 3 -based perovskite solar cells (PSCs). The best CH 3 NH 3 PbBr 3 -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 Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(I 0.83 Br 0.17 ) 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.