Surface passivation is key to the power conversion efficiency (PCE) of organic-inorganic lead halide perovskite solar cells (PSCs). Herein, we report a novel molecular concept of a C2-symmetric syn-type bifacial donor-π-donor (D-π-D) passivation molecule (a racemic mixture of enantiomers) with hydrophobic phenyls and hydrophilic tetraethylene glycol-substituted phenyls on each face of the indeno-[1,2-b]fluorene π-core. In addition to this bifacial amphiphilic π-core unit, triphenylamine, a well-established passivation donor, effectively passivated the PSC surface, facilitated hole transfer, and increased the maximum PCE from 18.43 to 19.74%. Another notable effect is the removal of remnant PbI2 and the change in the perovskite orientation on the surface by the syn-type molecule. In contrast, the anti-type isomer degraded its long-term stability. We characterized the electrostatic and electronic properties of these molecules and highlighted the advantage of molecular strategy based on a bifacial structure and its stereochemistry.
Keywords: amphiphilic; bifacial molecule; perovskite solar cells; stereochemistry; surface passivation.