Evolving bifacial molecule strategy for surface passivation of lead halide perovskite solar cells†

A variety of passivation molecules have enhanced the performance and stability of organic–inorganic lead halide perovskite solar cells (PSCs); however, the tailoring of the design of these molecules remains largely unexplored. In this work, we propose two new classes of passivation molecules: a C₂-symmetric syn-type bifacial donor–π–donor molecule and a C₃-symmetric syn-type bifacial truxene. The former (PM-syn) bears hydrophobic alkylphenols and hydrophilic diethylene glycol-substituted phenyls on each face of the indenofluorene π-core. Owing to the efficient hole transfer and surface passivation by the flanked donor units, PM-syn (a racemate of enantiomers) exhibited an improved power conversion efficiency (PCE) of 18.79% and long-term stability compared with the control device (17.98%). The latter, bifacial truxene (TRX-syn), appended with three carboxyl units on one face, exhibited an improved PCE (19.76%) and stability, demonstrating the general effectiveness of the bifacial molecular concept in the passivation of PSC. Comparative spectroscopic and time-resolved studies of bifacial molecules and their anti-type analogues support our claims and provide a rich area for the design of new molecules for the modification of perovskite layers.