Pi-Conjugated Small Molecules Modified Sno2 Layer For Perovskite Solar Cells With Over 23% Efficiency

SnO2 has been universally applied as electron transporting layer (ETL) towards the fabrication of highly efficient perovskite solar cells (PSCs), owing to its unique advantages including low-temperature solution-processability, high optical, transmittance and good electrical conductivity. Uncoordinated Sn-dangling bonds on SnO2 surface exist as deep traps to capture the photogenerated carriers, causing hysteresis and device instability. Fullerene derivatives, though being widely utilized as the passivator for SnO2, are highly prone to self-aggregate due to their pi-cage structures, which hampers passivation. Herein, pi-conjugated n-type small molecules with better film formation ability are innovatively designed, to improve passivation effectiveness. By exploring the interplay between molecular stacking of small molecules and charge transporting/recombination dynamics at the SnO2/perovskite interface, it is unveiled that a more compact molecular packing of the organic passivators yields superior interfacial characteristics, in terms of fewer trap states, lower charge recombination and higher electron transporting efficiency. An impressive PCE over 23% is achieved with the assistance of this new-type SnO2-passivator, which is among the highest reported value for triple-cation perovskite systems to date. This work offers an original concept for the design and synthesis of ETL passivators towards the development of high performance and stable PSCs.