Molecular Engineering of Spiro-Type Hole-Transporting Materials with N-Heterocycles for Robust Perovskite Photovoltaics

The use of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) for doping of Spiro-OMeTAD is a widely adopted strategy for improving the power conversion efficiency (PCE) of perovskite solar cells (PSCs). However, the migration of lithium ions (Li⁺) within the device will pose a threat to device stability. To address this limitation, three hole-transporting materials (HTMs) were designed through the modification of Spiro-OMeTAD. Among them, Spiro-PR26 features a donor-acceptor (D-A) structure using pyrazine group as the electron acceptor (A) and bis(4-methoxyphenyl)amine unit as the electron donor (D). This molecular design facilitates efficient charge carrier extraction along with a matched energy level. Furthermore, the nitrogen atoms with lone pair electrons in pyrazine exhibit a strong coordination effect, which effectively restricts the migration of Li⁺ and passivates the perovskite defects. As a result, the PSCs based on Spiro-PR26 achieved a champion PCE of 24.32% with excellent stability. Notably, the Spiro-PR26-based device retained 92% of its initial PCE after operating at 40-50% relative humidity (RH) for 1000 h, ranking among the best humidity-stable Spiro-type HTMs. This work delivers helpful revelations for the conception of stabilizing Li-TFSI-doped HTMs.