Simultaneous Interfacial Defect Passivation and Free-Volume Reduction by Fluorinated Hole Transport Materials for...

Interfacial defects and free-volume between the perovskite layer and hole transport material (HTM) critically impact the efficiency and stability of perovskite solar cells (PSCs). In this work, the interfacial issues of the PSCs are addressed by strategically modulating the fluorine distribution in N,N'-bicarbazole (BCz)-based HTMs, specifically, fluorine is introduced either asymmetrically and symmetrically on the peripheral groups, yielding AdF-BCz and SdF-BCz, respectively. Through a combination of experimental characterization and atomistic molecular dynamics simulations, it is demonstrated that AdF-BCz exhibits superior interfacial passivation stability against Pb²⁺ and I^- related defects, as well as stronger adhesion to the perovskite surface compared to the fluorine-free NF-BCz and symmetrically substituted SdF-BCz. Moreover, AdF-BCz reduces interfacial free-volume, promotes more intimate contact at the interface, and enhances the suppression of ion migration and perovskite degradation. Consequently, the PSCs fabricated with AdF-BCz achieved a notable efficiency of 25.35%, outperforming devices based on SdF-BCz (23.12%) and NF-BCz (24.2%). Furthermore, unencapsulated AdF-BCz based PSCs exhibit respectable stability, retaining 97% of their initial efficiencies after 2000 h under 30% relative humidity and 82% after 300 h of continuous heating at 85 °C.