A Multifunctional Fluorinated Polymer Enabling Efficient MAPbI₃-Based Inverted Perovskite Solar Cells

Exploring polymeric hole-transporting materials (HTMs) with passivation functions represents a simplified and effective approach to minimize the perovskite defect density. To date, most of reported polymeric HTMs were applied to fabricate n-i-p regular perovskite solar cells (PSCs). The polymers compatible for p-i-n inverted PSCs were very limited. Moreover, the passivation polymers were devoted to passivate the uncoordinated Pb²⁺. However, the MA⁺ cation defect has profound unwanted effect on device efficiency and long-term stability. In order to synchronously passivate the Pb²⁺ and MA⁺ defects in p-i-n inverted PSCs, a new nonfused polymer was intentionally explored via mild polymerization. The aromatic bridge instead of long alkyl chains enabled polymer BN-12 to achieve excellent thermal stability and good wettability of perovskite precursor. Furthermore, the incorporation of chemical anchor sites ("C═O" and "F") strongly controlled the crystallization of perovskite and restrained the MA⁺ ion migration. As a result, a significant fill factor (FF) of 82.9% and an enhanced power conversion efficiency (PCE) of 20.28% were achieved for MAPbI₃-based devices with the dopant-free BN-12, exceeding those with the commercial HTM PTAA (FF = 81.7%, PCE = 19.51%). More importantly, the unencapsulated devices based on BN-12 realized outstanding long-term stability, maintaining approximately 95% of its initial efficiency after stored for 85 days. By contrast, the PTAA-based device showed rapid decrease which retained only 50% of its initial value after 45 days.