Suppressing Ion Migration for 1.68 eV Wide‐Bandgap Perovskite Solar Cells via Bulk Crosslinking and Buried Passivation

ABSTRACT The 1.68 eV wide‐bandgap (WBG) perovskite plays a key role as the top cell in perovskite/silicon tandem solar cells (PSTSCs). However, ion migration in perovskites significantly limit its light stability and potential for tandem applications. Here, dual strategies of bulk crosslinking and buried passivation were introduced. First, the small molecule, N‐2‐hydroxyethylpiperazine‐N’‐2‐ethanesulfonic acid (HEPES), was used as a bulk additive to undergo an initiator‐free condensation reaction, forming a crosslinked network that inhibits ion migration. Second, a newly designed self‐assembling molecule with dual phosphonic acid groups, (3,8‐dichloroindolo[2,3‐a]carbazole‐11,12‐diyl)bis(propane‐3,1‐diyl)bis(phosphonic acid) (3‐BPIC‐Cl), was used to modify the buried interface. DFT simulations and characterizations showed that the configuration of 3‐BPIC‐Cl has one phosphonic acid group pointing downward to anchor to the substrate and one phosphonic acid group pointing upward to effectively passivate buried interfacial defects and further obstruct ion migration pathways. Consequently, inverted‐structure 1.68 eV WBG perovskite solar cells achieved a champion power conversion efficiency (PCE) of 23.69% with an exceptional fill factor of 84.98%. Unencapsulated single‐junction devices retained 96.0% of their initial performance after 1000 h of continuous operational testing in N 2 , confirming effective ion migration suppression by the dual strategies. The corresponding PCE of 4‐terminal (4T) PSTSCs reached 31.57%, verifying their potential for 4T tandem applications.