Researchers at Rice University, University of Cambridge, Artois University, Lawrence Berkeley National Laboratory, DirectH2, Rennes University, Northwestern University and Lille University have developed a chloride-based coadditive strategy that stabilizes the black phase of formamidinium lead iodide (FAPI) while preserving its excellent efficiency.
The formation and degradation pathways for extremely stable Cl-doped FAPI. Image credit: Science
FAPI is an attractive perovskite for single-junction solar cells because of its near-optimal 1.45-1.5 eV bandgap and strong thermal stability, but its photoactive cubic black α-phase (3C-FAPI) is unstable at room temperature and tends to reconstruct into a nonperovskite yellow hexagonal δ-phase (2H-FAPI), which lowers device performance. Previous attempts to stabilize 3C-FAPI by alloying with MA, Cs, and Br could suppress this transition, but they introduced phase segregation and long-term instability. The key challenge has been to lock in the black phase without sacrificing durability.
The team addressed this by using a coadditive recipe that both incorporates chloride and imposes a stabilizing lattice strain. They introduced 15 mol % formamidinium chloride (FACl) and 0.5 mol % BA2PbI4 (BA = butylammonium) into the precursor solution. FACl drives chloride into the lattice, while FACl and BA2PbI4 together create compressive lattice strain that favors a highly oriented (100) Cl-doped 3C-FAPI black phase. Synchrotron-based in situ wide-angle x-ray scattering showed that, instead of collapsing into yellow phases, the film evolves in a controlled way through 2H, 4H, 6H, and 8H polytypes before settling into the corner-sharing 3C structure.
Solid-state 35Cl NMR directly confirmed chloride incorporation in the perovskite lattice and revealed, together with modeling, that Cl reshapes the energetic landscape of both formation and degradation. The Cl-doped FAPI no longer follows the usual low-energy degradation path via the yellow phase or 2H-PbI2. Under extreme stress (15-sun illumination at 90 °C for more than 400 hours), it instead degrades through a more energetically uphill 3R-PbI2 pathway, effectively raising the barrier to failure.
These structural and energetic benefits translate into high device performance and durability. p–i–n solar cells based on the coadditive-treated FAPI (FAPI-CA) reached a 25.1% power conversion efficiency, with an average of 24.1% across 40 devices. Under 1-sun illumination, open-circuit conditions, and 85 ± 5 °C, the devices retained 98% of their initial efficiency after 1200 hours of operation.
The work shows that a carefully tuned FACl/BA2PbI4 coadditive strategy can simultaneously control crystal formation, stabilize the black 3C phase, and redirect degradation along a far less favorable path, moving FAPI perovskites closer to commercially relevant stability.
