The stability issue is still one of the main limitations of the commercialization of perovskite photovoltaics. The mixed cation FAxCs1−xPbI3 has shown great promise owing to its improved thermal and moisture stability. However, the study of FAxCs1−xPbI3 is concentrated on formamidine (FA)‐rich perovskite, whereas cesium (Cs)‐rich FAxCs1−xPbI3 perovskites are barely studied due to the inevitable phase separation when Cs > 30 mol%. Here, a Cs4PbI6‐mediated method is developed to synthesize Cs‐rich FAxCs1−xPbI3 perovskites. It is demonstrated that Cs4PbI6 intermediate phase has a low Cs cation diffusion barrier and therefore offers a fast ion exchange with the preformed FA‐rich perovskite phase to finally form the Cs‐rich FAxCs1−xPbI3 perovskite. The results indicate that ≈15% alloying with organic FA cations can sufficiently stabilize the perovskite phase with excellent phase and UV‐irradiation stability. The FA0.15Cs0.85PbI3 perovskite solar cells achieve a champion power conversion efficiency of 17.5%, showing the great potential of Cs‐based perovskites for efficient and stable solar cells.
Deciphering the atomic and electronic
structures of interfaces
is key to developing state-of-the-art perovskite semiconductors. However,
conventional characterization techniques have limited previous studies
mainly to grain-boundary interfaces, whereas the intragrain-interface
microstructures and their electronic properties have been much less
revealed. Herein using scanning transmission electron microscopy,
we resolved the atomic-scale structural information on three prototypical
intragrain interfaces, unraveling intriguing features clearly different
from those from previous observations based on standalone films or
nanomaterial samples. These intragrain interfaces include composition
boundaries formed by heterogeneous ion distribution, stacking faults
resulted from wrongly stacked crystal planes, and symmetrical twinning
boundaries. The atomic-scale imaging of these intragrain interfaces
enables us to build unequivocal models for the
ab initio
calculation of electronic properties. Our results suggest that these
structure interfaces are generally electronically benign, whereas
their dynamic interaction with point defects can still evoke detrimental
effects. This work paves the way toward a more complete fundamental
understanding of the microscopic structure–property–performance
relationship in metal halide perovskites.
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