Visualization of Long-Range Order in Self-Assembled Ion Barriers for Stable Perovskite Tandems

Abstract The arrangement of self-assembled molecules on perovskite surfaces has remained a black box, despite their widespread use across perovskite optoelectronics. All prior understanding has relied on computational simulations that default to random or short-range ordered packing. Here we report the direct visualization of this arrangement at atomic resolution. Scanning tunnelling microscopy reveals that a fully conjugated, planar molecule assembles into regularly repeating arrays of hundreds of molecular units, and complementary techniques confirm that this long-range order is conformally preserved on perovskite surfaces, including the textured silicon in tandems. This nearly square packing aligns with the cubic perovskite lattice and matches grain dimensions, enabling uniform coating. Four carbonyl groups, one at each corner of the molecule, form intermolecular coordination bonds that encircle each molecular unit, creating a supramolecular network that locks the ordered configuration while blocking metal ion migration. Incorporated into perovskite/silicon tandem solar cells, this self-assembled ion barrier achieves a power conversion efficiency exceeding 34.5% (certified steady-state 34.4%) and retains its performance under continuous illumination, light-dark cycling, damp-heat exposure, and thermal shock. The extracted structural parameters, including molecular orientation, intermolecular spacing, π-π stacking periodicity, and lamellar packing distances, provide an experimental benchmark to calibrate future simulations across the field.