Enhancing Vertical Orientation via Self‐Assembled Molecule Interlayer Enables Efficient Ruddlesden–Popper Perovskite Solar Cells

The typical anisotropic crystal orientation in Ruddlesden–Popper perovskites (RPPs) is not conducive to carrier transport, resulting in a reduced power conversion efficiency (PCE) compared to three‐dimensional perovskites. Here, we present a novel method for manipulating the crystal orientation by introducing a self‐assembled molecular layer, MeO‐2PACz ([2‐(3,6‐dimethoxy‐9H‐carbazol‐9‐yl)ethyl] phosphonic acid), as an interlayer between PTAA (poly[bis(4‐phenyl)(2,4, 6‐trimethylphenyl) amine]) and the perovskite. The phosphate group of MeO‐2PACz bonds with Pb 2+ in the RPP, promoting the vertical orientation formation of the perovskite and facilitating efficient charge transport within the RPP materials. Additionally, the grain size is increased, and grain boundary defects are passivated, which contributes to suppressed nonradiative recombination of carriers. The interlayer incorporation of significantly improves the PCE of the optimized device to 17.80%, compared to the device without MeO‐2PACz, which has an efficiency of approximately 15.68%. This presents the highest efficiency for an MA‐based RP perovskite solar cell (PSC) utilizing 4FPEA (4‐fluoro‐phenethylammonium) as the spacer cation. Furthermore, the unencapsulated devices demonstrate superior thermal stability. This proposed optimization offers new insights into the manipulation of RPP crystal growth orientation.