The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201901673.Low-dimensional Ruddlesden-Popper perovskites (RPPs) exhibit excellent stability in comparison with 3D perovskites; however, the relatively low power conversion efficiency (PCE) limits their future application. In this work, a new fluorine-substituted phenylethlammonium (PEA) cation is developed as a spacer to fabricate quasi-2D (4FPEA) 2 (MA) 4 Pb 5 I 16 (n = 5) perovskite solar cells. The champion device exhibits a remarkable PCE of 17.3% with a J sc of 19.00 mA cm −2 , a V oc of 1.16 V, and a fill factor (FF) of 79%, which are among the best results for low-dimensional RPP solar cells (n ≤ 5). The enhanced device performance can be attributed as follows: first, the strong dipole field induced by the 4-fluoro-phenethylammonium (4FPEA) organic spacer facilitates charge dissociation. Second, fluorinated RPP crystals preferentially grow along the vertical direction, and form a phase distribution with the increasing n number from bottom to the top surface, resulting in efficient charge transport. Third, 4FPEA-based RPP films exhibit higher film crystallinity, enlarged grain size, and reduced trap-state density. Lastly, the unsealed fluorinated RPP devices demonstrate superior humidity and thermal stability. Therefore, the fluorination of the long-chain organic cations provides a feasible approach for simultaneously improving the efficiency and stability of low-dimensional RPP solar cells. Perovskite Solar CellsOrganic-inorganic hybrid perovskites have attracted tremendous attention due to their high absorption coefficients, [1] high charge carrier mobility, [2] high defect tolerance, [3] and long diffusion lengths. [4] Although the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has reached 23.32% in the past few years, the intrinsic material instability of 3D perovskites still remain unresolved, which hinder the future commercialization of perovskite solar cells. [5] Compared

However, the intrinsic environmental instability of 3D PSCs become a big obstacle to future commercialization. Emerging 2D Ruddlesden-Popper (RP) perovskites with the general formulate of A 2 B n−1 Pb n I 3n+1 , where A is the bulky large cations (e.g., n-butylamine (BA), phenylethlammonium (PEA)), B is the small organic cations (e.g., methylammonium (MA)), and n is the number of confined lead halide octahedral, have recently shown the excellent inherent environmental stability. [5,6] In 2D RP perovskite, the hydrophobic PEA or BA bulky cations prevent the inorganic perovskite layers from oxygen and moisture corrosion, leading to long-term environmental stability. On the other hand, the bulky space cations can passivate the defects and increase the ion migration activation energy in 2D RP perovskite, contributing to the improvement of the thermal stability and photostability.Despite of the impressive stability, the photovoltaic performance of 2D RP PSCs is still lower as compared to their 3D counterparts. [6][7][8] The lower efficiency is ascribed to two major reasons. First, owing to the quantum confinement effect, the enlarged energy bandgap of 2D RP perovskite causes the insufficient light absorption especially at the near-infrared region, leading to the reduced short circuit current (J sc ). Second, 2D RP perovskite crystals prefer growing along the in-plane direction with respect to the substrate. Consequently, the insulating interlayer bulky cations of 2D perovskite impede the out-ofplane charge transfer between the conducting inorganic slabs, leading to insufficient interlayer charge transport. In principle, this poor charge transport capability can be improved by tuning the crystal orientation of 2D RP perovskite. Different strategies including processing, [7,9] solvent, [10] additives, [11,12] and cations engineering [13][14][15][16][17] have been proposed to control crystallographic orientation from the parallel to the vertical direction, and consequently improve the device efficiency. For example, Nie and co-workers first reported the hot-casting method to achieve over 12% efficiency from the 2D perovskite (BA 2 MA 3 Pb 4 I 13 ). [7] Zhao and co-workers developed a slow postannealing process to align the multiphases growth along the vertical direction to the substrate, and obtained a champion PCE of 17.26% for BA 2 MA 3 Pb 4 I 13 2D PSCs. [18] Chen and co-workers, reported the addition of ammonium thiocyanate (NH 4 SCN) can promote vertically oriented crystal growth, leading to the improvement of efficiency and lifetime. [11] Besides, cation engineering is another effective approach to improve the efficiency of 2D PSCs. Liu and co-workers reported a new type of 2D perovskite withOwing to their insufficient light absorption and charge transport, 2D Ruddlesden-Popper (RP) perovskites show relatively low efficiency. In this work, methylammonium (MA), formamidinum (FA), and FA/MA mixed 2D perovskite solar cells (PSCs) are fabricated. Incorporating FA cations extends the absorption range and enhances the li...

Substituting the hydrogen atoms on the conjugated side chain of a wide-bandgap polymer J52 with chlorine atoms can simultaneously increase the J sc , V oc , and FF of nonfullerene OSCs, leading to an efficiency boost from 3.78 to 11.53%, which is among the highest efficiencies for as-cast OSCs reported to date. To illustrate the impressive 3-fold PCE enhancement, the chlorination effect on the optical properties and energy levels of polymers, film morphology, and underlying charge dynamics is systematically investigated. Grazing incidence wide-angle X-ray scattering studies show that chlorinated J52-2Cl exhibits strong molecule aggregation, the preferred face-on orientation, and enhanced intermolecular π−π interactions, hence increasing the charge carrier mobility by 1 order of magnitude. Moreover, chlorination modifies the miscibility between the donor and acceptor and consequently optimizes the phase separation morphology of J52-2Cl:ITIC blend films. These results highlight chlorination as a promising approach to achieve highly efficient as-cast OSCs without any extra treatment.