Fluorinated Low-Dimensional Ruddlesden-Popper Perovskite Solar Cells with over 17% Power Conversion Efficiency and Improved Stability.

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 Pb5 I16 (n = 5) perovskite solar cells. The champion device exhibits a remarkable PCE of 17.3% with a Jsc of 19.00 mA cm-2 , a Voc 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.