Toward Efficient and Stable Ruddlesden-Popper Perovskite: Unraveling the Selection Rule of Binary Spacer Cations

2D Ruddlesden-Popper (RP) perovskites have excellent environmental stability and enhanced photostability compared with their 3D counterparts. However, the introduction of organic spacer cations induces disordered crystal growth and increased exciton binding energy, limiting the photoelectric performance. Here, a binary spacer cations engineering strategy is reported that incorporates aromatic and alkylamine spacer cations, in which fluorine-substituted hydrocarbons (4-TFBZAI) promotes carrier transport and alkylamine (BAI) assists the ordered growth of perovskite. The resulting binary spacer perovskite (4-TFBZA_1.6BA_0.4)FA₄Pb₅I₁₆ shows preferred orientational growth and reduced exciton binding energy, enabling an efficient photodetector with a detectivity exceeding 10¹³ Jones and a response time of 583 ns. The influence of the spacer cations with varying functional groups and chain lengths is examined. The results reveal that selecting long-chain alkylamine to mix with aromatic spacer cations shall be based on considerations of both functional groups and chain length. The aromatic spacer cation with suitable groups enhances carrier transport, while the flexible long-chain alkylamine facilitates perovskite crystal growth. Moreover, matched chain lengths between aromatic and alkylamine spacer cations are crucial to the photoelectric performance of RP perovskites. This work will guide researchers in selecting suitable binary spacer cations and designing new types of RP perovskites.