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Mixed 2D/3D perovskite solar cells (PSCs) show promising performances in efficiency and long-term stability. The functional groups terminated on a large organic molecule used to construct 2D capping layer play a key role in the chemical interaction mechanism and thus influence the device performance. In this study, 4-(trifluoromethyl) benzamidine hydrochloride (TFPhFACl) is adopted to construct 2D capping layer atop 3D perovskite. It is found that there are two mechanisms synergistically contributing to the increase of efficiency: 1) The TFPhFA + cations form a dipole layer promoting the interfacial charge transport. 2) The suppressed nonradiative recombination of perovskite through the coordination of TFPhFA + cations with Pb-I octahedron, as well as the recrystallization of 3D perovskite induced by Clions. As a result, the PSC delivers an efficiency of 24.0% with improved open-circuit voltage (V OC ) of 1.16 V, shortcircuit current density (J SC ) of 25.42 mA cm -2 , and fill factor of 81.26%. The device shows no decrease in efficiency after 1500 h stored in the air indicating the good stability. The utilization of TFPhFACl not only provides a facile way to optimize the interfacial problems, but also gives a new perspective for rational design of large spacer molecule for constructing efficient 2D/3D PSCs.

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