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Abstract CsPbI 3− x Br x ‐based organic‐free perovskite has emerged as a superstar photovoltaic material not only because of its superior photoelectronic properties but also its outstanding thermal and chemical stability. Unfortunately, the significant energy loss resulting from its nonradiative recombination has become a major obstacle to further improvement of device performance. Here, a 2D/3D multidimensional structure formed spontaneously at room temperature is developed. The results reveal that the formed Ruddlesden–Popper 2D ( n = 1) perovskite atop CsPbI 3− x Br x plays an active role in mediating carrier transport, maintaining a long‐life charge separation state on the nanosecond time scale and promoting the efficiency of carrier injection into the hole transport layer, and thus enhances the hole extraction efficiency, which greatly reduces severe interfacial nonradiative charge recombination. In addition, the undercoordinated Pb 2+ is effectively passivated, resulting in significantly reduced surface trap density and prolonged charge lifetime within the perovskite films. Consequently, the combination of the above increases the solar cell efficiency from 19.05% to 20.31%, with an open‐circuit voltage raised to 1.23 from 1.17 V, which corresponds to an energy loss reduction from 0.54 to 0.49 eV. Also, the optimized solar cells exhibit better long‐term and thermal stability.
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