Abstract
<h2>Summary</h2> The open-circuit voltage (<i>V</i><sub>OC</sub>) of cesium-based perovskite solar cells (Cs-PSCs) is severely limited by carrier recombination both in bulk and at the interface of the perovskite layer, which mainly roots in its elevated fabrication temperature. Enhancing the quality of perovskite film and optimizing the device structure are effective strategies to improve the performance of Cs-PSCs. Here, the <i>V</i><sub>OC</sub> is markedly promoted by constructing CsPbI<sub>3</sub>/Cs<sub>1-x</sub>DMA<sub>x</sub>PbI<sub>3</sub> bulk heterojunction (BHJ) structure, which was spontaneously formed by a precisely controlled spatially selective phase transition method. The perovskite BHJ structure not only facilitates the charge separation and collection process by enhancing the built-in potential but also obviously reduces the carrier recombination loss. Importantly, a maximum <i>V</i><sub>OC</sub> of 1.23 V was obtained as the <i>V</i><sub>OC</sub> deficit was 0.45 V, and the champion power conversion efficiency (PCE) reached 20.32% (certified PCE of 19.66%). Our finding indicates that junction engineering will be a new strategy for efficient perovskite devices.
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