Abstract
<h2>Summary</h2> Achieving high-quality cesium-formamidinium lead iodide (Cs<sub>x</sub>FA<sub>1−x</sub>PbI<sub>3</sub>) perovskites with tunable band gaps is highly desired for optoelectronic applications including solar cells and light-emitting diodes (LEDs). Herein, by utilizing an alkaline-interface-assisted cation-exchange method, we fabricate highly emissive Cs<sub>x</sub>FA<sub>1−x</sub>PbI<sub>3</sub> perovskite films with fine-tunable Cs-FA alloying ratio for emission-tunable near-infrared (NIR) LEDs. We reveal that the deprotonation of FA<sup>+</sup> cations and the formation of hydrogen-bonded gels consisting of CsI and FA facilitated by the zinc oxide underneath effectively removes the Cs-FA ion-exchange barrier, promoting the formation of phase-pure Cs<sub>x</sub>FA<sub>1−x</sub>PbI<sub>3</sub> films with tunable emissions filling the gap between that of pure Cs- and FA-based perovskites. The obtained NIR perovskite LEDs (PeLEDs) peaking from 715 to 780 nm simultaneously demonstrate high peak external quantum efficiencies of over 15%, maximum radiances exceeding 300 W sr<sup>−1</sup> m<sup>−2</sup>, and high power conversion efficiencies above 10% at 100 mA cm<sup>−2</sup>, representing the best-performing LEDs based on solution-processed NIR emitters in a similar region.
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