Double-Side Crystallization Tuning to Achieve over 1 μm Thick and Well-Aligned Block-Like Narrow-Bandgap Perovskites for High-Efficiency Near-Infrared Photodetectors

Solution-processed narrow-bandgap Sn-Pb perovskites have shown their potential in near-infrared (NIR) photodetection as a promising alternative to traditional silicon and inorganic compounds. To achieve efficient NIR photodetection, high-quality Sn-Pb perovskite thick films with well-packed, smooth, and pinhole/void-free features are highly desirable for boosting the spectral absorption. Understanding the crystallization kinetics and tuning the crystallization are fundamentally important to reach such high-quality thick Sn-Pb perovskite films, and have been limitedly explored. Herein, an approach of double-side crystallization tuning through low-temperature space-restricted annealing in methylammonium-free Sn-Pb perovskite films with over 1 μm thickness is proposed. More specifically, through simultaneously retarding the crystallization in the top of precursor films and promoting the crystal growth of the bottom of precursor films, high-quality and block-like thick FA_0.85Cs_0.15Sn_0.5Pb_0.5I₃ perovskite films with improved crystallinity, preferred out-of-plane orientation, and reduced trap density are achieved. Finally, photovoltaic-mode Sn-Pb perovskite NIR photodetectors show a high external quantum efficiency of ≈80% at 760-900 nm, a recorded responsivity of 0.53 A W^-1, and a high specific detectivity of 6 x 10¹² Jones at 940 nm. This study offers the fundamental understanding of the crystallization kinetics of thick perovskite films and paves the way for perovskite-based emerging NIR photodetection and imaging applications.