High‐Performance Flexible Narrowband NIR Photodetectors Enabled by Synergistic Surface Passivation and Plasmonic Coupling

Achieving simultaneously high sensitivity, spectral selectivity, and mechanical compliance in narrowband near‐infrared (NIR) photodetectors (PDs) is crucial for optoelectronic devices. Rare‐earth (RE) doped upconversion nanoparticles are promising photoactive materials for constructing high‐performance flexible narrowband NIR PDs, but their practical application is limited by low upconversion luminescence (UCL) efficiency. Herein, we demonstrate a substantial enhancement of UCL in Cs 2 NaGdCl 6 :Er 3+ double perovskite nanocrystals (DPNPs) by core‐shell coating and plasmon vertical gold nanorods (Au NRs) coupling. Compared to pristine Cs 2 NaGdCl 6 :Er 3+ DPNPs, the UCL improvement of 376‐fold in Cs 2 NaGdCl 6 :Er 3+ @Cs 2 NaGdCl 6 core‐shell DPNPs is realized with the optimized shell thickness of ∼3.5 nm. Moreover, more than four‐order enhancement (1.17 × 10 4 fold) was achieved by combining vertically aligned Au NRs with the core‐shell DPNPs. Consequently, we developed a flexible narrowband NIRPDs based on the core‐shell DPNPs/MoO 3 /vertically aligned Au NRs/PET/SnO 2 /MAPbI 3 / Spiro‐OMeTAD/Ag device, which exhibits a detectivity of 1.01 × 10 14 Jones and a responsivity of 196.5 A W −1 at 980 nm, respectively. Finally, this device was integrated as the photoplethysmography system, realizing real‐time and accurate heart rate monitoring.