Hot-assisted spin-coating for ultrasmooth PbS colloidal quantum dot film in high-performance NIR photodetectors

Lead sulfide (PbS) colloidal quantum dots (CQDs) exhibit significant promise for near-infrared (NIR) photodetectors owing to their size-tunable bandgap and facile solution processability. However, with increasing PbS CQD size, a higher fraction of nonpolar (100) facets is exposed, thereby increasing their susceptibility to agglomeration during both synthesis and film formation, which degrades photodetection performance. Here, we develop a hot-assisted spin-coating technique that precisely regulates the evaporation kinetics of the mixed solvent, enabling single-step deposition of smooth, densely packed CQD films. When performed at 60 °C, this method yields an ultrasmooth film with a root mean square roughness of only 0.947 nm. The resulting p-i-n photodiode exhibits outstanding performance at 1350 nm: a specific detectivity (D*) exceeding 2.5×1011 Jones, a responsivity (R) of 0.21 A/W, and a −3 dB bandwidth of ∼92 kHz. Additionally, the device demonstrates its ability to faithfully resolve fine arterial pulse features under NIR illumination, highlighting its promise for high-sensitivity, real-time NIR photodetection and practical integration into wearable biomedical sensors.