Breaking the Passivation-Band-Alignment Trade-Off in PbS Colloidal Quantum Dot Photodiodes via Gradient Doping

Solution-processed PbS colloidal quantum dots (CQDs) are promising materials for short-wave infrared (SWIR) photodetection, yet extending the response to longer wavelengths is commonly accompanied by more severe surface trapping and unfavorable band alignment, which together limit carrier extraction and device sensitivity. Here, we report a post-deposition gradient doping strategy based on antimony trifluoride (SbF 3 ) to simultaneously address these two bottlenecks in 1650 nm PbS CQD photodiodes. The SbF 3 treatment creates a depth-dependent n -type doping profile in the halide-passivated CQD film, while F − coordination with undercoordinated Pb 2+ sites suppresses deep surface trap states and nonradiative recombination. Meanwhile, the resulting graded electronic structure reduces the conduction-band offset at the CQD/electron-transport-layer interface and establishes a more favorable quasi-type-II junction for electron extraction. As a result, the optimized p-i-n photodiodes deliver an external quantum efficiency (EQE) of 56.5% at 1650 nm under zero bias, a low dark current density of 31 nA cm −2 at −0.1 V, and a specific detectivity ( D *) of 5.62 × 10 11 Jones. Furthermore, monolithic integration with a complementary metal-oxide-semiconductor (CMOS) readout integrated circuit enables high-contrast SWIR imaging. This work offers an effective route to simultaneously regulate surface chemistry and junction energetics, providing a practical design strategy for high-performance long-wavelength CQD optoelectronics.