## 3 Results and Discussion
This phenomenon can be attributed to two reasons: (I) When the light intensity reaches a certain level, the responsivity and detectivity of the photodetector itself have already reached their maximum values, and further increases in light intensity do not correspondingly enhance the responsivity and detectivity. (II) At high light intensities, photogenerated carriers in the photodetector undergo recombination, where carriers excited by light recombine, leading to a decrease in the photodetection performance of the photodetector. As the key figures of merit of photodetectors, the response speed (\(\tau_{\text{rise}}\): photocurrent increases from 10 to 90% of its final value)/decay time (\(\tau_{\text{decay}}\): photocurrent falls from 90 to 10% of its beginning value) are also characterized. As displayed in Figure 4e, \(\tau_{\text{rise}}\)/\(\tau_{\text{decay}}\) values of 0.105/0.651 s (V\({}_{\text{ds}}\) = 1 V). Compared to the 0.63/1.27 s of bare Bi\({}_{2}\)O\({}_{2}\)Se (Figure 2d), the response speed and decay time of the hybrid heterojunction have been significantly improved. Figure 4f shows the photoresponse spectra of the compared devices. Compared to bare Bi\({}_{2}\)O\({}_{2}\)Se, the hybrid photodetector exhibited a 20-fold improvement in responsiveness below 1000 nm due to the detection range of PbS quantum dots covering 405-1350 nm (Figure S4). At \(\lambda\) > 1000 nm, the responsiveness of bare Bi\({}_{2}\)O\({}_{2}\)Se is nearly undetectable, whereas the PbS/Bi\({}_{2}\)O\({}_{2}\)Se hybrid photodetector exhibits significant photoresponse. The charge transfers between Bi\({}_{2}\)O\({}_{2}\)Se and PbS QDs is determined by the band alignment at the interface. To obtain the band information of the prepared PbS QDs and Bi\({}_{2}\)O\({}_{2}\)Se, ultraviolet photoelectron spectroscopy (UPS) measurements were conducted. In Figure 4g, valence band spectra and the second cutoff energy of Bi\({}_{2}\)O\({}_{2}\)Se and PbS are presented. The work functions (W) of PbS and Bi\({}_{2}\)O\({}_{2}\)Se can be calculated from the second cutoff energy (E\({}_{\text{cut}}\)) using W= hv \(-\) E\({}_{\text{cut}}\), where hv = 21.22 eV represents the photon energy. The work functions of PbS and Bi\({}_{2}\)O\({}_{2}\)Se were found to be 4.2 eV and 4.35 eV, respectively. Additionally, the valence band positions of PbS and Bi\({}_{2}\)O\({}_{2}\)Se were determined to be 0.49 eV and 0.51 eV below the Fermi level (E\({}_{\text{F}}\)) of each material based on the valence band spectra, thus establishing the valence band positions of PbS quantum dots and Bi\({}_{2}\)O\({}_{2}\)Se. Therefore, based on the bandgaps of PbS (1.1 eV) and Bi\({}_{2}\)O\({}_{2}\)Se (0.8 eV), along with the estimated work functions and valence band positions, the energy band diagram shown in Figure 4h was constructed. PbS and Bi\({}_{2}\)O\({}_{2}\)Se form a type-II hybrid heterojunction, where electrons are transferred from PbS to Bi\({}_{2}\)O\({}_{2}\)Se due to the lower work function of PbS, forming an internal electric field that enhances the photodetection performance of Bi\({}_{2}\)O\({}_{2}\)Se. Figure 5a illustrates the charge flow diagram of the PbS/Bi\({}_{2}\)O\({}_{2}\)Se hybrid photodetector under a 1 V bias. When the device is illuminated, both PbS quantum dots and Bi\({}_{2}\)O\({}_{2}\)Se generate photogenerated carriers. Due to the type-II heterojunction formed between PbS quantum dots and Bi\({}_{2}\)O\({}_{2}\)Se, photogenerated electrons in the PbS quantum dots transfer to the Bi\({}_{2}\)O\({}_{2}\)Se. Bi\({}_{2}\)O\({}_{2}\)Se acts as the current transport layer throughout the device. The transfer of electrons from the PbS quantum dots enhances the electron mobility in Bi\({}_{2}\)O\({}_{2}\)Se, shortening the carrier transport time (\(\tau_{\text{transit}}\)), thereby increasing the photoconductive gain of the entire device [44].