Excess PbBr2 Passivation of Large PbS Colloidal Quantum Dots to Reduce Dark-Current Density for Near-Infrared Detection

Small-size $\mathrm{Pb}\mathrm{S}$ colloidal quantum dots (CQDs) have achieved excellent performance in photoelectric conversion devices through the ligand-exchange method of mixed lead-halide passivation. However, with the increase of $\mathrm{Pb}\mathrm{S}$ CQD diameter, the proportion of (100) facets on the CQD surface increases and the original proportion of mixed lead-halide ligand cannot passivate (100) facets completely, which will introduce deep defects and deteriorate device performance. Here, we demonstrate an excessive ${\mathrm{Pb}\mathrm{Br}}_{2}$ concentration ligand strategy to sufficiently passivate large-size $\mathrm{Pb}\mathrm{S}$ CQDs with an absorption peak at 1300 nm. The first-principles calculation results suggest that ${\mathrm{Br}}^{\ensuremath{-}}$ can passivate (100) facets more efficiently compared with ${\mathrm{I}}^{\ensuremath{-}}$. With the increase of ${\mathrm{Pb}\mathrm{Br}}_{2}$ concentration (0--0.464 mmol/mL), both optical and electrical measurements imply that defects are effectively passivated, while carrier lifetime increases and dark-current density decreases. Finally, a device with specific detectivity of $5.22\ifmmode\times\else\texttimes\fi{}{10}^{12}$ Jones is obtained. This passivation strategy can also be used in other large-size $\mathrm{Pb}\mathrm{S}$ CQDs (diameter >4 nm) to realize a better device performance.