Fig. 1. CSM enables tunable doping and passivation of CQD inks.

a CSM consists of Step 1 (halogenation): the oleic-acid ligands are exchanged with lead halide anions; Step 2 (functionalization): lead halide anions are re-exchanged with the functional ligands to render p-type character. For the conventional solution-phase exchange process, the functional ligands are unable to fill the surface due to steric hindrance, which retains surface defects. In the CSM case, the initial halogenation step infiltrate sites not readily accessed by the large organic ligands. b PLQY measurements of CQD inks exchanged using conventional vs. CSM methods. TG ligands were used in this particular study. Error bars represent standard deviation calculated from a sample of three CQD inks. c Chemical structure of various functional ligands employed herein. d Photographs showing phase transfer of CQDs from octane to dimethylformamide (DMF) upon ligand exchange with functional ligands. Conventional exchange exhibits precipitation of CQDs due to low colloidal solubility, while CSM exchange enables to form stable colloids in the DMF phase. e The energy levels of the CQD films fabricated using n-type CQDs (halogenated) and p-type CQDs (functionalized), measured using UPS taken together with UV/visible absorbance. Top edge, dashed line, and bottom edge represent conduction band edge, Fermi level, and valence band edge, respectively. f Two-dimensional KPFM potential image of the CQD film. Inset schematic shows the film structure that consists of bottom n-type CQD layer (red) and top p-type CQD layer (blue). The mapping was performed at the interface between the n-type layer and the p-type layer (rectangular area). The halogenated CQD inks and CTA-reprogrammed inks were used as n-type and p-type, respectively.