Solvent Engineering in PbS Colloidal Quantum Dot Inks: Enabling Colloidal Stability, Ligand Versatility, Large–Area Film Fabrication, and Multilayer Processing

ABSTRACT PbS colloidal quantum dots (CQDs) have attracted considerable attention for photovoltaics and infrared (IR) optoelectronics because their bandgap can be tuned across the near–infrared (NIR) and short–wavelength infrared (SWIR) regions. This review highlights solvent engineering as a central strategy for fabricating high–performance PbS CQD inks, rather than treating it simply as a dispersion step. We first discuss the key parameters governing CQD surface chemistry throughout the dispersion and film–formation processes. We then compare the conventional butylamine (BTA)–based system with emerging alternatives, including BTA–based co–solvents and BTA–free weakly polar solvents such as γ–butyrolactone (GBL) and 2,6–difluoropyridine (DFP) used with lead halide ligands. In addition, we summarize solvent platforms developed for other ligand systems, including 3–mercaptopropionic acid (MPA), aromatic ligands dispersible in weakly polar solvents for orthogonal hole–transport layer (HTL), self–assembled monolayer (SAM)–functionalized CQDs, and thiolate–capped CQDs processed in fluorinated alcohols. Overall, this review highlights how solvent engineering governs not only ink stability, but also interfacial compatibility, multilayer integration, large–area coating, and device performance. Finally, we emphasize the key directions that require further development and investigation to establish more sophisticated solvent–engineering strategies.