Colloidal quantum dots (CQDs) hold immense promise for next-generation optoelectronic devices, yet their performance remains fundamentally limited by surface defects and poor charge transport within the CQD solid film. Traditional ligand exchange strategies often focus on passivating cationic Pb²⁺ sites, leaving the exposed anionic S^2- sites on the {100} facets of larger PbS CQDs insufficiently coordinated, particularly in polar processing solvents. This deficiency results in high trap-state density, poor colloidal stability, disordered packing, and ultimately, short carrier diffusion lengths (< 150 nm), severely restricting the efficiency and stability of devices.

To address this challenge, we introduce a novel strategy based on Organic Cation Lewis Acid (OCLA) ligands, specifically designed to comprehensively passivate the CQD surface and induce highly ordered film formation. By judiciously manipulating the pKa value of the organic cation (e.g., Benzyl Hydrazine Cation, BH⁺), the OCLA ligand acts as an effective Lewis acid, coordinating with the anionic S^2- sites through strong H⁺-S^2- interactions. Concurrently, the associated anion (Cl^-) passivates the Pb²⁺ sites, achieving dual-site passivation.

Crucially, the aromatic structure incorporated in the OCLA ligand facilitates rapid, self-limiting self-assembly. Utilizing potential pi-pi interactions, this strategy guides the PbS CQDs into a remarkably ordered 3D rhombohedral superlattice structure during a simple, single-step spin-coating process, enabling the fabrication of thick (> 400 nm) films.

Structural and electrical characterization confirms the superiority of the OCLA-CQD films. We observed a significant reduction in trap-state density (from 6.6 × 10¹⁵ cm^-3 to 4.7 × 10¹⁵ cm^-3) and a dramatic sixteen-fold enhancement in photoluminescence lifetime (from 1.2 ns to 18.9 ns). Most significantly, the comprehensive passivation and superior ordering boost the carrier diffusion length to a record-breaking 256 nm, establishing a new benchmark for PbS CQD solids.

This high-quality material platform translated into extraordinary device performance. Infrared photodetectors achieved a record-high specific detectivity of 1.01 × 10¹³ Jones at 1560 nm. Furthermore, infrared light-emitting diodes (LEDs) demonstrated a record radiance of 22.4 W Sr^-1 m^-2—double the previous state-of-the-art—along with excellent operational stability (T₉₀ > 200 h). This work introduces a versatile ligand engineering pathway, also validated on CdSe/ZnS CQDs, paving the way for stable, high-performance solution-processed optoelectronics.