Researchers from Hanyang University, Ajou University and POSTECH have developed a hydrolysis-assisted ligand-exchange strategy that significantly improves charge transport and efficiency in metal halide perovskite nanocrystal (MHP NC) LEDs, achieving a record external quantum efficiency (EQE) of 31.7% for green-emitting devices.
Schematic illustration of the ligand-exchange and surface-functionalization process of MHP NCs. Image from: Advanced Materials
MHP nanocrystals are widely considered promising candidates for next-generation light-emitting diodes due to their excellent color purity and high radiative efficiency. However, their performance has been limited by the presence of long-chain native ligands on the nanocrystal surface. These ligands are weakly bound and electrically insulating, which hinders charge injection and transport, introduces trap states, and ultimately leads to energy losses in devices. To address these challenges, the researchers introduced a multifunctional π-conjugated pyridine carboxamide (PCA) ligand via a hydrolysis-assisted, one-step ligand-exchange process. This approach removes the original insulating ligands under mild conditions and replaces them with PCA, which provides multidentate, multisite surface coordination. The ligand acts as a strong anchoring group while simultaneously enabling enhanced electronic coupling between nanocrystals and inducing n-type surface functionalization.
This design leads to several improvements. First, the stronger binding and chelating nature of PCA enhances colloidal stability while preventing structural degradation typically associated with ligand exchange. Second, the π-conjugated structure improves electrical coupling, facilitating charge transport across the nanocrystal film. Third, the induced n-type surface character increases electron transport and reduces trap densities, as confirmed by surface analysis, DFT calculations, and electrical measurements including single-carrier devices and UPS.
A key outcome of this surface engineering is precise control over charge balance in the LED. By enhancing electron transport and passivating surface traps, the recombination zone - where electrons and holes meet to emit light - shifts toward the center of the emissive layer. This reduces interfacial quenching losses and increases the probability of radiative recombination.
As a result, LEDs based on the ligand-exchanged nanocrystals achieved a peak luminance of approximately 25,000 cd m−2, a current efficiency of 121.4 cd A−1, and an EQE of 31.7%, placing them among the highest-performing perovskite nanocrystal LEDs reported to date. The performance improvement directly addresses the longstanding issue of charge imbalance in these systems.
“This study is an example of directly controlling charge balance, which determines the core performance of LED devices, by precisely designing the electronic properties of the nanocrystal surface rather than relying on external structures,” stated Hanyang University's Professor Han Tae-hee.
Beyond efficiency gains, the work demonstrates that surface ligand design can be used not only for defect passivation but also as a tool to actively tune charge transport and recombination dynamics. The authors suggest that this strategy provides a general framework for engineering nanocrystal emitters and could be extended to other perovskite compositions and optoelectronic devices.
The results highlight the importance of molecular-level surface control in unlocking high-performance, charge-balanced LEDs, with potential applications in next-generation displays such as AR/VR, wearable devices, and foldable electronics.
