Amino Acid-Passivated Pure Red CsPbI₃ Quantum Dot LEDs

CsPbI₃ quantum dots (QDs) are promising light-emitting materials for pure-red display applications. Several methods have been tried for perovskite to obtain the red-light emission in the range 620-650 nm, including mixed-halide perovskite CsPbI_xBr_3-x, quazi-2D perovskite, and quantum-confined CsPbI₃. Among those methods, quantum-confined CsPbI₃ is the most exciting material as it is not subject to ion migration effects or contains organic cations that lead to spectral instability. Unfortunately, small colloidal quantum dots contain excess resistive ligands, negatively affecting the efficiency and stability of the resulting light-emitting diodes (LEDs). Here we developed a facile ligand-exchange method using amino acids to reduce native long-chain ligands on CsPbI₃ quantum dots. As a molecule with amine and carboxylate groups, amino acids can mildly exchange the oleate ligands and oleylammonium ligands on the surface of quantum dots. We also assessed a variety of related amino acids on their side-chain effects on LED performance. We found that in cysteine-exchanged QDs, both thiol and amino acid groups passivated the surface of quantum dots and led to the best LED performance. The optimized LEDs achieved an external quantum efficiency (EQE) of 18.0% and a T₅₀ of 87 minutes, comparable with many of the best-reported perovskite pure red LEDs.