Researchers from Guangxi University and Ningbo Institute of Engineering have made progress in the study of multifunctional ligand stabilized CsPbI3 quantum dots for efficient pure red LEDs

Metal halide perovskites have excellent photophysical properties such as tunable bandgap, high color purity, high photoluminescence quantum yield (PLQY), and cost-effective solution processing capability, making them promising for commercial optoelectronic applications. The progress in morphology control, composition engineering, and device engineering has led to significant advancements in perovskite light-emitting diodes (PeLEDs). Their external quantum efficiencies (EQEs) are now close to organic LEDs (OLEDs) and inorganic quantum dot LEDs (QLEDs). However, pure red PeLEDs with emission wavelengths between 620-650 nm can meet the recommended BT.2020 (Rec.2020), but progress in performance indicators such as efficiency, brightness, and stability is relatively slow. Therefore, there is an urgent need for in-depth and systematic research to overcome the obstacles of reducing the efficiency and stability of pure red light-emitting diodes.

  In order to achieve high-performance pure red PeLEDs, efficient and stable pure red perovskite emitters are essential. For this purpose, researchers have proposed and studied several methods. The mixed halide components (I and Br) can easily achieve pure red emission, but due to the unstable crystal structure of the mixed halide, it is prone to phase separation and defect formation under external electric fields, which seriously deteriorates the performance of pure red PeLEDs. Although low dimensional quasi two-dimensional CsPbI3 perovskite is expected to achieve efficient pure red emission, the widespread presence of multiphase in perovskite films results in low energy transfer efficiency and lower color purity than the standard. On the contrary, strongly constrained CsPbI3 perovskite quantum dots (QDs) are expected to achieve high-performance pure red emission and improve luminescence efficiency by avoiding halide separation and multiphase mixing. However, the phase stability of CsPbI3 quantum dots is poor, and they spontaneously transition from the black photoactive phase to the non emissive yellow phase during purification or storage, greatly limiting their optoelectronic applications. Compared with other perovskite quantum dots, strongly constrained CsPbI3 quantum dots are more prone to phase transition for two reasons. Firstly, their high surface to volume ratio amplifies the impact of surface defects on optical properties. Secondly, surface defects caused by weak binding ligands may lead to lattice distortion, indirectly causing phase transitions. Due to the aforementioned issues, the commercialization of pure red PeLEDs based on CsPbI3 quantum dots faces significant obstacles. It is highly necessary to develop new strategies to further improve the stability of pure red CsPbI3 quantum dots.

 In the traditional synthesis of CsPbI3 quantum dots, a combination of two capping ligands, oleic acid (OA) and oleylamine (OAm), is commonly used. However, the dynamic equilibrium between OA/OAm ligands and the surface of CsPbI3 quantum dots often leads to unstable binding on the quantum dot surface. This instability often leads to ligand detachment from the surface of quantum dots, followed by significant aggregation during the purification process of CsPbI3 quantum dots. Multiple strategies, including ligand treatment and alloying after doping synthesis, have been reported to improve the stability of CsPbI3 quantum dot perovskite phase. Unfortunately, most of them can only extend the shelf life of CsPbI3 quantum dots to 30 days. In previous studies, guanidine (Gu+) was found to be a highly stable small organic cation and an excellent ligand for stabilizing CsPbI3 quantum dots due to its effective resonance stabilization between its three amino groups. Cheng et al. used trioctylphosphine oxide (TOPO) and guanidine iodide (GUAI) as ligands to sequentially purify CsPbI3 quantum dots (~4 nm). This method can effectively maintain the size and perovskite structure of CsPbI3 quantum dots after two washes, and significantly improve the optoelectronic properties of CsPbI3 quantum dot thin films. However, the efficacy of single guanidine ligands (such as GUAI) is limited, and their performance is limited by insufficient protection against environmental moisture and oxygen invasion. Specifically, quantum dots processed by GUAI still face challenges, such as limited long-term stability under environmental conditions and incomplete defect suppression. These issues are mainly due to the relatively weak spatial hindrance of GUAI, which results in insufficient bonding strength to certain defect areas. Given these limitations, there is still an urgent need for a multifunctional ligand that synergistically passivates defects and resists environmental degradation.

 Zhao Jialong and Mo Xiaoming from Guangxi University, along with Zheng Jinju from Ningbo University of Engineering, demonstrated a simple and effective method for synthesizing highly stable red CsPbI3 quantum dots by introducing the multifunctional molecule PhenHCl as an additive ligand. When the biguanide functional group in PhenHCl forms multiple hydrogen bonds with lead halide octahedra, excess Cl - anions compensate for iodine vacancies and eliminate trap states in CsPbI3 quantum dots. The synergistic effect of biguanide functional groups and halogens significantly compensated for the surface defects of red CsPbI3 quantum dots, resulting in a photoluminescence (PL) quantum yield of 98.6% and good environmental stability, maintaining 90% PL intensity within 80 days. The results showed that the electroluminescence performance of pure red PeLEDs based on PhenHCl treated CsPbI3 quantum dots was significantly improved around 649 nm, with an external quantum efficiency of 13.38% and a maximum brightness of 2159 cd m-2. The findings in this work provide a way to improve the performance of pure red plasmonic light-emitting diodes by adjusting the surface chemistry of CsPbI3 quantum dots.