Perovskite halides are promising semiconductor materials for light-emitting diodes (PeLEDs) due to their tunable bandgaps, high charge carrier mobility, and long carrier diffusion length. However, blue perovskite emitters lag behind green and red counterparts in photoluminescence quantum yield (PLQY) and the external quantum efficiency (EQE) of their corresponding LEDs. Efforts to improve blue PeLEDs have focused on 3D, 2D, and quasi-2D perovskite films with mixed-Cl/Br halides, achieving EQEs up to 13.8%. Perovskite quantum dots (QDs), with their high PLQY, strong quantum confinement effect, and high monochromaticity, offer another potential pathway. Early QD-based PeLEDs showed low EQEs (0.07%). Various approaches, including ion doping (Mn²⁺, Sn²⁺, Cd²⁺, Zn²⁺, Cu²⁺, and trivalent lanthanide metal ions) and multiple-cation doping, have improved EQE to around 4.7%. Organic cation doping is another strategy offering improved thermal, moisture, and chemical stability. Formamidinium (FA) cations, used in perovskite solar cells and LEDs, can tune the perovskite tolerance factor, improving stability and suppressing ion migration. This research investigates the mechanism of FA cation-doped blue QDs and aims to fabricate high-efficiency pure blue QD LEDs via a room-temperature synthesis method suitable for large-scale production.

The development of efficient and stable blue perovskite quantum dot light-emitting diodes (PeLEDs) has been a significant challenge. While green and red perovskite LEDs have achieved high external quantum efficiencies (EQEs) exceeding 20%, blue PeLEDs have lagged behind, primarily due to the higher defect density and lower photoluminescence quantum yield (PLQY) of blue-emitting perovskites. Previous research has explored various strategies to enhance the performance of blue PeLEDs, including the use of 3D, 2D, and quasi-2D perovskite films with mixed halide compositions (Cl/Br). These approaches have yielded some improvements, with reported EQEs reaching up to 13.8%. However, perovskite quantum dots (QDs) offer unique advantages like high PLQY and size-tunable emission, making them attractive candidates for blue PeLEDs. Early attempts using perovskite QDs resulted in very low EQEs, but subsequent research incorporating techniques like ion doping and surface passivation has led to incremental increases in device performance. While inorganic cation doping has shown promise, the use of organic cations such as formamidinium (FA) offers the potential for improved stability and efficiency.

The researchers synthesized high-performance pure blue perovskite quantum dots (QDs) at room temperature using an organic cation composition modification strategy. Formamidine acetate (FAAc) was added as a precursor to improve the quality of the QDs, reduce defect density, and enhance the interaction between organic cations and the Pb-Br octahedron frameworks. The microstructure of the synthesized QDs was characterized using transmission electron microscopy (TEM), revealing cubic QDs with a size of ~11 nm. X-ray diffraction (XRD) confirmed the cubic CsPb(Br/Cl)3 phase, and the shift of diffraction peaks to lower angles with increasing FA cation concentration indicated lattice expansion due to the substitution of smaller Cs⁺ ions by larger FA⁺ ions. The optical properties of the QDs were investigated using UV-Vis absorption spectroscopy and photoluminescence (PL) spectroscopy. The PLQY increased significantly with FA doping, reaching 65% for the optimized sample, a six-fold improvement over the undoped QDs. Time-resolved PL (TRPL) measurements showed a significant increase in the fluorescence lifetime with FA doping, suggesting a reduction in nonradiative recombination pathways. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed the incorporation of FA⁺ ions into the QD lattice. Femtosecond transient absorption spectroscopy (TAS) was employed to study the carrier dynamics, revealing faster hot charge carrier relaxation and reduced nonradiative recombination in the FA-doped QDs. Density functional theory (DFT) calculations were performed to investigate the impact of FA cation doping on the band structure. Finally, pure blue perovskite QD LEDs were fabricated and characterized to evaluate their performance, including luminance and EQE.

The study successfully demonstrated a significant enhancement in the performance of pure blue perovskite quantum dot light-emitting diodes (PeLEDs) through formamidinium (FA) cation doping. The key findings include: 1. **High PLQY:** FA-CsPb(Cl₀.₅Br₀.₅)₃ QDs exhibited a remarkably high photoluminescence quantum yield (PLQY) of 65%, which is six times higher than that of the undoped samples. This substantial increase in PLQY is attributed to the reduction in defect density and the enhancement of radiative recombination. 2. **Improved LED Performance:** Pure blue PeLEDs fabricated using the FA-doped QDs achieved a maximum luminance of 1452 cd m⁻² and an external quantum efficiency (EQE) of 5.01% at an emission wavelength of 474 nm. This represents a substantial improvement over previously reported blue perovskite QD LEDs. 3. **Structural Characterization:** Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses confirmed the successful incorporation of FA cations into the perovskite lattice, leading to lattice expansion and improved crystal quality. 4. **Carrier Dynamics:** Time-resolved photoluminescence (TRPL) spectroscopy and femtosecond transient absorption spectroscopy (TAS) studies revealed that FA doping led to a longer carrier lifetime and faster hot carrier relaxation, further contributing to the enhanced PLQY and EQE. 5. **DFT Calculations:** Density functional theory (DFT) calculations provided insights into the changes in the electronic band structure caused by FA doping, explaining the improved carrier injection and radiative recombination efficiency. 6. **Room Temperature Synthesis:** The synthesis method employed in this study was performed at room temperature, making it a scalable and cost-effective approach for large-scale production of high-performance blue perovskite QDs.

The results of this study demonstrate that FA cation doping is a highly effective strategy for enhancing the performance of pure blue perovskite quantum dot light-emitting diodes (PeLEDs). The significant increase in PLQY and EQE achieved in this work surpasses many previous reports on blue perovskite LEDs. The observed improvements are attributed to a combination of factors, including the reduction in defect density, the enhancement of radiative recombination, the faster hot carrier relaxation, and the optimized band structure. The room-temperature synthesis method employed is particularly noteworthy, as it addresses the scalability challenge often associated with the production of high-quality perovskite QDs. The findings contribute significantly to the development of high-efficiency blue LEDs for applications in displays and solid-state lighting. The mechanistic insights gained from this study offer valuable guidance for future research in perovskite materials and device engineering.

This study successfully demonstrated a facile room-temperature synthesis approach for high-efficiency pure blue perovskite quantum dot light-emitting diodes (PeLEDs) using FA cation doping. The resulting devices exhibited significantly enhanced performance compared to their undoped counterparts, achieving a high PLQY (65%) and an EQE of 5.01%. This improved performance is attributed to reduced defect density, faster hot carrier relaxation, and an optimized band structure. The room-temperature synthesis strategy holds promise for scalable production of high-performance blue PeLEDs, paving the way for broader applications in display and lighting technologies. Future research could explore further optimization of the doping strategy and device architecture to achieve even higher efficiencies and longer operational lifetimes.

While this study demonstrates a significant improvement in blue perovskite QD LED performance, there are some limitations to consider. The achieved EQE of 5.01%, although promising, is still lower than the EQEs reported for green and red perovskite LEDs. Further research is needed to address this gap. The long-term stability of the devices under continuous operation also needs to be investigated more thoroughly to assess their practical viability. The study primarily focused on the impact of FA doping on the optical and electronic properties of the QDs. A more comprehensive investigation of the underlying mechanisms, potentially including detailed surface analysis and more advanced characterization techniques, could provide deeper insights into the observed improvements. Finally, while the room-temperature synthesis is advantageous, scaling up to industrial-level production requires further optimization and process control.