The research focuses on developing environment-friendly, ultra-broadband electroluminescent devices for healthy lighting. Current solid-state lighting technologies, relying on photoluminescence down-conversion, suffer from spectral instability, the use of rare-earth or toxic metals, and harmful blue-violet emissions. While some double perovskites and perovskite derivants exhibit broadband emissions, their electroluminescent performance is inadequate for lighting applications due to low carrier mobility and wide bandgaps. Ligand-stabilized nanoclusters offer a promising alternative due to their tunable luminescence spectra, rigidity, and photostability. Previous CuI nanocluster-based LEDs demonstrated broad EL spectra, but with inferior external quantum efficiency. This research aimed to overcome these limitations by developing a one-step solution-deposition method for fabricating high-performance ultra-broadband LEDs using CuI nanoclusters. The key innovation lies in the dedicated design of ligands and selection of solvents to ensure efficient nanocluster formation during film deposition, avoiding separate synthesis and purification steps.

The paper reviews existing technologies for ultra-broadband electroluminescence, highlighting the limitations of traditional solid-state lighting sources which utilize photoluminescence down-conversion techniques. These techniques often involve rare-earth or toxic metals and suffer from spectral instability. The authors discuss the shortcomings of alternative materials such as double perovskites and perovskite derivants, which, while exhibiting broadband emission, lack sufficient electroluminescent performance. The potential of ligand-stabilized nanoclusters is presented as a superior alternative, citing previous work on CuI nanocluster-based LEDs. However, the previous research highlighted the challenges associated with low external quantum efficiency and complex fabrication processes.

The researchers developed a novel one-step solution-deposition method to synthesize and integrate CuI nanoclusters directly into the LED. The process involves mixing solutions of CuI and a specifically designed ligand, 3,5-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (35DCzPPy), in chlorobenzene and acetonitrile, respectively. The choice of these solvents is crucial; chlorobenzene dissolves 35DCzPPy while acetonitrile dissolves CuI, and their combination avoids premature nanocluster formation in the solution. The mixture is spin-coated onto substrates, and the nanoclusters form in situ during the solvent evaporation process. The 35DCzPPy ligand is designed to act as both a chelating agent for the CuI and as a host material for the resulting nanoclusters, thus improving the film conductivity. The formation process was monitored using in-situ PL spectroscopy, revealing the rapid conversion of the precursor mixture into nanoclusters. The morphology of the resulting film was characterized using AFM and SEM, demonstrating its smoothness and uniformity. The composition and homogeneity of the film were further verified using TOF-SIMS and XPS depth profiling. The molecular structure of the [35DCzPPy]4Cu2I2 nanocluster was determined using DFT simulations and EXAFS. Photophysical properties were investigated using steady-state and time-resolved PL spectroscopy, temperature-dependent PL measurements, and TDDFT simulations to understand the dual-emissive mechanism.

The one-step solution-deposition process successfully produced highly uniform and smooth CuI nanocluster films. The nanoclusters exhibited broadband emission with an FWHM of ~123 nm and a high PL quantum yield of 60%. Time-resolved PL studies revealed dual-emissive modes: phosphorescence (32%) and temperature-activated delayed fluorescence (TADF, 68%) at room temperature. The low singlet-triplet splitting energy (ΔEST = 0.048 eV) facilitated efficient TADF. The resulting LEDs demonstrated exceptional performance: an FWHM of ~120 nm, a peak external quantum efficiency (EQE) of 13%, a maximum luminance of nearly 50,000 cd m⁻², and an operating half-life of 137 h at 100 cd m⁻². Notably, the LEDs showed consistent performance in both nitrogen and air atmospheres without encapsulation.

The high performance of the LEDs is attributed to the unique properties of the CuI nanoclusters and the optimized fabrication method. The dual-emissive mechanism, combining PH and TADF, efficiently harvests both singlet and triplet excitons, leading to high EQE. The high rigidity of the nanoclusters in the excited state minimizes non-radiative energy losses, enhancing the luminescence efficiency. The in-situ formation of nanoclusters in the film ensures uniform distribution and prevents aggregation, contributing to the high luminance and long operational lifetime. The insensitivity to the atmosphere further underscores the robustness of the device. These results significantly advance the field of broadband electroluminescence and pave the way for the development of next-generation healthy lighting technologies.

This study demonstrates a simple, efficient, and scalable method for fabricating high-performance ultra-broadband LEDs based on environment-friendly CuI nanoclusters. The optimized ligand design, solvent selection, and in-situ synthesis-deposition process resulted in devices with exceptional performance metrics. This work highlights the significant potential of CuI nanoclusters as emitters for next-generation lighting applications. Future research could explore different ligands and nanocluster compositions to further enhance the performance and expand the range of emission colors.

While the LEDs demonstrated impressive performance, potential limitations include the long-term stability under high-luminance operation and the scalability of the blade coating technique for mass production. Further investigation is needed to assess the long-term stability and to develop more scalable and cost-effective fabrication methods.