Efficient and bright broadband electroluminescence based on environment-friendly metal halide nanoclusters

Electroluminescent devices with ultrabroad emission are promising for next-generation healthy lighting because they can offer high CRI, high LER, and appropriate CCT without relying on down-conversion phosphors that often contain rare-earth or toxic metals and exhibit spectral instability. Conventional approaches using blue/UV LEDs with phosphors suffer from spectral shifts due to differential degradation and may include harmful blue-violet components. Environment-friendly, spectra-stable ultrabroadband electroluminescent devices are therefore highly desirable. Several perovskite-related materials (e.g., double perovskites such as Cs2AgInCl6, heterophase α/δ-CsPbI3, and derivants like CsCu2I3) can yield broadband emissions via self-trapped excitons, but device performance has lagged due to low carrier mobility and wide bandgaps, leaving efficient, spectra-stable ultrabroadband LEDs an unsolved challenge. Ligand-stabilized nanoclusters, particularly CuI-based nanoclusters, offer tunable spectra, rigidity, and photostability and thus are attractive for ultrabroadband LEDs. Prior CuI nanocluster LEDs achieved very broad EL (FWHM up to 180 nm) but low EQE (~0.73%), whereas higher efficiencies have been limited to narrower FWHM (<90 nm). Moreover, fabrication has been constrained by thermal/solution processability due to nanocluster stability and solubility issues. Here, the authors introduce a one-step solution synthesis-deposition strategy using tailored ligands and solvent selection to form bright, uniform CuI nanocluster films with excellent thermal and atmospheric stability. Resulting LEDs show nearly identical performance in nitrogen and air without encapsulation, with FWHM ~120 nm, peak EQE 13%, maximum luminance ~50,000 cd m−2, and T50 of 137 h at 100 cd m−2, advancing environment-friendly ultrabroadband lighting.

• Materials design: Selected 3,5-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (35DCzPPy) as a bipolar ligand/host with pyridine anchoring groups to coordinate CuI, expecting a dopant–host emissive film with in-situ formed CuI nanoclusters as dopants and excess ligands as host. Compared to mCPy, 35DCzPPy includes an extra phenyl to enhance conjugation and conductivity.
• Solvent engineering: Dissolved 35DCzPPy in chlorobenzene and CuI in acetonitrile after screening for solubility and volatility. Chlorobenzene dissolves ligand but not CuI; acetonitrile dissolves CuI but not ligand. Mixing avoids premature cluster formation, unlike DMSO/DMF (which dissolve both and cause immediate precipitation). Highly volatile solvents (THF, DCM) hinder film control. Optimized volume ratio CuI(acetonitrile):35DCzPPy(chlorobenzene) = 1:4; optimized ligand:CuI molar ratio = 2.7:1.
• One-step synthesis–deposition: Mixed the two solutions to form a precursor, then spin-coated onto substrates. Bright films formed without thermal annealing. In-situ PL monitoring during spin coating (excitation 310 nm; spectrometer with 0.1 s time resolution) tracked formation: first ~4.0 s thinning with PL decrease (no cluster emission), onset of nanocluster emission at ~4.6 s, and full conversion by ~7.2 s as precursor emission vanished and cluster emission saturated. Aggregate-induced reaction triggered by rapid solvent evaporation and supersaturation.
• Comparative ligands: Tested 26DCzPPy (pyridine), TmPyPB (unipolar), and DBFDP (diphosphine). 26DCzPPy’s steric hindrance weakened coordination and yielded dim films; TmPyPB acted as an emission quencher due to unipolar transport; DBFDP did not suppress triplet cluster-centered emission effectively, limiting PL QY.
• Film formation and scale-up: Achieved smooth, compact films via spin coating; also blade-coated large-area (10 × 10 cm2) films in air with bright uniform emission.
• Characterization: • Morphology: AFM (RMS roughness 0.27 nm); SEM showed dense, pinhole-free films. • Composition homogeneity: TOF-SIMS depth profiles of N, I, Cu uniform; depth-dependent XPS showed constant atomic percentages across thickness. • Structure and electronic properties: Proposed [35DCzPPy]4Cu2I2 cluster (Cu2I2 core chelated by two ligands per Cu) based on prior analogs and DFT; verified by EXAFS and XPS core-level spectra indicating Cu–pyridine coordination. UV–vis absorption of nanocluster films showed ligand-dominated absorption with MLCT tails (350–400 nm). HOMO/LUMO measured by UPS at −5.8/−2.3 eV. • Excited-state analysis: TDDFT for S1 and T1 states showed minimal Cu2I2 geometry change (high excited-state rigidity). FMO/NTO analyses indicated (M+X)LCT character with HOMO on Cu2I2 core and LUMO on pyridine/ligand. Temperature-dependent time-resolved PL from 80–300 K: lifetimes fitted to extract ΔEST and separate PH vs TADF contributions, demonstrating dual emission.
• Device context: Films intended as emissive layers for LEDs; 35DCzPPy provides bipolar transport to confine excitons and aid charge balance. Devices tested in nitrogen and air without encapsulation; performance and stability metrics reported.

• Achieved efficient, ultra-broadband CuI nanocluster LEDs with emission FWHM ~120 nm, peak EQE 13%, maximum luminance ~50,000 cd m−2, and operating half-lifetime (T50) 137 h at 100 cd m−2; performance nearly identical in nitrogen and air without encapsulation.
• In-situ formed CuI nanocluster films (using 35DCzPPy host/ligand) exhibit broadband PL centered ~550 nm with FWHM 123 nm and film PL QY up to 60%.
• Optimized processing: CuI(acetonitrile):35DCzPPy(chlorobenzene) volume ratio 1:4; ligand:CuI molar ratio 2.7:1.
• Formation kinetics: During spin coating, cluster emission appears at ~4.6 s and saturates by ~7.2 s; reaction initiated by solvent-evaporation-induced supersaturation (aggregate-induced reaction).
• Film quality and uniformity: AFM RMS roughness 0.27 nm; SEM shows dense, pinhole-free films; TOF-SIMS and depth XPS confirm uniform distribution of N, I, Cu across the film; large-area 10 × 10 cm2 blade-coated films show uniform brightness.
• Nanocluster structure and energetics: Proposed [35DCzPPy]4Cu2I2 cluster verified by EXAFS/XPS; absorption dominated by ligand with MLCT tail (350–400 nm); UPS-derived HOMO −5.8 eV, LUMO −2.3 eV.
• Excited-state photophysics: Temperature-dependent TRPL shows lifetime decreasing from 25.7 μs at 80 K to 7.9 μs at 300 K; fitted singlet–triplet energy gap ΔEST ≈ 0.048 eV enabling efficient TADF. Dual-mode emission at room temperature with PH ≈ 32% and TADF ≈ 68%.
• Ligand effects: 35DCzPPy yields brightest films and best PL QY; 26DCzPPy suffers from steric hindrance; TmPyPB quenches emission due to unipolarity; DBFDP insufficiently suppresses triplet cluster-centered emission.

The study addresses the challenge of creating efficient, spectra-stable ultrabroadband electroluminescent devices using environmentally friendly emitters by leveraging ligand-stabilized CuI nanoclusters formed in-situ during film deposition. By tailoring the ligand (35DCzPPy) and solvent system to trigger cluster formation only during rapid solvent evaporation, the authors overcome solubility and thermal stability bottlenecks that previously impeded processability and uniform film formation. The resulting dopant–host films exhibit high photoluminescent efficiency, exceptional smoothness, and compositional uniformity, which translate into LEDs with high EQE, very high luminance, and robust operation in air without encapsulation. Excited-state analyses reveal a small ΔEST (~0.048 eV) and dual PH/TADF emission (approximately 32%/68% at room temperature), which helps harvest both singlet and triplet excitons and mitigate exciton accumulation and quenching. This dual-emissive character, together with the rigid Cu2I2 core (suppressing non-radiative relaxation) and bipolar host environment, accounts for the observed efficiency and stability. Compared with earlier CuI nanocluster LEDs that offered either very broad spectra but low efficiency or high efficiency with narrower bandwidth, this work achieves a balanced ultrabroadband emission (FWHM ~120 nm) with high EQE (13%) and strong stability, demonstrating the viability of Cu halide nanoclusters for healthy lighting applications.

This work introduces a one-step solution synthesis–deposition approach to fabricate uniform, bright CuI nanocluster films serving as efficient ultrabroadband emitters in LEDs. Through ligand/solvent co-design, the authors realize in-situ formation of [35DCzPPy]4Cu2I2 nanoclusters within a bipolar host, yielding devices with FWHM ~120 nm, EQE up to 13%, maximum luminance ~50,000 cd m−2, and T50 of 137 h at 100 cd m−2, operating similarly in air and nitrogen without encapsulation. Photophysical analyses identify a small singlet–triplet gap (≈0.048 eV) and dual PH/TADF emission that, combined with excited-state rigidity and high-quality films, underpin the efficiency and stability. These findings establish copper halide nanoclusters as promising, environment-friendly emitters for next-generation healthy lighting. Future work could optimize ligand architecture to further tune bandwidth and efficiency, quantify lighting metrics such as CRI and LER, integrate into large-area device architectures, and explore device encapsulation and long-term operational stability under practical conditions.