Solution-processed light-emitting diodes (LEDs) are emerging as a promising technology for advanced solid-state lighting (SSL) and display industries, owing to their energy-efficient, low-cost, and straightforward manufacturing processes1-3. In this realm, lead-halide based perovskites4, organic semiconductors5, and colloidal core–shell quantum dots (QDs)6 have gained recognition as promising candidates. They have been utilized as emitting layers via solution processing and achieved impressive performance in green and lower energy LEDs. However, developing efficient blue, and particularly deep-blue, LEDs that align with the Rec. 2020 primary blue for full-color displays poses a significant challenge for these materials7. Conventional blue organic LEDs (OLEDs) and perovskite LEDs (PeLEDs) demonstrate compromised structural and spectral stability under elevated operational biases8,9. Additionally, lead-based perovskites10 and Cd-based quantum dots11 are subject to concerns over environmental issues due to their toxicity, while the deployment of low-toxic phosphorescence organic emitters12, InP- and ZnSe-based QDs13 is constrained by their high cost in complex design and synthetic process. These challenges limit the practical application of these materials and raise difficulties to cover the full color gamut. Consequently, developing cost-effective, eco-friendly, and efficient emitters for solution-processed deep-blue LEDs is vital for the advancement of comprehensive SSL and display technologies.
Copper halide-based emitters have recently attracted significant attention due to their earth-abundant nature, low toxicity, air/moisture stability, optical tunability, and high luminescent efficiency14-17. Efforts to employ these materials as the emissive layer (EML) in LEDs, encompassing ligand design for improved solution processability18-22 and optimization of host-dopant structures23, have resulted in notable advancements in device efficiency and stability. Specifically, in the domain of deep-blue LEDs, 0D-Cs 3 Cu 2 I 5 has attracted substantial research interest due to its optimal emission wavelength (445 nm), high photoluminescence quantum yield (PLQY) of 87-95%, and high heat/moisture resistance24. Nevertheless, the maximum external quantum efficiency (EQE max ) of 0D-Cs 3 Cu 2 I 5 -based blue LEDs remains very low, 1.02%25, a constraint primarily attributed to inefficient carrier injection stemming from its molecular (cluster) structure as a common drawback among cluster-based copper-iodide hybrids. Beyond the host-guest approach, overcoming this limitation could be achieved by developing/designing new emitters with higher dimensionality on the material side and advanced interfacial engineering at the device level.
Herein, we report a new one-dimensional (1D) hybrid copper iodide, synthesized using a bifunctional ionic ligand (Hdabco)I (Hdabco = 1,4-diazabicyclo-[2.2.2]octane-1-ium). The compound is composed of 1D-Cu 4 I 8 4- anionic chains coordinated to cationic ligand Hdabco+ with a chemical formular 1D-Cu 4 I 8 (Hdabco) 4 [also referred to as CuI(Hda)]. It exhibits strong photoluminescence (PL) in the deep-blue region (449 nm) with nearly unity (~100%) PLQY. We fabricated high-quality polycrystalline thin films of CuI(Hda) via a solution-process, which exhibits a highly preferred orientation and a PLQY identical to that of single-crystal samples. Temperature-dependent time-resolved emission study, alongside femtosecond/nanosecond transient absorption (TA) spectroscopic analysis, suggests a complex emission mechanism involving fluorescence, thermally activated delayed fluorescence (TADF), and phosphorescence, consistent with the results from density functional theory (DFT) calculations. We further investigated the electrical properties and electronic structure of CuI(Hda), including its anisotropic charge carrier dynamics and band structure using time-resolved microwave conductivity (TRMC) techniques and photoelectron spectroscopy, respectively.
We then fabricated high-quality deep-blue LEDs using the CuI(Hda) thin film as the sole active EML based on a dual interfacial hydrogen-bond passivation (DIHP) strategy. Our approach adopted a functionalized carbazole-phosphonic acid-based self-assembled monolayer (SAM)26,27 and an ultrathin PMMA capping layer28 as H-bond donor on the HTL/EML and EML/ETL (HTL: hole-transport layer; ETL: electron-transport layer) interfaces, respectively. The coverage/thickness and interfacial interactions were analyzed by angular-resolved X-ray photoelectron spectroscopy (ARXPS). We reasoned that the observed enhancement in device performance is attributable to defect passivation and optimized charge injection, facilitated by the H-bond coupled heterojunctions. The DIHP-CuI(Hda) deep-blue LED devices outperform the pristine and other control devices with non-H-bonded SAMs, achieving a record-high EQE max of 12.57% among metal halide-based blue LEDs and a high maximum luminance (L max ) of 3970.30 cd/m², with color coordinates (0.147, 0.091), closely approaching the Rec. 2020 standard. Furthermore, these devices exhibit exceptional operational stability, with a half-lifetime (T 50 ) of 204 hours under ambient conditions. We subsequently demonstrated a large-area DIHP-CuI(Hda) device (2 cm × 2 cm) that maintains a high EQE max of 7.87%, illustrating the reliability of the solution processability of hybrid copper iodide and our surface modification technique.
Design, thin film fabrication, and characterization of CuI(Hda)
We designed a mono-protonated aliphatic ligand, (Hdabco)I, from dabco, with an active monodentate coordination site and a high LUMO level, targeting the solution-processibility16 and an optimum band gap of the resultant hybrid material for deep-blue emission. High-quality single crystals of CuI(Hda) (Fig. 1a) were grown from a precursor solution using a facile recrystallization method. The precise crystal structure of 1D-Cu 4 I 8 (Hdabco) 4 was determined using the single crystal X-ray diffraction (SCXRD) technique (Fig. 1a, Supplementary Figs. S1 and S2 and Table S1). The refined crystal structure reveals that the compound comprises of anionic 1D-(Cu 4 I 8 )4- inorganic motif and organic cationic ligand (Hdabco)+, connected through both coordinate and ionic bonds. Each Cu(I) atom is coordinated to three iodine atoms and one nitrogen atom from (Hdabco)+, forming a distorted tetrahedron of (CuI 3 N). Each tetrahedron shares edges and corners with its adjacent tetrahedra, resulting in an infinite 1D chain. The organic cations form a H-bonded dimer, (Hdabco) 2 2+, as a result of the unique structure of (Hdabco)+, where one nitrogen atom is protonated and the other serves as a free binding site, enabling the formation of both the Cu-N coordination bond and intermolecular H-bond (Fig. 1a).
We performed first-principles DFT calculations using the Vienna ab initio Simulation Package (VASP) to understand the electronic structure of CuI(Hda). The result shows that it is a direct bandgap semiconductor (Fig. 1b). The band structure shows that the valence band (VB) includes a series of flat bands with relatively small dispersion, while the conduction band (CB) edge is noticeably dispersive, indicating a smaller electron effective mass and more favorable electron transport properties. To correctly estimated the DFT band gap, we used the screened hybrid functional of Heyd, Scuseria and Ernzerhof (HSE). The HSE band gap of CuI(Hda) was calculated to be 3.8 eV. The projected density of states (PDOS) analysis (Fig 1b) indicates that the atomic contributions to its valence band maximum (VBM) are primarily from the inorganic components, specifically Cu 3d and I 5p atomic orbitals. On the other hand, the conduction band minimum (CBM) is populated by atomic orbitals from both the inorganic motif and the organic ligand, specifically Cu 4s, I 5s, as well as C and N 2p atomic orbitals. This differs significantly from previously reported hybrid copper-halides4,15,19,29,30 or all-inorganic31,32 copper halides. The electron transfer process in CuI(Hda) is a combination of (metal/halide)-to-ligand charge transfer [(M/X)LCT] and all-inorganic-based charge transfer typically found in hybrid perovskites (VBM: Pb 6s and I 5p; CBM: Pb 6p and I 5s). This results in a distinct and intricate emission mechanism for CuI(Hda).
We fabricated CuI(Hda)thin films via single-step spin-coating of the precursor solution on ITO and larger area quartz substrates (Fig. 1c), followed by antisolvent dripping to initiate the crystallization and annealing at room temperature for a relatively long time. Pin-hole free thin-film samples were obtained with a thickness of 90 nm and R a of 0.177 nm, as measured by atomic force microscopy (AFM, Fig. 1c and Supplementary Fig. S3). Top-view scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) images (Supplementary Figs. S4-S8) demonstrate the good compactness, surface smoothness, and even elemental distribution of the as-fabricated CuI(Hda)thin films. We assessed the crystallinity and phase purity of the as-made thin films by comparing the powder X-ray diffraction (PXRD) patterns of a CuI(Hda) thin film sample and a sample of CuI(Hda) powders scratched from multiple thin films with the simulated pattern from single crystal data (Fig. 1d). The thin film sample exhibits only one prominent diffraction peak at 2θ = 12.9°, which corresponds to the (200) plane of the structure, while the pattern of the scratched powders matches well with the simulated one, confirming the formation of phase pure CuI(Hda). The GIWAXS patterns reveal the polycrystalline nature of the CuI(Hda) thin film and confirm the preferred orientation of (200) plane (perpendicular to the 1D-(Cu 4 I 8 )4- chain) in out-of-plane scattering profile (Fig. 1e and Supplementary Figs. S9 and S10). The combination of low surface roughness, high crystallinity, and an identical PLQY (99.4%, Supplementary Fig. S11) to that of single-crystal samples affirms the high-quality of the thin film samples, which is attributed to the excellent solution processability of this type of materials.
Photophysical properties of CuI(Hda)
Steady-state optical absorption and diffuse reflectance spectra of polycrystalline thin film (90 nm) and powder samples of CuI(Hda) were collected and analyzed using UV-vis spectroscopy at room temperature. The nearly identical absorption edges of the two types of samples suggest the direct band gap nature of this compound (Fig. 2a and inset). The estimated optical band gap is ~3.7 eV, aligned well with the calculated HSE band gap. Room temperature PL of the thin film sample reveals a strong single-band emission peak centered at 449 nm with a full width at half maximum (FWHM) of 97 nm and color coordinates (0.147, 0.091) (Fig. 2a). CuI(Hda) features a large Stokes shift and relatively broad emission band, similar to those observed in some pure inorganic copper halides and most hybrid copper halides. Both the single crystals and thin film samples of CuI(Hda)exhibit near-unity PLQY values of 99.4 ± 0.4% when excited at 285 nm, marking the highest value for blue emitters33.
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