Efficient doped and non-doped light-emitting diodes based on a TADF-emitting Cu₄Br₄ cluster

Abstract

Copper-halide clusters emerge as a promising category of photoluminescent materials, attributed to their rich structural and photophysical properties in solid states. However, their environment-dependent emission quenching and challenges in film formation pose obstacles to their application in electroluminescent (EL) devices. Herein, we present Cu₄Br₄(AcNP)₂, a sublimable copper-halide cluster chelated by a bidentate N^P ligand with a donor–acceptor configuration. The steric and electronic modification effects of the dimethylacridine group not only induce predominant (metal + halide)-to-ligand charge transfer and intra-ligand charge transfer characters in the excited states of the cluster, ensuring efficient and short-lived thermally activated delayed fluorescence, but also effectively prevent concentration quenching in films by avoiding intermolecular π–π stacking. As a consequence, 40 wt% doped and non-doped devices achieve maximum external quantum efficiencies of 12.8% and 10.2%, respectively. We anticipate that the current study offers insights into the design of copper-halide clusters for fabricating highly efficient EL devices, especially for non-doped EL devices.

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Introduction

Organic light-emitting diodes (OLEDs) have been leading innovation in the field of solid-state displays due to their ability to deliver high-quality and flexible products.^1–3 In order to attain high electroluminescence (EL) efficiency, it is imperative to utilize highly efficient emitters capable of utilizing both singlet (25%) and triplet (75%) excitons during device fabrication. Phosphorescence and thermally activated delayed fluorescence (TADF) emitters have emerged as the two most promising candidates for achieving highly efficient OLEDs. Phosphorescent transition-metal complexes with d⁶ and d⁸ electron configurations have the potential to achieve a theoretical internal quantum efficiency of 100% thanks to the robust spin–orbit coupling (SOC) interactions between the triplet state (T₁) and the singlet states (S₁ and S₀).⁴ TADF materials can efficiently harvest triplet excitons for fluorescence through reverse intersystem crossing (RISC) from the T₁ state to the energetically proximate S₁ state.^5,6 Many phosphorescence and TADF emitters typically exhibit relatively long exciton lifetimes, spanning from microseconds to milliseconds, and often suffer from severe aggregation-caused quenching (ACQ) in neat films.^7–9 To mitigate the concentration quenching and exciton annihilation in the emitting layer of OLEDs, these emitters must be employed in co-doping systems with precise control of the doping concentration. In comparison with doped OLEDs, non-doped OLEDs offer advantages, including process simplicity, reduced fabrication costs, and the avoidance of phase separation. The development of emitters for efficient non-doped OLEDs has gained significant attention recently. However, the performances of non-doped OLEDs still fall significantly short of those achieved in doped OLEDs.^10–12

Copper(I) complexes have shown promise as cost-effective EL emitters because of their diverse structural and photophysical properties, along with their capacity to efficiently harness both singlet and triplet excitons for emission. The versatile coordination chemistry of copper(I) ions allows them to create remarkable structural diversity in combination with both organic and inorganic ligands. Among them, copper(I)–halide clusters are well known for their rich photophysical properties. Owing to the low oxidation potentials of copper(I) and halide ions, copper(I)–halide clusters often exhibit (metal + halide)-to-ligand charge transfer (M + X)LCT character in their lowest excited states, endowing them with typical TADF properties.^13–21 Moreover, the heavy atom effect (HAE) exhibited by these clusters can lead to significant SOCs between the lowest triplet and singlet states, thereby facilitating intersystem crossing (ISC) and phosphorescence processes, and the high rigidity of the cluster structure effectively inhibits the photothermal decomposition of Cu(I) and the quenching of the excited state caused by the Jahn–Teller distortion.^22–25 These attributes of copper(I)–halide clusters make them potential high-efficiency EL emitters. Nonetheless, copper(I)–halide clusters often come with significant drawbacks when it comes to the fabrication of efficient EL devices. First, copper(I)–halide clusters frequently exhibit an intricate composition in their excited states, involving both organic ligands and inorganic cluster cores. The presence of non-radiative, low-lying cluster-centered triplet states (³CC) can potentially quench the emission.^26,27 In addition, a substantial number of copper(I)–halide clusters are insoluble in conventional solvents and exhibit instability during sublimation, which makes them unsuitable for fabricating cluster light-emitting diodes (CLEDs) via solution-based processes and vacuum deposition. Despite some progress,^10,28–35 the rational design of copper(I)–halide clusters for highly efficient CLEDs and the comprehension of structure–property relationships continue to pose challenges.

In this work, we report a new copper(I)–bromine cluster, Cu₄Br₄(AcNP)₂, supported by a bidentate N^P ligand featuring an electron donor (D)–acceptor (A) configuration. Cu₄Br₄(AcNP)₂ emits efficient EL in films benefiting from the nearly degenerate S₁ and T₁ states, both originating from (M + X)LCT and intra-ligand charge transfer (ILCT) transitions. Moreover, the highly twisted and bulky D–A configuration effectively prevents concentration quenching in films by effectively separating the emission centers of adjacent molecules and preventing intermolecular π–π stacking.^36–38 Cu₄Br₄(AcNP)₂ in 40 wt%-doped and neat films show PLQYs of 47% and 38% and decay times of 2.7 and 1.9 μs, respectively. The doped OLEDs employing Cu₄Br₄(AcNP)₂ as the dopant achieved peak EQEs reaching 12.8% with a small efficiency roll-off. Remarkably, the non-doped OLED utilizing Cu₄Br₄(AcNP)₂ maintains high performance with a maximum EQE of 10.2%, which is excellent among those attained by non-doped OLEDs based on Cu(I) complexes.

Results and discussion

Synthesis and characterization

The D–A type bidentate N^P ligand (AcNP) was synthesized in three steps, resulting in an overall yield of 38% (refer to the ESI† for detailed synthesis procedures). The copper(I)–bromine cluster, Cu₄Br₄(AcNP)₂, was prepared from the reaction of one equivalent of the AcNP ligand and two equivalents of CuBr in dichloromethane (as illustrated in Fig. 1a). After purification through recrystallization, accomplished by the slow diffusion of ether into its dichloromethane solution, the cluster Cu₄Br₄(AcNP)₂ was obtained as yellow single crystals with a moderate yield of 55%. The obtained cluster was further purified by vacuum sublimation before conducting characterization studies and device fabrications. The molecular structure of Cu₄Br₄(AcNP)₂ was determined with ¹H-NMR,¹³C-NMR, and single-crystal X-ray diffraction analysis. The CCDC number of Cu₄Br₄(AcNP)₂ is 2323855.†

Fig. 1 (a) Synthetic route of Cu₄Br₄(AcNP)₂; (b) single crystal structures of Cu₄Br₄(AcNP)₂; (c) single-crystal packing diagram of Cu₄Br₄(AcNP)₂; the red dotted lines indicate the intermolecular interaction and the red number shows the force distance.

Single-crystal structure analysis

The single crystal information in the X-ray structure analysis and the selected bond lengths and bond angles are listed in Tables S1 and S2,† respectively. As depicted in Fig. 1b, Cu₄Br₄(AcNP)₂ exhibits a C_i symmetric structure in the crystal. The four coordinated copper atoms form a parallelogram structure with an acute angle of 60.66°, and adjacent sides (Cu–Cu distances) of 2.81 and 2.91 Å. In addition, the two copper atoms positioned at the vertices of the obtuse angles of the parallelogram form a Cu–Cu bond at a distance of 2.89 Å. Each set of three copper atoms forming an acute angle is μ³-capped with a bromine atom, respectively. The other two bromine atoms form μ²-bonds at the two long sides of the parallelogram, each featuring a Cu–Br–Cu angle of 74.76°. Two AcNP ligands coordinate with copper atoms on the two shorter sides of the parallelogram in a C₂-symmetric arrangement. The ligand AcNP exhibits a D–A dihedral angle of 88.33° between the acridine unit and the pyridine ring. The highly distorted D–A structure of the ligand is expected to hinder the formation of intermolecular π–π stackings that typically lead to emission quenching, especially at high doping concentrations. As shown in Fig. 1c, only weak intermolecular C–H⋯π interactions (3.37–3.70 Å) were observed, and no π–π interactions were found in the crystal lattice of Cu₄Br₄(AcNP)₂. Moreover, the peripheral non-planar dimethylacridine units effectively separate the emission centers of adjacent molecules (Fig. 1c), thereby inhibiting short-distance Dexter energy transfer (DET), which is primarily responsible for concentration quenching.³⁸

Thermal and electrochemical properties

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to evaluate the thermal stability of the powder sample of Cu₄Br₄(AcNP)₂. As shown in Fig. S1,† the decomposition temperature (Td, 5% weight loss) was detected to be 365 °C, and no obvious signs of glass transition were observed in the temperature range of 50–360 °C. The good thermal stability of Cu₄Br₄(AcNP)₂ could ensure its ability to endure high thermal stress during the thermal evaporation process and Joule heating during the operation of OLEDs. Moreover, to ascertain whether high-temperature evaporation causes decomposition and validate the purity of the evaporated cluster film, we recorded the ¹H-NMR spectrum of a Cu₄Br₄(AcNP)₂ sample deposited through evaporation (Fig. S15†), and compared it with those of the ligand AcNP and a Cu₄Br₄(AcNP)₂ sample without evaporation (Fig. S16†). The results indicate the absence of an uncoordinated ligand or other impurities in the evaporated Cu₄Br₄(AcNP)₂ sample, confirming that thermal evaporation does not cause the decomposition of the cluster.

The electrochemical properties of both the ligand AcNP and the Cu₄Br₄(AcNP)₂ cluster were investigated by cyclic voltammetry (CV). The cyclic voltammograms for the oxidations of AcNP and Cu₄Br₄(AcNP)₂ in dichloromethane are shown in Fig. S2.† Using the E_onset values of the initial oxidative peaks, the HOMO energy levels of AcNP and Cu₄Br₄(AcNP)₂ were calculated to be −5.28 and −5.22 eV, respectively. In conjunction with the optical band gaps, which were determined as 3.17 and 2.86 eV from the UV-Vis absorption onset values (Fig. 3a and Fig. S3†), the LUMO levels of AcNP and Cu₄Br₄(AcNP)₂ were calculated to be −2.11 and −2.36 eV, respectively (Table S6†).

Theoretical calculations

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed at the B3LYP/6-31G* level to investigate the frontier molecular orbitals (FMOs) and excited states of Cu₄Br₄(AcNP)₂. As shown in Fig. 2 and Table S3,† the HOMO of this cluster is primarily situated on the Cu₄Br₄ cluster and extends partially to the arylphosphine unit of the ligand, whereas the LUMO is predominantly distributed over the ligand. The effective separation of FMOs is expected to result in a small singlet–triplet splitting (ΔE_ST), a crucial requirement for facilitating TADF emission.^39,40 Although a D–A type ligand was employed, DFT calculations revealed that the dimethylacridine unit on the ligand does not contribute to the HOMO of Cu₄Br₄(AcNP)₂. This could be attributed to the lower oxidation potential of the Cu₄Br₄ cluster compared to the dimethylacridine unit, which is confirmed from the CV measurement. TD-DFT calculations unveiled that the S₁ and T₁ states of Cu₄Br₄(AcNP)₂ are close in energy (ΔE_ST = 0.15 eV) and both featured with predominant intramolecular charge transfer (ICT) transitions (Table S4† and Fig. 2). According to orbital transition analysis (Table S5†), both the S₁ and T₁ states of Cu₄Br₄(AcNP)₂ comprise (M + X)LCT plus ILCT transitions.

Fig. 2 HOMO and LUMO orbital distributions and redistribution of electron density of the lowest excited states (S₁ and T₁) of Cu₄Br₄(AcNP)₂.

Photophysical properties

Photophysical properties of Cu₄Br₄(AcNP)₂ were investigated in dichloromethane, doped and neat films. Fig. 3a shows the ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) spectra of the Cu₄Br₄(AcNP)₂ cluster and the ligand AcNP in dichloromethane (c = 2 × 10⁻⁵ M) at room temperature. Cu₄Br₄(AcNP)₂ and AcNP exhibit similar absorption profiles, each consisting of a strong band below 330 nm and a weak shoulder band above 330 nm. The analogous high-energy absorption bands of both the cluster and the ligand can be assigned to the spin-allowed π–π* transitions localized at the NP ligand. The weak low-energy shoulder band of AcNP could be ascribed to the ILCT transition from the dimethylacridine donor unit to the pyridine acceptor unit. In contrast, the low-energy shoulder band of Cu₄Br₄(AcNP)₂ should stem from different CT transitions, as evidenced by its absorption onset at 445 nm, which represents a significant redshift compared to that of the ligand at 390 nm. According to theoretical calculations (vide supra, Table S4†), the S₀ → S₁ absorption bands of Cu₄Br₄(AcNP)₂ mainly involve the transitions from the Cu₄Br₄ cluster and P atoms to the ligand. Consequently, the low-energy CT absorption band of Cu₄Br₄(AcNP)₂ originates from a combination of (M + X)LCT and ILCT transitions. In degassed dichloromethane solution, the ligand AcNP emits a greenish-blue light with a featureless emission spectrum, peaking at 496 nm. In comparison, the Cu₄Br₄(AcNP)₂ cluster exhibits a notably weaker emission, significantly redshifted and broadened, with a peak at 707 nm. The weak emission of this cluster in solution is attributed to the flattening distortion in the MLCT excited state, exacerbating nonradiative decay.^41,42 Such an excited-state distortion is common in copper(I) complexes and can be effectively suppressed in a rigid environment.^43,44 Despite numerous reports of high-efficiency copper(I) complexes in the powder state,^45,46 addressing the excited-state distortion in solution and film states through molecular design remains a formidable challenge.

Fig. 3 (a) Absorption (dotted line) and emission (solid line) spectra of AcNP and Cu₄Br₄(AcNP)₂ in dichloromethane (c = 2 × 10⁻⁵ M); (b) photoluminescence spectra and decay curves (the inset) of Cu₄Br₄(AcNP)₂ in a 40 wt%-doped BCPO film and a neat film at room temperature; (c) photoluminescence decay curves of Cu₄Br₄(AcNP)₂ in a 40 wt%-doped BCPO film at different temperatures. The inset shows a local magnification of the decay curves; (d) steady-state photoluminescence spectra of Cu₄Br₄(AcNP)₂ in a 40 wt%-doped BCPO film and a neat film at 77 and 300 K; (e) temperature dependence of the decay times of Cu₄Br₄(AcNP)₂ in a 40 wt%-doped BCPO film; (f) diagram illustrating the photophysical processes of Cu₄Br₄(AcNP)₂.

As shown in Table 1, in doped (40 wt%-doped in BCPO host, BCPO = Bis-(4-(N-carbazolyl) phenyl)-phenylphosphine oxide) and neat films (deposited via thermal evaporation), Cu₄Br₄(AcNP)₂ exhibits efficient greenish-yellow emission with a peak maxima of 552 nm and 567 nm, photoluminescence quantum yields (PLQYs) of 47% and 38%, and short emission lifetimes of 2.7 and 1.9 μs, respectively. Aggregation-caused quenching and emission redshift with increasing doping concentration are not significant. This phenomenon may be attributed to the highly twisted D–A structure of the ligand, which impedes the formation of intermolecular π–π stackings. The radiative (k_r) and non-radiative rate constant (k_nr) are calculated to be 1.7 × 10⁵ and 2.0 × 10⁵ for the doped film, and 2.0 × 10⁵ and 3.3 × 10⁵ for the neat film, respectively. Evidently, the comparatively reduced PLQY of Cu₄Br₄(AcNP)₂ in the neat film, as opposed to the doped film, can be ascribed to more pronounced nonradiative decay in the neat film. To gain a deep understanding of the emissive mechanism, we examined the temperature dependence of both emission spectra and decay times. As depicted in Fig. 3c and d, Fig. S4 and S5,† Cu₄Br₄(AcNP)₂ in both doped and neat films exhibit slight red-shifted emission spectra and considerably prolonged lifetimes on decreasing the temperature from 300 to 77 K. This suggests that the emission of this cluster may stem from two thermally equilibrated excited states, namely S₁ and T₁. Assuming a rapid thermal equilibrium between the S₁ and T₁ states, the temperature dependence of the emission decay time (τ) is expressed by the following Boltzmann equation:^14,47–49

Here, k_B and T denote the Boltzmann constant and absolute temperature, respectively. τ_S1 and τ_T1 are the decay lifetimes of the thermally equilibrated S₁ and T₁ states, respectively. As shown in Fig. 3e, the experimentally obtained emission decay times (τ) at temperatures ranging from 77 to 300 K can be well fitted using eqn (1), yielding fitting results of τ_S1 = 166 ns, τ_T1 = 117.5 μs, and ΔE_ST = 0.055 eV for Cu₄Br₄(AcNP)₂ in the doped film. The fitted value for τ_T1 approximates to the experimentally observed value (112.3 μs) at 77 K, and the ΔE_ST value aligns well with the energy differences (0.059 eV) between the onsets of emission spectra recorded at 77 and 300 K. As illustrated in Fig. S6,† similar fitting results were obtained for the neat film of Cu₄Br₄(AcNP)₂. These results suggest that the emission of Cu₄Br₄(AcNP)₂ stems from two thermally equilibrated excited states (S₁ and T₁) which are very close in energy. At 77 K, the exciton population is predominantly frozen in the T₁ state, and thus Cu₄Br₄(AcNP)₂ emit phosphorescence. As the temperature rises, excitons can be thermally activated to the slightly higher-lying S₁ state. Finally, emission from the S₁ state becomes dominant at room temperature. Given that the emission at room temperature is characterized by a microsecond lifetime (with no detected nanosecond decay components), it can be attributed to TADF.

Table 1 Photophysical data of Cu₄Br₄(AcNP)₂ in a 40 wt%-doped BCPO film and a neat film at 300 K

Cu4Br4(AcNP)2	λ max /nm	Φ PL /%	τ /μs	k r /s^−1	k nr /s^−1	ΔEST^f/eV
a Emission maximum. b Photoluminescence quantum yield. c Emission decay time. d Radiative decay rate. e Nonradiative decay rate. f Energy gap between the S1 and T1 states determined from the emission spectra at 77 and 300 K.
40%	552	47	2.7	1.7 × 10^5	2.0 × 10^5	0.059
100%	567	38	1.9	2.0 × 10^5	3.3 × 10^5	0.050

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PL spectra and decay curves were also measured for a spin-coated neat film of Cu₄Br₄(AcNP)₂ and a thermal-deposited neat film of AcNP, and then compared with those obtained from the thermal-deposited Cu₄Br₄(AcNP)₂ neat film. As shown in Fig. S7,† both the PL spectrum and decay curve of the spin-coated cluster film closely resemble those of the thermal-deposited cluster film, both exhibiting an emission spectrum peak at 567 nm and an emission lifetime of around 1.9 μs. In contrast, the ligand AcNP in the neat film exhibits sky-blue emission with a spectrum peaking at 469 nm and an emission lifetime of 12.3 ns, which can be attributed to be prompt fluorescence. The absence of emission from the ligand further confirms the purity of the thermal-deposited cluster film.

Electroluminescence (EL) properties

To evaluate the EL properties of Cu₄Br₄(AcNP)₂, vacuum deposition OLEDs were fabricated with a structure of ITO/HATCN (10 nm)/TAPC (40 nm)/mCP (5 nm)/emitting layer (30 nm)/DPEPO (5 nm)/TmPyPB (30 nm)/Liq (1 nm)/Al (100 nm) (Fig. 4a). Here, Cu₄Br₄(AcNP)₂ was used as the emitter; BCPO served as the host material to disperse the emitter; HATCN, TAPC, mCP, DPEPO, TmPyPB and Liq act as the hole-injection, hole-transporting, electron-blocking, hole-blocking, electron-transporting, and electron-injection layers, respectively. The device energy-level diagram and molecular structures of the used organic materials are depicted in Fig. 4a and Fig. S9,† respectively.

Fig. 4 (a) Energy level diagram of the OLEDs; (b) current density–voltage–luminance characteristics; (c) EL spectra of devices with different doping concentrations at the same voltage and 40 wt%-doped Cu₄Br₄(AcNP)₂ devices at different voltages; (d) external quantum efficiency (EQE), current efficiency (CE) and power efficiency (PE) vs. luminance characteristics.

We explored the impact of the doping concentration on the EL performance, varying the emitter doping concentrations from 20 wt% to 100 wt% (non-doped). The EL performances are presented in Fig. 4b–d and Table 2. A progressive spectral redshift in EL was observed as the doping concentration increased, with spectral peaks red-shifting from 557 to 572 nm and the corresponding Commission Internationale de L'Eclairage (CIE) color coordinates from (0.39, 0.46) to (0.42, 0.49), when going from 20 wt% to 100 wt%. The doping concentration-dependent EL spectra of D–A emitters are commonly associated with the solid solvation effect.^50–52 Specifically, at elevated concentration levels, the emissive CT states of the emitters are more likely stabilized by the polar neighboring molecules. In comparison to the majority of reported D–A emitters, the redshift observed in the EL spectra of Cu₄Br₄(AcNP)₂ with increasing doping concentration is not substantial. This may be attributed to the presence of large steric ligands, which effectively separate the neighboring chromophores. All these OLEDs exhibited remarkably low turn-on voltages, ranging from 3.3 to 3.6 V, indicating efficient carrier injection and transport within the devices. The maximum EQEs of the OLEDs exhibited a gradual increase with the rise in the doping concentration from 20 wt% to 40 wt%, reaching a highest value of 12.8% in the devices doped at 40 wt%. As the doping concentration continues to increase, the maximum EQE values exhibit only a slight decrease, going from 12.8% in the 40 wt%-doped device to 10.8% in the non-doped device. The dependence of efficiency roll-off on the doping concentration can be explained as follows: At low doping concentrations (20 wt% and 30 wt%), the presence of a shoulder band in the high-energy region of the EL spectrum suggests insufficient host–guest energy transfer which causes low efficiencies and reduced luminance in the device. In these cases, under high current density, the high-concentration excitons of host molecules may undergo bimolecular quenching before diffusing into the guest molecules for radiative transition, leading to severe efficiency roll-offs.^53–55 This efficiency roll-off behavior could be alleviated by optimizing the doping-concentration for ensuring efficient host–guest energy transfer. The OLED doped at 40 wt% exhibits a significantly smaller efficiency roll-off compared to the devices doped at 20 wt% and 30 wt%. However, when the dopant concentration is excessively high (e.g., 100 wt%), the accumulation of dopant excitons under high current density inevitably intensifies bimolecular quenching, thereby exacerbating efficiency roll-off. In summary, 40 wt%-doped and non-doped OLEDs achieved the maximum EQE of 12.8% and 10.8%, a peak power efficiency (PE) of 32.0 and 25.3 lm W⁻¹, a maximum current efficiency (CE) of 34.0 and 27.0 cd A⁻¹, and a maximum luminance of 5904 and 1045 cd m⁻², respectively. It is uncommon to observe such minimal dependence of the EL performance on the doping concentration for copper(I)-based emitters.^{10,28,32,56–58} Notably, as summarized in Table S7,† the device performances of the non-doped OLED can be comparable to that of the state-of-the-art non-doped OLEDs based on copper(I) emitters.

Table 2 Summary of device performances

Cu4Br4(AcNP)2	λ EL /nm	V on /V	L max /cd m^−2	EQE^d^/%	CE^e/cd A^−1	PE^f/lm W^−1	CIE^g (x, y)
a The wavelength at EL maximum (recorded at 9 V). b Turn-on voltage at 1 cd m^−2. c Maximum luminance. d EQE maximum value, value at 100 cd m^−2. e Maximum current efficiency. f Maximum power efficiency. g Commission Internatinale de L'Eclairage coordinates measured at 9 V.
20 wt%	557	3.5	544	7.4/3.8	19.7	18.8	(0.39, 0.46)
30 wt%	558	3.6	1655	9.1/7.3	23.6	21.0	(0.40, 0.48)
40 wt%	561	3.3	5904	12.8/11.9	34.0	32.0	(0.41, 0.50)
50 wt%	562	3.4	3689	11.1/9.9	29.2	27.7	(0.41, 0.50)
60 wt%	562	3.3	4570	11.0/10.3	29.1	27.3	(0.41, 0.50)
100 wt%	572	3.5	1045	10.2/8.2	27.0	25.3	(0.42, 0.49)

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Conclusions

In summary, a novel yellow-emissive Cu₄Br₄ cluster, namely Cu₄Br₄(AcNP)₂, was synthesized utilizing a D–A type bidentate N^P ligand. At room temperature, Cu₄Br₄(AcNP)₂ exhibited efficient TADF with PLQYs of 47% and 38%, along with short decay times of 2.7 and 1.9 μs in 40 wt%-doped and neat films, respectively. OLEDs utilizing Cu₄Br₄(AcNP)₂ as the emitter achieved high EL performance. The 40 wt%-doped device and non-doped device achieved maximum EQEs of 12.8% and 10.2%, respectively. The high efficiencies and well-suppressed concentration quenching observed in both PL and EL for this cluster are outstanding among copper(I)-based emitters and can be attributed to the distinctive D–A ligand configuration. This study offers insights into the design and structure–property relationships of copper(I)-based emitters with the aim of achieving highly efficient non-doped OLEDs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 52073286 and 21805281), the Natural Science Foundation of Fujian Province (Grant No. 2006L2005), and the Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China (Grant No. 2021ZR132 and 2021ZZ115).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2323855. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi00210e

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