Research ArticleMATERIALS SCIENCE

Highly efficient sky blue electroluminescence from ligand-activated copper iodide clusters: Overcoming the limitations of cluster light-emitting diodes

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Science Advances  21 Jun 2019:
Vol. 5, no. 6, eaav9857
DOI: 10.1126/sciadv.aav9857

Abstract

Organic light-emitting diodes using cluster emitters have recently emerged as a flexible optoelectronic platform to extend their biological and optical applications. However, their inefficient cluster-centered excited states and deficient electrical properties limit device performance. Here, we introduce donor groups in organic ligands to form ligand-activated clusters, enabling the fabrication of the first cluster-based sky blue–emitting device with a record 30- and 8-fold increased luminance and external quantum efficiency up to ~7000 nits and ~8%, respectively. We show that the electron-donating effect of donor groups can enhance ligand-centered transitions and thoroughly eliminate cluster-centered excited states by delocalizing the molecular transition orbitals from the cluster unit to the ligand, leading to 13-fold increased photoluminescence quantum yield. In turn, the excellent rigidity and photostability of the cluster unit improve the color purity and efficiency stability of the devices. These results will motivate the further development of high-performance optoelectronic clusters by ligand engineering.

INTRODUCTION

Ligand-stabilized clusters have attracted extensive attention because of their various structures and unique optical properties (14). The high rigidity of clusters not only can suppress excited-state relaxation-induced energy loss but also gives rise to their excellent thermo- and photostability, which is indispensable for versatile applications in bioimaging (5), biosensing/chemosensing (69), stimuli response (1013), and emissive coating (1416). However, facing increasing performance requirements, the inherent radiative limitation of the mixed counterion-to-metal charge transfer (3XMCT) and d→s,p metal-centered transitions, namely, the triplet cluster-centered (3CC) excited state, has impeded further improvement of their optical properties (6, 17). Only few clusters have a photoluminescence quantum yield (PLQY; ϕPL) beyond 50% (1821). Meanwhile, the emission colors for most of them are from green to near-infrared, revealing the real challenge in constructing blue-emitting clusters. Moreover, the shortage of ligand-passivated clusters in electrical properties restrains their applications in optoelectronic devices (22), such as organic light-emitting diodes (OLEDs). Up to now, only three electroluminescent (EL) metal cluster emitters were demonstrated, however, with a maximum luminance (Lmax) of approximately 1000 nits and an external quantum efficiency (EQE; ηEQE) below 1%, which lag far behind the OLEDs based on other kinds of EL emitters (2325). We recently demonstrated the activation effect of aromatic phosphine ligands on the electrical performance of Cu4I4 clusters, while the incorporation of conjugated ligands simultaneously complicated the excited-state transitions (24). Nonetheless, we reason that ligand-activated clusters can integrate the advantages of their organic and inorganic components for the optimized EL process on the basis of ligand engineering.

In general, except for 3CC, all the other excited-state transitions of clusters would involve the organic ligands, including metal-to-ligand charge transfer (nMLCT, n = 1 for singlet and 3 for triplet), counterion-to-ligand charge transfer (nXLCT), ligand local excited state (nLLE), and intraligand charge transfer (nILCT) (Fig. 1A) (17). The metal-metal bonding in the 3CC state is substantially enhanced for slight distortion, giving rise to not only the remarkably bathochromic emission and the large Stokes shift but also the suppressed radiative transition (26). As a component of the lowest triplet state (T1), the 3CC state may converge the excited energy by intersystem crossing (ISC) between n(M+X)LCT and the triplet energy migration from ligand-involved triplet excited states (2628). Despite intensive efforts to enhance the radiative transition of the 3CC state (12), it is still inferior to the other excited states in this respect; therefore, it actually serves as the quenching site of excited energy (29). In this case, the 3CC proportion in the T1 state should be reduced to the maximum extent, making the contributions of ligand-centered triplet excited states become overwhelming.

Fig. 1 Transition characteristics of clusters.

(A) Typical structure and excited-state transitions of ligand-stabilized clusters. X refers to counterion. (B) Excited-state deactivation channels of clusters. IC, internal conversion; n(M+X)LCT, mixed metal and counterion-ligand charger transfer transition; 3LLE, ligand local excited transition. Superscripts “1” and “3” refer to singlet and triplet, respectively.

Learning from the experience of luminescent mononuclear complexes, whose emission color and radiation process can be accurately modulated by introducing electron-donating or withdrawing groups in their ligands to adjust the π-π* gap and intramolecular charge transfer (3032), it was demonstrated that the ligands with electron-rich atoms can effectively enhance emissions from the cluster by facilitating nMLCT (33). The recent study on dual-emissive and chromic characteristics of clusters further indicates that the ratio of low-energy (LE) and high-energy (HE) emissions that respectively originated from the 3CC and n(M+X)LCT/nILCT transitions is remarkably dependent on organic ligands, indicating the feasibility of optimizing excited-state transitions by ligand engineering (11). Nonetheless, because the cluster unit is simultaneously involved in the 3CC state and n(M+X)LCT, how to thoroughly suppress the 3CC state without weakening n(M+X)LCT is still a big challenge.

Here, we report a molecular design strategy of high-performance EL clusters that manifests the remarkable enhancement of ligand-centered excited states and effective suppression of the 3CC state of phosphine-chelated Cu4I4 clusters by involving donor groups in the ligands. Delocalization of the triplet highest occupied natural transition orbital (HONTO) from the cluster unit to the organic ligands induces the 3CC-free triplet state composed of 17% 3(M+X)LCT, 21% 3LLE, and 62% 3ILCT. As a result, the cluster achieves 10-fold increased PLQY (ηPL = 65%). Lmax and ηEQE of its solution-processed sky blue–emitting OLEDs are markedly improved by 30- and 8-fold to ~7000 nits and ~8%, respectively.

RESULTS

Structures and excited-state characteristics

4,6-Bis(diphenylphosphino)dibenzofuran (DBFDP) was chosen as the bidentate coordination unit, whose P···P distance of 5.75 to 5.85 Å is flexible to enough accommodate Cu4I4 (24). The donor-acceptor (D-A) ligands DCzDBFDP and DtBCzDBFDP were prepared by introducing carbazole (Cz) and 3,6-di-tert-butylcarbazole (tBCz) groups with consecutively increased electron-donating ability at the 2,8-positions of DBFDP (see the Supplementary Materials). These three ligands, namely, DArDBFDP (Ar = H, Cz, and tBCz), reacted with 2 equiv of CuI to afford greenish crystals (Fig. 2A). The single-crystal x-ray diffraction analysis shows the zero-dimensional distorted pseudocubic structures of the formula [DArDBFDP]2Cu4I4, in which each DArDBFDP is linked to two coplanar Cu(I) ions (Fig. 2B). The two DArDBFDP ligands are approximately orthogonal, because the same atom locates at the opposite corner of the same face in their Cu4I4 units and each Cu(I) ion has a distorted tetrahedral configuration with three iodide ions and one P atom. It is noted that, in contrast to the other two clusters, [DCzDBFDP]2Cu4I4 reveals the symmetrical coordination geometry with two identical DCzDBFDP-chelated Cu2I2 faces, which would undoubtedly give rise to its different electronic structure. Furthermore, in comparison to [DBFDP]2Cu4I4, the Cu···Cu distances in [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 are gradually decreased by ~0.05 and ~0.08 Å, respectively. Simultaneously, the Cu···P distances in [DtBCzDBFDP]2Cu4I4 are increased by 0.01. It manifests that the variation of the electron-donating ability of the ligands remarkably influences the Cu4I4 geometry and the ligand-metal interactions, which respectively determine the potential surface position and transition probability of the 3CC state and (M+X)LCT (34, 35). With regard to [DtBCzDBFDP]2Cu4I4, the shortened Cu···Cu distances would restrain further Cu4I4 contraction and thereby hamper the formation of the 3CC state. Moreover, the potential surface of its 3CC state would be narrowed and shifted away from the potential surfaces of its ligand-centered HE excited states, which further weakens the couplings between them and consequently suppresses intramolecular triplet migration to its 3CC state.

Fig. 2 Molecular design and structures of [DArDBFDP]2Cu4I4.

(A) Synthetic procedure for Cu4X4 clusters of [DBFDP]2Cu4I4 with unipolar ligand and [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 with Cz-DBF–based D-A–type ligands. (B) Single-crystal structures of [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4. (C) DFT and TDDFT analysis of the ground state (S0) characteristics and the singlet and triplet transitions of [DArDBFDP]2Cu4I4. HONTO and LUNTO are the highest occupied and lowest unoccupied NTOs, respectively. S0, S1, and T1 refer to the ground state and the first singlet and triplet excited states. f is the oscillator strength of S0→S1 excitation.

A density functional theory (DFT) simulation of the ground states (S0) shows that the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) of these clusters are basically separated, with the locations on Cu4I4 units and dibenzofuran (DBF) groups in the ligands accompanied by overlaps on phosphorus coordination sites, suggesting the incorporation of (M+X)LCT components in the lowest excited states (Fig. 2C). Furthermore, Cu4I4 units make the major contributions to the three highest unoccupied molecular orbitals but are thoroughly excluded from the four lowest unoccupied molecular orbitals (figs. S1 to S3). Notably, the HOMO-3s of [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 are localized on their Cz and tBCz groups, rather than on the Cu4I4 unit for [DBFDP]2Cu4I4. The incorporation of these donor groups further gradually decreases the energy difference between HOMO and HOMO-3 from 0.54 and 0.39 to 0.16 eV, which closes the gap between the ligand and the Cu4I4 unit in the electron-donating transition and facilitates the ligand-centered excited-state transitions.

The transition characteristics of these clusters are even more diverse, as shown by a time-dependent DFT (TDDFT) simulation (Fig. 2C). The natural transition orbital (NTO) analysis of the S0→S1 excitation for [DBFDP]2Cu4I4 reveals the highest occupied and the lowest unoccupied NTOs (HONTO and LUNTO) mainly localized on Cu4I4 and one DBF group, which overlapped on P atoms with a relatively low overlap integral (<ΨHL>) of 0.19 (fig. S4). Despite the identical singlet HONTO location, the LUNTO of [DCzDBFDP]2Cu4I4 is dispersed to both ligands, owing to its symmetrical coordination geometry, which increases the overlap integral to 0.24, accompanied by an almost doubled oscillator strength (f) of 0.0169 (fig. S5). As expected, the strong electron-donating effect of tBCz groups delocalizes HONTO of [DtBCzDBFDP]2Cu4I4 from Cu4I4 to DBF and tBCz, which further improves the overlap integral and f to 0.28 and 0.0246, respectively, in the case of its DBF-localized LUNTO (fig. S6). Consequently, in contrast to the other two clusters, besides the predominant 1(M+X)LCT, 1ILCT and 1LLE also contribute to the singlet excited-state transition of [DtBCzDBFDP]2Cu4I4, with considerable proportions of 17% and 4%, respectively.

For the S0→T1 excitations, the situations of [DBFDP]2Cu4I4 and [DCzDBFDP]2Cu4I4 are similar to their S0→S1 transition, except for their triplet HONTOs slightly dispersed to the DBF groups, with the increased triplet <ΨHL> of 0.31 and 0.35, respectively. Significantly, owing to the stronger electron-donating effect of tBCz groups, the triplet HONTO of [DtBCzDBFDP]2Cu4I4 is mainly localized on one tBCz group and one phenyl of the DBF group, accompanied by the minor contribution from its Cu4I4 unit, which gives rise to its triplet <ΨHL> reaching 0.48. As a result, the triplet state of [DtBCzDBFDP]2Cu4I4 includes 3(M+X)LCT, 3ILCT, and 3LLE, with proportions of 17%, 62%, and 21%, respectively. In particular, in contrast to the triplet HONTOs uniformly distributed on the whole Cu4I4 unit in [DBFDP]2Cu4I4 and [DCzDBFDP]2Cu4I4, the contribution from the Cu4I4 unit in [DtBCzDBFDP]2Cu4I4 to the triplet HONTO is extremely limited on the single Cu(I) ion bonded with the triplet-involved specific P atom, as well as the corresponding three I atoms in the same tetrahedron, which manifests the exclusion of its Cu4I4-based 3CC state from the triplet transition, owing to the appropriately weakened metal-ligand interactions in [DtBCzDBFDP]2Cu4I4. The negative influence on 3(M+X)LCT is then remedied by enhancing 3ILCT through the strengthened electron-pushing and drawing interactions between tBCz and DBF groups.

The donor groups in the organic ligands, with their suitable electron-donating ability, can lead to appropriately weakened metal-ligand interactions and enhanced ILCT effect, giving rise to the modulation effect on excited-state characteristics by changing Cu4I4 unit geometry and transition orbital distributions, which establishes the basis of luminescent performance optimization by ligand engineering.

Photophysical properties

The electronic absorption characteristics of these clusters were investigated in dilute dichloromethane solutions (10−5 M) (Fig. 3A). Compared to [DBFDP]2Cu4I4, besides the ligand-attributed bands, the distinct charge transfer (CT)–featured absorption tails from 350 to 400 nm (ε ~ 103 cm2 mol−1) can be observed in the spectra of [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 with the enhanced MLCT transitions, which is consistent with TDDFT simulation results. Moreover, the relative intensity of this CT absorption band from [DtBCzDBFDP]2Cu4I4 is the strongest, owing to the additional contribution from its ILCT transition (inset in Fig. 3A). The time-resolved phosphorescence (PH) spectrum of [DBFDP]2Cu4I4 reveals that about 30% of its PH emission is ascribed to the LE 3CC band centered at ~620 nm, which is thoroughly involved in its broad unstructured room temperature photoluminescence (PL) spectrum with a full width at half maximum (FWHM) of 95 nm. Because of the decreased Cu···Cu distances, the 3CC band in the PH spectrum of [DCzDBFDP]2Cu4I4 shifts red by 80 nm due to the transition from the lowest vibration level of 3CC to the higher lying vibrational level of the ground state, accompanied by a reduced proportion of ~10%. The thorough separation of its (M+X)LCT-, ILCT-, and LLE-featured HE bands and LE 3CC band renders the exclusion of the latter from its PL spectrum, giving rise to the sky blue emission at 491 nm with an FWHM as small as 60 nm. More significantly, the PH spectrum of [DtBCzDBFDP]2Cu4I4 consists of only ligand-involved HE bands, which validates the effectiveness of ligand engineering on excited-state transition modulation. As indicated by TDDFT simulation, the delocalization of the triplet HONTO from the Cu4I4 unit to donor groups in ligands can enhance ligand-centered transitions and simultaneously suppress the formation of the 3CC state, thereby leading to the 3CC-free sky blue emission of [DtBCzDBFDP]2Cu4I4 that peaks at 480 nm. Its slightly wider PL spectrum with an FWHM of 67 nm implies the multicomponent emission. Nevertheless, the remarkably higher emission color purity of [DtBCzDBFDP]2Cu4I4 than blue-emitting Cu(I) complexes (FWHM of >100 nm) (36) verifies the superiority of rigid cluster-type emitters in suppressing excited-state structural relaxation.

Fig. 3 Photophysical properties of performance of [DArDBFDP]2Cu4I4.

(A) Electronic absorption (Abs.) spectra and time-resolved PH spectra in dilute CH2Cl2 (10−5 M) and room temperature PL in the film of the clusters. The absorption edges corresponding to CT transitions in the range of 300 to 400 nm are amplified as insets. The emission characteristics of 3CC are distinguished by red fill color. (B) Temperature-dependent fractional contributions of TADF and PH to HE emissions of the clusters simulated according to the experimental data from time-correlated transient emission spectra in fig. S7. (C) Temperature-dependent emission spectra (left) and the corresponding emission peak variation of the clusters in the range of 25° to 200°C. (D) PLQY (η) variations of the clusters under the UV exposure (302 nm) for a month (left) and after heating at temperatures of 25° to 200°C (right). η0 refers to the original η before UV radiation and heating. a.u., arbitrary units.

The similar CT-predominant S1 and T1 excited states of [DCzDBFDP]2Cu4I4 inducing its singlet-triplet splitting (ΔEST) are remarkably reduced from 0.16 eV of [DBFDP]2Cu4I4 to 0.07 eV, which can markedly enhance thermally activated delayed fluorescence (TADF) by facilitating reverse intersystem crossing (RISC). Thus, for [DCzDBFDP]2Cu4I4, the temperature dependence of its long emission lifetime (τL) is remarkably weakened, revealing the predominant thermally activated population of its S1 state (figs. S7 and S8). In contrast to dual-emissive [DBFDP]2Cu4I4, [DCzDBFDP]2Cu4I4 is basically mono-emissive, with the fractional intensity of 1(M+X)LCT-originated TADF component on its room temperature emission beyond 90% (Fig. 3B). In this case, as the minor triplet component, the involvement of its 3CC state in transitions is negligible. In turn, besides triplet CT components as 3(M+X)LCT and 3ILCT, the significant contribution of 3LLE to the T1 state of [DtBCzDBFDP]2Cu4I4 not only increases the probability of its PH radiation but also elevates its ΔEST to 0.10 eV for appropriate RISC limitation. Therefore, the transient emission characteristics of [DtBCzDBFDP]2Cu4I4 become strongly temperature dependent, with remarkable τL variation from ~1 ms at 77 K to 10 μs at 300 K, reflecting the increased population of the S1 state for spin-allowed TADF at high temperature. As a result, among these clusters, [DtBCzDBFDP]2Cu4I4 achieves the most balanced dual emission, with TADF and PH proportions of 76 and 24%, respectively. Furthermore, it can be noticed that the time decays of HE and LE emissions for these clusters are thoroughly synchronous, reflecting the further weakened couplings between the corresponding excited states (fig. S9). Therefore, despite their likely similar energy levels, the intramolecular energy transfer from ligand-centered excited states to 3CC states in these clusters is restrained, which originates from the huge differences of these states in Cu4I4 geometry distortion (4, 26). In this sense, [DtBCzDBFDP]2Cu4I4, with the most compact Cu4I4, is the unit undoubtedly superior in preventing the triplet migration to the 3CC state.

The optimized excited-state transition characteristics endow [DtBCzDBFDP]2Cu4I4 with a high ηPL of 65% in the solid state, which is 1.4- and 13-fold of those of [DCzDBFDP]2Cu4I4PL = 46%) and [DBFDP]2Cu4I4PL = 5%), respectively. Therefore, the suppression of the 3CC state can markedly improve the emission efficiency of the clusters. Note that the triplet states of [DCzDBFDP]2Cu4I4 are basically excluded from radiation as a whole to minimize the negative influence of its 3CC state. In contrast, only the 3CC state of [DtBCzDBFDP]2Cu4I4 is specifically eliminated through HONTO delocalization. This selective optimization is undoubtedly more efficient, because it can suppress nonradiation without sacrificing the dual-emissive characteristics.

Thermogravimetric analysis shows that the thermal stability of these phosphine-chelated clusters is outstanding among the reported Cu4I4 clusters (fig. S10). The decomposition temperatures (Td) of [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 are beyond 450°C, which are ~150°C higher than that of [DBFDP]2Cu4I4, because Cz and tBCz groups enhance the electron-donating ability of the ligands and thereby strengthen the Cu···P bonds (14). To further investigate the photostability of these clusters, they were heated in air at gradually increased temperature without protection. The temperature-irrelevant PL profiles of [DCzDBFDP]2Cu4I4 manifest its monocomponent TADF emission, while [DBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 reveal the themochromic behaviors due to the equilibrium of their TADF and PH processes (Fig. 3C). The differences in the quenched excited states for [DBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 are LE 3CC and HE 3LLE states, respectively, rendering their opposite emission shifts. Nonetheless, after heating at 200°C for 24 hours, the decreases of ηPL for these clusters are less than 1% (Fig. 3D). A long-term photostability test was then performed by measuring the ηPL variation of these clusters before and after ultraviolet (UV) exposure in air for a month. The maximum ηPL reduction ratios of these clusters were about 30, 14, and 8, respectively. The introduction of donor groups enhances the electron-donating ability of the ligand and suppresses the photo-oxidation of Cu(I) ions and photobleaching.

As a consequence, the emission efficiency of [DtBCzDBFDP]2Cu4I4 is markedly improved by delocalizing HONTO with donor groups to achieve full ligand-centered excited states. Accompanied by an excellent photostability far beyond other Cu(I) materials, [DtBCzDBFDP]2Cu4I4 would be of great potential to support high-performance OLED applications.

OLED performance

Last, we verified the EL performance of these three clusters on the basis of solution-processed OLEDs with a bilayer device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS; 70 nm), 9-(4-tert-butylphenyl)-3, 6-bis(triphenylsilyl)-9H-carbazole (CzSi)/[DArDBFDP]2Cu4I4 [10 weight % (wt %), 40 nm], 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB; 60 nm), and lithium 8-quinolinolate (Liq; 1 nm)/Al (100 nm), in which PEDOT:PSS and Liq serve as hole and electron-injecting layers, TmPyPB is the electron-transporting layer, and CzSi is used as a host matrix of the clusters as dopants (Fig. 4A). Despite their equivalent HOMO energy levels, the LUMO energy gaps of 0.5 to 0.6 eV between the clusters and CzSi are big enough to induce the electron capture and exciton confinement by the clusters (fig. S11). In this case, the EL mechanism of these devices should be direct charge recombination with the advantage in suppressing energy losses during the host-dopant energy transfer, which is believed to be more effective for blue-emitting OLEDs.

Fig. 4 EL performance of [DArDBFDP]2Cu4I4-based OLEDs.

(A) Configuration and energetic diagram of the devices and the chemical structures of the used CzSi as the host and TmPyPB as the electron-transporting layer. (B) Luminance–current density–voltage (L-J-V) characteristics and EL spectra of the devices. (C) Efficiency-luminance relationships of the devices.

EL spectra of the devices are in good agreement with the solid-state PL data of the clusters, indicating the thorough exciton confinement on these cluster emitters (Fig. 4B, inset). In comparison to the bluish white emission from [DBFDP]2Cu4I4-based devices with FWHM as large as 142 nm, the EL spectra of [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 correspond to remarkably narrowed sky blue emissions with almost halved FWHMs and Commission Internationale de L’Eclairage (CIE) coordinates of (0.22, 0.43), which verified the exclusion of the LE 3CC transition from their EL processes. It is known that EL Cu(I) materials are inferior in emission color purity, with FWHM of more than 100 nm, which is attributed to their multiple CT transitions (37, 38). However, on the basis of 3CC suppression, [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 achieve greatly improved EL color purity by virtue of the unique superiority of cluster-type emitters in structural rigidity.

The peripheral hole-transporting Cz and tBCz groups improve electrical properties of [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 to facilitate the carrier injection and transportation in the emissive layers under a high doping concentration of 10 wt %, giving rise to the remarkably increased current density (J) of their devices (Fig. 4B). At the same voltages, J of [DCzDBFDP]2Cu4I4-based devices was the highest, owing to the intermolecular interactions between Cz groups of adjacent [DCzDBFDP]2Cu4I4 molecules, which was restrained by the steric effect of tert-butyl groups in [DtBCzDBFDP]2Cu4I4 (fig. S12). Nonetheless, the maximum luminance of [DtBCzDBFDP]2Cu4I4-based devices was markedly improved to ~7000 cd m−2, which was about 7- and 25-fold of those of the other cluster-based analogs. Moreover, at the same J, the luminance of [DtBCzDBFDP]2Cu4I4-based devices was always the largest, accompanied by the unique linear luminance-J relationship in logarithmic scale, manifesting the balanced carrier flux for efficient charge recombination (fig. S13).

As designed, owing to the effective suppression of 3CC-induced exciton wastage, the EL efficiencies of [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4 were successfully elevated by about seven- and fivefold, compared to those of [DBFDP]2Cu4I4. [DtBCzDBFDP]2Cu4I4 endowed its devices with record efficiencies of 20.2 cd A−1 for current efficiency and 7.9% for EQE as the maxima, which are the highest values to date for cluster-based OLEDs (Fig. 4C). The maximum efficiencies of [DCzDBFDP]2Cu4I4-based devices of 14.7 cd A−1 and 6.0% were relatively lower, stemming from the inferiority of its mono-emission in exciton utilization.

The Cu(I) ion has significant sensitivity toward photo-oxidation and photobleaching, leading to serious efficiency roll-offs for most of the Cu(I) emitter-based devices (39). In contrast, the excellent photostability of the clusters is successfully transformed to the efficiency stability of their device, e.g., the remarkably reduced EQE roll-offs of [DtBCzDBFDP]2Cu4I4-based devices are 22% at 100 cd m−2 and 37% at 1000 cd m−2. Device lifetime investigation was not included in this work because, compared to vacuum processing, the lifetime optimization of solution-processed devices is too difficult and time consuming in the present because of the limited alternatives of charge transport layers and the formidable challenge in controlling layer morphology and interfacial effects. Nevertheless, the main criticism of the EL Cu(I) emitters is that the poor stability of the Cu(I) ion leads to the reduction of device lifetime. In this sense, the excellent photostability and EL efficiency stability of [DtBCzDBFDP]2Cu4I4 suggest that Cu(I) cluster materials would be a promising solution to this issue.

To directly demonstrate the advantages of cluster materials in OLED application, we also prepared the binuclear Cu(I) complex [DtBCzPDP]2Cu2I2 with the tBCz-substituted ligands similar to [DtBCzDBFDP]2Cu4I4 (fig. S14A). On the basis of the same device structure, in comparison to its cluster counterpart, this binuclear complex reveals the yellowish green EL emission with a bigger FWHM of ~150 nm, accompanied by nearly halved luminance and efficiencies (fig. S14). Despite the preliminary study of EL Cu(I) clusters, the comprehensive EL performance of [DtBCzDBFDP]2Cu4I4 is already beyond those of the conventional solution-processable blue-emitting Cu(I) complexes (38). Meanwhile, the superiorities of the EL Cu(I) clusters in color purity and stability are also displayed. The possibility of improving luminous efficiency and electrical properties by ligand engineering is so high that the EL performance of the Cu(I) clusters could reach the state-of-the-art level in the near future. Related investigations is already underway in our laboratory.

CONCLUSION

We attain efficient sky blue solution-processed OLEDs using cluster emitters for the first time. The excited-state transitions of the clusters are selectively optimized using a ligand engineering approach to facilitate radiation. Delocalization of the HONOTs from the cluster unit to the organic ligands is realized by the electron-donating effect of the donor groups in the ligands, which simultaneously suppresses the formation of the inefficient 3CC state and enhances the efficient ligand-centered radiative transitions. In this way, the superiorities of the cluster unit in structural rigidity and photostability and organic ligands in luminescence are perfectly integrated to form high-performance EL clusters. As a consequence, the extremely stable sky blue emission with 13-fold increased PLQY of [DtBCzDBFDP]2Cu4I4 transforms to the record EQE up to ~8%, extraordinary color purity with halved FWHM (82 nm), and favorable efficiency stability of its solution-processed sky blue OLEDs, which is already beyond the performance of conventional blue Cu(I) materials but still leaves a huge space for further improvement. This substantial progress will motivate and accelerate the development of optoelectronic cluster materials.

MATERIALS AND METHODS

Synthesis and characterization

[DBFDP]2Cu4I4 was prepared according to our previous work (24). The ligands DCzDBFDP and DtBCzDBFDP were synthesized and characterized following the procedure described in the “Experimental section” of the Supplementary Materials.

Coordination reaction procedure

CuI (0.38 g, 2 mmol) and the ligand (1 mmol) were dispersed in dichloromethane (20 ml) and stirred for 24 hours at room temperature. After removal of the solvent, the residue was further purified by recrystallization from dichloromethane/diethyl ether to afford greenish crystals with a yield of more than 90%.

[DCzDBFDP]2Cu4I4

1H nuclear magnetic resonance (NMR) [tetramethyl silane (TMS), CDCl3, 400 MHz]: δ = 8.259 (s, 4H), 8.119 (d, J = 7.2 Hz, 8H), 7.746 (t, J = 8.4 Hz, 16H), 7.634 (d, J = 6.8 Hz, 4H), 7.461 to 7.379 (m, 24H), 7.332 to 7.251 (m, 16H), and 7.148 parts per million (ppm) (d, J = 7.6 Hz, 8H). Matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry (MALDI-TOF): mass/charge ratio (m/z) (%) 2496 (100) [M+]. Elemental analysis for C120H80Cu4I4N4O2P4: C, 57.75; H, 3.23; N, 2.24; found: C, 57.77; H, 3.24; N, 2.26.

[DtBCzDBFDP]2Cu4I4

1H NMR (TMS, CDCl3, 400 MHz): δ = 8.223 (s, 4H), 8.095 (s, 8H), 7.743 (t, J = 8.8 Hz, 16H), 7.626 (d, J = 6.8 Hz, 4H), 7.463 (m, 32H), 7.080 (d, J = 8.8 Hz, 8H), and 1.460 ppm (s, 72H). MALDI-TOF: m/z (%) 2944 (100) [M+]. Elemental analysis for C152H144Cu4I4N4O2P4: C, 62.00; H, 4.93; N, 1.90; found: C, 62.01; H, 4.93; N, 1.92.

The cif and structure factor data are available from the Cambridge Structural Database (CCDC nos. 1835656 and 1835660).

Fabrication and characterization of OLEDs

The PEDOT:PSS layer was spin-coated on the patterned ITO-coated glass substrate after oxygen plasma treatment and then baked at 120°C for 20 min in the glove box. The emitting layer was spin-coated from chlorobenzene at a concentration of 10 mg ml−1. Then, the sample was transferred to a high-vacuum evaporation system. The layers of TmPyPB, Liq, and Al were deposited through a shadow mask in the chamber under a base pressure of ~10−4 Pa. Last, all the devices were encapsulated with UV epoxy resin in the glove box before carrying out the luminance-current-voltage measurement. The emission intensity was measured with a calibrated Si photodiode. EQE was calculated with the assumption of a Lambertian distribution. EL spectra were determined by a calibrated Ocean Optics USB4000 spectrometer.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/6/eaav9857/DC1

Experimental section

Fig. S1. Contours and energy levels of the first four highest occupied (HOMO to HOMO-3) and lowest unoccupied molecular orbitals (LUMO to LUMO+3) of [DBFDP]2Cu4I4.

Fig. S2. Contours and energy levels of the first four highest occupied (HOMO to HOMO-3) and lowest unoccupied molecular orbitals (LUMO to LUMO+3) of [DCzDBFDP]2Cu4I4.

Fig. S3. Contours and energy levels of the first four highest occupied (HOMO to HOMO-3) and lowest unoccupied molecular orbitals (LUMO to LUMO+3) of [DtBCzDBFDP]2Cu4I4.

Fig. S4. Contours of HONTO and HONTO-1 and LUNTO and LUNTO+1, transition weight (σ) and oscillator strength (f), and overlap integral (<ΨHL>) of singlet and triplet transitions of [DBFDP]2Cu4I4.

Fig. S5. Contours of HONTO and HONTO-1 and LUNTO and LUNTO+1, transition weight (σ) and oscillator strength (f), and overlap integral (<ΨHL>) of singlet and triplet transitions of [DCzDBFDP]2Cu4I4.

Fig. S6. Contours of HONTO and HONTO-1 and LUNTO and LUNTO+1, transition weight (σ) and oscillator strength (f), and overlap integral (<ΨHL>) of singlet and triplet transitions of [DtBCzDBFDP]2Cu4I4.

Fig. S7. Time-decay curves of HE emissions for [DArDBFDP]2Cu4I4 in the temperature range from 88 to 300 K.

Fig. S8. Emission lifetime variation of HE emissions for [DArDBFDP]2Cu4I4 in the temperature range from 88 to 300 K.

Fig. S9. Time decay curves of HE and LE emissions for [DArDBFDP]2Cu4I4 in CH2Cl2 at room temperature.

Fig. S10. Thermogravimetric curves of [DArDBFDP]2Cu4I4.

Fig. S11. Cyclic voltammogram of [DArDBFDP]2Cu4I4 measured at room temperature with a scanning rate of 100 mV s−1.

Fig. S12. Single-crystal packing diagram of [DCzDBFDP]2Cu4I4 and [DtBCzDBFDP]2Cu4I4.

Fig. S13. Luminance versus current density (J) relationship of the [DArDBFDP]2Cu4I4-based devices.

Fig. S14. EL performance of [DtBCzPDP]2Cu2I2.

Fig. S15. 1H NMR spectrum of 2,8-dibromodibenzofuran in CDCl3.

Fig. S16. 1H NMR spectrum of 2,8-di(carbazol-9-yl)dibenzofuran in CDCl3.

Fig. S17. 1H NMR spectrum of 2,8-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)dibenzofuran in CDCl3.

Fig. S18. 1H NMR spectrum of DCzDBFDP in CDCl3.

Fig. S19. 1H NMR spectrum of DtBCzDBFDP in CDCl3.

Fig. S20. 1H NMR spectrum of [DCzDBFDP]2Cu4I4 in CDCl3.

Fig. S21. 1H NMR spectrum of [DtBCzDBFDP]2Cu4I4 in CDCl3.

Table S1. Physical properties of [DArDBFDP]2Cu4I4.

Table S2. EL performance of OLEDs based on the reported clusters and [DArDBFDP]2Cu4I4.

References (4043)

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

REFERENCES AND NOTES

Acknowledgments: H.X. appreciates the great assistance of R. Chen (Nanjing University of Posts and Telecommunications) with the theoretical simulations. Funding: This study was supported by the Changjiang Scholar Program of Chinese Ministry of Education (Q2016208), NSFC (21672056, 61605042, 21602048, and 51873056), the Natural Science Foundation of Heilongjiang Province (QC2016072), the Harbin Science and Technology Bureau (2015RAYXJ008), and the National Postdoctoral Program for Innovative Talents (BX201600048). Author contributions: H.X. conceived the project. M.X., G.X., C.H., Q.L., and J.Z. performed the experiments. G.X., M.X., C.H., and H.X. analyzed the data and wrote the paper. All authors commented on the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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