White light-emitting diodes are important for the development of high-resolution displays and lighting. However, complicated multi-emissive layers (EML) structures undoubtedly increase cost, and limit production yield. In recent years, single-EML white light-emitting devices have been developed rapidly, the composition of the EML are mainly divided into: (i) single-molecular white emitters: which is easy to prepare the device, but the molecular structure design difficult; (ii) multi-doping white emitter systems: the structure of molecular are more flexible and diversified, but the excited states of the hosts and the emitters are similar, as a result, the performance of the device is highly sensitive to the concentration of host-emitter doping, and excessive doping of long-wavelength emitter will reduce the color purity and efficiency of the white light-emitting device, which markedly decreased fabrication repeatability and performance stability.

However, copper clusters exhibit completely different excited state properties due to their unique structure, and the energy transfer and exciton allocation processes between the host and emitter are completely different. By optimizing the excited states of copper clusters, the energy transfer and exciton allocation between host and emitter can be precisely regulated. In this sense, copper clusters can be competent for yellow/orange emitters in both single doped and heavily doped white light-emitting systems.

Moreover, on the basis of ligand engineering, we have developed a series of blue-green EL Cu₄I₄ cubes clusters with the most advanced external quantum efficiency (EQE, η_EQE) of over 20% (Nat. Commun., 2023, 14: 2901). However, although single-molecule luminescent Cu₄I₄ clusters have also been confirmed, no white cluster light-emitting devices have been reported so far because the maximum η_EQE of yellow Cu₄I₄ cubes is still less than 10%.

Figure 1. Structures and Photophysical properties of the Cu₄I₄ clusters. a Chemical structures of [Dppy]₂Cu₄I₄ and [tBCzDppy]₂Cu₄I₄ and the design of ligand functionalization with antenna effect in carrier and energy transfer. CT and ET refer to charge and energy transfer, respectively. b Single-crystal X-ray diffraction results of [Dppy]₂Cu₄I₄ and [tBCzDppy]₂Cu₄I₄. The bond angles of Cu₄I₄ in coordination skeletons are highlighted from top and front views. c Proposed mechanisms of facilitated charge (above) and energy transfer (below) between CzAcSF and [tBCzDppy]₂Cu₄I₄ mediated by tBCz-contributed energy levels, in comparison to [Dppy]₂Cu₄I₄. Data out and in parenthesis are experimental and simulated values, respectively. d Doping concentration dependence of key transition parameters for CzAcSF:x% [tBCzDppy]₂Cu₄I₄ films.

In that sense, in this contribution, based on 2-(diphenylphosphanyl)pyridine (Dppy), we introduced the di-(tert-butyl)-carbazole (tBCz) group with electron donor effect, obtained the tBCzDppy ligand with the donor-acceptor structure, and then coordinated with CuI, successfully constructed the copper cluster [tBCzDPPy]₂Cu₄I₄ (Figure1a and b). Compared with the parent [Dppy]₂Cu₄I₄, [tBCzDPPy]₂Cu₄I₄ has more suitable highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) energy levels than CzAcSF, which can directly form excitons by capturing holes and electrons simultaneously. On the other hand, the first singlet (S₁) and triplet (T₁) energy levels of the clusters are about 2.2 eV. In comparison to those of CzAcSF (2.9 eV), the large energy gaps reaching ~0.7 eV prevents host-to-cluster energy transfer. However, tBCz-centralized high-lying S₉/S₁₀ and T₉/T₁₀ states are intraligand charge transfer (ILCT) predominant, which are similar to intramolecular CT excited states of CzAcSF. Moreover, the S₉/S₁₀ and T₉/T₁₀ energy levels of ~2.6 eV are just located in the middle of the S₁ and T₁ energy levels of CzAcSF and [tBCzDPPy]₂Cu₄I₄, therefore they can serve as the intermediate energy levels to facilitate exciton allocation to the cluster through a ladder-like process: energy transfer from S₁/T₁ of CzAcSF to S₉/T₉ (or S₁₀/T₁₀) of [tBCzDPPy]₂Cu₄I₄, then to S₁/T₁ of [tBCzDPPy]₂Cu₄I₄ by internal conversion (Figure 1c). Therefore, the introduction of tBCz group can optimize the charge and energy transfer from the host to the cluster, so that the photoluminescence (PL) quantum yield (PLQY, ϕ_PL) of the doped film can reach more than 80% (Figure 1d), and the prepared white light-emitting device can achieve a maximum EQE of 22.3% when the cluster doping concentration is as high as 30%, which are the record values of white cluster light-emitting devices (CLED) reported so far, and also among the best results for all-color CLEDs (Figure 4).

Figure 4. Electroluminescence (EL) performance of spin-coated CzAcSF:x% cluster based devices. a Device configuration of the cluster light-emitting diodes (CLED), and chemical structure of CzAcSF serving as blue TADF host. b Commission Internationale de lEclairage (CIE) coordinates of the devices on CIE 1931 chromatic panel and corresponding EL spectra of CLEDs (insets). Different to x = 30 for [DPPy]₂Cu₄I₄, arrow indicates the variation of CIE coordinates at x = 10-40 for [tBCzDPPy]₂Cu₄I₄. The CIE coordinates of spin-coated device based on neat CzAcSF are included for comparison. c Current density-voltage-luminescence curves and d efficiencies vs. luminance relationships of CzAcSF:30% cluster based devices. The data of neat CzAcSF based control device are also presented for comparison. e Summary of external quantum efficiencies (EQE) and optimal doping concentrations (x%) for representative singly doped white light-emitting devices. Solid and hollow symbols indicate the device fabrication through spin coating and vacuum evaporation, respectively. For detailed comparison on EL performance, please see Table S5.

These results not only demonstrate the advantages of cluster luminescent in practical white-emitting applications, but also show that the excited state properties of cluster molecules and exciton processes in organic-cluster hybrid systems can be flexibly manipulated by ligand engineering.