Thermally activated delayed fluorescence dendrimers achieving 20% external quantum efficiency for solution-processed OLEDs

Abstract

Two multi-carbazole-encapsulated TADF dendrimers were obtained by coupling tercarbazole (tBuTCz or MeOTCz) and di(pyridin-3-yl)methanone (DPyM) units. Both tBuTCz-DPyM and MeOTCz-DPyM show small singlet–triplet energy gap (ΔE_ST) values (0.01 vs. 0.02 eV) and high photoluminescence quantum yield (PLQY) values (66.2 vs. 55.0%). High-performance solution-processed OLEDs based on tBuTCz-DPyM and MeOTCz-DPyM as the emissive layer were fabricated. The tBuTCz-DPyM-based solution-processed device exhibited a maximum current efficiency (CE_max) of 52.6 cd A⁻¹, a maximum external quantum efficiency (EQE_max) of 20.4% and a maximum luminance (L_max) of 6165 cd m⁻² with a low turn-on voltage of 3.2 V. The MeOTCz-DPyM-based solution-processed device also showed an efficient performance with a CE_max of 27.0 cd A⁻¹, an EQE_max of 9.2%, an L_max of 8169 cd m⁻² and a low turn-on voltage of 3.4 V. The tBuTCz-DPyM-based device (I) and the MeOTCz-DPyM-based device (II) show Commission internationale de l'éclairage (CIE) coordinates of (0.25, 0.48) and (0.37, 0.54), respectively. Both tBuTCz-DPyM and MeOTCz-DPyM demonstrate a high performance among solution-processed OLEDs with TADF dendrimers.

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1. Introduction

Due to their wide viewing angle, high response speed, high brightness and lightweight properties, organic light-emitting diodes (OLEDs) had been developed as commercial technologies for next-generation displays and lighting.^1–5 Generally, the fabrication of OLEDs can be divided into vacuum-deposited and solution-processed devices. The construction of multiple organic functional layers for vacuum-deposited OLEDs has been proved to be efficient.^6–8 However, due to the limited deposited space, low material utilization and high manufacturing costs, the development of vacuum-deposited OLEDs has been restricted. Therefore, it is more important to develop solution-processed OLEDs. At this stage, great progress has been made with solution-processed OLEDs.^9–16 Nonetheless, the performance of solution-processed OLEDs still needs to be greatly improved, and comparison with that of vacuum-deposited OLEDs is difficult. In recent years, as a third-generation light-emitting material in OLEDs, based on their advantages of being heavy metal-free, low cost and achieving a 100% internal quantum efficiency (IQE) through efficient reverse intersystem crossing (RISC) procedures, thermally activated delayed fluorescence (TADF) emitters have received great attention.^17–22 Thus, it is particularly important to develop high-performance solution-processed TADF materials.

Small-molecule TADF emitters have the disadvantages of being low-molecular-weight compounds, with poor solution-processability and a high crystallization tendency, which are not suitable for solution-processed OLEDs.^23,24 In recent years, polymers and dendritic TADF molecules have been widely developed as solution-processed OLED materials.^25–30 In particular, due to their definite structural composition, good film-forming properties and high purity, dendrimers are a type of ideal solution-processing OLED material.^31–34 Because multi-carbazole-encapsulated molecules have good solubility, a good hole-transport capacity, amorphous film-forming properties and good thermal stability, these are suit for solution-processed OLEDs. Therefore, a suitable design strategy for multi-carbazole-encapsulated molecules for the construction of TADF dendrimers could be beneficial for solution-processed OLEDs.^35–39 In this paper, we report two TADF dendrimers with multi-carbazole encapsulation, named tBuTCz-DPyM and MeOTCz-DPyM, which comprise di(pyridin-3-yl)methanone (DPyM) as the acceptor core coupled with 3,3′′,6,6′′-tetra-tert-butyl-9′H-9,3′:6′,9′′-tercarbazole (tBuTCz) or 3,3′′,6,6′′-tetramethoxy-9′H-9,3′:6′,9′′-tercarbazole (MeOTCz) as the donor units. The solution-processed devices based on tBuTCz-DPyM and MeOTCz-DPyM as the doped emissive layer show a high performance with maximum current efficiency (CE_max) values of 52.6 and 27.0 cd A⁻¹, maximum external quantum efficiency (EQE_max) values of 20.4 and 9.2%, maximum luminance (L_max) values of 6165 and 8169 cd m⁻² at 8 V, and low turn-on voltages of 3.2 and 3.4 V, respectively. In addition, the full-width at half-maximum (FWHM) of the EL spectrum for tBuTCz-DPyM is narrow, at 84 nm.

2. Results and discussion

2.1. Synthesis and structural characterization

As shown in Scheme S1 (ESI†), the target compounds for tBuTCz-DPyM and MeOTCz-DPyM were prepared via the Ullmann C–N coupling reaction of DPyM with tBuTCz or MeOTCz, respectively.^40,41 The chemical structures of the two dendrimers were confirmed using ¹H NMR and ¹³C NMR and high-resolution MS.

2.2. Thermal properties and film-forming properties

The two materials showed good thermal stability, which was indicated by their high decomposition temperature (T_d, corresponding to 5% weight loss) values of 443 and 446 °C for tBuTCz-DPyM and MeOTCz-DPyM, respectively, and the glass transition temperature (T_g) for tBuTCz-DPyM and MeOTCz-DPyM reached 260 and 268 °C, respectively (Fig. S1, ESI†). Moreover, in order to evaluate the morphological stability of the target compounds, atomic force microscopy (AFM) was utilized to explore the surface images of their neat films. As shown in Fig. S2 (ESI†), the surfaces of the two films were smooth and pinhole-free, and the root-mean-square (RMS) roughness was 0.44 nm and 0.46 nm for tBuTCz-DPyM and MeOTCz-DPyM, respectively. Therefore, the two emitters can provide uniform amorphous films for solution-processed devices, and the introduction of the tert-butyl group into the molecule provides better film-forming properties than for the methoxy-containing emitter.

2.3. Electrochemical and computational properties

The electrochemical behavior of the dendrimers was investigated using cyclic voltammetry (CV). As shown in Fig. S3 (ESI†), both of the compounds displayed one oxidation wave. According to the onset potentials, the highest occupied molecular orbital (HOMO) energy levels of tBuTCz-DPyM and MeOTCz-DPyM were estimated to be −5.47 and −5.12 eV, respectively. According to the optical bandgaps, the lowest unoccupied molecular orbital (LUMO) energy levels of tBuTCz-DPyM and MeOTCz-DPyM were calculated to be −2.70 eV and −2.52 eV, respectively. To gain insight into the electronic nature and geometry of the emitters, density functional theory (DFT) calculations were performed at the B3LYP/6-31G* level. As shown in Fig. 1, the HOMO and LUMO of the two compounds were located on the electron-donating multi-carbazole unit and the electron-withdrawing dipyridyl ketone unit, respectively. It was shown that the two molecules have a small HOMO–LUMO overlap, which indicates strong charge-transfer (CT) character and a small electron-exchange energy.^42,43 The calculated HOMO levels of tBuTCz-DPyM and MeOTCz-DPyM were −5.17 and −4.88 eV, while the calculated LUMO levels were −2.41 and −2.40 eV, respectively. Due to the stronger electron-donating ability of the methoxy group over the tert-butyl group, the HOMO level of MeOTCz-DPyM is shallower than that of tBuTCz-DPyM.¹⁶ This matches the experimental values obtained from the CV curves.

Fig. 1 Chemical structures (top), B3LYP/6-31G*-optimized molecular orbitals for the HOMO (lower middle) and LUMO (upper middle), and the energy levels (bottom) of the tBuTCz-DPyM (left) and MeOTCz-DPyM (right) emitters.

2.4. Photophysical properties

The two materials displayed similar absorption spectra. The weak absorption bands in the range of 320–450 nm assigned to the CT transitions from the tBuTCz or MeOTCz as donor units to DPyM as the acceptor unit. The tBuTCz-DPyM and MeOTCz-DPyM materials showed emission peaks at 521 and 554 nm in solution, respectively. Fig. 2b shows the fluorescence and phosphorescence spectra of the two compounds in their film states. According to the onsets of the fluorescence and phosphorescence spectra, ΔE_ST was calculated to be 0.01 eV for tBuTCz-DPyM, and 0.02 eV for MeOTCz-DPyM. The small ΔE_ST values indicated that an up-conversion process of the triplet excitons into their singlet state could occur efficiently through RISC, which results in the TADF phenomenon.^42,43 The corresponding data are summarized in Table 1.

Fig. 2 UV-Vis absorption and PL spectra of tBuTCz-DPyM and MeOTCz-DPyM in (a) dilute toluene solution, and (b) the film state (8 wt% doped in mCBP). Fluorescence spectra at room temperature (300 K) and phosphorescence spectra at 77 K for tBuTCz-DPyM and MeOTCz-DPyM in doped films (8 wt% doped in mCBP).

Table 1 Photophysical, electrochemical and thermal stability data of tBuTCz-DPyM and MeOTCz-DPyM

Compound	λ abs (nm)	λ PL /λPL^b (nm)	Φ PL (%)	HOMO^c/HOMO^d (eV)	LUMO^e/LUMO^d (eV)	ΔEST^f/ΔEST^d (eV)	k RISC (s^−1)	τ p/τd^h [ns]/[μs]	T d/Tg^i (°C)
a Measured in toluene solution. b Measured in the film state. c HOMO energy levels deduced from the equation HOMO = −(4.8 + E^oxons). d Obtained from quantum calculations using B3LYP/6-31G*. e LUMO energy levels obtained from the equation LUMO = HOMO + E^gopt. f Estimated from the onset of fluorescence spectrum at 300 K and phosphorescence spectrum at 77 K. g k RISC: the rate constant for RISC from the triplet excited state to the singlet excited state. h Measured in thin films on a quartz substrate at 300 K. i Measured via TGA/DSC at a heating rate of 10 °C min^−1.
tBuTCz-DPyM	392	521/495	63.5	−5.47/−5.17	−2.70/−2.41	0.01/0.07	1.8 × 10^5	7.0/9.5	443/260
MeOTCz-DPyM	405	554/514	55.0	−5.12/−4.88	−2.52/−2.40	0.02/0.03	2.4 × 10^5	15.7/7.9	446/268

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Moreover, the two dendrimers also showed small calculated ΔE_ST values of 0.07 and 0.03 eV for tBuTCz-DPyM and MeOTCz-DPyM, respectively. The dendrimers of tBuTCz-DPyM and MeOTCz-DPyM have a high PLQY in the films of 63.5 and 55.0%, respectively, which indicated that, compared with MeOTCz-DPyM with methoxy groups, tBuTCz-DPyM with tert-butyl groups can better reduce the concentration quenching caused by molecular aggregation. As shown in Fig. 3, transient PL measurements were carried out to prove the TADF features of the two dendrimers in the film state at room temperature. Both target compounds showed a multi-exponential decay with a short lifetime of 7.0 ns and a long lifetime of 9.5 μs for tBuTCz-DPyM, and a short lifetime of 15.7 ns and a long lifetime of 7.9 μs for MeOTCz-DPyM, which is attributed to TADF.³⁶ In order to gain a deeper insight into the RISC process, the rate constants of the two materials were calculated using reported methods.⁴⁴ As shown in Table S1 (ESI†), the rate constant for radiative decay from the S₁ state (k_r, _S) of tBuTCz-DPyM and MeOTCz-DPyM was calculated to be 5.0 × 10⁷ and 1.9 × 10⁷ s⁻¹, respectively. This is equivalent to the value of conventional fluorescent materials. Moreover, the rate constant of non-radiative decay from the S₁ state (k_nr,S) of tBuTCz-DPyM and MeOTCz-DPyM was estimated to be 2.9 × 10⁷ and 1.6 × 10⁷ s⁻¹, respectively. The ratio of k_r, _S to k_nr,S is 1.7 for tBuTCz-DPyM and 1.2 for MeOTCz-DPyM. Therefore, the ratio of k_r, _S to k_nr,S for tBuTCz-DPyM is slightly higher than that of MeOTCz-DPyM, which leads to a slightly higher PLQY value for tBuTCz-DPyM compared with MeOTCz-DPyM.⁴⁵ Furthermore, the k_RISC values of the two emitters were relatively fast, with the corresponding values of 1.8 × 10⁵ s⁻¹ for tBuTCz-DPyM and 2.4 × 10⁵ s⁻¹ for MeOTCz-DPyM, which exhibited an effective RISC process, leading to small ΔE_ST values.

Fig. 3 Transient decay curves of tBuTCz-DPyM (a) and MeOTCz-DPyM (b) in doped films (8 wt% doped in mCBP) at 300 K. Inset: prompt fluorescence.

2.5. Electroluminescent properties

As shown in Fig. 4, the relative energy-level alignments, the electroluminescence (EL) spectra, current density–voltage–luminance (J–V–L) characteristics, and the external quantum efficiency–current density curves of the optimized devices are displayed, and the corresponding parameters are summarized in Table 2. Both of the solution-processed devices were fabricated with the following structures: ITO/PEDOT:PSS (40 nm)/PVK (5 nm)/mCBP: emitter (8 wt%, 60 nm)/TPBI (40 nm)/Yb (5 nm)/Ag (120 nm). PEDOT:PSS is poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonic acid), which act as the hole-injection layer; PVK is poly(N-vinylcarbazole), which acts as the hole-transporting layer; and TPBI is 1,3,5-tris(N-phenyl-benzimiazole-2-yl)benzene, which serves as the electron-transporting layer. The EQE–current density curves show CE_max = 52.6 cd A⁻¹, and EQE_max = 20.4% for the tBuTCz-DPyM-based device (device I), and CE_max = 27.0 cd A⁻¹, and EQE_max = 9.2% for the MeOTCz-DPyM-based device (device II). In addition, the full-width at half-maximum (FWHM) of the EL spectrum of device I is only 84 nm, narrower than that of device II (115 nm), and the EL spectrum for devices I and II exhibits, respectively, bluish-green and green emission with a maximum at 503 and 530 nm and Commission internationale de l'éclairage (CIE) coordinates of (0.25, 0.48) and (0.37, 0.54). Both emitters have small ΔE_ST and high PLQY values, which can not only ensure an effective RISC process but also provide high-efficiency emission. Hence, both materials could be used to obtain high-performance devices.³⁷ Moreover, because the tert-butyl group has good solubility and electrochemical stability, the tBuTCz-DPyM emitter, which contains tert-butyl groups, is more suitable for solution-processed OLEDs than MeOTCz-DPyM, which will lead to a better performance.^38,39

Fig. 4 (a) Relative energy-level alignment of the devices (left) and chemical structures of the materials (right); (b) EL spectra of the devices; (c) current density–voltage–luminance (J–V–L) characteristics of the devices; and (d) external quantum efficiency–current density characteristics of the devices.

Table 2 EL characteristics of devices I and II

Device	λ ELmax (nm)	FWHM (nm)	V on (V)	L max (cd m^−2)	CEmax^d (cd A^−1)	EQEmax^e (%)	CIE^f (x, y)
a EL emission peak. b Turn-on voltage recorded at 1 cd m^−2. c Maximum luminance at 8 V. d The maximum current efficiency. e The maximum external quantum efficiency. f Commission internationale de l'éclairage coordinates.
I	503	84	3.2	6165	52.6	20.4	0.25, 0.48
II	530	115	3.4	8169	27.0	9.2	0.37, 0.54

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3. Conclusions

In conclusion, we have developed two new TADF dendrimers based on tercarbazole as the donor and di(pyridin-3-yl)methanone as the accepter. Both tBuTCz-DPyM and MeOTCz-DPyM have excellent thermal stability, good film-forming properties, a small ΔE_ST value, and a relatively high PLQY. Two solution-processed devices based on tBuTCz-DPyM and MeOTCz-DPyM as the emitter showed relatively high values, respectively, for CE_max of 52.6 and 27.0 cd A⁻¹, EQE_max of 20.4% and 9.2%, L_max of 6165 and 8169 cd m⁻² at 8 V, and with low a turn-on voltage of 3.2 and 3.4 V. In particular, the tBuTCz-DPyM-based solution-processed device showed a bluish-green emission with CIE coordinates of (0.25, 0.48), which possess a narrow FWHM of 84 nm. By contrast, the MeOTCz-DPyM-based solution-processed device showed a green emission with CIE coordinates of (0.37, 0.54). These are high efficiencies among solution-processed TADF dendrimer OLEDs. High-performance, solution-processable TADF dendrimers have been developed that are beneficial for the practical application of large-area manufacturing based on solution processing.

4. Experimental section

4.1. General information

Unless otherwise stated, all commercial chemicals are used directly without further purification. All solvents are analytical grade and freshly distilled before use. Anhydrous toluene was refluxed with sodium and anhydrous dichloromethane was refluxed with calcium hydride, followed by fresh distillation before use. ¹H NMR and ¹³C NMR spectra were recorded in CDCl₃ with tetramethylsilane (TMS) as the internal standard using a Bruker Avance 600 MHz spectrometer. HRMS (Q-TOF) spectra were obtained using an Impact II spectrometer (Bruker Daltonics Inc.). UV-Vis absorption spectra were collected using a Mapada UV-1800PC recording spectrophotometer. Photoluminescence (PL) spectra were recorded using a PerkinElmer LS-55 fluorescence spectrophotometer. The absolute PLQY values of the emitters in the solid state were measured using an Edinburgh FLS 980 spectrophotometer with an integrating sphere under ambient conditions. TGA was carried out using a Hitachi STA7300 instrument at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere. DSC was carried out using a Mettler Toledo (DSC1) instrument at a heating rate of 10 °C min⁻¹ under a nitrogen flow. CV was measured at room temperature using a CorrTest electrochemical workstation. The CV system was constructed using a platinum plate as the working electrode, a platinum wire as the auxiliary electrode and Ag/AgNO₃ (0.1 mol L⁻¹ in acetonitrile) as the quasi-reference electrode with ferrocenium/ferrocene (Fc⁺/Fc) as the external standard. Tetrabutylammonium perchlorate (0.1 M) was used as the supporting electrolyte.

The synthesis of tBuTCz, MeOTCz and DPyM was completed according to the reference methods.^36,40,46

Synthesis of tBuTCz-DPyM. tBuTCz (346 mg, 0.48 mmol), DPyM (80 mg, 0.23 mmol), CuI (9.2 mg, 0.05 mmol), K₃PO₄ (1.0 g, 4.8 mmol) and 1,2-diaminocyclohexane (8.0 mg, 0.07 mmol) were added to the system, which was then purged with argon 3 times. The dioxane (4 ml) was added to the system. The reaction was stirred at 110 °C for 12 h. After the reaction, the mixture was filtered using a filter paper. The crude product was obtained after removing the solvent, and was purified via column chromatography (silica, hexane/ethyl acetate, v/v 5 : 1) to afford the tBuTCz-DPyM compound as a yellow solid with a yield of 52%. ¹H NMR (600 MHz, CDCl₃) δ 9.38 (d, J = 1.8 Hz, 2H), 8.64 (dd, J = 8.4, 1.8 Hz, 2H), 8.33 (d, J = 8.4 Hz, 4H), 8.26 (s, 4H), 8.17 (s, 8H), 8.10 (d, J = 8.4 Hz, 2H), 7.74 (dd, J = 9.0, 1.8 Hz, 4H), 7.48 (d, J = 9.6 Hz, 8H), 7.38 (d, J = 9.0 Hz, 8H), 1.47 (s, 72H). ¹³C NMR (150 MHz, CDCl₃) δ 190.65, 154.78, 151.55, 142.81, 140.12, 139.83, 138.33, 132.83, 129.69, 126.36, 125.89, 123.67, 123.26, 119.13, 117.76, 116.30, 113.36, 109.03, 34.75, 32.03, 29.71. HRMS (APCI) m/z: [M + H]⁺ calcd for C₁₁₅H₁₁₅N₈O⁺, 1623.9188; found, 1623.9267.

Synthesis of MeOTCz-DPyM. A procedure similar to that used for tBuTCz-DPyM was followed but using MeOTCz instead of tBuTCz to afford the compound MeOTCz-DPyM as a yellow solid with a yield of 56%. ¹H NMR (600 MHz, CDCl₃) δ 9.35 (d, J = 1.8 Hz, 2H), 8.61 (dd, J = 8.4, 1.8 Hz, 2H), 8.31 (d, J = 9.0 Hz, 4H), 8.26 (d, J = 1.8 Hz, 4H), 8.06 (d, J = 8.4 Hz, 2H), 7.70 (dd, J = 8.4, 1.8 Hz, 4H), 7.58 (d, J = 2.4 Hz, 8H), 7.33 (d, J = 9.0 Hz, 8H), 7.04 (dd, J = 9.0, 2.4 Hz, 8H), 3.95 (s, 24H). ¹³C NMR (150 MHz, CDCl₃) δ 190.57, 154.67, 154.00, 151.49, 140.09, 138.29, 136.94, 132.79, 129.66, 126.37, 125.82, 123.49, 119.12, 117.66, 115.23, 113.42, 110.50, 102.99, 56.12. HRMS (APCI) m/z: [M + H]⁺ calcd for C₉₁H₆₇N₈O₉⁺, 1415.5026; found, 1415.5074.

OLED device fabrication and characterization. ITO patterned glass substrates (size 12 mm × 8 mm) were cleaned in an ultrasonic bath using acetone, isopropyl alcohol, and DI water for 15 min, respectively. After UV-ozone treatment for 3 min, a 40-nm-thick PEDOT:PSS layer was spin coated onto the ITO substrate, which was then heated at 150 °C for 15 min. Then, a 5-nm-thick PVK interlayer was spin coated on top of the PEDOT:PSS layer under nitrogen and annealed at 110 °C for 10 min. Subsequently, an emissive layer of the TADF dendrimer was spin coated from a chlorobenzene solution and annealed at 100 °C for 5 min. Finally, TPBI (40 nm), Yb (5 nm) and silver (120 nm) cathodes were deposited via thermal evaporation under vacuum conditions (1 × 10⁻⁷ torr). Characterization of the device performance was measured using a Keithley 2400 meter with a BM-7AS luminance colorimeter in an F-star optical measurement system. The EL spectra and CIE coordinates were recorded using a PR-788 photometer.

Author contributions

Cheng Zhang: methodology, data curation, and writing – review and editing. Hao Yan: OLED device fabrication and characterization and writing – review and editing. Yuting He: conceptualization, investigation, and writing – original draft. Yongshuai Chai: software, data curation, and validation. Deyun Zhou: project administration, supervision, and writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2021JQ-664), and the Natural Science Foundation of Shaanxi Provincial Department of Education (Program No. 19JK0379).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qm00833e

‡ These authors contributed equally to this work.

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