Comparative study on electroluminescence efficiencies and operational stability of blue TADF materials based on xanthone or triphenyltriazine acceptors

Abstract

The development of blue thermally activated delayed fluorescence (TADF) materials holds high importance for next-generation display and lighting technologies, yet their device stability remains a critical challenge for commercialization. In this work, we report two blue TADF materials incorporating the same stable pyridine-derived donor but different acceptors of xanthone (XT) and triphenyltriazine (TRZ), respectively. They exhibit excellent thermal stability and efficient blue TADF properties. When employed as emitters in organic light-emitting diodes (OLEDs), they show similar maximum external quantum efficiencies (EQE_maxs) of about 24%. However, the XT-based TADF emitter exhibits a longer operational lifetime than the TRZ-based emitter. What's more, when these two materials serve as sensitizers for a green multi-resonance TADF emitter, the XT-based sensitizer still provides a higher EQE_max and a longer operational lifetime than the TRZ-based sensitizer. These findings underscore the high potential of XT-based acceptors in the design of stable TADF emitters and sensitizers for fabricating high-performance OLEDs.

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Introduction

With the maturity of organic light-emitting diode (OLED) technology, it has been gradually commercialized and appeared in people's lives.^1–7 The emergence of thermally activated delayed fluorescence (TADF) materials has greatly promoted the development of OLEDs, because they are able to utilize triplet excitons through the reverse intersystem crossing (RISC) process without the aid of heavy metals, achieving a theoretical exciton utilization efficiency of 100%.^8,9 Purely organic TADF materials have the merits of low cost, high efficiency and easy synthesis, and thus, developing high-performance TADF materials has become a cutting-edge research direction in the field of luminescent materials and devices in recent years.^10–13 On the other hand, a long device operational lifetime is a critical parameter determining whether the material can be commercialized. However, most TADF materials face the issue of unsatisfactory operational stability, especially for blue OLEDs.^14–18 At present, there are various explanations for the device degradation, while the stability of the functional layers as well as luminescent emitters plays a key role in influencing the operational lifetimes of the devices.^19–21 One of the prerequisites for achieving high device stability is the structural stability of the organic molecules, which requires that the building blocks hold high bond degradation energies (BDEs).

Recently, a great number of carbonyl-containing TADF emitters have been developed, which exhibit impressive electroluminescence (EL) efficiencies. The fast RISC process benefitting from enhanced spin–orbit coupling (SOC) due to the n–π* transition of the carbonyl group is considered to contribute greatly to the excellent EL performances. But due to the presence of the carbonyl group, researchers normally believe that the operational stability of the devices adopting these emitters could decline. In our opinion, whether the carbonyl-containing emitters will cause poor device stability or not is highly dependent on the specific molecular structures. And some previous studies^22,23 had demonstrated that certain carbonyl-containing emitters can improve device stability instead.

Amongst various carbonyl-containing electron acceptors, xanthone (XT) is a representative one and has been widely used in constructing efficient TADF emitters.^24–27 In this work, we design and synthesize a new blue TADF material (PytBuCz-XT) using XT as an acceptor, 3,6-di-tert-butyl-9H-carbazole as a donor, and pyridine as a bridge linking donor and acceptor (Fig. 1A). In order to comparatively evaluate the EL performance and operational stability, a control blue TADF material (PytBuCz-TRZ) is prepared. PytBuCz-TRZ has the same structure as PytBuCz-XT except for employing triphenyltriazine (TRZ) as an electron acceptor, which is regarded as a stable electron acceptor with not only good electron transport ability but also high BDE.^28–34 PytBuCz-XT and PytBuCz-TRZ show blue EL emissions, with similar maximum external quantum efficiencies (EQE_maxs) of about 24%. But interestingly, the device using PytBuCz-XT as an emitter has better operational stability, with about two-fold longer operational lifetime than that of the device using PytBuCz-TRZ as an emitter. Moreover, when using these two materials as the sensitizers for a multi-resonance (MR) emitter of BN2,³⁵ the device employing PytBuCz-XT as a sensitizer has a longer operational lifetime as well. These results strongly indicate that the XT acceptor has high potential to construct efficient TADF materials with high EQE_max and excellent device stability.

Fig. 1 (A) Molecular structures and (B) frontier molecular orbital distribution and energy levels of PytBuCz-XT and PytBuCz-TRZ.

Results and discussion

Synthesis and characterization

The synthetic routes of PytBuCz-XT and PytBuCz-TRZ are presented in Scheme S1 in the ESI.† Their molecular structures are confirmed by ¹H and ¹³C NMR spectra and high-resolution mass spectrometry, and the detailed characterization data are described in the ESI.† As shown in Fig. S1 (ESI†), PytBuCz-XT has better thermal stability with a higher decomposition temperature (466 °C) than PytBuCz-TRZ (422 °C), determined by thermal gravimetric analysis (TGA), indicating that XT acceptor could be beneficial for improving the thermal stability of the materials.^36–40 The differential scanning calorimetry (DSC) curves of the two molecules show no clear glass-transition temperatures, which implies that they are morphologically stable. To study the electrochemical properties of both molecules, cyclic voltammetry (CV) measurements are performed in dichloromethane solutions. According to the onset oxidation and reduction potentials relative to Fc/Fc⁺ (Fig. S2, ESI†), the energy levels of the highest occupied molecular orbitals (HOMOs) are calculated to be −5.5 and −5.5 eV for PytBuCz-XT and PytBuCz-TRZ and those of the lowest unoccupied molecular orbitals (LUMOs) are calculated as −3.0 and −2.9 eV for PytBuCz-XT and PytBuCz-TRZ, respectively. These results suggest that PytBuCz-XT and PytBuCz-TRZ have similar HOMO and LUMO energy levels.

Theoretical calculation

To further research the geometrical and electronic structures of both molecules, theoretical simulation is performed via density functional theory (DFT) and time-dependent density functional theory (TD-DFT) at the B3LYP/6-31g(d,p) level. The results show that PytBuCz-XT and PytBuCz-TRZ have similar molecular conformations with close dihedral angles of 35.1°, and 34.7° between pyridine and XT and pyridine and TRZ, respectively. The dihedral angles between carbazole and pyridine in both molecules are similar as well (33.0–35.2°) (Fig. 1B). The HOMOs of PytBuCz-XT and PytBuCz-TRZ are mainly distributed on carbazole and pyridine moieties, while the LUMOs are mainly concentrated on XT and TRZ acceptors and pyridine bridge. The HOMOs and LUMOs are basically separated, which are beneficial for the reduction of energy difference between the lowest triplet and singlet states (ΔE_STs). The calculated ΔE_STs of PytBuCz-XT and PytBuCz-TRZ are as low as 0.10 eV, which can ensure the occurrence of the RISC process. Meanwhile, PytBuCz-XT and PytBuCz-TRZ still have certain overlaps of the HOMOs and LUMOs on the pyridine bridge, which is conducive to improving oscillator strength and facilitating radiative decay.

To distinguish the difference in BDEs between these two molecules, the degradation energies of some chemical bonds are calculated by DFT at the B3LYP/def2svp level. As displayed in Fig. S3 (ESI†), the BDEs of the bonds between the donor and acceptor of PytBuCz-XT and PytBuCz-TRZ are the same at the neutral state, which are both 5.0 eV. But these BDEs of PytBuCz-XT at cation and anion states are 6.9 and 5.1 eV, respectively, which are higher than those of PytBuCz-TRZ (6.6 and 3.6 eV). Moreover, the BDEs of XT acceptors in PytBuCz-XT are 33.4 and 18.7 eV, respectively, which are greatly higher than the BDEs of TRZ acceptors in PytBuCz-TRZ (15.4 and 4.9 eV). In addition, the BDEs of the C–N bond between the carbazole and pyridine units of PytBuCz-XT and PytBuCz-TRZ are almost the same at neutral, cation and anion states. These calculation results of BDEs indicate that PytBuCz-XT has a higher structural stability than PytBuCz-TRZ, which is beneficial for improving the operational stability of the device.

Photophysical property

As shown in Fig. 2A and B, PytBuCz-XT and PytBuCz-TRZ in toluene solutions with a concentration of 10⁻⁵ M have a maximum absorption at 391 and 379 nm, respectively, which stems from the charge transfer (CT) state. PytBuCz-XT and PytBuCz-TRZ exhibit blue photoluminescence (PL) emissions in toluene solutions with PL peaks at 474 and 460 nm, and PL quantum yields (Φ_PLs) of 45% and 38%, respectively (Table 1). The redder emission of PytBuCz-XT can be attributed to the stronger electron-withdrawing effect of the XT acceptor. When PytBuCz-XT and PytBuCz-TRZ are doped in 2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) host at a concentration of 15 wt%, their PL peaks are slightly red-shifted to 487 and 474 nm, respectively (Fig. 2C), due to the high polarity of the PPF host and thus stronger CT effect. The Φ_PLs of PytBuCz-XT and PytBuCz-TRZ are significantly enhanced to 89% and 82%, respectively, which can be partially attributed to the restriction of intramolecular motions in the condensed phase.^41–46 Moreover, the low-temperature fluorescence and phosphorescence spectra of the doped films are measured (Fig. S4, ESI†), and the calculated ΔE_STs of PytBuCz-XT and PytBuCz-TRZ are 0.12 and 0.17 eV, respectively, basically consistent with the experimental simulation results. The small ΔE_STs indicate that these molecules could have TADF properties and the triplet excitons can be converted to singlet excitons through the RISC process by using them as emitters or sensitizers. Subsequently, the transient PL decay spectra of PytBuCz-XT and PytBuCz-TRZ in doped films are tested. As can be seen in Fig. 2E and F and Table S1 (ESI†), their doped films show obvious delayed components, which are promoted when the temperature increases from 77 to 300 K, confirming the TADF feature.^47,48 The delayed fluorescence lifetimes of PytBuCz-XT and PytBuCz-TRZ are 19.4 and 33.5 μs (Fig. S5, ESI†), corresponding to the RISC rates of 4.6 × 10⁵ and 2.4 × 10⁵ s⁻¹, respectively. The faster RISC process of PytBuCz-XT should be conducive to improving EL efficiencies and the operational stability of the devices. Additionally, neat films of these two molecules show similar temperature-dependent PL decay trends (Fig. S6 and Table S2, ESI†), further confirming their TADF property.

Fig. 2 (A) Absorption and (B) PL spectra in toluene solutions (10⁻⁵ M) and (C) in doped films with PPF host (15 wt%) of PytBuCz-XT and PytBuCz-TRZ. (D) Overlap of the absorption of BN2 and PL of PytBuCz-XT and PytBuCz-TRZ in toluene solutions. Temperature-dependent transient decay spectra of the doped films with PPF host (15 wt%) of (E) PytBuCz-XT and (F) PytBuCz-TRZ, measured under nitrogen atmosphere.

Table 1 The photophysical properties of PytBuCz-XT and PytBuCz-TRZ

Molecule	λ abs (nm)	λ em Tol/DF	Φ PL (%) Tol/DF	τ delayed (μs)	R delayed (%)	k ISC (×10^7 s^−1)	k RISC (×10^5 s^−1)	ΔEST^h (eV)
a Measured in oxygen-free toluene solution (10^−5 M) at room temperature. b PL peak in toluene (Tol) solution and doped film (DF) in PPF (15 wt%) at room temperature. c Absolute PL quantum yield determined by a calibrated integrating sphere under nitrogen at room temperature. d Delayed fluorescence lifetimes. e Ratio of the delayed component. f Rate constant of intersystem crossing process. g Rate constant of the reverse intersystem crossing process measured 300 K under nitrogen in doped films with PPF host (15 wt%). h Singlet–triplet energy splitting calculated from the onsets of the fluorescence and phosphorescence spectra of the doped films with PPF host (15 wt%) at 77 K.
PytBuCz-XT	391	474/487	45/89	19.4	88.8	2.34	4.6	0.12
PytBuCz-TRZ	379	460/474	38/82	33.5	87.3	1.68	2.4	0.17

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Electroluminescence performances

Based on the apparent TADF property of PytBuCz-XT and PytBuCz-TRZ, their EL performances are further investigated. The devices are constructed with the configuration of ITO/HATCN (5 nm)/TAPC (50 nm)/TCTA (5 nm)/mCP (5 nm)/EML (20 nm)/PPF (5 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al. Thereof, dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN), 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline] (TAPC), PPF and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) work as hole-injection, hole-transporting, hole-blocking and electron-transporting layers, respectively. Tris[4-(carbazol-9-yl)phenyl]amine (TCTA) and 1,3-bis(carbazol-9-yl)benzene (mCP) perform as exciton-blocking and electron-blocking layers, respectively (Fig. 3). The doped films of 15 wt% PytBuCz-XT and 15 wt% PytBuCz-TRZ in PPF host work as light-emitting layers (EMLs) for devices A1 and A2, respectively. Devices A1 and A2 have the same turn-on voltages (V_on) of 3.1 V and exhibit sky-blue lights, with EL peaks at 496 and 482 nm, respectively. As listed in Table 2, device A1 attains good EL performance, with maximum luminance (L_max), maximum power efficiency (PE_max), maximum current efficiency (CE_max) and EQE_max of 19 570 cd m⁻², 62.2 Im W⁻¹, 63.2 cd A⁻¹ and 24.5%, respectively, which are slightly better than those of device A2 (11 140 cd m⁻², 53.7 Im W⁻¹, 54.7 cd A⁻¹, and 24.0%).

Fig. 3 (A) Device structure and energy level diagram. (B) Molecular structures of the materials employed in the devices. (C) EL spectra, and plots of (D) external quantum efficiency–luminance spectra and (E) current density and luminance versus voltage of the devices A1 and A2.

Table 2 EL performances the OLEDs using PytBuCz-XT and PytBuCz-TRZ as emitters or sensitizers

Device^a	λ EL (nm)	V on (V)	L max (cd m^−2)	CEmax (cd A^−1)	PEmax (lm W^−1)	EQEmax (%)	CIE (x, y)	LT50/LT90 (h)
				Maximum value/at 1000 cd m^−2
a Abbreviations: Von = turn-on voltage at 1 cd m^−2; Lmax = maximum luminance; CE/PE/EQE = current efficiency/power efficiency/external quantum efficiency at maximum value and 1000 cd m^−2; CIE = Commission Internationale de l’Eclairage coordinates at 1000 cd m^−2; LT50/LT90 = device operational lifetimes from initial luminance to 50%/90%.
A1	496	3.1	19 570	63.2/48.1	62.2/33.6	24.5/19.0	(0.209, 0.418)	—
A2	482	3.1	11 140	54.7/26.7	53.7/17.9	24.0/11.9	(0.188, 0.349)	—
LT1	482	2.3	31 370	21.2/13.7	31.7/10.8	10.1/6.1	(0.176, 0.372)	210/20
LT2	468	2.3	20 050	16.5/6.9	19.9/4.7	7.4/3.8	(0.174, 0.258)	120/16
LT3	546	2.7	56 840	85.0/27.6	95.3/14.0	21.1/6.8	(0.375, 0.604)	948/140
LT4	546	2.7	35 180	70.7/31.5	67.1/14.1	17.1/7.7	(0.364, 0.613)	864/101
LT5	544	3.9	22 880	38.1/14.9	24.2/5.7	8.9/3.5	(0.335, 0.627)	5.4/0.9

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The device's operational lifetime is a key parameter for assessing the potential of OLEDs for practical application.⁴⁹ To evaluate the operational stability of the devices using PytBuCz-XT and PytBuCz-TRZ as emitters, optimized devices with a configuration of ITO/HATCN (5 nm)/BPBPA (30 nm)/BCzPh (5 nm)/EML (20 nm)/CzPhPy (5 nm)/DPPyA (30 nm)/LiF (1 nm)/Al are fabricated, in which the doped films of 20 wt% PytBuCz-XT and 20 wt% PytBuCz-TRZ in mCBP host, with Φ_PLs of 76% and 71%, respectively, work as EMLs for devices LT1 and LT2, respectively. Here, N,N,N′,N′-tetra(4-biphenylyl)-4,4′-biphenyldiamine (BPBPA) is applied as a hole-transporting layer, 9,9′-diphenyl-9H,9′H-3,3′-bicarbazole (BCzPh) works as a stable hole-transporting layer as well as an electron-blocking layer, and 4,6-bis(3-(9H-carbazol-9-yl)phenyl)pyrimidine (CzPhPy) and 9,10-bis(6-phenylpyridin-3-yl)anthracene (DPPyA) perform as hole-blocking layer and electron-transporting layer, respectively. As shown in Fig. 4 and Table 2, the EL performances of the devices are declined to some extent in comparison with those of devices A1 and A2, because of these more stable functional materials, but the device LT1 using PytBuCz-XT as an emitter still shows better EL performance than device LT2 based on PytBuCz-TRZ. For example, device LT1 exhibits an ultra-low V_on of 2.3 V, a L_max of 31 370 cd m⁻², a PE_max of 31.7 Im W⁻¹, a CE_max of 21.2 cd A⁻¹ and an EQE_max of 10.1%, better than those of device LT2. Additionally, the operational lifetime to 50% (LT₅₀) of device LT1 at an initial luminance of 1000 cd m⁻² can reach 210 h (Fig. 4C), which is nearly two-fold larger than that of device A2 (120 h). These results suggest that, under a similar molecular structure, the XT acceptor may provide not only higher EL efficiencies but also a better device lifetime than TRZ. Due to the presence of the carbonyl group, the SOC of PytBuCz-XT is enhanced, resulting in a faster RISC process. This can reduce the accumulation of triplet excitons and alleviate adverse triplet–triplet annihilation.^50–52 In addition, the higher structural stability of PytBuCz-XT than PytBuCz-TRZ also contributes to the longer device lifetime.

Fig. 4 (A) Device structure and energy level diagram. (B) Molecular structures of the materials used in the devices. (C) Operational lifetime of devices LT1–LT5. (D) EL spectra, and plots of (E) external quantum efficiency–luminance spectra and (F) current density and luminance versus voltage of the devices LT1 and LT2. (G) EL spectra, and plots of (H) external quantum efficiency–luminance spectra and (I) current density and luminance versus voltage of the devices LT3–LT5.

In addition to emitters, the operational stability of the devices using PytBuCz-XT and PytBuCz-TRZ as sensitizers for MR-TADF molecule BN2³⁵ is further studied. The absorption spectrum of BN2 is largely overlapped with the PL spectra of PytBuCz-XT and PytBuCz-TRZ (Fig. 2D), indicating efficient energy transfer can occur from PytBuCz-XT and PytBuCz-TRZ to BN2. To further decipher the energy transfer process between the sensitizers and BN2, the Förster energy transfer rates (k_FETs) and Förster energy transfer efficiencies (Φ_FETs) are calculated by the following equations:⁵³

Here, R₀ is the Förster energy transfer radius and R_DA is the radius between the donor and acceptor. The estimated k_FETs and Φ_FETs are 1.2 × 10⁹ s⁻¹ and 97% for PytBuCz-XT, and 8.1 × 10⁸ s⁻¹ and 96% for PytBuCz-TRZ. The high Φ_FETs indicate that the energy transfer from PytBuCz-XT and PytBuCz-TRZ to BN2 is very sufficient.

The fabricated devices have the same configuration as the devices LT1 and LT2 except for EMLs, which are composed of 3 wt% BN2: 20 wt% PytBuCz-XT: mCBP for device LT3 and 3 wt% BN2: 20 wt% PytBuCz-TRZ: mCBP for device LT4. A control device LT5 with EML of 3 wt% BN2: mCBP is also prepared for comparison. From Fig. 4G–I and Table 2, it can be seen that device LT3 using PytBuCz-XT as a sensitizer can achieve an excellent EQE_max of 21.1%, which is better than that using PytBuCz-TRZ as a sensitizer (17.1%). Moreover, device LT3 also has a better operational lifetime than device LT4, with a fitted LT₅₀ of 948 h at an initial luminance of 1000 cd m⁻², longer than that of the device LT4 (864 h). On the other hand, the operational lifetimes of devices LT3 and LT4 are much longer than the sensitizer-free device LT5, which has a poor LT₅₀ of 5.4 h, suggesting the introduction of sensitizers is helpful for improving device stability based on MR-TADF emitter. These results indicate that, in comparison with PytBuCz-TRZ, PytBuCz-XT can realize better operational stability not only as an emitter but also as a sensitizer.

Conclusions

In conclusion, two efficient blue TADF materials PytBuCz-XT and PytBuCz-TRZ comprised of the same 3,6-di-tert-butyl-9H-carbazole donor and pyridine bridge but different acceptors of XT and TRZ are prepared, and comparatively investigated. It is found that given the same donor, the introduction of XT acceptor endows the molecule with better thermal stability, larger BDEs, smaller ΔE_ST, higher Φ_PL, and faster RISC process. In comparison with PytBuCz-TRZ, PytBuCz-XT provides not only higher EQE_maxs but also much longer operational lifetimes for the devices employing them as emitters or sensitizers. These findings strongly demonstrate that XT could be a promising and competitive building block for constructing efficient and stable TADF materials.

Author contributions

Z. Liu designed, prepared and measured OLED devices. T. Guo and J. Fang synthesized and characterized molecules. T. Guo conducted theoretical calculations. Z. Liu and T. Guo analyzed the results and wrote the manuscript. B. Z. Tang and Z. Zhao supervised the experiments. Z. Zhao revised and finalized the manuscript. All the contributors have given their approval for the submitted version of the manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

Z. L. and T. G. contributed equally to this work. This work was financially supported by the National Natural Science Foundation of China (U23A20594 and 22375066), the GuangDong Basic and Applied Basic Research Foundation (2023B1515040003) and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology.

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Footnote

† Electronic supplementary information (ESI) available: Materials and instruments, synthesis and characterization data, TGA and DSC thermograms, cyclic voltammograms, BDE calculation, fluorescence and phosphorescence spectra and transient PL decay curves. See DOI: https://doi.org/10.1039/d5tc01399b

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