Abstract

Energy level alignment of frontier molecular orbital (FMO) is essential for controlling charge carrier and exciton dynamics in organic light-emitting diodes (OLEDs). However, multiple resonance (MR) emitters with exceptional narrowband luminescence typically suffer from inadequate FMO levels. Herein, a conventional blue MR prototype with a shallow highest occupied molecular orbital (HOMO) level of −5.32 eV is initially employed to reveal the charge carrier and exciton dynamics. Severe hole trapping by its shallow HOMO significantly hinders its transport. More importantly, trapped carriers induce direct exciton formation and recombination at MR emitters in a hyperfluorescent system, leading to triplet accumulation in MR emitters. To resolve these issues, a proof-of-concept wavefunction perturbation strategy is proposed by incorporating cyano motifs at peripheral sites of MR backbone to adjust the energy levels. This approach significantly shifts HOMOs of 0.36 and 0.51 eV without compromising colour purity. The derivative substituting meta-boron position (mCNDB) exhibits a pure-blue emission peaking at 459 nm with a narrow bandwidth of 13 nm. The detrimental carrier trapping effect is eliminated, enhancing external quantum efficiency to exceeding 23%, maintaining around 20% at 1000 cd m⁻², and improving the device stability.

Introduction

Comprehending and manipulating the energy level alignments of functional molecules is essential for determining the behaviours of charge carriers and excitons in optoelectronic devices. This is particularly crucial for blue organic light-emitting diodes (OLEDs). Simultaneously achieving high efficiency, stability, and colour purity has been the most challenging topic since the first two-layer OLED was reported by C. W. Tang and S. A. VanSlyke in 1987¹. Regarding efficiency, the fundamental breakthrough of the charge-transfer (CT) state has allowed the creation of numerous efficient emitters, induced by metal-ligand CT and thermally activated delayed fluorescence (TADF) molecules^2,3,4. By versatilely utilizing spin-forbidden triplets for radiation, a unity internal quantum efficiency, as well as a high external quantum efficiency (EQE), has been realized with adequate device architectures^5,6,7,8. Furthermore, by leveraging the multiple resonance (MR) effect, an extremely narrow spectral width of less than 20 nm can be achieved by MR-TADF molecules by suppressing the intramolecular vibrations and relaxation in their excited states^9,10,11,12.

Nevertheless, device durability under continuous operation, which is highly dependent on charge carrier and exciton dynamics, remains the primary obstacle. Substantial efforts have been devoted to extending its longevity by optimizing the intrinsic properties of each component material. These efforts include reinforcing molecular rigidity to enhance photostability¹³; differentiating orbital angular momentum or incorporating heavy atoms to accelerate the triplet upconversion rate^14,15,16; deuterium substitution to suppress high-frequency vibrations^17,18; and constructing a sterically shielded structure to reduce detrimental Dexter energy transfer¹⁹. Unlike direct photon absorption under photoexcitation, the performance of electrically-driven devices is heavily associated with complicated charge carrier and exciton behaviours, e.g., single carrier injection²⁰, transport²¹, trapping^22,23, recombination^24,25, Förster/Dexter energy transfer²⁶, as well as multiple channels of exciton annihilations^27,28,29. In blue OLEDs with wide energy gaps (E_g > 2.6 eV), the alignment of frontier molecular orbitals (FMOs) energy levels between adjacent layers plays a dominant role in determining the carrier dynamics and recombination pathways. For the representative blue MR prototypes like DABNA-2 and ν-DABNA, featuring single- and binary-boron units, respectively, their highest occupied molecular orbital (HOMO) levels are localized at around −5.4 eV^9,30. These energy levels are much shallower than those of traditional blue phosphorescent or TADF emitters, as well as those of host matrices or assistant dopants (around −6.0 eV)³¹, leading to significant carrier traps under electrical excitation. To gain a comprehensive view, the HOMO levels of the reported blue boron-containing MR emitters are summarized in Fig. 1a and Supplementary Table 1. It is evident that the HOMOs of most MR emitters are shallower than −5.6 eV, highlighting the dilemma of inadequate FMOs energy levels. Although given a few examples of blue MR emitters with HOMOs deeper than −5.6 eV, these studies primarily focused on structural modifications aimed at modulating photophysical properties, like reverse intersystem crossing process, emission wavelength, and spectral bandwidth^32,33. Charge carrier trapping or blocking by inadequate energy levels has been found to impact device performance, while the underlying mechanism and effective strategies for adjusting the energy levels have not received sufficient attention^34,35.

Fig. 1: Chemical structures, theoretical calculation, and photophysical properties of studied molecules.

Results

Molecular design and energy level tuning

As illustrated in Figs. 1b and 1c, double-boron-embedded CzDB is selected as the MR prototype because of its blue narrowband emission (λ_peak = 471 nm, FWHM = 17 nm)³⁶. The structural and electronic properties of CzDB were initially investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations at the PEB0/6-31 G(d,p) level (Fig. 1d). As expected, HOMO and the lowest unoccupied molecular orbital (LUMO) wavefunctions are mainly distributed at the discrete nitrogen and boron atoms, resulting in short-distance MR effect within the robust skeleton, i.e., narrowband emission. Moreover, the atomic HOMO/LUMO separation induces a small experimental singlet-triplet splitting (ΔE_ST = 0.11 eV), triggering the TADF property (Supplementary Fig. 1). However, the simulated low-lying HOMO level of CzDB is localized at −5.12 eV, which is consistent with traditional MR molecules (summarized in Fig. 1a and Supplementary Table 1). To resolve this dilemma, electron-accepting cyano motifs are employed to substitute at the meta- and para-positions of the central boron atoms in the periphery BN backbone. As depicted in Fig. 1d and Supplementary Fig. 2, the simulation suggests that the limited size of the cyano fragments would barely influence the molecular rigidity and the wavefunction distribution at the BN core. Thus, the MR effect still governs the electronic properties of both mCNDB and pCNDB. These molecules exhibit an extremely small root mean square displacement (RMSD) of around 0.030 Å between the optimized singlet excited state (S₁) and ground state (S₀) configurations, indicating negligible structural relaxation under the excited state with cyano substituents. Furthermore, Huang?Rhys (HR) factors for the S₁ → S₀ transition are calculated. The dominant vibration modes of these molecules are primarily from the out-of-plane distortion of the CzDB core at low frequency, whereas cyano units barely contribute to the structural vibrations (Supplementary Fig. 3). As a result, the spectral widths of both mCNDB and pCNDB are as narrow as 13 and 15 nm, respectively, preserving the high colour purity in pure blue (459 nm) to sky blue (483 nm) emission (Supplementary Fig. 4). Gaining deeper insight into the function of cyano-units, the varying substitution positions lead to different effects on their electronic characteristics. In mCNDB, cyano is localized at the meta-position of boron, as well as the para-position of nitrogen, primarily contributing to the HOMO distribution. Thus, this weakens the electron-donating ability of the skeleton, resulting in a deeper HOMO level (−5.67 eV) than that of the pristine CzDB (−5.12 eV). In contrast, incorporating the cyano unit in pCNDB enhances the conjugation coupling with the para-B atoms, thus mainly participating in the LUMO distribution. The reinforced electron-accepting strength induces a much deeper LUMO level (−2.09 eV) than that of CzDB (−1.44 eV), along with a deeper HOMO level (−5.64 eV), due to the electronic effect in the highly conjugated molecular system³⁷. Noteworthy, despite the significant shifts in their HOMO/LUMO levels, wide bandgaps of 3.68, 3.76, and 3.55 eV are reserved for CzDB, mCNDB, and pCNDB, respectively, with narrowband blue luminescence. To validate the accuracy of the simulated HOMO/LUMO energy levels, cyclic voltammetry (CV) and ultraviolet photoelectron spectroscopy were used to measure the energy levels in the solution and the solid state, respectively (Supplementary Figs. 5?7). The HOMO levels of CzDB, mCNDB, and pCNDB are determined to be −5.32, −5.68, and −5.83 eV, respectively, which are consistent with the prediction of the DFT calculation.

The photophysical characteristics of CzDB, mCNDB, and pCNDB in solid-state films are successively investigated. The blend films are composed of 1.5 wt% emitters in a 3,3’ -bis(carbazol-9-yl)biphenyl (mCBP) host matrix. The photoluminescent quantum yields (PLQYs) of the mCNDB- and pCNDB-based films are 86% and 89%, respectively, which are slightly higher than that of the CzDB-based film (85%). Furthermore, distinct delayed components are observed in these blend films from the transient PL decay profiles, confirming the TADF nature of the three emitters (Supplementary Figs. 8 and 9). In specific, the larger ΔE_ST values of mCNDB (0.16 eV) and pCNDB (0.15 eV) than that of CzDB (0.10 eV) in the solid-state films (Supplementary Fig. 10), which is consistent with the singlet-triplet energy difference measured in toluene. It can be partially explained by a larger wavefunction overlap integral (0.53, 0.57, and 0.60 for CzDB, mCNDB, and pCNDB, respectively) after incorporating cyano-units. Therefore, the reverse intersystem crossing (RISC) rates of the mCNDB- and pCNDB-based films (2.7 × 10³ and 3.3 × 10³s⁻¹, respectively) are slightly slower than that of the CzDB-based film (6.4 × 10³s⁻¹). Additionally, the photostability of the three molecules in a degassed toluene solution was evaluated. The comparable PL intensity degradation under a constant excitation power of 5 mW cm⁻² indicates that the incorporation of cyano units barely affects the molecular stability (Supplementary Fig. 11). The detailed photophysical data of the three molecules are summarized in Supplementary Table 2. Although the photophysical characteristics of mCNDB and pCNDB are comparable to those of CzDB, their energy level variation would sufficiently influence the electrical properties.

Charge carrier dynamics in non-sensitized OLEDs

To evaluate the impact of varied energy levels on their EL properties, OLEDs incorporating a emitting layer (EML) with the host-guest system are fabricated as the following architecture (Supplementary Fig. 12): indium tin oxide (ITO), 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN, 10 nm), N⁴,N^4’-di(naphthalen-1-yl)-N⁴,N^4’-diphenyl-[1,1’-biphenyl]−4,4’-diamine (NPD, 20 nm), 9-phenyl-3,6-bis(9-phenyl-9H-carbazol-3-yl)−9H-carbazole (Tris-PCz, 10 nm), mCBP (5 nm), 1.5 wt% emitters: mCBP (30 nm), SF3-TRZ (10 nm), 30 wt% 8-quinolinolato lithium (Liq): SF3-TRZ (30 nm), Liq (2 nm), and aluminium (Al, 100 nm). Specifically, mCBP is employed as the electron-blocking layer and host matrices, which governs the dynamic behaviours of charge carriers. Moreover, the adjacent SF3-TRZ serves as the hole-blocking layer, which dominates electron injection. The HOMO and LUMO levels of the compounds in the charge carrier blocking layers and EML are illustrated in Fig. 2a. The characteristics of current density-driving voltage-luminance (J-V-L) are exhibited in Fig. 2d. In contrast of mCNDB- and pCNDB-based OLEDs, the CzDB-based device demands a much larger driving voltage. Noticeably, HOMO/LUMO levels (−5.32/−2.74 eV) of CzDB are much shallower than those of either mCNDB (−5.68 and −3.07 eV) or pCNDB (−5.83 and −3.34 eV), which are responsible for the electrical properties. To clarify the mechanism of different J-V characteristics, electron-only and hole-only devices (EOD and HOD) are respectively designed to independently reveal the single charge carrier dynamics under electrical excitation without the hole-electron recombination process (Fig. 2b, c, and Methods). At the electron side, LUMO levels of SF3-TRZ and the adjacent mCBP matrix were reported to be −3.10 and −2.74 eV, respectively, leading to a distinct energy gap (0.36 eV) for insufficient electron injection²¹. Consequently, EOD based on the mCBP neat film exhibits a large electron injection voltage (V_ei, ~5.5 V) and poor electron transport, as shown in Fig. 2e. After doping 1.5 wt% CzDB in mCBP matrix, the V_ei (~5.0 V) is slightly reduced, and the electron transport capability is enhanced. In contrast, mCNDB- and pCNDB-based EODs show the opposite characteristic of a reduced V_ei (~3.0 V), while a much poorer electron flow upon increasing the driving voltage. The variation in EODs can be ascribed to the LUMO alignment of the CzDB derivatives, where the LUMO levels of mCNDB (−3.07 eV) and pCNDB (−3.34 eV) are equivalent to that of SF3-TRZ ( −3.10 eV), leading to a non-barrier electron injection. Conversely, CzDB, with a shallow LUMO (−2.74 eV), is close to mCBP, maintaining a relatively large energy gap with SF3-TRZ. In addition, HOD undergoes different dynamic behaviours (Fig. 2c and f). Either mCNDB- or pCNDB-based HODs have an identical J-V property to that of mCBP-based HOD, suggesting the hole transport is governed by mCBP matrices. However, the CzDB-based HOD shows much-suppressed hole transport, as shown in Fig. 2f. Notably, CzDB has the shallowest LUMO level of −5.32 eV, and the substantial energy gap (?E_HOMO = 0.76 eV) with mCBP makes it act as the deep wells to trap the mobile holes. Upon applying a reverse bias voltage (−10 V) after switching off the pulsed EL excitation (pulse width of 100 μs), an intense EL spike appears in the CzDB-based OLED, indicating the hole trapping effect in CzDB molecules with shallow energy levels (Fig. 2g)^38,39.

Fig. 2: Device performance and charge carrier dynamic of non-sensitized OLEDs.

Discussion

Numerous efforts have been made to stabilize blue OLEDs. In particular, in advanced HF systems, excitons are expected to initially generate efficient phosphors or TADF sensitizers to manage triplet excitons. Therefore, optimizing the intrinsic photophysical properties of phosphors, TADF assistant dopants, and terminal emitters, such as increasing their triplet energy levels, reinforcing their molecular stability, and accelerating the triplet upconversion rate, has become mainstream. However, unlike photoexcitation, the pathways for exciton formation under electrical excitation are critical yet often overlooked.

This work demonstrated that charge carrier trapping by terminal emitters significantly impacts the charge carrier mobility and determines the pathways of exciton generation. A blue MR molecule, CzDB, with a shallow energy level (E_HOMO = −5.32 eV), is chosen as the prototype because its distinct delayed lifetime difference from that of HDT-1 makes it easier to distinguish the sources of the delayed components. In CzDB-based HF devices, it is concluded that the free charge carriers would directly recombine at the terminal emitters rather than the TADF sensitizers, resulting in unsatisfactory device performance. To resolve this issue, a strategy of wavefunction perturbation in blue MR backbones is proposed. The incorporation of electron-withdrawing cyano units maintains the narrowband blue emission (FWHM = 13 and 15 nm, respectively) while substantially deepening the HOMO/LUMO levels (−5.68 and −5.83 eV, respectively). Consequently, severe hole trapping and direct exciton recombination issues caused by MR emitters are circumvented, leading to a higher EQE exceeding 23%, a suppressed efficiency rolloff, and optimized operational durability in blue OLEDs.

As discussed, the prototype molecule CzDB that we chose is aimed at clarifying the pathways of exciton generation and recombination. However, its intrinsic limitations, like a slow RISC rate (~10³s⁻¹), imped the realization of the high performance. Given that the energy levels of MR molecules are generally shallow (Fig. 1a), charge carrier trapping-induced drawbacks prevalently occur in the corresponding devices. Thus, we believe that the wavefunction perturbation strategy enables us to feasibly modulate the energy levels of other advanced MR platforms, such as ν-DABNA⁹, V-DABNA⁴¹, and ω-DABNA⁴², to advance state-of-the-art blue OLEDs.

Methods

Material

All reagents were purchased from commercial sources and used without further purification (Supplementary Fig. 21). ¹H NMR and¹³C NMR spectra were recorded on a Bruker 400/101 MHz spectrometer in deuterium reagent at room temperature. MALDI-TOF mass data were recorded on a Bruker ultrafleXtreme instrument. High-resolution mass spectrometry (HRMS) data were recorded on Thermo Scientific QE plus (Supplementary Figs 22–27).

Photophysical measurement

UV?vis absorption spectra were recorded on a Hitachi U-3900 spectrophotometer. Photoluminescence (PL) spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer. Transient PL decays were measured on an integrated streak camera system (C4334, Hamamatsu Photonics). The absolute PLQY was recorded on a Hamamatsu Quantaurus-QY quantum yield spectrometer (C13534-11). The angle-dependent PL spectra of the doped films were measured via a molecular orientation characteristic measurement system (C14234-11, Hamamatsu Photonics). The photostability in solution was estimated by using a multichannel spectrometer (PMA-12, Hamamatsu Photonics) under 300–400 nm light (5 mW cm⁻²) irradiation via xenon light (MAX-303, Asahi Spectra) with a UV light intensity feedback control unit. The photostability of the HF blend films was measured in a nitrogen atmosphere by using a multichannel analyser (C14631, Hamamatsu Photonics, Japan) under 300–400 nm light (25 mW cm⁻²) irradiation from a filtered Xe lamp (MAX-350, Asahi Spectra, Japan).

Energy level measurements

The highest occupied molecular orbital (HOMO) energy levels of the emitters in the films were determined by atmospheric ultraviolet photoelectron spectroscopy using a photoelectron emission spectrometer (Riken Keiki AC-3). Cyclic voltammetry (CV) curves were recorded on a CHI660e electrochemistry station in an oxygen-free dichloromethane (DCM) solution with tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) at a scan rate of 0.1 V s⁻¹.

Theoretical calculation

All calculations were performed with the Gaussian 16 programme package⁴³. The equilibrium geometry of the ground state was determined by density functional theory (DFT) using the PBE0 density functional method with a basis set of 6-31 G(d,p). The Huang?Rhys (HR) factors for the S₁ → S₀ transition were determined with the DUSHIN module in MOMAP (Molecular Materials Property Prediction Package)⁴⁴. All calculations were performed in the gas phase. The molecular structure, frontier molecular orbital, and electron distribution models were rendered by Multiwfn and VMD^45,46.

Hole-only and electron-only devices (HODs and EODs)

Hole-only device structure: ITO/HAT-CN (10 nm)/NPD (20 nm)/Tris-PCz (10 nm)/mCBP (5 nm)/EML (50 nm)/mCBP (5 nm)/TrisPCz (10 nm)/NPD (20 nm)/HAT-CN (10 nm)/Al (100 nm). Electron-only device structure: ITO/Liq (2 nm)/SF3-TRZ: Liq (30 wt%, 30 nm)/SF3-TRZ (10 nm)/EML (50 nm)/SF3-TRZ (10 nm)/SF3-TRZ: Liq (30 wt%, 30 nm)/Liq (2 nm)/Al (100 nm).

Device characterization

The electrical characteristics of the devices were measured using a source metre (Keysight B2911A, Keysight Technologies) and a luminance metre (CS-2000, Konica Minolta, Japan). The lifetimes of all devices are measured using a luminance metre (SR-3AR, TOPCON, Japan) under constant current density driving conditions with an initial luminance of 1000 cd m⁻².

Transient electroluminescent (EL) radiation measurements

Pulsed voltages were applied by using a function generator, the light signals were detected via a photomultiplier tube (PMT) module (H10721-01, Hamamatsu Photonics, Japan), and the current signals from the PMT were amplified using a current amplifier (DHPCA-100, Femto, Germany).