Solution-processable triplet exciton harvesting blue and orange emitters for a high-efficiency, color-stable, simple-structured white fluorescent organic light-emitting diode

Abstract

The utilization of triplet excitons has attracted considerable attention for high-efficiency fluorescent organic light-emitting diodes (OLEDs). Here, two solution-processable triplet-exciton-harvesting emitters (CPhCN and CBzTPA) are reported. They are designed and synthesized by connecting a highly soluble tercarbazole moiety to a donor–acceptor fragment of 4-(triphenylen-2-yl)benzonitrile for CPhCN and 4-(benzo[c][1,2,5]thiadiazol-4-yl)-N,N-diphenylaniline for CBzTPA. Photophysical studies and theoretical calculations prove that CPhCN is a triplet–triplet annihilation (TTA)-based sky blue emitter and CBzTPA is a hybridized local and charge-transfer (HLCT)-based orange emitter with decent solid-state photoluminescence quantum yields. Due to their commendable hole mobility, high solubility, and thermal stability, these compounds are effectively utilized as non-doped emitters in solution-processed blue and orange OLEDs, achieving satisfactory device performances. Notably, the white light-emitting OLED utilizing a compatible blend film of CPhCN and CBzTPA (99.24 : 0.76) as a single emissive layer successfully produces white light emission with the CIE coordinates of (0.315, 0.372) and exceptional color stability across various applied voltages and achieves outstanding electroluminescent performance with a maximum external quantum efficiency (EQE_max) of 9.43%. This work offers valuable guidance for the development of a high-efficiency, color-stable and solution-processed white fluorescent OLED.

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Introduction

White organic light-emitting diodes (WOLEDs) have emerged as a promising technology for modern lighting and display applications due to their ability to produce high-quality white light, offering wide-ranging advantages such as excellent efficiency, large-area device fabrication, mechanical flexibility, and low energy consumption.^1–3 In general, white light emission in OLEDs is achieved by combining different colors of emitted light generated from multiple emitters to realize light emission collectively covering the whole visible spectrum (380–780 nm). A simple and highly effective approach requires the use of two complementary colors, specifically blue and orange,^4,5 to generate white light. This system can lessen the complexity of the device architecture while maintaining the necessary spectral balance to achieve high color quality and efficiency. It is well known that traditional fluorescent OLEDs have an internal quantum efficiency (IQE) limit of 25% because only singlet (S₁) excitons can be utilized for light emission,⁶ while the remaining 75% of triplet (T₁) excitons decay non-radiatively, hence restricting their electroluminescence (EL) performances. In recent years, fluorescent organic materials with triplet–triplet annihilation (TTA), hybridized local and charge transfer (HLCT), and thermally activated delayed fluorescence (TADF) properties have been extensively studied as singlet- and triplet-exciton-harvesting emitters for realizing high-performance fluorescent OLEDs.^7–11 TTA and TADF are two upconversion phenomena that concern the harvesting of the lowest-energy triplet excitons (T₁) by means of a reverse intersystem crossing (RISC) of T₁ to S₁ in TADF and an annihilation of two T₁ excitons to form a higher-energy S₁ in TTA. Meanwhile, HLCT involves a conversion of high-lying triplet (T_n, n ≥ 2) excitons into radiative singlet (S_m, m ≥ 1) states via a high-lying reverse intersystem crossing (hRISC) process. These processes help to overcome the 25% IQE limit of fluorescent emitters with a theoretical IQE limit of 100% for TADF and HLCT emitters and an upper IQE limit of 62.5% for TTA emitters, as two electrogenerated T₁ excitons can fuse into one S₁ exciton. Recently, TTA-based materials have been developed as a new class of emitters or host materials for highly efficient and stable blue OLEDs because of their attractive photophysical properties, including the capability to generate emissive S₁ excitons via TTA process, high photoluminescence quantum yield, and simple molecular design.^10,12–14 Besides, the TADF process is initiated from a charge transfer state, which makes it hard to obtain blue emission. The long T₁ lifetimes in TADF emitters can also cause several issues in the devices, as well as incorporate them as dopants within a suitable host matrix to avoid unwanted interactions and enhance device efficiency.^15,16 However, unlike TADF materials, the HLCT emitters with fast hRISC rate constant and ultra-short lifetimes of the high-lying triplet excitons are favourable for the fabrication of cheap high-efficiency non-doped OLEDs.^17–19 In this regard, the combination of a TTA-based emitter with sky-blue fluorescence and an HLCT-based emitter with orange fluorescence could be a perfect candidate in formulating a bicomponent white light-emitting material, where the TTA serves as both a sky-blue emitter and host matrix, while the HLCT acts as an orange dopant.

Thermal vacuum evaporation has been widely used to fabricate highly efficient WOLEDs, as it offers precise control over the film thickness and composition without damaging the neighboring layers. However, the method faces several serious issues, such as high energy consumption and ineffective material utilization, which are hurdles for the large-scale production of affordable WOLEDs. Alternatively, solution processing offers a low-cost and large-scale manufacturability, leading to it being widely regarded as a promising choice for the mass production of economical WOLEDs.^20–22 So far, a large number of small-molecule fluorescent emitters have been investigated and applied as solution-processable non-doped emitters in single emissive color OLEDs.^23–30 Some small molecule-based solution-processed WOLEDs have been reported.^27,31,32 Meanwhile, only a few numbers of solution-processable triplet exciton harvesting emitters and their solution-processed non-doped OLEDs have been studied.^33–38 Recently, our group reported on some solution-processable HLCT emitters and their electroluminescent devices. G1FTPI and G2FTPI as HLCT emitters bearing triphenylamine-phenanthroimidazole (PI) as an HLCT core substituted with fluorenyl carbazole dendrons exhibited a deep-blue emission and solution-processed OLED with an EQE_max of 5.30%,³⁹ while the TPA-NZF as an HLCT emitter comprising naphthothiadiazole (NZ) functionalized with triphenylamine (TPA) and dioctylfluorene showed a deep-red fluorescent emission and solution-processed device with an EQE_max of 3.59%.⁴⁰ A series of solution-processable HLCT green emitters, including CBFPhC, CBFTPA, CPBzFC, CBzF, and CBzFC, were also synthesized using 7-(4-(carbazol-N-yl)phenyl)benzothiadiazole as an HLCT fragment decorated with either 9,9′-bis(8-(3,6-di-tert-butylcarbazol-N-yl)octyl)fluorene or 9,9′-bis(octyl)fluorene. Their solution-processed OLEDs achieved EQE_max values of 5.59–6.74%.^37,41,42 Usta et al. in 2020 reported a solution-processed HLCT emitter based on oligo(p-phenyleneethynylene).³⁸ The non-doped OLED produced green emission with an EQE of 5.5%. Alam et al. recently synthesized HLCT emitters of KCPhDPA and KCPhCz, and their solution-processed non-doped devices emitted blue and green lights with EQEs of 5.7% and 7.4%, respectively.⁴³ For the solution-processed TTA emitters, our group prepared FAnCN and FCsCN bearing (anthracen-9-yl)benzonitrile (AnCN) and (chrysen-6-yl)benzonitrile (CsCN) as TTA emissive cores modified with 9,9′-bis(8-(carbazole-N-yl)octyl)fluorene.⁴⁴ Their solution-processed non-doped TTA OLEDs showed intense blue emissions with EQE_max values of 5.47–6.84%. Ding et al. in 2023 reported on TbuPyB based on benzene substituted with three tert-butylpyrene arms.⁴⁵ The non-doped TTA device exhibited an impressive EQE of 10.65%.

Hence, this work aims to develop solution-processable triplet exciton harvesting blue and orange emitters for efficient two-component single-layer WOLEDs. In the molecular design, as described in Fig. 1, CPhCN comprises 4-(triphenylen-2-yl)benzonitrile as a TTA emissive core, while CBzTPA consists of 4-(benzo[c][1,2,5]thiadiazol-4-yl)-N,N-diphenylaniline as an HLCT emissive fragment. Both units are modified with a highly soluble and electron-rich 3′′,6′′-di-tert-butyl-9,9′-didodecyl-9H,9′H-3,3′:6′,9′′-tercarbazole.^28,46,47 Both 4-(triphenylen-2-yl)benzonitrile and 4-(benzo[c][1,2,5]thiadiazol-4-yl)-N,N-diphenylaniline have been successfully employed as part of the molecules in the construction of blue TTA emitters and orange HLCT emitters, respectively.^36,48–50 Their derivatives were effectively applied as emitters in thermally evaporated OLEDs, which showed decent EL performances. Meanwhile, the tercarbazole moiety holding electron-donating, hole-transporting, and solubility-enhancing properties has been effectively utilized as a functionalizing section to many fluorescence cores in the preparation of solution-processable emissive materials.^27,36,46,51 The presence of long alkyl chains and tert-butyl groups offers high solubility in organic solvents. The carbazole units not only improve the hole-transporting and morphological properties, but also lower the ionization potential (or HOMO level) of the molecule, giving rise to an effective hole injection and transport in the device.⁵² The terminal tert-butyl substituents on carbazole will offer electrochemical stability to the molecule.⁵³ Indeed, CPhCN and CBzTPA exhibited solution-processable triplet exciton harvesting features with acceptable OLED device performances (EQE_max of 4.54% for CPhCN and 3.94% for CBzTPA). Their blend film as a single emissive layer realized a simple-structured WOLED with CIE coordinates of (0.315, 0.372) and a high EQE_max of 9.43%.

Fig. 1 Molecular structures of the charge transfer emitters (CBzTPA and CPhCN).

Results and discussion

Materials synthesis

The designed molecules (CPhCN and CBzTPA) were synthesized as outlined in Scheme S1. The Suzuki cross-coupling reaction of 2,7-dibromotriphenylene (1) with 4-cyanophenylboronic acid and 4,7-dibromobenzo[c][1,2,5]thiadiazole (3) with 4-(diphenylamino)phenyl)boronic acid was performed in the presence of Pd(PPh₃)₄/K₂CO₃ as a catalytic system to obtain the bromo intermediates 2 and 4 in good yields. Then, under the same conditions, the reactions of each of the resultants 2 and 4 with an available 3′′,6′′-di-tert-butyl-9,9′-didodecyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H,9′H-3,3′:6′,9′′-tercarbazole⁴⁷ afforded CPhCN as pale yellow solids and CBzTPA as orange solids in 70–75% yields, respectively. The structural characteristics of all the compounds were unambiguously confirmed through multinuclear NMR (¹H/¹³C) analysis and high-resolution mass spectrometry, with the results agreeing well with their molecular structures (Fig. S7). Owing to the existence of dodecyl and tert-butyl substituents, both CPhCN and CBzTPA exhibited excellent solubility in most organic solvents (THF, chloroform, chlorobenzene, and toluene), which could allow them to be cast into good-quality thin films by simple, scalable solution processing techniques such as spin-coating, slot-die coating, doctor blading, and ink-jet printing.

Photophysical properties

The photophysical properties of CPhCN and CBzTPA were systematically investigated in dilute solutions and film states. As shown in Fig. 2a, UV-vis absorption spectra in toluene displayed two prominent absorption bands in the regions of 270–300 nm and 325–520 nm. The absorption peaks at around 300 nm were attributed to π–π* electronic transitions of the carbazole moieties, while weaker absorption bands at longer wavelengths (>340 nm) were ascribed to the intramolecular charge transfer (ICT) transition of the ground state between the donor (tercarbazole and triphenylamine) and acceptor (benzonitrile and benzothiadiazole). The latter bands appeared at 348 nm for CPhCN nm and 455 nm for CBzTPA, indicating a much stronger donor–acceptor interaction in CBzTPA than in CPhCN. Accordingly, CPhCN showed a strong blue PL emission with a peak at 420 nm accompanied by a high absolute PL quantum yield (Φ_F) of 76% in the toluene solution, whereas the CBzTPA solution emitted an intense orange PL emission peaking at 590 nm with an Φ_F of 90%. Their UV-vis absorption and PL spectra in the neat films showed similar profiles to those in toluene solutions, as shown in Fig. 2b. According to the absorption onset in the thin films, the optical bandgaps (E_gs) were estimated to be 3.01 and 2.26 eV for CPhCN and CBzTPA, respectively. Upon photoexcitation, CPhCN displayed sky-blue emission at 470 nm in a neat film with a redshift of 50 nm compared to that in toluene solution, and its corresponding Φ_F was reduced to 52%, indicating the presence of a certain degree of intermolecular interactions in the solid state. Meanwhile, CBzTPA in a neat film retained an orange emission with a slight redshift (18 nm) peaking at 608 nm and a small decrease of Φ_F to 76%, suggesting weak intermolecular interactions in the film state. To attain the S₁ and T₁ energy levels of CPhCN and CBzTPA, the room-temperature triplet state spectroscopic (TRES) measurement method in poly(methyl methacrylate) shielded with EXCEVAL^TM film was performed (Fig. 2c and d).⁵⁴ According to the prompt PL and delayed PL (phosphorescence) emission peaks, the S₁ and T₁ values could be calculated as 3.31 and 2.63 eV for CPhCN, and 2.50 and 2.33 eV for CBzTPA, respectively. Hence, the production of extra singlet excitons in the device through the TTA process is possible for CPhCN owing to its low T₁, and the energy levels of S₁ and T₁ also follow the principle of 2E_T1 > E_S1 for the fusion of two low-energy T₁ excitons into one high-energy exciton. Besides, the sizable energy gap between the S₁ and T₁ energy levels (0.17–0.68 eV) proved that the TADF mechanism for both CPhCN and CBzTPA could be excluded. Additionally, the transient PL decay spectra analyzed in the neat film (Fig. S1) illustrated monoexponential decay profiles with single lifetime values (τ) in the nanosecond (3.4–8.0 ns), further ruling out the TADF process.

Fig. 2 UV-vis absorption and PL spectra in (a) dilute toluene solutions (∼1.5 × 10⁻⁵ M) and (b) thin films coated on fused silica substrates (insets: fluorescence images of thin films under UV-365 nm illumination). The integrated TRES slices of the prompt PL spectrum and delayed PL spectrum @14 ms for (c) CBzTPA and (d) CPhCN.

To further study the excited state natures of CPhCN and CBzTPA, the solvation effects on the UV-vis absorption and PL emission were examined in different polar solvents ranging from the non-polar solvent hexane to the highly polar solvent dimethylformamide (DMF). As shown in Fig. 3a and d, the UV-vis absorption plots remained nearly unchanged with the variation in of solvent polarity, indicating that solvent polarity had a negligible effect on the molecular ground state, i.e., slight alterations in the ground state dipole moment (μ_g) across different environments. In contrast, the PL spectra of CPhCN and CBzTPA changed markedly in different polar solvents. As the solvent polarity increased, the PL spectra were gradually red-shifted and broadened. From hexane to DMF, the emission wavelength red-shifted by 168 nm for CPhCN and 105 nm for CBzTPA, proving a clear ICT feature. According to the plots of the Stokes shift as a function of the solvent orientation polarizibility (f(ε,η)) following the Lippert–Mataga model (Fig. 3b and d), a single good linear relationship with the excited state dipole moment (μ_e) of 38.46 D for CPhCN and 20.91 D for CBzTPA was attained, which agreed with the typical ICT natures of both compounds. Additionally, the transient PL decay spectra of the two compounds in different solvents showed single exponential decay profiles and short lifetimes (Fig. 3c and f), confirming that the excited state responsible for the PL emission arose from one excited state.

Fig. 3 (a) and (d) Normalized UV-vis absorption/PL spectra in different solvents. (b) and (e) Lippert–Mataga plots of Stokes shift vs. solvent polarity function (f(ε,η)). (c) and (f) Transient PL decay traces in various solvents.

Theoretical calculations

To evaluate the structure–property relationships of CPhCN and CBzTPA, density functional theory (DFT) and time-dependent (TD) DFT calculations were performed using Gaussian 16 software at the B3LYP/6-31G(d,p) level for exploring the molecular electronic properties in the ground and excited states. As depicted in Fig. 4a, the ground-state optimized molecular geometries of CPhCN and CBzTPA revealed non-planar and bulky structures, which can effectively limit the intermolecular interactions to a certain extent in order to preserve the pure PL emission and high luminous efficiency of both compounds in the film states. The lowest unoccupied molecular orbital (LUMO) of CPhCN was mostly delocalized on the 4-(triphenylen-2-yl)benzonitrile backbone and the attached phenyl ring, while the LUMO of CBzTPA was mainly localized on the benzothiadiazole unit with a negligible distribution on the adjacent phenyl rings. The highest occupied molecular orbital (HOMO) of CPhCN was largely delocalized on the carbazole units of the tercarbazole moiety, whereas the HOMO of CBzTPA was distributed over the entire conjugated backbone of the molecule. Accordingly, there was a large separation between the HOMO and LUMO distributions in CPhCN, indicating that the molecule predominantly exhibits a CT state. In contrast, the partial overlap between the HOMO and LUMO orbitals in CBzTPA suggests a mixing of both locally excited (LE) and CT characters within the same excited state or HLCT state, which could be beneficial to the high luminous efficiency and the effective carrier transport ability.

Fig. 4 (a) The HOMO and LUMO distributions calculated by the B3LYP/6-31G(d,p) method. (b) The natural transition orbits (NTOs) of the S₀ → S₁ transition calculated using the TD-B3LYP/6-31G(d,p) function. The percentage is the proportion of transitions.

To further explore the excited state properties, the natural transition orbitals (NTOs) were calculated and analyzed. As illustrated in Fig. 4b, the hole and particle of the S₀→S₁ transition in CBzTPA showed a substantial overlap in the central benzothiadiazole ring and partially extends into the neighboring benzene rings, while a certain separation was observed in the triphenylamine and carbazole rings. This hole and particle distribution of CBzTPA replicates typical HLCT features with the potential to utilize triplet excitons. Notably, the high oscillator strength (f) of 0.446 would essentially facilitate high luminescence efficiency. For the S₀ → S₁ transition in CPhCN, the hole and particle exhibited a considerable separation between the 4-(triphenylen-2-yl)benzonitrile conjugated backbone and tercarbazole moiety with a fractional overlap of the two moieties on the benzene ring of the triphenylene unit. Such hole and particle distribution of CBzTPA confirms the HLCT feature with a sizable domination of CT character (f = 0.055). The extensive orbital overlap signifies a strong LE-type transition from S₁ to S₀, which likely contributes to the high radiative transition rate. Conversely, the partial charge separation indicates the existence of CT behavior, potentially enabling the generation of multiple singlet excitons. The large separation between the two orbitals (CT feature) could offer possible transport channels for holes and electrons, and thus favor bipolar properties.⁵⁵ In addition, the NTO analysis revealed prominent singlet and triplet excited states with the corresponding energy shown in Fig. S2. Markedly, high-energy HLCT states were witnessed with a small energy splitting between S₁ and T₂ of 0.12 eV for CPhCN and 0.23 eV for CBzTPA. These features support the feasibility of reverse intersystem crossing (RISC), which may enhance the fluorescence and boost the external quantum efficiency (EQE) in OLED applications. Obviously, the large energy gaps between T₂ and T₁ in both molecules (0.67 eV for CPhCN and 1.09 eV for CBzTPA) can greatly suppress interconversion (IC) decay from T₂ to T₁, according to the energy-gap law. Meanwhile, a big energy gap between S₁ and T₁ (0.55 eV for CPhCN and 0.96 eV for CBzTPA) also limits a reverse intersystem crossing (RISC) process (T₁–S₁ transition) via the TADF process. Notably, the calculated T₁ (0.93 eV) of CPhCN was relatively deep, and the two-T₁ fusion energy levels (2E_T1) follow the condition of 2E_T1 (1.86 eV) > E_S1 (1.78) for promoting the annihilation of the triplet excited states to singlet excited states via a TTA mechanism.

Thermal, morphological, electrochemical, and hole-transporting properties

The thermal properties of CPhCN and CBzTPA were analyzed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown in Fig. S3, both compounds had a higher thermal stability with decomposition temperatures (T_5d) at 5% initial weight loss over 418 °C. Meanwhile, the glass transition temperature (T_g) related to the endothermic baseline shift in the DSC trace was observed at 195 °C for CPhCN and 289 °C for CBzTPA. The morphology of the neat films of CPhCN, CBzTPA, and CPhCN blended with CBzTPA (99.24 : 0.76) was examined by atomic force microscopy (AFM) and thin film X-ray diffraction. The films were prepared by spin coating from their chlorobenzene solutions onto glass substrates, followed by thermal annealing at 100 °C for 20 min. As depicted in Fig. 5a, the non-contact mode AFM images of the thin films displayed a smooth surface with no pinholes and crystalline islands, indicative of an amorphous morphology with a root-mean-square (rms) roughness of 0.554 nm for CPhCN and 0.278 nm for CBzTPA. In particular, the blended film has a rms value (0.533 nm) similar to the CPhCN film according to the dominant CPhCN amount in the film, signifying a good compatibility between the dopant (CBzTPA) and CPhCN. This result suggests that the physical mixing of CBzTPA and CPhCN is suitable for the fabrication of a bicomponent single-emissive layer WOLED using solution processing. Moreover, the XRD patterns of the thin films showed a broad diffraction peak in a wide 2θ range of 15–25° with no clear diffraction peaks being noticed, proving their amorphous characteristic, as depicted in Fig. 5b.⁵⁶ The properties of high thermal stability and good film-forming capability of these emitters and their blending could be beneficial for the fabrication of OLED devices by solution-based methods.

Fig. 5 (a) Non-contact mode AFM images of the spin-coated thin films. (b) XRD patterns of the thin films. (c) Current density–voltage (J–V) plots of the hole-only devices (HODs).

The electrochemical properties of CPhCN and CBzTPA were analyzed by cyclic voltammetry (CV) measurement performed using 0.1 M n-Bu₄NPF₆ as a supporting electrolyte. As shown in Fig. S4, the CV traces exhibited multiple quasi-reversible redox processes with the half-wave potentials (E_1/2) being listed in Table 1. Two compounds had virtually the same first oxidation process, appearing at E_1/2 values around 0.9 V, which was ascribed to the oxidation of the tercarbazole conjugated fragment to form the corresponding radical cations.^27,51 Markedly, only CBzTPA presented a quasi-reversible reduction process at E_1/2 of −1.49 V, which was related to a radical anion formation of the benzothiadiazole unit.⁵⁷ The repetitive CV scans of CPhCN and CBzTPA displayed unchanged voltammograms, which evidenced that they were electrochemically stable emitters. The HOMOs of CPhCN and CBzTPA were measured in thin films using an ultraviolet photoelectron spectrometer in air (PESA) (Fig. S5), and their LUMOs were deduced from E^opt_g and HOMO values (LUMO (eV) = HOMO − E^opt_g). As tabulated in Table 1, the HOMO and LUMO levels were determined to be −5.46 and −2.45 eV for CPhCN and −5.27 and −3.01 eV for CBzTPA, respectively. Importantly, both emitters reveal high HOMO levels, signifying that they are able to be used as a hole-transporting layer (HTL)-free emissive material. Moreover, these HOMO levels align well with the work function of the ITO/PEDOT:PSS anode (5.20 eV) (Fig. 6a), supporting that the emitters can be fabricated next to this electrode layer without any supporting HTL, resulting in a simple-structured solution-processed OLED. The hole-transporting property of the compounds was further analyzed using a hole-only device (HOD). The HODs were fabricated with a structure of ITO/PEDOT:PSS (40 nm)/emitters (100 nm)/MoO₃ (10 nm)/Al (110 nm), where molybdenum trioxide (MoO₃) served as a hole injection layer for blocking electrons from the opposite electrode. A thick layer of the emitters will enable the prevention of charge-carrier accumulation.⁵⁸ Following the space-charge-limited current (SCLC) theory, hole mobility was calculated from the current density–voltage (J–V) plots of the HOD (Fig. 5c) using the Mott–Gurney equation and Frenkel effect.⁵⁹ The hole mobilities of CPhCN, CBzTPA, and CPhCN : CBzTPA (99.24 : 0.76) blend films were 4.74 × 10⁻⁷, 2.92 × 10⁻⁷, and 6.75 × 10⁻⁷ cm² V⁻¹ s⁻¹, respectively. It should be noted that the hole mobility of the blended film was comparable to that of its primary compounds, likely owing to its well-homogeneously mixed film, which is credited to the high compatibility of both emitters to be blended. Such decent hole mobilities can contribute to a broadening of the recombination zone in the emissive layer, resulting in a low driving voltage and long lifetime, as well as high luminance efficiency.⁶⁰

Table 1 Optical and physical data of CPhCN and CBzTPA

Compd	λ abs (nm) sol^a/film^b	λ ^maxPL (nm) sol^a/film^b	Φ F (%) sol^a/film^b	τ (ns) sol^a/film^b	S1/T1^f (eV)	E 1/2 vs. Ag/Ag^+^e (V)	T g/Td^f (°C)	E ^optg/HOMO/LUMO^g (eV)
a Measured in dilute toluene. b Measured in a thin film coated on fused silica substrates. c Absolute PL quantum yield measured using an integrating sphere. d Transient PL decay lifetime. e Obtained from CV and DPV measurements in CH2Cl2 with a scan rate of 50 mV s^−1. f Measured by DSC (2nd scan) and TGA under N2 flow at a heating rate of 10 °C min^−1. g Estimated from E^optg = 1240/λonset, where λonset is the onset of the absorption spectra in the thin film. HOMO obtained from PESA measurements of the thin films, LUMO (eV) = HOMO − E^optg.
CPhCN	348/350	420/470	76/52	3.7/3.4	3.31/2.63	0.92, 1.06, 1.17, 1.38	195/414	3.01/−5.46/−2.45
CBzTPA	455/470	590/608	90/76	6.7/8.0	2.50/2.33	−1.49, 0.94, 1.14, 1.46	287/442	2.26/−5.27/−3.01

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Fig. 6 (a) Energy diagram of the OLEDs and organic material used in the devices. (b) EL spectra at maximum luminance (insets: photographs of the OLEDs under operation). (c) Current density–voltage–luminance (J–V–L) characteristics. (d) Current efficiency–luminance–external quantum efficiency (CE–L–EQE) plots. (e) Transient EL plots at different applied voltages (7–10 V) of the CPhCN-based OLED. (f) Compared transient EL traces of the CPhCN and CBzTPA-based OLEDs at 8 V. (g) Log–log scale plots of luminance vs. current density.

Electroluminescent properties

Inspired by their excellent photophysical and solution-processed HTL-free emissive features, the electroluminescence (EL) properties of CPhCN and CBzTPA as the emitting layer (EML) for a simple solution processed OLED were first investigated with the device configuration of ITO/PEDOT:PSS (40 nm)/EML (40 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (110 nm) (Fig. 6a). Herein, indium tin oxide (ITO) and aluminum (Al) were employed as electrodes, lithium fluoride (LiF) served as an electron injection layer, and 1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB) acted as both electron transport layer and hole blocking layer. EML was spin-cast from the solutions of either CPhCN or CBzTPA (2% w/v) in chlorobenzene. The EL characteristics and performances of the fabricated devices are shown in Fig. 6 and listed in Table 2. Both solution-processed OLEDs unveiled low turn-on voltages (V_on at 1 cd m⁻²) of 3.0–3.2 V, indicating the efficient charge injection and transporting properties in the devices. As illustrated in Fig. 5b, the OLED using the CPhCN emitter produced a sky-blue emission color with an EL peak at 472 nm and the Commission Internationale de l’Eclairage (CIE) coordinates of (0.159, 0.220), while the CBzTPA-based device emitted an orange color with an EL peak at 595 nm and the corresponding CIE coordinates of (0.555, 0.441). The EL spectra matched well with their correlated PL spectra in the thin films without unwanted emission bands from the TmPyPB emission (@<400 nm)⁶¹ and the EML/TmPyPB exciplex emission (@>500 nm),⁶¹ signifying effective recombination of electron and hole carriers in the EMLs and purely EML emissions. The EL spectra recorded over the operating voltages (5–9 V) of the OLEDs showed consistent EL emission from the EML with no change in the spectral shape (Fig. S6), reflecting that the emitter and device emission properties remain the same despite the voltage change. These simple-structured non-doped OLEDs demonstrated acceptable EL performances. The CPhCN-based device achieved a brightness of (L_max) of 6268 cd m⁻², a maximum external quantum efficiency (EQE_max) of 4.54%, a maximum current efficiency (CE_max) of 6.50 cd A⁻¹, and a maximum power efficiency (PE_max) of 6.44 lm W⁻¹. The CBzTPA-based OLED showed a slightly inferior performance with the L_max, EQE_max, CE_max, and PE_max, values of 3436 cd m⁻², 3.94%, 7.36 cd A⁻¹, and 6.63 lm W⁻¹, respectively. It is anticipated that such good EL performances of both devices could be attributed to good thin film PL efficiency, hole mobility, as well as appropriate HOMO/LUMO levels of their emitters. To further explore the utilization of excitons in the EL process, the singlet exciton utilization efficiency (η_s) was calculated using η_s = EQE/(η_out × η_rec × Φ_PL), where η_out represents the light outcoupling efficiency, approximately 20% in glass substrates, Φ_PL denotes the absolute PL quantum yield of the emissive layer in thin film (Φ_PL = 52% for CPhCN and Φ_PL = 76% for CBzTPA), and η_rec symbolizes the exciton formation efficiency from injected charge carriers, assuming 100% for ideal charge recombination. Consequently, the η_s of CPhCN- and CBzTPA-based devices were calculated to be 43% and 27%, respectively. This result indicates that the radiative exciton yield in these OLEDs surpasses the 25% typically seen in conventional fluorescent OLEDs, highlighting the effectiveness of CPhCN and CBzTPA as solution-processable hot exciton emitters. To discover the TTA process in the non-doped devices, the transient EL measurements were carried out with a voltage pulse of 20 μs at a frequency of 1 kHz and a negative voltage pulse of −5 V shortly after the termination of an applied positive voltage pulse to eliminate the delayed fluorescence (DF) generated from charge trapping. As depicted in Fig. 6e, the transient EL profile of the CPhCN-based OLED showed the typical behavior of the TTA process, as the spectra consisted of two distinct decay components: the first component represents a prompt decay emission from a rapid radiation of the initially generated singlet excitons, and the second component denotes a microsecond-scaled delayed emission from the TTA-induced singlet excitons. Under the varying voltage, the ratio of the delayed component increased gradually from 7 to 10 V, resulting from the TTA process, which was consistent with the transient EL profiles of typical TTA OLEDs.^14,49,62,63 Triphenylene derivatives have previously been employed as TTA emitters to realize high-efficiency blue TTA OLEDs.^36,49 In contrast, the transient EL trace of the CBzTPA-based device showed a clear prompt decay component with little, if any, of the delayed part. A comparison of the transient EL decay profiles of both devices at the same voltage (@8 V) (Fig. 6f) reveals that the delayed component in the CPhCN-based OLED is significantly larger than that in the device of the CBzTPA emitter. Fig. 6g shows the comparison of luminance vs current density in a log–log scale of the OLEDs. The CPhCN-based device displayed a linear correlation between the luminance and current density, suggesting a linear relationship between the output photons and excitons generated, which is a characteristic of typical fluorescent OLEDs. In contrast, the CBzTPA-based device showed a non-linear relationship with two slopes at low current density and high current density regimes, indicative of the TTA process.⁶⁴ These findings suggest that the TTA phenomenon is a mechanism contributing to the superior EL performance of the CPhCN-based OLED, while the improved EL result in the CBzTPA-based device is rather associated with the HLCT character of the CBzTPA emitter.

Table 2 Hole mobility and electroluminescent data of CPhCN and CBzTPA as emitters^a

EML	Hole mobility^b (cm^2 V^−1 s^−1)	V on (V)	λ EL (nm)	L max (cd m^−2)	J max (mA cm^−2)	EQEmax^d (%)	CEmax^e (cd A^−1)	PEmax^f (lm W^−1)	CIE (x, y)
a ITO/PEDOS:PSS (40 nm)/EML (40 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (110 nm). b Obtained from HOD (ITO/PEDOT:PSS (40 nm)/EML (100 nm)/MoO3 (10 nm)/Al I(110 nm). c A maximum brightness. d A maximum external quantum efficiency. e A maximum current efficiency. f A maximum power efficiency.
CPhCN	4.74 × 10^−7	3.2	472	6286	361	4.54	6.50	6.44	(0.159, 0.220)
CBzTPA	2.92 × 10^−7	3.0	595	3436	243	3.94	7.36	6.63	(0.555, 0.441)
CPhCN + CBzTPA (99.24 : 0.76)	6.75 × 10^−7	3.0	471, 561	3927	127	9.43	8.83	8.28	(0.315, 0.372)

---

To take advantage of the complementary emission colors of the blue-emitting CPhCN and orange-emitting CBzTPA, their excellent compatible-blended film, as well as the excessive EL performance of their non-doped devices, the solution-processed white OLED (WOLED) utilizing a bicomponent blend film of CPhCN and CBzTPA as a white-emitting EML was investigated. To realize a pure white light emission from the device, the blending ratios of CPhCN (blue) and CBzTPA (orange) were tested. It was found that by blending CPhCN and CBzTPA in a ratio of 99.24 to 0.76, the PL spectrum of the mixing thin film displayed a broad emission profile covering the wavelength range from 400 to 800 nm with a Φ_F of 100% (Fig. 7a). Two emission peaks were noticed at 466 nm and 580 nm, corresponding to the emissions from CPhCN and CBzTPA, respectively. The intensity of the first peak was significantly weaker than the second one, despite a much higher content of CPhCN than CBzTPA in the mixture. This is due to an effective Förster resonance energy transfer between them, evidenced by a strong overlap between the CT absorption peak of CBzTPA (orange) with the PL of CPhCN (blue) (Fig. 2b).⁶⁵ No excimer emission was observed in the PL, indicating that the intermolecular interactions are minimal and effectively suppressed due to the highly steric structure of both emitters. The WLOED was then fabricated using a CPhCN : CBzTPA (99.24 : 0.76) thin film as an EML with the structure of ITO/PEDOT:PSS (40 nm)/EML (40 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (110 nm). EML was prepared by the spin-coating of a CPhCN:CBzTPA (2% w/v) solution in chlorobenzene. The EL characteristic curves and relevant device performance data are shown in Fig. 7 and Table 2, respectively. Fig. 7b displays the EL spectra under several voltages with white light emission. Each EL spectrum showed a broad emission profile covering the wavelength range from 400 to 750 nm with two peaks located at 471 and 561 nm, corresponding to the emissions from CPhCN and CBzTPA, respectively. This EL spectrum looks like its PL profile, except the intensity of the blue emissive peak of the EL is much stronger than that in the PL, owing to the incomplete energy transfer from CPhCN to CBzTPA in the device. The EL spectrum at 10 V displayed a white-light emission with the corresponding CIE coordinates of (0.315, 0.372), which are close to the CIE values of pure white (0.33, 0.33). Because of the relatively balanced blue and orange components in the white light spectrum, the color-rendering index (CRI) was calculated to be 72, which is quite high for two-component single-layer WOLEDs. The correlated color temperature (CCT) was estimated to be 6186 K, which falls between that of mid-morning daylight (CCT = 5500 K) and standard average daylight (CCT = 6500 K). As the driven voltage increases from 5 V to 10 V, the EL spectra remain consistent, as evidenced by the negligible changes in the CIE coordinates (x = 0.343, y = 0.401) at 5 V, (x = 0.333, y = 0.390) at 6 V, (x = 0.326, y = 0.381) at 7 V, (x = 0.319, y = 0.379) at 8 V, (x = 0.316, y = 0.375) at 9 V, and (x = 0.315, y = 0.372) at 10 V. Such minor variations indicate that this WOLED maintains good color stability across different voltage levels, likely due to the emission coming from all the emitters and the well-matched blend that facilitates effective energy transfer among them. In addition, the angular dependencies of WOLED displayed a slight change compared to the Lambertian source and kept a nearly identical spectral shape with increasing viewing angle, denoting a consistent white color (Fig. 7c). The solution-processed WOLED exhibited outstanding performance with a low V_on of 3.0 V and excellent EL characteristics, achieving L_max, EQE_max, and CE_max and PE_max values of 3927 cd m⁻², 9.43%, 8.83 cd A⁻¹, and 8.28 lm W⁻¹, respectively. This EQE value is exceptionally high for a fluorescent WOLED device manufactured through a solution process. Markedly, the WOLED showed a small efficiency roll-off with the EQE of 6.50% at the brightness of 1000 cd m⁻². The singlet exciton utilization efficiency (η_s) of the solution-processed WOLED was calculated based on its Φ_PL of 100% to be 47%. To realize the origin of the high efficiency of the WOLED, the transient EL measurement was performed, as shown in Fig. 7f. The EL decay profiles showed a multiexponential decay within a 20 μs window, similar to what was observed in the CPhCN-based device. The intensity of the delayed EL component slightly decreases when applying the voltage from 9.0 to 13.0 V, which could be attributed to the excitons being quenched by singlet–triplet annihilation and triplet-polarons annihilation processes at high exciton and polaron density. As depicted in Fig. 7g, the log–log scale plots of the transient EL intensity vs. time (@10 V) revealed two regimes with a slope of ≈ −1 in the initial time region and a slope of ≈ −2 in the long-life region, consistent with many literature reports of TTA OLEDs.^66–69 The evidence of a long-life decay with a slope close to −2 signifies that the conversion of triplet excitons into singlet excitons through the TTA process certainly occurred in this solution-processed WOLED. Accordingly, the combination of the TTA process in CPhCN, the hRISC process in CBzTPA, and the Förster energy transfer from CPhCN to CBzTPA contributes to the high EQE of 9.43% in the CPhCN : CBzTPA (99.24 : 0.76)-based WOLED, representing a great achievement, as described in Fig. 8. Moreover, the strong PL efficiency of the blend film, along with good hole mobility and film compatibility between the emitters, could play a part in the high performance of this bicomponent solution-processed WOLED.

Fig. 7 (a) PL spectrum of the blend film (inset: picture of the thin film under UV-365 nm illumination). (b) EL spectra at different applied voltages (inset: picture of WOLED under operation @10 V). (c) Normalized angle-dependent emission intensities from WOLED compared with a typical Lambertian light source. (d) Current density–voltage–luminance (J–V–L) characteristics. (e) Current efficiency-luminance-external quantum efficiency (CE–L–EQE) plots. (f) Transient EL plots at different applied voltages (9–13 V) of WOLED. (g) Log–log scale plot of the transient EL intensity vs. time of the WOLED at 10 V.

Fig. 8 A proposed mechanism for the white-light emission of the WOLED.

Experimental

Materials and methods

The chemicals and solvents were supplied by Thai or overseas dealers, and used as received unless mentioned. NMR spectra (¹H and ¹³C) were recorded using a Bruker AVANCE III HD 600 MHz NMR spectrometer with CDCl₃ as a solvent containing tetramethylsilane (TMS) as an internal standard. High-resolution mass spectrometry (HRMS) measurements were obtained using either a Bruker compact LC-Quadrupole-Time-of-Flight Tandem Mass Spectrometer (LC-QTOF-MS/MS) or Bruker Autoflex Speed^TM MALDI-TOF mass spectrometer. UV-Vis absorption spectra were recorded on a PerkinElmer model Lambda 1050 spectrophotometer. Photoluminescence emission and lifetime measurements were carried out using an Edinburgh FLS980 spectrophotometer. The absolute fluorescence quantum yield (Φ_F) was measured using an integrating sphere equipped with an Edinburgh FLS980 spectrophotometer. Atomic force microscopy (AFM) analysis was performed on a Park system model NX-10 using the standard non-contact mode. Thin film X-ray diffraction (XRD) was carried out on a Bruker D8 ADVANCE diffractometer at 40 kV, 40 mA for Cu Kα, (λ = 1.5418 Å) from the two-theta range of 2° to 50° with a step size of 0.01° and scanning rate of 0.6° min⁻¹. The ultraviolet photoelectron spectrometer in air (PESA) measurement was made on a Riken AC-2 photoelectron spectrometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis were carried out using Rigaku model Thermoplus EV02 TG-DTA8122 and PerkinElmer model DSC8500 instruments, respectively, with a heating rate of 10 °C min⁻¹ under N₂ flow. The cyclic voltammetry (CV) experiment was performed using an Autolab potentiostat PGSTAT 101 equipped with three electrodes (platinum counter electrode, glassy carbon working electrode, Ag/AgCl reference electrode) in CH₂Cl₂ containing 0.1 M n-Bu₄NPF₆ as a supporting electrolyte under an argon atmosphere at a scan rate of 50 mV s⁻¹. Melting points were recorded using a Krüss KSP1N melting point meter. All density functional theory (DFT) calculations were performed using the Gaussian 16, Revision C.01 package.⁷⁰ The ground state geometries, HOMO/LUMO distributions and energy levels were calculated with the B3LYP functional with 6-31G(d,p) basis sets in the gas phase. The excited state geometries and energy levels were computed using time-dependent (TD) DFT calculations using the B3LYP/6-31G(d) level of theory.

Device fabrication and characterization

The solution-processed OLEDs using CPhCN, CBzTPA, and CPhCN : CBzTPA (99.24 : 0.76) as emissive layers (EML) were fabricated on an indium tin oxide (ITO)-coated glass substrate (12 Ω sq⁻¹) with a structure of ITO/PEDOT:PSS (40 nm)/EML (40 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (110 nm). Hole-only devices were fabricated with a structure of ITO/PEDOT:PSS (40 nm)/EML (100 nm)/MoO₃ (10 nm)/Al (100 nm). The substrate was cleaned with Liquinox^TM, followed by a stepwise washing process with deionized water, acetone, and isopropyl alcohol. After flushing with N₂ gas and surface-treating with UV-ozone for 20 min, the substrate was then spin-coated on top with a hole injection layer (HIL) of poly(3,4-ethylenedioxythiophene:poly(4-styrenesulfonate) (PEDOT:PSS) at a spin speed of 5000 rpm for 30 seconds and dried at 120 °C for 30 min. After that, the solution of each EML (2% w/v) in chlorobenzene was spin-coated on top at a spin speed of 3000 rpm for 30 seconds, followed by heating at 70 °C for 15 min. It was then transferred to a vacuum deposition system (Kurt J. Lasker mini SPECTROS 100 thin film deposition) with a base pressure of ∼5 × 10⁻⁶ Pa for organic and metal depositions. The layer of 1,3,5-tris(3-pyridyl-3 phenyl)benzene (TmPyPB) was evaporated on top at a rate of 0.2–0.3 Å s⁻¹. Lithium fluoride (LiF) was then deposited at a rate of 0.2 Å s⁻¹, followed by a deposition of aluminum (Al) at a rate of 0.5-1 Å s⁻¹. The thickness of all spin-coated films was measured using a Dektak XTL stylus profilometer. The thickness of all evaporated films was monitored and controlled by a quartz crystal microbalance (QCM) integrated with the instrument. Current density–voltage–luminance (J–V–L) data were measured simultaneously using a Keithley 2400 source meter and a Hamamatsu Photonics PMA-12 multi-channel analyzer. The absolute external quantum efficiency (EQE) was measured using an integrating sphere equipped with a Hamamatsu Photonics C9920-12 external quantum efficiency measurement system. All measurements were performed under an ambient atmosphere at room temperature. The transient electroluminescence (EL) data were acquired using an arbitrary function generator (AFG1022, Tektronix) and a Si photodiode (OSD15-E, Centronic), connected to a variable-gain high-speed current amplifier (DHPCA100, Femto). A positive pulse voltage with a pulse width of 20 μs and a frequency of 1 kHz was applied to the devices. To remove the partially delayed emission from the recombination of trapped charges, a negative pulse voltage of −5.0 V was applied immediately after applying a positive pulse voltage. The delayed EL signals were collected using a mixed domain oscilloscope (500 MHz, MDO32, Tektronix).

Synthesis and characterization

4-(7-(3′′,6′′-Di-tert-butyl-9,9′-didodecyl-9H,92′H-[3,3′:6′,9′′-tercarbazol]-6-yl)triphenylen-2-yl)benzonitrile (CPhCN). A mixture of 2 (0.15 g, 0.37 mmol) and 3′′,6′′-di-tert-butyl-9,9′-didodecyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H, 9′H-3,3′:6′,9′′-tercarbazole 5 (0.47 g, 0.44 mmol), Pd(PPh₃)₄ (23 mg, 0.02 mmol), and 10% K₂CO₃ (aq) (5 mL) in dried THF (45 mL) was degassed with N₂ for 10 min, and then heated at reflux under N₂ atmosphere for 12 h. After being cooled to room temperature, the reaction mixture was diluted with water and then extracted with dichloromethane (3 × 50 mL). The combined organic layer was washed with water (50 mL), brine solution (2 × 30 mL), dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo to dryness. The crude was purified by column chromatography on silica gel, eluting with hexane:dichloromethane (4 : 1), followed by reprecipitation in a mixture of dichloromethane and methanol to give yellow solids (332 mg, 70%). M.p. 147-148 °C; ¹H NMR (600 MHz, CDCl₃) δ 8.97 (s, 1H), 8.83 (d, J = 14.5 Hz, 2H), 8.75–8.70 (m, 3H), 8.59 (s, 1H), 8.52 (s, 1H), 8.42 (s, 1H), 8.33 (d, J = 1.9 Hz, 1H), 8.18 (d, J = 1.9 Hz, 2H), 8.05 (d, J = 8.3 Hz, 1H), 7.96–7.84 (m, 6H), 7.81 (d, J = 7.8 Hz, 2H), 7.74–7.67 (m, 2H), 7.64–7.50 (m, 5H), 7.46 (dd, J = 8.6, 1.9 Hz, 2H), 7.37 (d, J = 8.6 Hz, 2H), 4.41 (dt, J = 26.3, 7.3 Hz, 4H), 1.99 (dp, J = 28.9, 7.4 Hz, 4H), 1.48 (s, 25H), 1.26 (d, J = 11.9 Hz, 29H), 0.87 (td, J = 6.8, 4.6 Hz, 6H); ¹³C NMR (151 MHz, CDCl₃) δ 145.72, 142.39, 141.42, 140.70, 140.40, 140.25, 140.16, 139.86, 137.41, 133.66, 133.38, 132.74, 131.95, 130.45, 130.35, 130.11, 130.06, 129.73, 129.44, 127.94, 127.69, 127.63, 127.43, 126.96, 126.14, 126.01, 125.43, 125.12, 124.28, 124.00, 123.94, 123.81, 123.66, 123.47, 123.28, 123.19, 123.07, 122.15, 121.71, 119.36, 119.27, 119.10, 119.04, 116.17, 111.04, 109.66, 109.25, 43.57, 43.44, 34.74, 32.09, 31.92, 29.65, 29.63, 29.61, 29.57, 29.54, 29.49, 29.47, 29.35, 29.33, 29.18, 29.12, 27.43, 27.38, 22.68, 14.10; HRMS MALDI-TOF (m/z): calcd for C₉₃H₁₀₀N₄, 1273.7982; found: 1273.7965 [M⁺].

4-(7-(3′′,6′′-Di-tert-butyl-9,9′-didodecyl-9H,9′H-[3,3′:6′,9′′-ter carbazol]-6-yl)benzo[c][1,2,5]thiadiazol-4-yl)-N,N-diphenyl aniline (CBzTPA). It was synthesized similarly to CPhCN using compounds 4 and 5 and obtained as orange solids (248 mg, 75%). M.p. 114–115 °C; ¹H NMR (600 MHz, CDCl₃) δ 8.76 (d, J = 1.7 Hz, 1H), 8.48 (d, J = 1.7 Hz, 1H), 8.39 (d, J = 1.7 Hz, 1H), 8.31 (d, J = 1.8 Hz, 1H), 8.18 (s, 2H), 8.14 (d, J = 8.4, 1H), 7.94–7.87 (m, 4H), 7.85 (d, J = 8.4, 1H), 7.79 (d, J = 7.3 Hz, 1H), 7.63–7.55 (m, 4H), 7.52 (d, J = 8.5 Hz, 1H), 7.47 (dd, J = 8.6, 1.9 Hz, 2H), 7.36 (d, J = 8.6 Hz, 2H), 7.31 (t, J = 7.7 Hz, 4H), 7.24–7.20 (m, 6H), 7.08 (t, J = 7.3 Hz, 2H), 4.41 (dt, J = 24.8, 7.2 Hz, 4H), 1.98 (dp, J = 29.1, 7.4 Hz, 4H), 1.48 (s, 18H), 1.45–1.22 (m, 32H), 0.86 (q, J = 6.6 Hz, 6H); ¹³C NMR (151 MHz, CDCl₃) δ 154.64, 154.22, 147.90, 147.57, 142.38, 140.92, 140.38, 140.21, 140.11, 139.84, 133.65, 133.34, 131.82, 131.29, 129.92, 129.39, 129.36, 128.42, 127.88, 127.60, 126.10, 124.87, 123.93, 123.72, 123.47, 123.25, 123.16, 123.06, 123.04, 119.34, 119.09, 116.17, 109.23, 108.85, 43.56, 43.43, 34.74, 32.09, 31.92, 29.65, 29.63, 29.60, 29.57, 29.53, 29.48, 29.35, 29.33, 29.17, 29.12, 27.42, 27.36, 22.69, 14.11; HRMS MALDI-TOF (m/z): calcd for C₉₂H₁₀₂N₆S, 1322.7887; found: 1322.7887 [M⁺].

Conclusions

In summary, the solution-processable triplet harvesting blue and orange emitters (CPhCN and CBzTPA) were developed as an efficient bicomponent emissive layer for a solution-processed white fluorescent OLED. A series of experiments and theoretical calculations established that CBzTPA possessed suitable HLCT characters with an orange emission, while CPhCN was a sky-blue emitter capable of TTA upconversion. Both emitters exhibited admirable thin film photophysical properties with decent hole mobility, good film-forming capability, and high thermal stability, facilitating OLED fabrication via solution processing. Due to the hRISC channel from T₂ to S₁, the CBzTPA-based orange solution-processed OLED exhibited satisfactory EL performances with the EQE_max of 3.94%. Meanwhile, due to the TTA process, the CPhCN-based sky-blue solution-processed OLED achieved the EQE_max of 4.54%. More importantly, the single-emissive-layer two-color hybrid solution-processed WOLED based on CPhCN : CBzTPA (99.24 : 0.76) attained a brilliant EL performance with the EQE of 9.43%, CIE coordinates of (0.315, 0.372), and CRI of 72. These findings provide a valuable approach for developing high solid-state fluorescent solution-processable triplet harvesting emitters, which will critically reduce the complexity of the WOLED fabrication process, as well as improve the EL performance and color stability of solution-processed WOLEDs.

Author contributions

T. L. synthesized and characterized the material, fabricated the device, discussed the data, and prepared the original draft. R. W. performed the DFT calculations and analyzed the data. N. P., P. W., W. W., and T. W. partially performed the material synthesis, characterization, device fabrication, and data analysis. V. P. conceived and supervised the project, reviewed and edited the final draft. All authors approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and/or its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5tc02980e.

Acknowledgements

This work is funded by the National Research Council of Thailand (NRCT) (grant no. N42A650196).

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