Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2022Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes Xiaojie Chen, Dongyu Ma, Tiantian Liu, Zhu Chen, Zhan Yang, Juan Zhao, Zhiyong Yang, Yi Zhang and Zhenguo Chi Xiaojie Chen PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Dongyu Ma PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Tiantian Liu PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Zhu Chen PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Zhan Yang PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Juan Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Zhiyong Yang PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Yi Zhang PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author and Zhenguo Chi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100900 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Hybridized local and charge-transfer (HLCT) excited-state fluorophores, which enable full exciton utilization through a reverse intersystem crossing from high-lying triplet states to singlet state, have attracted increasing attention toward organic light-emitting diodes (OLEDs) application. Herein, we report three D–π–A–π–D-type isomers o-2CzBT, m-2CzBT, and p-2CzBT by adjusting the donor (D) units from ortho-, meta-, to para-substituted positions with the acceptor (A) core unit, respectively. The HLCT properties of the three compounds are evidently confirmed by theoretical calculations, solvatochromic behaviors, and transient decay lifetimes analyses. As the substituted position changes from the ortho-, meta-, and para-positions, the reduced steric hindrance brings about decreased torsional angle between D and A moieties, resulting in increased oscillator strength. Accordingly, the para-substituted p-2CzBT is endowed with a more locally excited component that accounts for faster radiative decay, leading to a higher fluorescent efficiency than that of o-2CzBT and m-2CzBT. As expected, p-2CzBT enables its nondoped and doped OLEDs with higher external quantum efficiencies (EQEs) of 12.3% and 15.0%, respectively, which are among the state-of-the-art efficiencies of HLCT-based OLEDs. Moreover, o-2CzBT and m-2CzBT are also utilized as host materials for high-performance OLEDs, thus extending the application of HLCT materials. Download figure Download PowerPoint Introduction Organic light-emitting diodes (OLEDs) have attracted widespread interest because of their promising applications in the fields of flat-panel display, solid-state lighting, and so on. To promote practical applications of OLEDs, great efforts have been paid to the development of high-efficiency luminescent materials, which are of vital importance to device performance. As known, the first-generation conventional fluorescent emitters have an upper limit of internal quantum efficiency (IQE) of 25% and fail to utilize triplet excitons which account for 75%. Although second-generation phosphorescent emitters can achieve an IQE of 100%, they suffer from high cost due to the incorporation of noble metal, as well as shortage of efficient blue phosphorescent emitters.1,2 In this regard, third-generation pure organic emitters including thermally activated delayed fluorescence (TADF)3–7 and hybridized local and charge-transfer (HLCT)8–11 excited-state emitters, which render a 100% exciton utilization efficiency while avoiding noble metal, have been considered as potential candidates. Among these, TADF emitters have been widely explored, as their intrinsic long exciton lifetimes tend to cause serious efficiency roll-off when the devices are driven under high currents.12,13 In comparison, the much short exciton lifetimes of HLCT emitters benefit to suppress exciton annihilation and thus improve efficiency roll-off.14,15 Given the fact that the HLCT concept was proposed a few years ago, to date the development of HLCT emitters still lags behind TADF emitters.16–18 In particular, device performances of most HLCT-based OLEDs remain restricted as their external quantum efficiencies (EQEs) are generally <10%,10,15,19–23 leaving much room for efficiency improvement. Seen in this light, there is an urgent need for simple and effective strategies to develop high-efficiency HLCT emitters and related OLEDs. With respect to the design of HLCT molecules, a key point is to improve their photoluminescence quantum yield (PLQY). The adjustments of donor (D) and acceptor (A) moieties have pivotal roles in PLQY of HLCT materials through the regulation of locally excited (LE) and charge-transfer (CT) components. For instance, the weaker D can help to increase the LE component and decrease the CT component,24 leading to HLCT materials with high PLQYs.25 In further consideration that the torsional angle between D and A moieties can also exert an influence on the PLQY of organic molecules,26–28 this is generally adopted in twisted D–A-type TADF molecules by introducing large steric hindrance to reinforce CT characteristics.5,29 Inspired by this design principle for TADF materials, exploring HLCT materials with high PLQYs is feasible through the adjustment of LE and CT components by virtue of tuning the torsional angle between D and A moieties. With decreasing steric hindrance in HLCT molecules, the LE component can be increased as the torsional angle decreases, and consequently high PLQYs can be realized. In light of this, we designed three D–π–A–π–D-type positional isomers o-2CzBT, m-2CzBT, and p-2CzBT (Figure 1), wherein the D (carbazole) units were ortho-, meta-, and para-substituted with the A (benzothiadiazole) core unit, respectively. Herein, benzothiadiazole is a universally recognized A in the HLCT system as it possesses high luminous efficiency, a large lowest triplet excited state (T1)–high-energy triplet state (T2) energy difference, but the small lowest singlet excited state (S1)–T2 energy gap, which conforms the molecular principles of HLCT materials.10 For these HLCT materials, as the substituted position between D and A units varies, the modified steric hindrance brings about a change in the torsional angle. As expected, the reduced steric hindrance endows p-2CzBT with a smaller torsional angle, leading to higher LE component accounting for high PLQY, and, accordingly, performance doped OLEDs with an EQE of 15.0% are achieved. Figure 1 | Design strategy and molecular structures of o-2CzBT, m-2CzBT, and p-2CzBT. Download figure Download PowerPoint Experimental Methods Compounds o-2CzBT, m-2CzBT, and p-2CzBT were synthesized by the Suzuki coupling reaction of 4,7-dibromobenzo[c]-[1,2,5]-thiadiazole and different boronic acid or borate ester with carbazole unit ( Supporting Information Scheme S1). The three compounds were characterized by proton (1H) nuclear magnetic resonance (NMR), carbon (13C) NMR, electron impact-mass spectrometry (EIMS), and high-resolution mass spectrometry (HRMS) ( Supporting Information Figures S1–S12). We noted that p-2CzBT was previously reported with attention paid to spectroscopic and electrochemical properties,30 whereas the luminescence mechanism and OLED application deserve further studies. Results and Discussion Theoretical calculations Density functional theory (DFT) calculations of o-2CzBT, m-2CzBT, and p-2CzBT were performed at the B3LYP/6-31G(d) level. DFT-optimized molecular geometry is displayed in Figure 2a, and it can be seen that the torsional angles between D and A (θ1 and θ2) are obviously altered by changing the linking positions of the carbazole Ds, while θ1 and θ2 gradually decrease from o-2CzBT and m-2CzBT to p-2CzBT due to decreased steric hindrance. With respect to the orbital distributions of the three molecules, Figure 2b shows that the lowest unoccupied molecular orbitals (LUMOs) are mainly located on the benzothiadiazole A, and the highest occupied molecular orbitals (HOMOs) are distributed on carbazole Ds, while the HOMO of p-2CzBT is also partially delocalized on the benzothiadiazole, which facilitates HOMO and LUMO overlaps, indicating extended π-conjugation as a result of decreased torsional angles. Then, to evaluate the excited states and transition characters of the excited singlet state (S1) and triplet states (T1, T2), natural transition orbitals (NTOs) calculations were carried out based on time-dependent DFT (TD-DFT) by Multiwfn software.31 As shown in Figure 2c and Supporting Information Figures S13 and S14, for S0 → S1 transition of the three compounds, the partial separation and partial overlap of hole and particle distributions demonstrate hybrid features of LE and CT, verifying a distinct HLCT character of the excited states. As known that, the LE state is more efficiently radiative than the CT state as it is beneficial for promoting PLQY.32 In comparison, more LE components are observed in para-linked p-2CzBT for S0 → S1 excitation, which can be quantitatively evidenced by a larger hole–particle overlap integral (〈ΨH|ΨP〉, Supporting Information Table S1).33,34 Meanwhile, as the torsional angle decreases, the oscillator strengths of the S1 → S0 transition (fS) increase, which are 0.0702, 0.2940, and 0.7970 for o-2CzBT, m-2CzBT, and p-2CzBT, respectively. These results indicate that the change of the linking positions of the carbazole Ds can afford the excited state (S1) of para-linked p-2CzBT with more LE components, leading to faster radiative decay and a higher PLQY.35 Figure 2 | (a) Optimized geometry structures, and (b) HOMO and LUMO distributions of o-2CzBT, m-2CzBT, and p-2CzBT. (c) NTOs of p-2CzBT. (d) Energy level diagrams of the first 10 singlet and triplet excited states of o-2CzBT, m-2CzBT, and p-2CzBT. Download figure Download PowerPoint In a further set of experiment, the energy levels of the first 10 singlet and triplet excited states of the three compounds investigated by TD-DFT are plotted in Figure 2d. It was found that all three compounds present low energy levels of T1 and clearly large energy gaps between T2 and T1 (ΔET2–T1), which can greatly suppress interconversion (IC) decay from T2 to T1 as the rate of IC is in an inverse relation with ΔET2–T1.36 In contrast, the three compounds exhibit small energy gaps between T2 and S1, which can trigger spin-flip at high-lying excited states and boost the high-lying reverse intersystem crossing (hRISC) process occurring from T2 to S1, according to the Fermi golden rule.37–39 Moreover, the three compounds exhibit large spin-orbital coupling (SOC) constants between T2 and S1 states (〈S1|ⒽSO|T2〉, Supporting Information Table S1), which are in favor of fast hRISC.40,41 As a result, due to a much larger rate of hRISC than the rate of IC, the three compounds can make efficient utilization of triplet excitons for radiative emission through hRISC process, corresponding to the “hot exciton” mechanism.42,43 It is worth noting that the large energy gap between T1 and S1 prevents the occurrence of RISC process from T1 to S1, excluding the existence of the TADF process.44–46 Photophysical properties The UV–vis absorption spectra and photoluminescence (PL) spectra of o-2CzBT, m-2CzBT, and p-2CzBT neat films are displayed in Figure 3a and summarized in Table 1. In the UV–vis absorption spectra of the three compounds, broad and featureless absorption bands around 350–470 nm assigned to intramolecular CT (ICT) absorption can be observed, along with a red-shift of the absorption band from o-2CzBT and m-2CzBT to p-2CzBT. As seen from the PL spectra of o-2CzBT, m-2CzBT, and p-2CzBT, the PL emission peaks are located at 504, 516, and 563 nm, respectively, showing spectral red-shift by changing the carbazole units from ortho- and meta-positions to para-substituted positions. The red-shifted emission is mainly associated with the relative planarization of p-2CzBT when compared with o-2CzBT and m-2CzBT with twisted molecular conformations, which interrupt intramolecular π-conjugation.19,47,48 The PLQYs of o-2CzBT, m-2CzBT, and p-2CzBT neat films are measured to be 16.9%, 26.6%, and 64.4%, respectively, and the highly increased PLQY in p-2CzBT is benefited from the enhanced LE components and is consistent with its larger oscillator strength.25,49 Then, transient PL decay curves of the three compounds were recorded to investigate their fluorescent behaviors. As plotted in Figure 3b, o-2CzBT, m-2CzBT, and p-2CzBT neat films exhibit short excited lifetimes without delay, the nanosecond-scaled lifetimes are fitted to be 10.7, 11.8, and 5.3 ns, respectively, and temperature-dependent transient decay curves ( Supporting Information Figure S15) show the absence of long lifetimes, helping to rule out the possibility of TADF in the compounds.25,50 Besides, no phosphorescence was observed from the three compounds at 77 K, which may be caused by low ratios of T1 excitons in the HLCT compounds. Thereafter, the radiative transition rates (kr) of o-2CzBT, m-2CzBT, and p-2CzBT are estimated to be 1.58 × 107, 2.25 × 107, and 1.22 × 108 s−1, respectively, and the increasing trend of kr agrees well with that of their fS.51 These results affirm the important influence of the linking position on the compounds. Moreover, photophysical properties of doped films by dispersing the compounds into a common host 4,4′-bis(9H-carbazol-9-yl)biphenyl (CBP) were examined ( Supporting Information Figures S16 and S17), and a bathochromic shift of PL emission as well as nanosecond-scaled lifetimes are also observed in the doped films. Besides, the PLQYs of o-2CzBT, m-2CzBT, and p-2CzBT-doped films were collected to be 38.5%, 61.6%, and 98.5%, respectively, which are improved compared with that of the corresponding nondoped films. Figure 3 | (a) UV–vis absorption spectra (inset: neat films under UV-light illumination) and PL spectra, and (b) transient PL decay curves of o-2CzBT, m-2CzBT, and p-2CzBT neat films. (c) Linear fitting of the Stokes shift (νa-νf) vs solvent polarity (f) of o-2CzBT, m-2CzBT, and p-2CzBT. Download figure Download PowerPoint Table 1 | Photophysical, Thermal, and Electrochemical Properties of o-2CzBT, m-2CzBT, and p-2CzBT Compound λUV (nm)a λPL (nm)b τ (ns)c PLQY (%)d kr (s−1)e Td (°C)f Tg (°C)g HOMO (eV)h LUMO (eV)i Nondoped/Doped o-2CzBT 361 504 10.7 16.9/38.5 1.58 × 107 341 110 5.2 2.4 m-2CzBT 390 516 11.8 26.6/61.6 2.25 × 107 454 127 5.2 2.5 p-2CzBT 423 563 5.3 64.4/98.5 1.22 × 108 441 / 5.6 3.1 aAbsorption peak in films. bPL emission peak in films. cLifetime in films. dPL quantum yield. ekr = PLQY/τ. fThermal decomposition temperature corresponding to 5% weight loss. gGlass-transition temperature. hHOMO levels were calculated from the oxidation onset potentials in CV curves. iLUMO levels estimated by the empirical equation ELUMO = EHOMO + Eg. The solvatochromic effects of the three compounds were analyzed in solvents with different polarities from hexane to acetone, as illustrated in Supporting Information Figure S18. It was found that the PL spectral peaks of the three compounds gradually red-shifted with increased solvent polarity. Consequently, the Stokes shift (νa-νf) versus the solvent polarity (f) is fitted (Figure 3c) according to the Lippert–Mataga relation.52 As seen, o-2CzBT, m-2CzBT, and p-2CzBT present two independent slopes, and the lower slope in low-polarity solvents is ascribed to the LE state while the larger slope in high-polarity solvents is attributed to the CT state, which is a typical behavior of HLCT materials.32,53–55 Therefore, based on the aforementioned theoretical calculations (electronic structures and properties of excited states) and photophysical studies (nanosecond-scaled lifetimes and solvatochromic effects), the HLCT feature is verified for o-2CzBT, m-2CzBT, and p-2CzBT through the coupling and intercrossing between the LE and CT states.56 Thermal and electrochemical properties The thermal properties of the three compounds were examined with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) ( Supporting Information Figure S19). On the one hand, the decomposition temperatures (Td: corresponding to 5% weight loss) for o-2CzBT, m-2CzBT, and p-2CzBT are found to be 341, 454, and 441 °C, respectively. On the other hand, the glass-transition temperatures (Tg) of o-2CzBT and m-2CzBT were found to be 110 and 127 °C, respectively, while there was no Tg observed for p-2CzBT before its melt temperature (Tm) of 334 °C. These results suggest that all compounds exhibit high thermal stability beneficial for practical applications in OLEDs. Furthermore, cyclic voltammetry (CV) measurements were carried out ( Supporting Information Figure S20) to investigate energy levels of the compounds. The HOMO levels of the o-2CzBT, m-2CzBT, and p-2CzBT were shown to be 5.2, 5.2, and 5.6 eV, respectively, and their LUMO levels were calculated to be 2.4, 2.5, and 3.1 eV with the band gaps (Eg) evaluated from corresponding absorption spectra. Electroluminescent properties Device characterizations of OLEDs To examine the electroluminescent (EL) properties of o-2CzBT, m-2CzBT, and p-2CzBT, doped OLEDs were constructed with a configuration as: indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (30 nm)/m-bis(N-carbazolyl)benzene (mCP) (20 nm)/emitting layer (EML) (30 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) (40 nm)/LiF (1 nm)/Al (100 nm), as plotted in Figure 4a, wherein PEDOT:PSS and LiF act as charge-carrier injection layers, mCP and TPBi serve as charge-carrier transporting layers, and EML is CBP:10 wt % o-2CzBT, CBP:10 wt % m-2CzBT, or CBP:10 wt % p-2CzBT. The device characteristics are demonstrated in Figures 4b and 4c and summarized in Table 2. It is noted that the luminance of the three doped devices increased less than linearly as the current density gradually increased ( Supporting Information Figure S21), especially at higher current density. Therefore, the contribution of triplet–triplet annihilation (TTA) upconversion to high efficiencies can be excluded, because in TTA-mechanism-based OLEDs the luminance is inclined to increase more than linearly with increasing current density, considering the second-order TTA process.57,58 These results further support the HLCT property of the molecules that accounts for high EQEs achieved from their devices. As shown from the EQE–current density curves, o-2CzBT and m-2CzBT-based devices achieve a maximum EQE of 5.3% and 8.4%, respectively. These EQE values are higher than that obtained from traditional fluorescent OLEDs, benefiting from their HLCT property. Remarkably, the p-2CzBT-based device exhibits a maximum EQE of 15.0%, which is amongst the state-of-the-art efficiencies of HLCT-based OLEDs reported so far. In comparison with o-2CzBT and m-2CzBT-based devices, p-2CzBT-based device demonstrates low efficiency roll-off. To explore the efficiency roll-off behaviors, single carrier devices were constructed ( Supporting Information Figure S22), and the results confirm much more balanced electron and hole transporting abilities of p-2CzBT than that of o-2CzBT and m-2CzBT. Moreover, the EQE curves are also fitted in the presence of either TTA or singlet–triplet annihilation (STA), as plotted in Supporting Information Figure S23. It is found that, the TTA or STA fitted curves could not satisfactorily model the experimental data due to considerable deviations exist between them, implying TTA or STA involved exciton quenching should not play a critical role on the efficiency roll-off. Therefore, these results certify that the low efficiency roll-off in p-2CzBT-based device is mainly related to well-balanced charge carriers, while the charge-carrier unbalance causes efficiency roll-off in o-2CzBT and m-2CzBT-based devices.59,60 As seen from the EL spectra of the devices (inset of Figure 4c), when the substitution position changes from ortho- or meta-, to para-, the EL spectral peaks of o-2CzBT, m-2CzBT, and p-2CzBT are gradually red-shifted from 488 and 490, to 508 nm, respectively, which is consistent with the observations from their PL spectra. In addition, nondoped OLEDs by adopting o-2CzBT, m-2CzBT, and p-2CzBT neat films as EMLs were also with a device as that of the doped In comparison with their corresponding doped OLEDs, the nondoped o-2CzBT, m-2CzBT, and p-2CzBT devices present red-shifted EL emission with spectral peaks at and nm, (inset of Figure The EQE–current density curves of the nondoped OLEDs (Figure demonstrate that due to a much higher PLQY of the p-2CzBT neat than that of o-2CzBT and m-2CzBT, the p-2CzBT-based nondoped device a higher EQE of 12.3% ( Supporting Information Figure which is reported among based on a ( Supporting Information Table Figure | (a) Device (b) current curves, and (c) EQE–current density curves of doped OLEDs (inset: EL (d) EQE–current density curves of nondoped OLEDs (inset: EL spectra and device Download figure Download PowerPoint Table 2 | EL of OLEDs on o-2CzBT, m-2CzBT, and p-2CzBT Device EQE o-2CzBT 488 m-2CzBT 508 p-2CzBT 508 maximum current EL peak at a luminance of Device of OLED It is that o-2CzBT and m-2CzBT have potential as host materials for and organic emitters, considering their charge-carrier transporting abilities with In this regard, (inset of Figure which is an HLCT we previously was as a is considerable overlap between the UV–vis absorption of and the PL spectra of o-2CzBT and m-2CzBT ( Supporting Information Figure efficient resonance energy from to OLEDs were (Figure It was found that o-2CzBT and m-2CzBT enable devices with maximum EQEs of and respectively, which are higher than the EQE obtained from a nondoped the important role of In addition, these two devices present the EL spectra with one emission peak around nm from (inset of Figure energy from host to based on the process ( Supporting Information Figure These results demonstrate it is an effective strategy to achieve high-efficiency OLEDs by HLCT host and HLCT materials, which are to high exciton utilization efficiency and device Figure | (a) curves (inset: of and (b) EQE–current density curves of OLEDs (inset: EL Download figure Download PowerPoint fluorophores, o-2CzBT, m-2CzBT, and p-2CzBT, through regulation of D and A torsional angles were designed with HLCT as evidenced by theoretical and photophysical studies. the D (carbazole) units were ortho-, meta-, and para-substituted with the A (benzothiadiazole) core unit in o-2CzBT, m-2CzBT, and p-2CzBT, respectively, the steric hindrance was gradually reduced and about a decrease in torsional angle between D and A accounting for the LE component and oscillator that to high along with red-shifted PL Accordingly, the para-substituted p-2CzBT with a smaller torsional angle was endowed with a higher PLQY than o-2CzBT and m-2CzBT. the p-2CzBT-based OLEDs the device as a high EQE of 12.3% in nondoped device and an EQE of 15.0% in doped which are among the state-of-the-art efficiencies of HLCT-based OLEDs reported so far. Furthermore, o-2CzBT and m-2CzBT were adopted as the host materials for an HLCT resulting in HLCT OLEDs with high light on molecular design principles for high-efficiency HLCT materials, helping to promote its to with the by TADF materials. Supporting Information Supporting Information is and OLEDs and materials and of is no of interest to Information was by the Science of and and the Guangdong Science and Technology and 1. Yang of Efficient from for Organic Light-Emitting