Suppressing the Undesirable Energy Loss in Solution‐Processed Hyperfluorescent OLEDs Employing BODIPY‐Based Hybridized Local and Charge‐Transfer Emitter

Hyperfluorescent organic light‐emitting diodes (HF‐OLEDs) approach has made it possible to achieve excellent device performance and color purity with low roll‐off using noble‐metal‐free pure organic emitter. Despite significant progress, the performance of HF‐OLEDs is still unsatisfactory due to the existence of a competitive dexter energy transfer (DET) pathway. In this contribution, two boron dipyrromethene (BODIPY)‐based donor‐acceptor emitters (BDP‐C‐Cz and BDP‐N‐Cz) with hybridized local and charge transfer characteristics (HLCT) are introduced in the HF‐OLED to suppress the exciton loss by dexter mechanism, and a breakthrough performance with low‐efficiency roll‐off (0.3%) even at high brightness (1000 cd m−2) is achieved. It is demonstrated that the energy loss via the DET channel can be suppressed in HF‐OLEDs utilizing the HLCT emitter, as the excitons from the dark triplet state of such emitters are funneled to its emissive singlet state following the hot‐exciton mechanism. The developed HF‐OLED device has realized a good maximum external quantum efficiency (EQE) of 19.25% at brightness of 1000 cd m−2 and maximum luminance over 60 000 cd m−2, with an emission peak at 602 nm and Commission International de L'Eclairage (CIE) coordinates (0.57, 0.41), which is among the best‐achieved results in solution‐processed HF‐OLEDs. This work presents a viable methodology to suppress energy loss and achieve high performance in the HF‐OLEDs.


Introduction
Organic light-emitting diodes (OLEDs), a promising technology in lighting and display applications, have attracted significant attention due to their tremendous advantages, such as lightweight, minimal energy consumption, efficient electroluminescence, and so on. [1,2]Due to the ongoing demand for OLEDs, many efforts have been devoted to improving the device's external quantum efficiency (EQE). [3,4]The conventional OLEDs based on organic fluorophores utilize only singlet excitons, limiting the maximum internal quantum efficiency (IQE) up to 25%. [5]Owing to this dilemma, OLEDs based on phosphorescent and thermally activated delayed fluorescence (TADF) emitting materials have been developed to maximize the usage of generated excitons. [6,7]TADF material is considered the best contender for OLEDs due to its ability to transform dark triplet excitons into an emissive singlet state via a unique spin polarization process termed reverse intersystem crossing, ensuring the 100% IQE. [8][11] The poor stability of the TADF-OLEDs due to the long lifetime of the generated exciton is another major hurdle toward practical applications. [12]he development of highly efficient, cost-effective OLEDs with negligible efficiency roll-off and pure color is currently desired for display technology. [13]Thanks to the "hyperfluorescence (HF) mechanism", a TADF sensitization strategy proposed by Nakanotani and coworkers that enabled the achievement of excellent device performance.[16] Compared to traditional TADF and phosphorescent OLEDs, the HF-OLEDs have the advantage of minimum efficiency roll-off because it enables rapid conversion of long-lived triplet excitons into fast radiative singlet excitons, thereby reducing singlet-triplet and triplet-triplet annihilation (STA and TTA) within the device. [17,18]But unfortunately, a primary undesirable energy loss channel, dexter energy transfer (DET) from the triplet state of TADF to the dark triplet state of the emitter, still exists in the HF-OLEDs that adversely affect the device's performance. [19,20] Nie, Dr. Z. Mahmood, Prof. D. Hu, Dr. W. Chen, L. Xing, Prof. Y. Huo, Prof. S. Ji School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China E-mail: smji@gdut.edu.cnD. Liu, Dr. M. Li, Prof. S. Su State Key Laboratory of Luminescent Materials and Devices and Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, China E-mail: mssjsu@scut.edu.cn The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/eem2.12597.DOI: 10.1002/eem2.12597Hyperfluorescent organic light-emitting diodes (HF-OLEDs) approach has made it possible to achieve excellent device performance and color purity with low roll-off using noble-metal-free pure organic emitter.Despite significant progress, the performance of HF-OLEDs is still unsatisfactory due to the existence of a competitive dexter energy transfer (DET) pathway.In this contribution, two boron dipyrromethene (BODIPY)-based donor-acceptor emitters (BDP-C-Cz and BDP-N-Cz) with hybridized local and charge transfer characteristics (HLCT) are introduced in the HF-OLED to suppress the exciton loss by dexter mechanism, and a breakthrough performance with low-efficiency roll-off (0.3%) even at high brightness (1000 cd m −2 ) is achieved.It is demonstrated that the energy loss via the DET channel can be suppressed in HF-OLEDs utilizing the HLCT emitter, as the excitons from the dark triplet state of such emitters are funneled to its emissive singlet state following the hot-exciton mechanism.The developed HF-OLED device has realized a good maximum external quantum efficiency (EQE) of 19.25% at brightness of 1000 cd m −2 and maximum luminance over 60 000 cd m −2 , with an emission peak at 602 nm and Commission International de L'Eclairage (CIE) coordinates (0.57, 0.41), which is among the best-achieved results in solution-processed HF-OLEDs.This work presents a viable methodology to suppress energy loss and achieve high performance in the HF-OLEDs.
Previously, multiple efforts have been made to maximize the device performance using various approaches without knowing about the energy loss mechanism. [21,22]25] For instance, Yasuda and coworkers suggested an encapsulated anthracene-based dendritic fluorophore to suppress the triplet exciton funneling from TADF host to the emitter. [23]In addition, an electronically inert terminal substituent-protected fluorophore and tert-butyl encapsulated TADF dopant is introduced to block the DET in HF-OLEDs. [24,25]Recently, the Su research team made an important contribution and successfully reduced the DET process by introducing the phenyl-fluorene periphery substituent on the TADF assistant host. [24]hese are good attempts to suppress the DET process; however, a nearly identical approach is adopted, i.e., increasing the intermolecular distance between the TADF host and emitting dopant.Further, the performance of HF-OLED is still far away from satisfaction, especially for red and NIR OLEDs.Another alternative and effective approach which can completely inhibit the energy loss of DET channel and improve the efficiency of HF-OLEDs is highly desired.
Boron dipyrromethene (BODIPY) represents an outstanding class of fluorophore and extensively investigated in several photochemical applications due to its intriguing photophysical properties such as a large molar absorption coefficient (ε ≈ 100 000 M −1 cm −1 ), facile derivatization, high fluorescence quantum yield, and narrow emission (full width at half maximum (FWHM) ≈ 20-40 nm) with a small Stokes shift. [26,27]These fascinating features can also be beneficial to achieving the pure emission color with minimum non-radiative energy loss in OLED.30] Recently, Duan and Kwon's research team employed the tert-butylprotected BODIPY material in HF-OLEDs as emitting dopant, and high EQE was achieved with green and red emissions, respectively. [31,32][33] However, a simple and cost-effective fabrication strategy such as solution processing is desired for large-scale applications.But such sterically protected fluorophores are with poor solubility and are not suitable for a solution-processing approach.According to our knowledge, limited reports on solution processing hyperfluorescent OLEDs are currently available. [34,35]Thus, it is currently demanding to explore new strategies which can suppress DET energy loss and ensure high efficiency in solution-processed HF-OLED.
Herein, we proposed a new tactic to suppress the energy loss of the DET process in HF-OLEDs by utilizing HLCT fluorophore as an emitting dopant and developed a solution-processed TADF-sensitized fluorescent system that demonstrates efficient performance even at high brightness.Two BODIPY emitters with bipolar donor-acceptor-donor (D-A-D) structures (BDP-C-Cz and BDP-N-Cz) are developed by affixing bulky carbazole donor to the BODIPY core from the different positions, [36] that exhibited orange-to-red emission with HLCT (hybridized local and charge-transfer) characteristics.The fabricated HF device based on these emitters showed superior performance with an EQE of 19.25% at brightness of 1000 cd m −2 , an electroluminescence peak at 602 nm, and a small FWHM of 63 nm.Benefiting from the HLCT feature of the emitter, the excitons lost by DET channel are funneled to the emissive singlet state of the emitter following the hot exciton mechanism, and excellent performance was realized with negligible efficiency roll-off (only around 0.3% at 1000 cd m −2 ) in the solution-processed device.These features make it completely different from the previously reported BODIPY-based HF-OLED. [32]To the best of our knowledge, it's the best device performance among reported solution-processed red OLEDs based on the pure organic emitter.

Molecular Design and Geometry of the Emitter
As aforementioned, the target is to develop the HLCT emitter with a high radiative ability to achieve the best performance by suppressing the DET energy loss.In this scenario, BODIPY's fascinating luminescence properties draw our attention.Note that unsubstituted BODIPY is highly fluorescent in the solution phase but suffers from the drawback of aggregation in the film state due to the planar core structure. [37,38]wo D-A-D BODIPY derivatives (BDP-C-Cz and BDP-N-Cz) were prepared, which may achieve the red emission with a high quantum yield. [5,39,40]Such a molecular motif is proposed to reduce nonradiative energy loss by suppressing intramolecular rotation on one side and achieving a bathochromic shift in emission by establishing electronic contact between donor and acceptor units on the other side (See Scheme 1). [40]Further, by varying the attachment position of carbazole to the BODIPY core, different mutual chromophore orientations and dihedral angles among donor and acceptor units are attained, and its effect on the electroluminescence properties will be explored.We expected that different mutual chromophore orientations in the two molecules will lead to different extent of excited-state electronic coupling among donor and acceptor moieties, thus resulting in different photophysical properties and especially will affect their electroluminescence (EL) properties, for instance, EL peak and FWHM.
The geometric configuration and electronic properties of the designed BODIPY emitters were preliminarily investigated using density functional theory (DFT) simulations.Figure 1 depicts the frontier molecular orbitals and optimized geometry of BODIPY derivatives.Due to the mild repulsive interaction of hydrogen to the methyl substituents, the BODIPY derivatives are not with optimal coplanar geometry.The carbazole at the 2nd and 6th positions sterically shielded the BOD-IPY moiety, which might be beneficial to avoid aggregation.For BDP-C-Cz, the derivative in which carbazole is linked from the 3rd position to the BODIPY core, a dihedral angle of 57°was observed among the BODIPY and carbazole units, indicating that the conformation restriction is weak.In the other compound BDP-N-Cz, a phenyl group was inserted between carbazole and BODIPY unit to reduce the ground state coupling, which may be advantageous to improve the photoluminescence quantum yield (PLQY).The meso-phenyl substituent in both BODIPY derivatives attained the orthogonal configuration regarding the BODIPY core due to the large steric repulsion, which can avoid the non-radiative energy loss by suppressing the intramolecular rotation and excited-state relaxation process.The frontier molecular orbitals (FMO) analysis showed that the lowest unoccupied molecular orbital (LUMO) is confined only to the BODIPY core.While the highest occupied molecular orbital (HOMO) is spread on both carbazole and BOD-IPY moieties (Figure 1).Such overlap between HOMO and LUMO may facilitate the radiative transition process. [41]Further, such an overlap of FMO suggests the involvement of intercrossed CT transition and typical HLCT character (Verify in the later section).

Photophysical Properties of BODIPY Emitter and Confirmation of HLCT Feature
To reveal the electronic coupling between donor and acceptor moieties, the photophysical properties of the BODIPY emitters were investigated.
The steady-state absorption spectra of BODIPY emitters in toluene are presented in Figure 2.Both derivatives exhibited a sharp absorption band around 540 nm, which arises due to the S 0 →S 1 transition of the BODIPY chromophore.The small bathochromic shift (ca.40 nm) in the absorption profile of the BOD-IPY derivatives compared to that of the parent unsubstituted BODIPY chromophore, which shows absorption at around ca. 504 nm may be due to the extended π-conjugation network via substitution of carbazole at the 2nd and 6th positions of the core. [42]Note that no obvious charge transfer absorption band was observed; however, the broadening and redshifting in the absorption of BODIPY emitters predicts the presence of a weak ground state coupling among carbazole and BODIPY units.
The photoluminescence study of BODIPY derivatives in solution and film shows that both compounds emit in the orange-to-red region of the spectrum (Figure 2).Compared to the parent BODIPY's emission, the current BODIPY derivatives showed approximately 60-100 nm bathochromic shift, probably due to the intramolecular charge transfer (ICT) feature.For BDP-C-Cz, a sharp characteristic emission at ca. 601 nm, with a small FWHM of 61 nm, was observed in the toluene, while BDP-N-Cz showed comparatively a narrow emission (FWHM ≈ 50 nm) around 573 nm.The hypsochromic shift of ca.27 nm in the emission profile of BDP-C-Cz compared to the other derivative, BDP-N-Cz, is probably due to the different extent of excited-state electronic coupling among donor and acceptor moieties, caused by the different mutual chromophore orientation in the two triads.Note that both emitters showed a Stokes shift of ~33-61 nm, much larger than the parent BODIPY and other simple BODIPY derivatives; thus, it can reduce self-quenching and be beneficial to improving the PLQY.Though there still exists a smaller self-absorption in the current BODIPY's derivatives, but it may be ignored and will not affect the overall hyperfluorescent device performance, as in HF device, the energy/excitons are majorly absorbed by the TADF molecule, that later on transferred to the emitter via FRET.Further, both compounds showed high PLQY (78%) in doped film.In a recent report by Kwon et al., a red-emitting BODIPY material was introduced for HF-OLED, in which alkyloxyphenyl group was incorporated on the meso-position to achieve the redshifting in emission profile while tertiary butyl-protected phenyl groups were included on the periphery position of BODIPY core to avoid the aggregation, which make the synthesis little complex.Herein, we introduced the simple D-A-D molecular design with orange-to-red  emission. [32]The photophysical data of both emitters are summarized in Table 1, indicating that mutual chromophore orientation significantly affects compounds' excited state properties and dynamics.
For the further verification of the emissive state, the effect of solvent polarity on the emission of BODIPY derivatives was studied in detail, data were analyzed using the Lippert-Mataga plot, [43,44] and the excited-state dipole moment (μ e ) of the BODIPY derivatives was estimated.A linear relationship showing the general solvent effect was obtained for both derivatives.The excited-state dipole moment (μ e ), estimated based on ground state optimized geometry, 10.97 and 7.40 D were observed for BDP-C-Cz and BDP-N-Cz, respectively, which confirmed the HLCT feature of the emissive (S 1 ) state. [44]This assumption was further supported by the NTO(natural transition orbitals) analysis, which revealed the mixing of locally-excited (LE) and chargetransfer (CT) states (Figure S8, Supporting Information).For S 0 →S 1 transition, NTO showed obvious CT character, "hole" distributed on the BODIPY moiety while "particles" confined on the whole molecule.The oscillator strength of the S 1 excited state for BDP-C-Cz and BDP-N-Cz are 0.5076 and 0.2644, respectively, suggesting the efficient radiative transition process in line with their high PLQY.Further, T 3 excited state, which is energetically the same as S 1 , also has CT characteristics.A small energy gap of 0.08 and 0.04 eV and a large spin-orbit coupling (SOC) value of 0.71 and 0.35 cm −1 were observed among S 1 /T 3 for BDP-C-Cz and BDP-N-Cz, respectively.These results suggest the existence of high reversed intersystem crossing (hRISC) between S 1 and T 3 excited states, which may be beneficial in reducing the energy loss in HF-OLED by maximizing the triplet exciton utilization (See Scheme 2).Further evidence of the HLCT feature is provided by the fluorescence lifetime study, which showed monoexponentially decay for both derivatives in different solvents (See Figure 2). [43,44]The time resolved electroluminescence (TREL) analysis also suggests the HLCT characteristic of BODIPY emitters, since no significant variation in TREL of undoped devices was observed under varied applied voltages (See Figure S15, Supporting Information).
The cyclovoltammetry measurements were performed to investigate the electrochemical properties and estimate the energy of HOMO/ LUMO orbitals.Both BODIPY compounds exhibited nearly similar electrochemical properties with a slight change (few mV) in the redox potential.The thermodynamic feasibility of the intramolecular electron transfer process from carbazole donor to BODIPY acceptor in both derivatives was confirmed in solvents of various polarities, employing Rehm−Weller's equation (Figure S13, Supporting Information). [45,46]Based on the redox potential and band gap (2.28 eV), the HOMO and LUMO value for BDP-C-Cz was determined to be −5.37 and −3.09 eV, respectively.While for the BDP-N-Cz, the measured HOMO and LUMO values are −5.32 and −3.13 eV, respectively.The nanosecond transient absorption study was also conducted which confirmed the triplet excited state population of the BODIPY derivatives.Both compounds showed moderate ISC efficiency and BODIPY localized triplet state was observed (See Figure S14, Supporting Information).

Energy Transfer Analysis
Among various factors controlling the performance of hyperfluorescent OLEDs, the energy transfer from TADF host to emitting material is crucial as it not only reduces the exciton lifetime but also promotes its utilization.Figure 3 depicts the luminescence spectra of 4CzIPN TADF material plotted together with the absorption spectra of BODIPY emitter, showing the excellent overlap; thus, efficient energy transfer from TADF to BODIPY emitter is expected.
To validate this assumption and optimize the dopant concentration, different 4CzIPN concentrations with the combination of varying BOD-IPY emitter concentrations were studied.Later, the 4CzIPN concentration is fixed at 13 wt% and the concentration of BODIPY emitter (0.9, 1.3, and 1.7 wt%) is varied (Figure S17 and Table S3, Supporting Information).In all the spin-coated films majorly BODIPY emission along with some residual TADF emission was observed, implying the efficient FRET process among the selected system.
Based on the initial results, 0.9 wt% is found to be the optimum concentration of BODIPY emitter.However, Figure 3e shows that the transient decay lifetime of the 1.3 wt% doped film is shorter than the 0.9 wt% doped film, indicating that there is a more efficient energy transfer from TADF molecule to the emitter at this doping concentration.But the EQE of the device is not only controlled by the energy transfer, but there are also some other factors, such as the doping concentration (TADF/emitter), which may affect the device's performance.It is worth mentioning here that the fluorescent emitter usually has a narrower band gap than TADF molecules and can more easily capture the charge in the EL part.Thus, a higher doping concentration can improve the FRET, but it may also cause more charge trapping on the fluorescent emitter, thus reducing the utilization of excitons and affecting the overall device's performance. [32]This assumption was further confirmed in the later section by studying the performance of devices with different dopant concentrations; 0.9 wt% is found to be the best concentration of BODIPY emitter.
For further insight, the rate of FRET and DET was calculated at different dopant concentrations employing Equations ( 1) and ( 2). [32] (1) where R 0 is the Förster radius, and R denotes the intermolecular distance between the energy donor and acceptor.Φ D and τ D are the PLQY and prompt fluorescence lifetime of the energy donor.к 2 is a configurational factor.N A is Avogadro's number, and n represents the refractive index of the medium.F D λ ð Þ is the normalized emission spectra of the donor, and ε A λ ð Þ is the molar absorption coefficient of the acceptor.K is the specific orbital interaction, J T is the normalized spectrum overlap integral, and L is the electron tunneling distance.Equation (1)  shows that the FRET is proportional to the R 0 .Thus, for an efficient FRET process, the value of R 0 should be greater than the intermolecular distance (R).The calculated R 0 value for BDP-C-Cz (4.3 nm) is slightly higher than BDP-N-Cz (4.2 nm), implying that the FRET and spectral overlap will be large for BDP-C-Cz.We observed a better spectral overlap between the 4CzIPN emission and BDP-C-Cz absorption (6.14 × 10 15 mol −1 dm 3 cm −1 nm 4 ) than the BDP-N-Cz (5.25 × 10 15 mol −1- dm 3 cm −1 nm 4 ).The calculated FRET and DET rate (from T 1 state of TADF to T 1 state of BODIPY) constants at different dopant concentrations are listed in Table S4, Supporting Information and presented in Figure 3.The FRET rate of BDP-C-Cz and BDP-N-Cz at 0.9 wt% is 2.45 × 10 7 and 2.24 × 10 7 s −1 , while the DET rate is 1.59 × 10 5 and 0.92 × 10 5 s −1 , respectively.Upon increasing the dopant concentration of BODIPY emitter from 0.9 to 1.3 wt%, the rate of both FRET and DET increased.But the growth trend in DET rate is slower, indicating the existence of some other competitive pathway.Previous studies have shown that if the energy gap between the lower triplet excited state (T 1 ) of TADF sensitizer and the higher triplet state (T n ) of the emitter is small, it may lead to efficient DET process through this channel. [46]Thus, we expect the major DET process will occur from T 1 of 4CzIPN to T 3 of BODIPY emitter (as the energy gap between them is negligible).However, considering HLCT characteristics of BODIPY emitter, it is expected the excitons from the T 3 state of emitter may recycle to its emissive singlet state (S 1 ) following the hot-exciton mechanism and can contribute in improving the overall device performance.

Electroluminescence Properties
To investigate the potentials of BDP-C-Cz and BDP-N-Cz as emitting dopants for electroluminescence application, the solution-processed   Further, we optimized the EML dopants' concertation, and devices with varying TADF sensitizer/emitter concentrations were fabricated to achieve the best performance.In all the devices, the EL mainly emerges from the BODIPY (around 600 nm), and contribution from the 4CzIPN (weak shoulder band at 450 nm) is relatively insignificant (<3%), thus, confirming the efficient FRET process from TADF sensitizer (4CzIPN) to the BODIPY emitter.The highest performance (EQE of 19.2 AE 0.3%) with an EL emission peak around 602 nm was achieved for the device with 4CzIPN (13 wt%) and BODIPY emitters (0.9 wt%) composition as EML layer.
A further increase or decrease in the 4CzIPN doping concertation lowers the device performance with a significant drop of EQE due to the reduction of FRET and the imbalance of charge carrier transport. [32]Figure 4 depicts the HF-OLED device structure and EL spectra of BDP-C-Cz and BDP-N-Cz, which is almost identical to the PL emission profile of these emitters and located at 602 and 589 nm, respectively.Corresponding photographs, J-V-L curves, EQE plots, and CIE coordinates of working devices are presented and summarized in Figure 4 and Table 2.A low turn-on voltage V on of 4.6 V, due to matched energy level and effective charge injection, was observed for the fabricated devices.A high current efficiency of 41.60 cd A −1 , EQE maximum of 19.25%, and a max brightness of 42 719 cd m −2 were realized for BDP-C-Cz HF device.Another emitter, BDP-N-Cz also showed the comparative device performance with an EQE of 19.12%, a current efficiency of 59.24 cd A −1 , and ultra-high brightness of 62 353 cd m −2 .Noticeably, negligible efficiency-roll off and EQE drop (~0.3%) was observed at high brightness, EQE remains as high as 18.08% even at high luminance of 5000 cd m −2 , which is comparable with the reported HF vacuum evaporated device, showing the effectiveness of this strategy. [32]o gain a better insight of the charge transfer in the fabricated devices, the device with different EML was fabricated and analyzed.The HF device showed a much higher current density compared to the device without TADF material, revealing that the charge transport takes place through the TADF material (Figure S17 and Table S3, Supporting Information).This was further confirmed by the comparing J-V curves of devices with varying concentrations of the 4CzIPN TADF material, which showed the different performances (Table S3, Supporting Information); thus indicating the charge recombination happens on the TADF material. [31]urther, we improved the device structure by changing the transporting layer, which interestingly showed better stability and performance compared to the previous HF devices.For BDP-C-Cz, the operational lifetime of 41.87 h (LT 50 , define as the corresponding time when the luminance declined to 50% of the initial luminance) and 1.44 h (LT 90 , define as the corresponding time when the luminance declined to 90% of the initial luminance) were found while BDP-N-Cz showed 42.88 h (LT 55 , define as the corresponding time when the luminance declined to 55% of the initial luminance) and 2.51 h (LT 90 ) lifetime at 1000 cd m −2 , the detailed data are shown in Figures S18,   S5, Supporting Information, which is the best achieved result till date in solution-processed HF-OLED.

Conclusion
In summary, this work describes the new strategy to suppress the undesirable energy loss of dexter energy transfer (DET) channel in hyperfluorescent organic light emitting diodes (HF-OLEDs) taking advantage of HLCT characteristic of the emitter.Two BODIPY-based donor-acceptor emitters (BDP-C-Cz and BDP-N-Cz) with HLCT characteristics are prepared for use in the HF OLEDs.BODIPY-derived D-A-D emitters showed orange-red emission with emission maxima around 601 nm, an FWHM of 63 nm, and high fluorescence quantum yield of 78% in toluene.By employing BODIPY emitter in HF-OLED, it has shown that the emitter with HLCT characteristic can significantly suppress the energy loss of dexter channel.Benefiting from the efficient FRET process and HLCT feature of the emitter, a high maximum EQE of 19.25% at around 1000 cd m −2 with EL peak around 602 nm and a negligible efficiency roll-off (0.3%) at 1000 cd m −2 was achieved.Overall, the fabricated device showed the best device performance among the previously reported solution-processed orange-red HF-OLEDs.The current finding will pave the way for the future development of highly efficient full-color hyperfluorescent OLED with reduced energy loss.

Experimental Section
Synthesis and characterization of BDP-C-Cz: Details of the synthesis protocol and characterizations are given in the Supporting Information.General methods: Reagents and solvents were purchased as of analytical grade and used without further purification.The NMR spectra were recorded on a Bruker AVANCE 400 spectrometer, using tetra-methyl-silane (TMS) as the internal standard.Mass spectra (MS) were obtained on a Thermo Fisher TSQ Endura Mass spectrometer.Ultraviolet-visible absorption spectra of the compounds were recorded using Shimadzu UV-2700 UV-vis absorption Spectrometer.An Edinburgh FLS980 spectrometer was used to acquire the photoluminescence (PL) spectra, fluorescence lifetime (τ F ) and absolute photoluminescence quantum yield (PLQY, Ф PL ) of the compounds.
Device fabrication and measurement: The EL devices were fabricated as follows: Firstly, the glass substrates precoated with a 95 nm thin layer of ITO substrates were carefully cleaned by acetone, isopropyl alcohol, detergent, deionized water, and isopropyl alcohol under the ultrasonic bath and treated with O 2 plasma for 2 min sequentially.Secondly, PEDOT: PSS was spin-coated onto indium tin oxide (ITO, 95 nm) at 3000 rpm (g = 1000 rpm/min 2 ) for 40 s.Then backed them at 150 °C for 15 min.After cooling, a 50 nm emitter layer was spin-coated from the chlorobenzene solution.Then 10 nm of PPF, 40 nm of TmPyPB, 1.3 nm of CsF, and 120 nm of aluminum were evaporated with a shadow mask under a high vacuum ( < 1 Â 10 À4 Pa).And the thickness of the evaporated ETL and cathode was monitored by a quartz crystal thickness monitor (SQC-310; Inficon).Deposition rates are 1 Å s −1 for TmPyPB, 0.1 Å s −1 for CsF, and 4 Å s −1 for Al, respectively.The overlapping area between the anode and cathode were defined the emission area as a pixel size of 10 mm 2 .All the fabrication processes were carried out inside a controlled atmosphere of nitrogen drybox containing <1 ppm oxygen and moisture.Current density (J) − luminance (L) − voltage (V) characteristics and EL spectra were collected by using the system XPQY-eq E350-1100 (Guangzhou Xi Pu Optoelectronics Technology Co. Ltd.).The transient EL dynamics were measured with a digital oscilloscope (AFG3152C; Tektronix) to provide rectangular pulse voltages (repetition rate 20 kHz, width 10 μs) to drive the devices, and detected by Edinburgh FL980 fluorescence spectrophotometer.EQEs of the devices were calculated from the current density, luminance, and EL spectrum, assuming a Lambertian distribution. [47]efore the measurement in the atmosphere, all the devices were sealed with a UV-cured epoxy resin.

Scheme 1 .
Scheme 1.The molecular design strategy for the BODIPY-appended carbazole emitters.

Figure 1 .
Figure 1.The optimized geometry and frontier molecular orbitals of BODIPY derivatives, computed by DFT with a B3LYP/6-31G(d) basis using the Gaussian 09 program.

Figure 2 .
Figure 2. a) Absorption and emission spectra of BODIPY derivatives in toluene, c = 1 × 10 −5 M. b) The Lippert-Mataga model in various solvents.c) Normalized PL spectra of BDP-C-Cz.d) Fluorescence emission decay traces of compounds in different solvents.

Scheme 2 .
Scheme 2. Energy transfer process in the device based on BDP-C-Cz.

Figure
Figure 3. a) The spectral overlap between the absorption of BODIPY derivatives and PL emission of 4CzIPN in toluene, c = 1 × 10 −5 M. b) The energy transfer rate versus doping concentration.The emission intensity in different doped film of c) BDP-C-Cz and d) BDP-N-Cz.e) Fluorescence lifetime decay curves of BDP-C-Cz in different doped film, λ exc = 365 nm, 20 °C.f) TREL of different doped device based on BDP-C-Cz.
Figure 3. a) The spectral overlap between the absorption of BODIPY derivatives and PL emission of 4CzIPN in toluene, c = 1 × 10 −5 M. b) The energy transfer rate versus doping concentration.The emission intensity in different doped film of c) BDP-C-Cz and d) BDP-N-Cz.e) Fluorescence lifetime decay curves of BDP-C-Cz in different doped film, λ exc = 365 nm, 20 °C.f) TREL of different doped device based on BDP-C-Cz.

Figure 4 .
Figure 4. a) Energy level diagram of the device configuration.b) Molecular structures used for device.c) Photograph and d) corresponding CIE coordinates of working devices.e) EL spectra.f) Current density-voltage-luminance (J-V-L).g) Current efficiency-current density (CE-J) and h) external quantum efficiencyluminance (EQE-L) characteristics of devices.

Table 1 .
Photophysical parameters of the BODIPY emitters.