Endowing TADF luminophors with AIE properties through adjusting flexible dendrons for highly efficient solution-processed nondoped OLEDs

Construction of core–dendron TADF emitters systematically: changing the behaviour of chromophores from aggregation-caused quenching to aggregation induced emission to develop high-performance fully solution-processed nondoped OLEDs.


Introduction
The reduction of the manufacturing cost of organic light-emitting diodes (OLEDs) remains one of the key challenges in the commercialization of OLED technology. 1 Compared with vacuum deposition, solution-process techniques are more promising for large-area fabrication because of their high material utilization rate, simple manufacturing process, and easy operation. [2][3][4] However, the efficiencies of solution-processed OLEDs are generally low, so there is an urgent need to develop more efficient luminescent materials. 5,6 Aer the pioneering work by Adachi and co-workers, thermally activated delayed uorescent (TADF) materials have been explored as the most promising third generation emitters followed by conventional uorescent materials, and phosphorescent heavy-metal complexes. [7][8][9] In addition, several small-molecule TADF emitters have been employed in solution-processed OLEDs and achieved relatively high device performance. [10][11][12] It is vital to emphasize that the majority of TADF emitters are doped in appropriate host matrices to weaken the intermolecular interactions and exciton quenching. 13,14 As is known, strong luminescence of conventional organic uorophores in dilute solution is normally weakened or quenched in their aggregated states, and there is no exception for most TADF materials. [15][16][17] Although long-lived triplet excitons in TADF molecules can be up-converted into radiable singlet excitons through the reverse intersystem crossing (RISC) process, the lower RISC rate (k RISC ) inevitably results in quenching of many triplet excitons in the aggregated state by triplet-triplet annihilation (TTA), singlettriplet annihilation (STA) and triplet-polaron annihilation (TPA). 18,19 The aggregation-caused quenching (ACQ) effect seriously limits their application and reduces the device performance. Fortunately, Tang and co-workers have reported a series of new luminogens with aggregation-induced emission (AIE) properties since 2001. [20][21][22] These AIE compounds effectively overcome the drawbacks of ACQ and achieve efficient solid-state luminescence. The enhanced photoluminescence quantum yield (PLQY) in the aggregation state enables the performances of nondoped OLEDs to be improved. [23][24][25] Based on the above research, TADF materials can harvest both singlet and triplet excitons to achieve theoretical 100% internal quantum efficiency, and AIE luminogens are favored to attain efficient emission in their condensed solid states. Therefore, integrating TADF emitters with the AIE nature could be a feasible strategy to develop efficient solution-processed nondoped OLEDs. [26][27][28] With this design concept, various types of AIE-TADF materials have been reported in recent years by Tang and Chi, Despite the nondoped devices exhibiting improved electroluminescence efficiencies, further utilization of these emitters in the solution process has been rarely explored. 32 This is mainly due to many inevitable challenges that need to be overcome. (i) Most of the AIE-TADF materials have poor solubility, making them unsuitable for nondoped solution processes. (ii) High crystallization of AIE-TADF materials in lm states can lead to the generation of dark current and exciton traps, eventually reducing device performance. (iii) In a long run, the common AIE-TADF materials cannot resist the erosion of solvent used for the processing of upper layers, which limited their further development in fully solution-processed OLEDs. Therefore, there is room for further development to extend the structural diversity of solution-processable AIE-TADF materials and enhance their device performances.
In this contribution, a novel type of AIE-TADF molecule was developed by constructing a core-dendron structure, which has not been reported in previous research. We designed and synthesized three AIE-TADF molecules named 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP (Fig. 1). The core molecule was 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzBN), which exhibits TADF characteristics. 33 The exible dendrons were different numbers of alkyl chain-linked spirobiuorene. Owing to increased molecular weight and solubilizing alkyl chains, these materials showed a good lm morphology and are suitable for solution processes. More interestingly, although the core 5CzBN and the dendron spirobiuorene displayed ACQ properties, core-dendron materials demonstrated the AIE phenomenon. With increasing the number of exible branches, the compounds showed better solubility, a more smooth surface morphology, more obvious AIE features and higher photoluminescence quantum yields (PLQYs). By employing these AIE-TADF materials (5CzBN-SSP, 5CzBN-DSP, and 5CzBN-PSP) as emitters, fully solution-processed nondoped OLEDs achieved high external quantum efficiencies (EQE) of 7.3%, 13.9% and 20.1%, which far exceed those of the fully solution processed OLEDs based on 5CzBN. Furthermore, it is worth mentioning that the OLED based on 5CzBN-PSP showed a record-breaking external EQE of 20.1% in the area of solutionprocessed nondoped OLEDs based on AIE emitters so far. The core-dendron system will provide a good candidate for highly efficient solution-processed nondoped OLEDs.
The thermal stability properties of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP were characterized by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere at a heating rate of 10 C min À1 . As is shown in Fig. S1, † the decomposition temperatures (T d ) with 5% weight loss of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP were 406.2 C, 394.7 C and 393.8 C, which demonstrated that all three emitters have high thermal stability. The glass transition temperatures (T g ) of 5CzBN-SSP, 5CzBN-DSP and 5CzBN- PSP were 166.8 C, 163.7 C and 144.2 C, respectively. And no crystal domains were formed during the thermal annealing process. The high T g values ensured that the three materials can form uniform amorphous lms during the solution process even though the values of T d and T g decrease to some extent when the number of branches gradually increases. Besides, we also investigated the morphology of the nondoped lms by AFM and SEM measurements ( Fig. S2 and S3 †). The lms of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP are very smooth and the rootmean-square values are 0.50, 0.43 and 0.38 nm, respectively. The values are better than that of 5CzBN (0.74 nm). 34 In addition, the SEM images of the emission layers were collected from various parts of the lms (at various magnications of 1000 and 4000). In Fig. S3, † an obvious trend can be observed which showed that the spin-coated emissive layer exhibited a more homogeneous lm with fewer defects as the number of dendrons increased. However, the solution-processed 5CzBN lms showed pinholes and partially crystalline granulates over the entire lm surface. This is also consistent with the AFM measurements with gradually reduced roughness. Furthermore, the SEM images of the powder were also obtained to further support the above views; 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP showed gradually uniformly distributed spherical particles, while 5CzBN exhibited aggregated bulk particles tending to have a crystalline state. The XRD plots also conrm the amorphous nature of the dendron derivatives in the solid state ( Fig. S4 †). This indicated that all three materials can form uniform amorphous lms through the solution process. And it is also convincing that the exible chain-linked spirobiuorene units can remarkably decrease the crystallization tendency and suppress the aggregation between the molecules. The better solubility and morphology are more benecial for the fabrication of highly efficient fully solution processed OLEDs.
The electronic properties of 5CzBN, 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP were researched using density functional theory (DFT) calculations at the B3LYP theory level with the 6-31G(d) as the basis set. The calculated results are shown in Fig. 2; the highest occupied molecular orbital (HOMO) electron clouds of all four materials are mainly located on the carbazolyl group of the emissive core, while the lowest unoccupied molecular orbital (LUMO) electron clouds are mainly located on the electron-decient benzonitrile units. Compared with the 5CzBN emitter (À5.54 eV), the HOMO levels and the calculated energy gap between S 1 and T 1 (DE ST ) of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP are À5.36 eV/0.15 eV, À5.35 eV/0.13 eV and À5.25 eV/0.11 eV, respectively. They changed regularly with the increasing of the number of electron-donating spirobiuorene groups. Thus, it is concluded that more spirobiuorene dendrons can make the levels of HOMO shallower. In addition, the smaller DE ST ensures efficient RISC and subsequently efficient TADF emission. The electrochemical properties were measured by using cyclic voltammetry (CV). As is shown in Fig. S5, † the energy levels of the HOMO of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP are À5.25, À5.23 and À5.22 eV, respectively. And the results matched well with those of the theoretical simulations. According to the equation E LUMO ¼ E HOMO + E g , where E g is the optical band gap calculated from absorption spectra, the energy of the LUMO of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP is À2.52, À2.48 and À2.52 eV, respectively (Table 1). Therefore, the shallower HOMO levels of these 5CzBN derivatives are close to the energy level of the hole transport layer PEDOT:PSS (À5.2 eV), which would facilitate the hole injection into the emitting layer. Furthermore, we measured the redox curves aer 100 cycles ( Fig. S5 †), and the peak potential and peak current remain unchanged. So these compounds show good electrochemical stability, [35][36][37] which is benecial to long-term device operation.
The photophysical properties of the three TADF materials were measured using an ultraviolet-visible (UV-Vis) absorption and uorescence spectrometer. As is depicted in Fig. 3 and S6, † the absorption bands at 300-400 nm can be attributed to the p-  Table 1 Basic thermal, photophysical and electrochemical parameters of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP  39 The PL spectra and UV-vis spectra of the spin-coated lms are shown in Fig. S6; † the spin-coated lms exhibited emission peaks at 502 nm, 504 nm and 504 nm. This demonstrated that alkyllinked spirobiuorene (SP) can effectively reduce the intermolecular interaction of emissive cores and then further maintain the colour of the 5CzBN core. In addition, the FWHM values of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP are 81 nm, 78 nm and 71 nm, respectively. The narrower FWHM of 5CzBN-PSP is attributed to sufficient encapsulation by the peripheral alkyllinked spirobiuorenes. Furthermore, all three materials exhibit bathochromic shis accompanied by the increase of solvent polarity (Fig. S7 †). The large solvatochromic shi indicates that the three compounds exhibit typical ICT characteristics. 40 Notably, the wavelength of 5CzBN-PSP is shied by a smaller amount than those of 5CzBN-SSP and 5CzBN-DSP due to sufficient encapsulation by large numbers of spirobiuorene dendrons. More SP dendrons which acted as the steric shield, weakened the response to environmental polarity. In order to explore the AIE features of these materials, the PL intensity in the mixture of tetrahydrofuran (THF) and deionized  water with various deionized water ratio fractions was measured and is depicted in Fig. 4 and 5a. For 5CzBN, the PL intensity showed an overall slow decline when water fractions (f w ) increased from 0% to 90%, even if there is a slight rise between 20-40% and 60-70%. This ACQ phenomenon can be attributed to the increased ISC rate and twisted intramolecular charge transfer (TICT) process, which makes the exciton quenching non-radiative. In pure THF solution, these molecules show weak uorescence. And when the water fractions f w were increased, with a rise in polarity the preferential solvation of the TICT state decreases the energy gap between the TICT state and the triplet. In addition, the rate of ISC from the TICT singlet state increases as the singlet-triplet energy gap decreases, and therefore the dark TICT state exhibited weaker emission, that is, PL quenching. However, by increasing the number of exible dendrons, the PL intensity of the three compounds showed different trends (AIE trends), especially for 5CzBN-PSP. 5CzBN-PSP achieved a signicant uorescence enhancement from 20% to 90%, and the PL intensity in 90% mixtures is more than 4 times stronger than the initial intensity (I 0 ). 5CzBN-SSP and 5CzBN-DSP also exhibited a slight enhancement in the PL intensity when f w was increased from 40% to 90%. Thus, these results provide signicant evidence to prove the AIE properties of the three compounds, though the degree of AIE enhancement differed for each molecule. [41][42][43] More interestingly, the peak of 5CzBN underwent a red-shi with increasing addition of water in the mixed solvent until molecular aggregation was induced to form nano-aggregates. Thereaer, the emission demonstrated a blue-shi until the concentration was increased to 90%. The same phenomenon can be observed in 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP. And this is also caused by a TICT process. 44 Before the generation of aggregates, solvatochromism accounts for the red-shi of the emission peak with increasing solvent polarity. In contrast, aer the generation of nano-aggregates, the intramolecular rotation is restricted and the local environment becomes less polar, thereby resulting in a blueshi in the emission color. The curves of the emission wavelength versus the water fraction are shown obviously in Fig. 5a. Some differences can also be found in this picture. With the increase in the number of spirobiuorene branches, the change in the emission wavelength becomes smaller and smaller. This suggested that with increasing the number of dendrons, the TICT process is suppressed efficiently, and the inuence of the environmental polarity on the emission color of the materials becomes weak. Simultaneously, the AIE feature of dendrimers is more outstanding. From these results, it was conrmed that the AIE phenomenon that does not exist in 5CzBN is emerging and enhanced in 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP. Furthermore, it can also be anticipated that the great AIE character for 5CzBN-PSP should be more favourable for its use as an emitter for highly efficient solution-processed nondoped OLEDs. 45 To better understand the TADF features of the three compounds, the transient PL decay proles were measured for the lms. As is depicted in Fig. 5, the PL decays of the three molecules exhibit bi-exponential decays which show prompt and delayed lifetimes of the components, fully demonstrating that 5CzBN, 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP are apparent TADF materials. 41 And the data of the three compounds are presented in Table 1. The transient lifetime of the three compounds is due to the conventional uorescence from S 1 to S 0 . The delayed lifetime is attributed to the reverse intersystem crossing (RISC) process from the non-radiative T 1 state to the radiative S 1 state and nally to S 0 because of the small DE ST . Thus, the PL decay revealed that the three molecules have TADF features. The delayed uorescence lifetimes became longer with increasing the number of exible dendrons. Besides, the PLQYs of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP in the neat lms were measured to analyse the emission features. As shown in Table 2, the PLQYs of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP are 38.0%, 45.7%, and 69.6%, respectively. 5CzBN-PSP has a higher PLQY than the other materials. This is also consistent with the better AIE phenomenon of 5CzBN-PSP. This depicted that increasing the number of alkyl chain-linked dendrons can improve the PLQY by suppressing the core's collision and is more benecial for the fabrication of excellent nondoped fully solution-processed OLEDs.
Before fabricating fully solution processed OLEDs, alcohol resistance was measured by UV-Vis absorption spectroscopy. 46 Fig. 5 (a) The plots of the fluorescence ratio (I/I 0 ) and peak wavelength versus the water volume fraction. (b) Transient PL decay spectra in the neat film measured at room temperature for 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP.  Fig. 6 shows the variations in the absorption intensity of 5CzBN, 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP before and aer spin-rinsing with isopropanol alcohol; isopropanol alcohol would be used to process the adjacent electron-transport layer (ETL). The absorption intensity of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP remained nearly constant while the 5CzBN lm is rinsed thoroughly with isopropanol alcohol. So this proves that the core would be encapsulated more sufficiently by introducing more alkyl chain-linked spirobi-uorene dendrons. The compounds are more resistant to isopropanol alcohol. In other words, by increasing the number of dendrons, the materials can adequately prevent the redissolution by isopropanol.
The devices with the three compounds showed green emission with a peak positioned at 508 nm. Fig. 7 shows the luminance-voltage-current density (L-V-J) and other EL properties of these devices, and the data of the devices are summarized in Table 2. Fig. S9 † shows that the EL spectra of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP have similar emission. The FWHM values are 90 nm, 88 nm and 84 nm, respectively. 5CzBN-PSP exhibited a narrower FWHM because of weaker interactions between 5CzBN cores. The turn-on voltages of 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP are 3.4 V, 3.2 V and 3.1 V. The maximum luminance, CE, PE, and EQE of 5CzBN-PSP are 13 700 cd m À2 ,58.7 cd A À1 , 46.2 lm W À1 and 20.1%, respectively (Table 2), which were the highest efficiency in the reported solution-processed AIE OLEDs. [47][48][49][50][51][52][53][54][55][56][57][58] All reported solution process nondoped AIE OLED performances are summarized in Fig. 8 and Table S1; † it is worth mentioning that there are very few reports on fully solutionprocessed AIE OLEDs and our work breaks the record of efficiencies of solution-processed nondoped AIE OLEDs. It indicated that AIE-TADF dendrimers can suppress exciton quenching of the emitter core, 59 and good lm-forming properties and high efficiencies can be achieved by increasing the number of branches to the core of the materials. All in all, regulating peripheral dendritic branched chains provided a new method for design of novel AIE-TADF materials. And inspired by the excellent device performance, this work would provide critical guidelines for design of solution-processable materials used in the fully wetprocessed optoelectronic led. Fig. 6 Absorption spectra before and after rinsing with isopropanol of 5CzBN (a), 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP (b). Fig. 7 (a) The plots of the fluorescence ratio (I/I 0 ) and peak wavelength versus the water volume fraction. (b) Transient PL decay spectra in the neat film measured at room temperature for 5CzBN-SSP, 5CzBN-DSP and 5CzBN-PSP.

Conclusions
In summary, a series of new core-dendron materials with AIE-TADF characteristics have been synthesized and studied by adjusting the number of alkyl chain-linked spirobiuorene dendrons. With the increasing number of exible dendrons, 5CzBN-PSP exhibited both signicant TADF and AIE features. Furthermore, it showed better resistance to isopropyl alcohol than 5CzBN-SSP and 5CzBN-DSP; the device with 5CzBN-PSP achieved a maximum external quantum efficiency of 20.1%, and current and power efficiencies of 58.7 cd A À1 and 46.2 lm W À1 and showed more efficient performance than fully solutionprocessed OLEDs based on traditional TADF materials. Our research indicated that adjusting the number of the alkyl-chain linked spirobiuorene dendrons attached to the TADF cores can transform common uorophores even ACQ molecules into new AIE molecules. This work thus opens up a new route to design new AIE-TADF emitters with efficient performance in optoelectronic applications.

General methods
All solvents and materials were used as received from commercial sources without further purication. Anhy-drication of THF solvent was carried out according to standard procedures. 1 H NMR and 13 C NMR spectra were recorded on a BRUKER AMX 600 MHz instrument. Elemental analysis was carried out using an Elementar Vario EL CHN elemental analyzer. Mass spectrometry was performed with a Thermo Electron Corporation Finnigan LTQ mass spectrometer. The UV-Vis absorption spectra of the compounds were measured using a SHIMADZU UV-2450. The absolute PLQYs of these materials were measured with a Hamamatsu Quantaurus-QY C11347 spectrometer. The photoluminescence emission spectra were recorded on a HORIBA FLUOROMAX-4 and liquid nitrogen was placed into an optical Dewar ask for low temperature (77 K) photophysical measurements. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves were recorded with a Netzsch simultaneous thermal analyzer (STA) system (STA409PC) and DSC 2910 modulated calorimeter under a dry nitrogen gas ow at a heating rate of 10 C min À1 . Cyclic voltammetry (CV) was performed on a CHI750C voltammetric analyzer in a typical three-electrode cell with a platinum plate working electrode, a platinum wire counter electrode and a silver wire reference electrode. The supporting electrolyte was tetrabutylammonium hexauorophosphate (0.1 M) and ferrocene was selected as the internal standard. AFM (Seiko Instruments, SPA-400) was used to measure the lm surface morphology. The measured pure lm were formed by spincoating and the solvent is 1,2-dichloroethane. The optimized structure was calculated using Gaussian 09 at the B3LYP functional with 6-31G(d) basis sets. The molecular orbitals were visualized using Gaussview 5.0.

Device fabrication and measurements
ITO-coated glass substrates were rinsed with deionized water and then ultrasonicated sequentially in acetone and ethanol. Before device fabrication, the ITO substrate was treated in a UVozone oven for 20 min. Then a 40 nm thick poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) lm was rst deposited on the pre-cleaned ITO glass substrates and baked at 150 C for 10 min. Then, an EML with a thickness of about 40 nm was spin-coated from 1,2-dichloroethane solution onto the PEDOT:PSS layer and annealed at 100 C for 30 min to remove the residual solvent in a N 2 atmosphere. PO-T2T was spin-coated from isopropanol solution as the electron transporting layer, respectively. Finally, 2 nm thick Cs 2 CO 3 and 100 nm thick Al layers were evaporated for use as the cathode. The EL spectra were measured using a PR655 spectra colorimeter. The current density-voltage and brightness-voltage curves of the devices were plotted using a Keithley 2400 source meter calibrated using a silicon photodiode. All the measurements were carried out at room temperature with no protective encapsulation. The EQE was calculated from the brightness, current density and EL spectrum assuming a Lambertian distribution.