Crystalization-enhanced Stability by Effectively Suppressing Pho-tooxidation Defect for Lasing Emission and Electroluminescent Device

Effectively protection against photooxidation of wide bandgap semiconductors remains a challenge that become the bottleneck on the road to commercialization. Herein, we demonstrated the crystallization effects of uorene-based blue emitters on long-wavelength green band (g-band) defect with the well-dened organic micro/nanocrystals obtained via the self-assembly of surfactant-assisted reprecipitation methods. For the key molecule model 2,2'-bi(9,9-dipropyl)uorene (DDC3F), the self-assembled nanorods lm ex-hibits an ultrastable spectral stability without the presence of g-band emission (530 nm), and even main-tains the deep-blue emission under ultraviolet exposure (~ 3 h). In contrast, the spin-coated amorphous lm shows a poor color purity with a green-index of ~ 4. Our results offer a robust evidence on the princi-ple in aggregate-enhanced stability, which is severely ignored by the general application of amorphously glassy molecules in OLED devices. In addition, the single nanorod as Fabry-Pérot (FP) optical resonator exhibits size-dependent dual wavelength lasing at 392 nm (threshold: 102 mW/cm2) and 412 nm (threshold: 216 mW/cm2), and generates the deep-blue electroluminescence in solution-processing OLED devices. The supramolecular self-assembled micro/nanocrystals strategy provides a potential platform to maintain ultrastable color purity in wide-bandgap optoelectronic device.


Introduction
Organic wide-bandgap semiconductors (OWBGS) are the fascinating and promising materials with a numerous of potential application, such as exible electronics, 1-3 organic light-emitting diodes (OLEDs), 4,5 organic solid-state lasers (OSSLs), 6,7 organic photovoltaics (OPVs), 8,9 nanogenerators 10,11 as well as memristive neurocomputing, 12,13 even arti cial intelligence. 14 In the past 30 years, the thirdgeneration OWBGS have received wide attention due to their advantages over atom-precise synthesis and superior performances. Compared with inorganic counterparts such as GaN, 15,16 SiC, 17,18 ZnO, 19 as well as diamonds, however, the essential breakthrough of the third-generation OWBGS has not been made in terms of molecular design, structure-property relationships, and photoelectronic device applications. It is signi cant but still elusive to excavate the inherent rules and deeply investigate the association between molecular structures, aggregate morphologies and optoelectronic performances of OWBGS. Salbeck groups have successfully introduced spirobi uorenes to suppress crystalline state and afford high electroluminescence performances in OLED devices, which offer a key cornerstone for a disruptive and transformative paradigm of amorphous and glassy molecular states. 20 Since 1990s ~ 2000s, uorene-based building blocks have been indispensable candidates in OWBGSbased systems and photoelectronic devices. 21,22 Generally, amorphous molecules with high glassy transition temperature (Tg) serve as the stable active layers, where molecular arrangements are disordered and loose. In this case, the generation of massive defects from the O 2 /O. intrusion in air environment (Fig. 1A) is detrimental to luminescence e ciency, stability, color purity as well as poor reproducibility in photoelectronic devices. 23,24 In uorene-based OWBGS systems, defects are generally originated from the presence of ketone sites and aggregation behaviors, where the former has been demonstrated but is rather di cult to be overcome. 25,26 In order to stabilize the uorene-based semiconductors, various strategies have been explored, including rational molecular design, 27 aggregate modulation, 28 device packaging 29 and so on. For example, in terms of electronic structures, the highest occupied molecular orbital (HOMO) energy level should not be high to defend H 2 O/O 2 attacking. On this basis, in terms of molecular structures, aryl substituents are more stable than that of alkyl groups, which plays a key role of steric hindrance in suppressing molecular packing. 30 Furthermore, bilateral side chain and chain encapsulation in macrocycles also offer a novel method to improve stabilization. However, the mechanisms of tailoring stabilization based on aggregated thin lm and other solid states are still unclear and have some contradiction due to some facts.
Recently, we created the intriguing self-assembly of OWBGS into organic micro/nanocrystals through synergistically molecular attractor-repulsor theory (SMART). 31,32 We previously discovered that the crystalline nanosheets affords the effect of crystal-induced luminescence and/or stability enhancement (CLoSE) on rac-tactic ter uorenes, while the absence of CLoSE effect in meso-con gurational ter uorenes (maintaining amorphous state) results in the g-band. 33,34 Inspired by this, we used such crystalline strategy to create "molecular packing wall" that favors the suppression of oxidation process and chemical ketone defects (Fig. 1B).
Herein, we investigate the generalization of aggregate-enhanced stability and demonstrate the effect of crystallization on preventing photoodixation behaviors, through the blue-emitting model molecule 9,9,9',9'tetrapropyl-9H,9'H-2,2'-bi uorene (DDC 3 F). The spin-coated amorphous lm (as the control state) was found to exhibit a low spectral stability with the presence of g-band emission (530 nm) and the I green /I blue value up to 4 under UV irradiation (3 h). In contrast, the 1D nanorods self-assembled via reprecipitation methods show an ultrastable deep-blue emission with an ultrahigh photoluminescence quantum yield (PLQY) of 87% and a high spectral stability under the same irradiation conditions, suggesting that the photooxidation defect is effectively suppressed. On this basis, the single nanorod as a Fabry-Pe´rot (FP) optical resonator shows a size-controlled dual wavelength lasing at 392 nm (threshold: 102 mW/cm 2 ) and 412 nm (threshold: 216 mW/cm 2 ), respectively. Finally, an inverted crystalline OLED device with solution processing in three layers is fabricated with a low turn-on voltage of 3.7 V.

Results And Discussion
We selected the 9,9-propyldiaryl uorene dimer (DDC 3 F) as the model building block to examine the role of crystallization in improving photostability (Figs. S1 and S2) The bulk-shaped single crystal was luckily obtained via the facile volatile process under the conditions of THF/ethanol mixed solvents (1:3 volume ratio) (Fig. S3). According to the single-crystal data, the DDC 3 F crystal exhibits the monoclinic space  Table S1). By further study the di uorenyl backbone exhibits anti-conformation, where the torsional angle between two uorenyl moieties is approximate to 180°. The propyl chains are oriented on two sides of di uorene plane, which shows massive sites of Van der Waals force in symmetric distribution (Fig. S6). Based on above conformational feature, the intermolecular packing of DDC 3 F building blocks were performed in a herringbone manner with the angle of 58.57°. In such packing architecture, multiple C-H…π intermolecular interactions (in the distance of 2.895 Å) between two sterically adjacent molecules connecting the alkyl chains and the uorene planes. In addition, compared with other pendant groups, the propyl chains also possess the role of steric hindrance to increase the distance (8.29 Å and 8.98 Å) between two adjacent di uorene plane (Fig.   S7). 30,35 Then, we investigated the surfactant assisted self-assembly of DDC 3 F into micro/nanocrystals to explore its crystalline features. Figs crystal planes on the lateral surfaces, which are completely correspondence with theoretically predicted growth morphology (Fig. S8). On this basis, we investigated the crystalline effect on the photophysical properties and photo-stability of DDC 3 F. Fig. 3A shows the normalized steady-state absorption and PL spectra of DDC 3 F in different states. Two absorption band centered at 270 and 330 nm were observed on the dilute solution and the spin-coated amorphous lm, the latter of which is assigned to the π-π* transition on di uorenyl moiety. For PL spectra, DDC 3 F in the dilute solution exhibits a clearly vibronic progression of 0-0, 0-1, 0-2 and 0-3 transitions at 373, 388, 408 and 438 nm, respectively. The DDC 3 Fbased amorphous lm exhibits a slightly red-shifted with the vibronic progression of 0-0, 0-1, 0-2 and 0-3 transitions at 375, 390, 410 and 440 nm, respectively, where the relative intensity of 0-0 emission is diminished. However, the DDC 3 F-based nanorods lm only shows 0-1, 0-2 and 0-3 emission peaks at 392, 412 and 443 nm, respectively, according to the tting of Lorentzian line shapes (Fig. S9), while the 0-0 emission is completely vanished. This deeply-blue emission feature is probably consistent with the highly ordered H-aggregate emission. 36 Deeply, the photoluminescence quantum yield (PLQY) of DDC 3 F is 97% in dilute solution, 57% on amorphous lm and 87% on crystalline nanorods lm (Fig. 3B), respectively. These results reveal that such nanorod architectures possess the crystallization-induced luminescence enhancement (CIEE) effect as approximately the highest level of PLQY. 37 Meanwhile, the corresponding photophysical data were shown in Table 1 and Figs. S10-12. According to the k r = Φ F /τ FL equation, the calculated radiative deactivation rate (k r = 1.23× 10 9 S −1 ) of nanorods lm is enhanced, while the nonradiative decay rate (k nr =1.83× 10 8 S −1 ) is reduced compared to that of DDC 3 F in amorphous lm. This is probably because of the inhabitation of quenching effect in crystal through the introduction of pendant propyl-alkyl chains.
The thermostability and photostability were examined for spin-coating amorphous lm and drop-casting nanorods lm. Initially, DDC 3 F powder displays excellent thermostability with a high thermal decomposition temperature of 320°C (Fig. S5). After thermal annealing at 100°C under ambient conditions for 10 min ~ 3 h, the PL spectra of both lm states are maintained without the addition of green emission band, indicating the excellent spectra stability. It is noted that, probably owing to the presence of crystallization behaviors, the intensity of 388 nm (0-0 band) on amorphous lm were reduced (Fig. S13). Further, phtotostability of lms was investigated under UV irradiation (room temperature, RH: 70%, light: 80 W). In Fig. 3C, after UV exposure for 10 min, the amorphous lm generates a new green emission band at 500-600 nm. Further exposed for 1~3 h, such green emission at 530 nm is gradually enhanced, with the green index I green /I blue increasing from 1 to 4. These results indicate the presence of photooxidation process on amorphous lm, which affords oxidative defects and results in the spectra instability (Top of Fig. 3C). In contrast, the nanorods lm is capable of maintaining deep-blue emission (with I green /I blue = 0), almost without the generation of green-emission band even under the same or longer UV irradiation (Fig. 3D). The excellent photostability was also con rmed by the patterning experiment through brushing process, as shown in Fig. S14. Under the same UV exposure, the crystalline nanorod region "CMSOD" can still maintain the blue-emission, while the amorphous background region shows the yellow-green luminescence.
To gain insight into the photostability mechanism of nanorods lm, FT-IR spectra and transient spectra were used to detect the oxidative defects (Fig. 3E). We observed that the FT-IR spectrum of amorphous lm (undergoing UV irradiation) generates an infrared absorption at 1720 cm −1 , assigned to the stretching vibration of C=O on ketone moieties. Meanwhile, the lifetime at 530 nm emission was measured as ~ 5.4 ns, con rming the ketone emission with partly forbidden transition feature (Fig. 3F). 38 In contrast, no C=O stretching vibration (at ~1720 cm −1 ) was observed on nanorod lms after the same UV exposure, along with no emission behaviors with the lifetime of ~ 5 ns, both of which indicate the nonexistence of ketone defects. These results reveal that the ketone defects during the exudation process are majorly formed on the amorphous lm, while the nanorod lm has a high-e cient in uence on suppressing the oxidation process for the formation of ketone defects (Fig. S15). Therefore, the closely molecular packing in crystalline nanorod architectures, analogous to molecular wall guard, allows the excellent ability to isolate oxygen (in air) and protect the 9-position of uorenyl moieties from undergoing exudation. It is noted that the high photostability from such crystalline molecular wall strategy is signi cant to e ciency, reproducibility and color purity of photoelectronic devices.
In order to further the verify the strategy of crystalization-enhanced stability, the deep blue emission of DDC 3 F nanorod as Fabry-Pe´rot (FP) optical resonator was also tentatively examined through micro/nanocrystals nanosecond lasing. In Figs. 4A and S17, the lasing spectra of nanorod architecture with the length of 42.8 µm possesses the violet emission at 392 (0-1) and 412 (0-2) nm under the pump density of 187 to 234 mW/cm 2 . However, the shorter nanorod structure (~16 µm length) only shows a stimulation emission at 392 nm (Fig. 4B). According to the variation of output intensity as a function of pump density (Fig. 4C), the lasing threshold was evaluated as ~102 mW/cm 2 , and the maximum width at half intensity (FWHM) was calculated as FWHM = 0.14 nm, along with the quality factor Q of 2800 (Fig.  S18A). Moreover, if increasing the nanorod length excessing ~ 70 µm, it is surprising that the simulation emission switches into the 412 nm (0-2 channel) (Fig. 4D), along with the lasing threshold of 216 mW/cm 2 (Fig. 4E), FWHM = 0.46 nm and the quality factor Q = 896 (Fig. S18B). In contrast, the amorphous lm only exhibited ampli ed self-emission (ASE) feature at 390 nm, with the pump energy threshold of ~ 99 mW/cm 2 (Fig. S19). These results reveal that that such simulation emission switching is closely associated with the nanorod architectures of size-dependent from reabsorption effect, 35,39 which provide the stable platform of e ciently propagating and manipulating miniaturized optics chip on subwavelength scale or microscale.
Finally, we tentatively examined the electroluminescent feature of self-assembled crystalline lm based on the inverted OLED con guration of ITO/ZnO (10 nm)/Cs 2 CO 3 (20 nm)/Crystals lm (150 nm) /NPB (40 nm)/ MoO 3 (8nm)/LiF (1 nm) /Al (100 nm) in Figs. 5A and S20. As the nanorods lm is too rough to exhibit the electroluminescence performance, we used thermal annealing method to afford the dendritic-like crystalline lm (Figs. S21 and S22). The turn-on voltage (VT) was measured as 3.7 V, indicating e cient charge injection into the crystalline device. When the driving voltage is 7.8 V, the luminance of crystalline device is up to 15 cd /cm 2 (Fig. 5B). Besides, the maximum current e ciency of device display is 0.013 cd/A at 4.5 V (Fig. 5C). Fig. 5D shows the corresponding EL spectra under different driving voltages and the emission peak is centered at 445 nm. It is worth mentioning that three layers were processed by the solution method based on crystalline OLED, which totally different from previous reported. 40 Therefore, the crystalization strategy demonstrated it is a unique technology for electroluminescent properties, which can be exploited for the whole solution OLED displays.

Conclusions
To summarize, we pioneering propose the crystalization-enhanced stability strategy for suppressing photooxidation defect of uorene-based OWBGS. The self-assembled nanorods of the uorene dimer DDC 3 F exhibit excellent ultraviolet stability. The nanorods lm still keeping the deep blue emission under UV irradiation, which e ciently suppress g-band (530 nm) of photooxidation defect compared with the amorphous lm using the unparalleled superiority of molecular dense packing. Besides, the single nanorod shows dual wavelength deep blue emission laser of dependent-size at 392 nm and 412 nm with FP optical resonator, respectively. Finally, the inverted crystalline OLED device with solution processing of three layers was fabricated with a low turn-on voltage of 3.7 V. The fabrication of crystalline OLED device based on solution processing technology further con rmed the enormous potential of our crystalization strategy like-molecular wall.
Structural and optical characterizations of organic nanorods SEM images were obtained by a scanning electron microscopy (Hitachi S-4800) with accelerating voltage of 5 kV. TEM images and selected-area electron diffraction (SAED) were obtained from a transmission electron microscopy (JEOL JEM 2010F, accelerating voltage 100 kV). One drop of the solution was dropped on a carbon-coated copper grid, and the solvent was evaporated at room temperature. Using the Bruker's Dimension Icon Atomic Force Microscope with trapping mode, the AFM images were measured under ambient conditions and the nanorods were dispersed on silicon substrate. The X-ray diffraction (XRD) of samples was performed on a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ = 1.54050 Å); the operating 2θ angle ranges from 5 to 30°C, with a step length of 0.02°C. The single-crystal data was performed at around 298 K on a Bruker 2000 CCD area detector using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). All structures were solved by direct methods using a SHELXS-2015 and re ned against F 2 using a SHELXL-2015. The steady-state absorption spectra of the samples were measured by a PerkinElmer Lambda 35 spectrophotometer at room temperature. Meanwhile, the emission spectra were measured using RF-6000 Plus uorospectrophotometer. Time-resolved uorescence decay lifetime data were obtained using an Edinburgh FLSP920 lifetime spectrometer with a 375 nm laser with the time correlated single-photon counting methods (pulse width: 55 ps; pulse repetition frequencies: 20 MHz). Absolute Photoluminescence quantum yield (PLQY) measurement was used for solution, amorphous lm and nanorods lm sample by using an Edinburgh Instruments FLS-920 in an integrating sphere. The uorescence microscopy images of nanorods were measured by an inverted uorescence microscope (FV1000-IX71, Tokyo, Japan).

Ampli ed spontaneous emission (ASE) and microcrystals laser measurement
The excitation source is a liquid-nitrogen-cooled laser (Janis ST-500, Microscope Cryostat) with 325 nm femtosecond pulse laser at a repetition of 1000 Hz and pulse duration of 150 fs. The lasing spectra of the microrods were excited by a focused 325 nm laser through a confocal µ-PL system (OLYMPUS BX53). The size of light spot was about 20 µm in diameter. The luminescent light was collected through an optical multichannel analyzer (Princeton, Acton SP2500i) and the spectral resolution of the spectrometer was 0.167 nm. All measurements were performed at ambient atmosphere. The pump uence (P) = (P 0 /f)/A, where P 0 is pump power, which is measured via pow meter, f is the repetition frequency, and A is light spot area.

The preparation of crystals OLED device
The ITO substrate was cleaned by acetone, ethanol, and deionized water, subsequently. The amorphous lms were prepared by spin-coating with a spin-coating speed of 2000 rpm for 30 s and an accelerating speed of 500 rpm/s in toluene solution. Then, the blended lm was dried at 60°C for 30 min in a warm table to remove the residual toluene solvent. After that, the crystals lm was obtained by annealing at heating stage for 20 min at 120°C. The EL spectra and CIE coordinates of the devices were recordedby a spectra-scan PR655 spectrophotometer. The emission area of the devices is 12 mm 2 . The current-voltage luminescence characteristics were measured using a combination of a keithley source meter (model 2602) and a luminance meter.

Data availability
The authors declare that the data that support the ndings of this study are available within the article and its Supplementary Information les. All other relevant data are available from the corresponding author upon reasonable request.