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Abstract
Organic integrated devices (OIDs) with ultraviolet (UV) photodetective and electroluminescent (EL) properties were fabricated using an aggregation-induced emission (AIE) featured material of 1,1,2,2-Tetraphenylethene (TPE) as host and a thermally activated delayed fluorescent (TADF) featured material of 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyanobenzen (4CzIPN) as dopant (doping concentration includes: 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%) in a doping system as the active layer. The 15% 4CzIPN doping OID yielded a maximum luminance of 2995 cd/m2, a relatively high detectivity of 2.8 × 1011 Jones under an illumination of 365 nm light with an intensity of 0.9 mW/cm2. The current efficiency and power efficiency of the doped device were 3.26 fold and 3.17 fold higher than those of the non-doped device, respectively. The performance improvement was analyzed by using the theory of emission quenching suppression in the AIE process, energy transfer from host to dopant, and simulation of section energy distribution variations in the active layer with the increase of doping concentration. Thus, combining AIE materials with TADF materials as an active layer is an effective way to enhance the performance of OIDs.
© 2017 Optical Society of America
1. Introduction
In recent years, organic photoelectronic devices, such as organic light-emitting diodes (OLEDs), organic photodetectors (OPDs), organic solar cells (OSCs), organic thin film transistors, organic integrated devices (OIDs) [1–6] have drawn wide interests for their potential applications in small size, low-cost, flexible, large-area and other commercial devices. Among them, OLEDs have been widely used in full-color displays and solid state lighting [7–10], while researches involving OSCs have been comprehensively conducted in device structure, mechanism, interface and morphology systematically. Meanwhile, OPDs have attracted a large amount of concerns owing to its excellent performance used in chemical/biological sensing, solar astronomy, environmental sensing and video imaging, etc [11–15]. Moreover, OIDs, such as dual function OID with ultraviolet (UV) detective and electroluminescent (EL) properties, have drawn great attentions for their irreplaceable roles in interior decoration, intelligent control and wearable electronic products [8,16].
Among the previous works, the study on OIDs is retarded for the complexity of device structure and work principles. When forward voltage is applied on, an OID works in the OLED mode, and with reverse bias, it works in the UV-OPD mode. When the OID works as an OLED, holes and electrons injected and transported from anode and cathode would form excitons in the active layer to light [11,17]. In contrast, when OIDs operate in UV-OPD mode, the photo-activated excitons are generated and dissociated into electrons and holes in the active layer. Then they would be gathered by the cathode and anode under the reverse bias, respectively. Thus, it seems extremely difficult to obtain an OID with efficient EL and high UV-detective properties simultaneously. Facing with such hard situations, Li demonstrated that a multilayer integrated device reached a luminance of 4182 cd/m2 and a detective response of 135 mA/W [11]. Recently, both delayed fluorescence (DF) materials with triplet-triplet annihilation (TTA) properties and thermally activated delayed fluorescence (TADF) materials with the reverse intersystem crossing (RISC) property have been introduced into OIDs. It brings a significant enhancement of luminance and detectivity to OIDs [18,19]. But the efficiency of OIDs, which is one of the most important factors restraining the carrying forward of OIDs, is still haunting researchers for intrinsic aggregation-caused quenching (ACQ) and low utilization of excitons in the active layer [20].
In this case, aggregation-induced emission (AIE) materials reported by Tang [21], become a natural choice for improving the efficiency of OIDs as an active material, since they are widely used in OLEDs [22], sensing, optical waveguides [23], bio-imaging [24,25] for utilizing the AIE behavior to improve the efficiency of devices. However, the internal quantum efficiency (IQE) of the AIE emitters with fluorescent emission can achieve only 25% [26]. While the IQE of TADF emitters is nearly 100%, as all excitons can participate in the EL process [27]. Therefore, using AIE materials doped with TADF materials may be a useful way to improve the efficiency of OIDs, and the doping system can also optimize the optical field distribution in the active layer, which is hard to realize in the film of materials with the feature of AIE and TADF compounds.
Thus, OIDs with active layers using a doping system composed of an AIE featured material of 1,1,2,2-Tetraphenylethene (TPE) as a host and a TADF featured material of 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyanobenzene (4CzIPN) as a dopant are fabricated, then both EL and UV detective properties of OIDs with various doping concentrations of 0 wt%, 5 wt %, 10 wt %, 15 wt %, 20 wt %, 25 wt% are studied. To illuminate the mechanism of the doping system for the improvement of both EL and UV detective properties, the principle of spectral alteration, energy transfer, UV-light absorption, doping concentrations and energy distribution simulation are analyzed.
2. Experimental
All the devices were deposited on pre-cleaned indium tin oxide (ITO) glass substrates, which were successively cleaned by detergent, acetone, deionized water, and isopropanol for 15 minutes at each ultrasonic step. Then they were treated by oxygen plasma for 5 minutes to decrease the work function. After that, organic functional layers were vacuum deposited at a rate of 0.5–2 Å/s under a pressure of 2 × 10−4 Pa [19], followed by the deposition of metallic cathode of Mg: Ag (weight ratio is 10: 1) at a rate of approximately 10 Å/s under a vacuum degree of 1.5 × 10−3 Pa without breaking the vacuum to form a device area of 0.2 cm2. During the deposition process, thicknesses and deposition rates of materials were monitored by oscillating quartz crystal monitor.
The OIDs were fabricated with a basic structure of ITO/ molybdenum trioxide (MoO3) (8 nm)/ 4,4’-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) (20 nm)/ 1,1-bis((di-4- tolylamino)phenyl)cyclohexane (TAPC) (10 nm)/TPE: 4CzIPN (10 nm, X wt%)/ 4,7-diphenyl-1,10-phenanthroline (Bphen) (30 nm)/Mg: Ag (100 nm). The energy level diagram of the OIDs along with the molecular structures of TPE and 4CzIPN are shown in Fig. 1. Therein, MoO3 was acted as a hole injection layer. While NPB, TAPC and Bphen were functioned as the hole transport layer (HTL), exciton adjusting layer (EAL), and electron transport layer (ETL), respectively. The blend film of 4CzIPN doped in TPE host was used as an active layer, where X wt% including 0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt% and 25 wt% represented for device I, device II, device III, device IV, device V, and device VI, respectively. The PL peaks of TPE are 440 nm and 539 nm, and the absorption region of 4CzIPN is 340 nm ~500 nm, which is beneficial for energy transfer from host to dopant materials, so TPE is an ideal host material.
Current density-voltage-luminance (J-V-L) characteristics were measured with a Keithley 4200 programmable semiconductor characterization system, EL spectra of devices were tested by an OPT-2000 spectrometer. UV–Vis absorption spectra of films were recorded with a Shimadzu UV-1700 spectrophotometer. Current density-voltage (J-V) characteristics in dark and under illumination were recorded with a Keithley 4200 source [18], and the 365 nm UV light source with an incident power of 0.9 mW/cm2 was used to illuminate the UV-PDs through ITO side. Section energy distributions of devices are simulated by FDTD Solution Microsoft. All the devices without capsulation were tested in the atmospheric environment at room temperature [28].
3. Results and discussion
From UV absorption spectra in Fig. 2(a), it can be seen that the absorption at 365 nm of doped film is slight higher than that of pristine TPE and 4CzIPN films, which ensures a more efficient exciton forming in the doped film when the OID works in the UV-OPD mode. The PL and EL spectra of TPE, 4CzIPN and TPE: 4CzIPN films are shown in Fig. 2(b). The concentration of doped film is 15 wt%. It is obvious that both of PL and EL spectra of TPE contain two peaks. The long-wave peaks locate at 539 nm and 589 nm, while the short-wave peaks are at 440 nm and 463 nm. The PL peaks of TPE are 440 nm, 539 nm and the EL peaks are 463 nm, 589 nm separately. And the PL and EL peaks of 4CzIPN situate at 551 nm and 561 nm. The TPE: 4CzIPN film reveals a slight blue shifted emission peak at 544 nm comparing with pristine 4CzIPN film, which is commonly observed in many TPE-containing AIE materials [29]. Both TPE and 4CzIPN peaks are found in the TPE: 4CzIPN film, which indicates inefficient energy transfer from host to dopant materials. Take the location of these peaks into consideration, it is possible for the OID to emit white light when it works in the OLED mode.
Fig. 2 (a) Absorption spectra of pristine TPE, 4CzIPN and TPE: 4CzIPN films. (b) EL and PL spectra of 4CzIPN doped and non-doped devices. (c) EL spectra of OIDs with 4CzIPN doping concentrations ranged from 0% to 25%. (d) J-V-L characteristics of the OIDs with 4CzIPN doping concentrations ranged from 0% to 25%.
To verify the PL difference between these films with different doping concentrations, the transient PL decay curves of these films as a function of doping concentration are measured, and the results are shown in the Inset of Fig. 2(a). It is clear that the decay lifetime of delayed emission increases along with doping concentration, i.e., 5 wt% concentration of dopant shows much shorter decay lifetime than that of 25 wt%. It is well known, the delayed emission lifetime of singlet excitons on TADF dopant molecules is on the order of µs, due to the up conversion of long-lived triplet excitons. In a low doping concentration of 5 wt%, the host-guest energy transfer of triplet excitons is not complete due to the short radius of Dexter energy transfer, so that parts of triplet excitons are lost on the non-radiative triplet excited states of AIE host. In contrast, in the case of a high doping concentration of 25 wt%, the host-guest Dexter energy transfer of triplet excitons is efficient. Consequently, the triplet excitons formed on hosts are completely transferred to dopants, and participate in up conversion and light emission processes, which prolongs the delayed emission lifetime.
Figure 2(c) shows the EL intensity changing along with the increase of doping concentration in the doped film. When the doping concentration lower than 5%, the EL peak at 589 nm increases, while the EL peak at 463 nm decreases. As the doping concentration higher than 5%, the peaks at 589 nm and 463 nm disappear, while a new peak at 561 nm occurs when doping concentration reaches 15%, indicating that the new peak is the emission of 4CzIPN. Then, the spectra peaks show no shift as doping concentration continues to increase. Moreover, when the doping concentration reaches 15%, the color of the device IV is warm white [30], with a CIE coordinates of (0.43, 0.46) at 7 V. It is mainly attributed to the broad emission band [31], which caused by the inefficient energy transfer from host to dopant materials. And the spectral of device IV is relatively stable when driven by different voltages, suggesting that energy transfer is more balanced in device IV than in other devices.
In forward bias, the OID works as an OLED. The J-V-L curves of OIDs with different doped concentrations are shown in Fig. 2(d). It is obvious that the OIDs with different doped concentrations have different EL performance. With the increase doping concentration of 4CzIPN from 0%, 5%, 10%, to 15%, the luminance of these devices continues to increase, then the luminance decreases as the doping concentration surpasses 15%. The L-V curves display that the 15% doping concentration based OID shows the maximum luminance of 2995 cd/m2 at 10.05 V. While the device with pristine TPE film (device I) has the maximum luminance of 479 cd/m2 at 9.2V. Compared with device I, the maximum luminance of the doped devices shows a significant improvement. Moreover, the detailed OLED parameters of these devices are listed in Table 1.
Table 1. EL property of OIDs with different doping concentrations
Figure 3(a)-(d) show the current density-current efficiency (J-CE), current density-power efficiency (J-PE), luminance-current efficiency (L-CE), and luminance-power efficiency (L-PE) of these devices, respectively. These curves of the OIDs exhibit first increased and then decreased tendency as the increase of doping concentration. Device I contains the lowest efficiency. As shown in Table 1, the CE and PE of it are only 0.158 cd/A and 0.103 lm/W at a current density of 100 mA/cm2, respectively. In comparison, the performance of other devices shows obviously improvement, especially for device IV, exhibiting an improvement of 4.2 folds for CE and 3.4 folds for PE. Similarly, the CE and PE of device I are 0.122 cd/A and 0.052 lm/W at a luminance of 300 cd/m2, respectively. So these doped devices achieve the dramatic performance improvement, and device IV also has a significant improvement of 5.3 folds for CE and 7.4 folds for PE. These data remarkably reveal that the doping system is beneficial for improving CE and PE of OIDs. Actually, the TPE with AIE feature can potentially solve the aggregation-caused quenching problem, so the doping system can effectively suppress the ACQ effect of emission, which should be a main reason for the improved EL efficiency of doped devices [32,33]. It is also noteworthy that, the AIE materials can suppress the quenching of singlet exciton, but they show no effect on triplet exciton [34]. When the doping concentration of 4CzIPN surpasses 15%, both of CE and PE decrease. This is mainly attributed to TTA process in the emission layer, which is evidenced by the performance of devices V and VI.
Fig. 3 (a) J-CE (b) J-PE (c) L-CE (d) L-PE characteristics of OIDs with stepped dopant concentrations ranged from 0 wt% to 25 wt%.
When reversed bias voltages applied, the OID works as an UV-OPD, the UV detective process of OID can be illuminated as Eqs. (1a-d):
where the ground state and the excited state of TPE (or 4CzIPN) are marked as M and M*,respectively. In detail, the UV detective process can be described as following four steps: (1) TPE and 4CzIPN absorb incident UV light and excitons form in the active layer. (2) The excitons are dissociated into holes and electrons under the applied reverse bias [35]. (3) The holes and electrons are transported through TAPC layer and Bphen layer to anode and cathode, respectively. (4) Holes and electrons are finally collected by anode and cathode.As the figure-of-merit for exhibiting the performance of UV-OPD, the detectivity (D*) is widely used to characterize the sensitivity of a photodetector [11]. Both of the photocurrent and dark current are taken into account of contribution. If we suppose that shot noise resulting from dark current is the main contribution of the noise, the D* of the photodetector can be given as below:
where Jdark and Jlight are the dark current density and photocurrent density, respectively. P is the incident optical power density, and q is the absolute value of electron charge.The current density-voltage (J-V) characteristics of OIDs in UV-OPD mode are shown in Fig. 4(a) and Fig. 4(b), which reveals the dependence of Jdark and Jlight upon the reverse bias under UV light illumination through the ITO side, the photocurrent density of Jlight is greatly enhanced along with the increase of reverse bias. It can be seen that these curves about Jdark of the OIDs exhibit first decreased and then increased tendency as the increase of doping concentration. On the contrary, the Jlight curves of the OIDs exhibit first increased and then decreased tendency. Table 2 lists the detailed photodetection parameters of OIDs. Device I shows no detective character, because only 4CzIPN can generate excitons under UV-365 nm light illumination. As the doping concentration increases from 5% to 25%, the Jdark decreases from 2.08 × 10−8 A/cm2 to 3.27 × 10−9 A/cm2 and then increases to 1.14 × 10−8 A/cm2. Conversely, the Jlight increases from 9.34 × 10−7 A/cm2 to 3.31 × 10−6 A/cm2 and then decreases to 8.77 × 10−7 A/cm2. It can be seen that the 4CzIPN doped devices have lower Jdark and larger Jlight, indicating that the doping system is beneficial to reduce the dark current and enhance the photocurrent of OIDs. As a consequence, both of the photocurrent and dark current of the doped devices show higher performance, resulting in that the OIDs consisted of a doping system have an improved detection property.
Fig. 4 (a) Current density-voltage characteristics of OIDs in dark with stepped dopant concentrations from 5 wt% to 25 wt%. (b) Current density-voltage characteristics of OIDs under UV light with stepped dopant concentrations from 5 wt% to 25 wt%. (c) UV detectivity of the OIDs as a function of reverse bias with stepped dopant concentrations from 5 wt% to 25 wt%. (d) Ion/Ioff versus bias of OIDs with stepped dopant concentrations from 5 wt% to 25 wt%.
Table 2. UV detective property of OIDs with different doping concentrations (photocurrent measured at −0.5 V, 0.9 mW/cm2 365 nm UV light)
The Ion/Ioff versus applied voltage characteristics are displayed in Fig. 4(d). It can be seen that the Ion/Ioff ratios of 4CzIPN doped devices in the reverse bias region show first increased and then decreased tendency as the increase of doping concentration. In addition, the detectivity characteristics of 4CzIPN doped OIDs with stepped dopant concentrations of 5 wt% to 25 wt% are shown in Fig. 4(c). It is obvious that the detectivity of 4CzIPN doped devices exhibit the same trend as the Ion/Ioff ratios. Furthermore, the device IV reaches the highest average detectivity of ~4.0 × 1011 Jones between −2V and −0.5V, indicating that the detective properties of 4CzIPN doped devices are all significantly improved.
To further innovate the effect of doping system to UV-detective property of OIDs, a simulation about energy distribution in non-doped and doped film when a UV-365 nm source irradiates on the active layers is carried out. The section energy distribution simulation of non-doped, doping concentration of 5%, 10%, 15%, 20% and 25% are collected in Fig. 5. It is clear that the energy distributes more uniformly in the 15% and 20% doping concentration films compared with that of others. In this situation, excitons generated by the incident source can separate more effectively than others for the decrease of trapping centers caused by the uneven electric field in the films. Thus, it may be the reason of higher light current densities occurred in 15% and 20% doping concentration films, which is further contributed to the high detectivity of these two devices.
Fig. 5 Section energy distribution simulation of (a) non-doped (b) 5% (c) 10% (d) 15% (e) 20% (f) 25% doping concentration.
4. Conclusion
In conclusion, a series of 4CzIPN doped OIDs with different doping concentrations ranged from 0% to 25% have been fabricated. On one hand, the effect of doping system on the EL performance of OIDs is investigated. Due to the suppression of the emission quenching in TPE with AIE feature and balanced energy transfer from TPE host to 4CzIPN dopant, the EL performance of OIDs improves remarkably, the maximum luminance of OIDs achieves a significant improvement from 479 cd/m2 to 2995 cd/m2. The maxima CE and PE of the non-doped OID are only 0.209 cd/A and 0.143 lm/W, respectively, while the 15 wt% 4CzIPN doped OID has a great improvement of 3.26 folds for CE and 3.17 folds for PE, respectively. In the aspect of UV-detective properties, the non-doped OID shows no detective character. With the doping concentration continues to increase, the 4CzIPN doped OIDs have an increased detectivity from 2.78 × 1010 Jones to 2.8 × 1011 Jones under a 0.9 mW/cm2 365 nm UV source. Meanwhile, the Ion/Ioff ratios of 4CzIPN doped OIDs also show an improvement from 44.8 to 1011.0 at −0.5 V for absorption enhancement and electric field optimization in the 15% doping active layer. In addition, an OID with a maximum luminance of 2995 cd/m2 warm white light and a detectivity of 2.8 × 1011 Jones is realized.
Funding
The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (NSFC) (Grant No. 61675041), the Foundation for Innovation Research Groups of the NSFC (Grant No. 61421002), Science & Technology Department of Sichuan Province via Grant No. 2016HH0027 & 2016FZ0100.
References and links
1. A. J. Heeger, “25th anniversary article: bulk heterojunction solar cells: understanding the mechanism of operation,” Adv. Mater. 26(1), 10–27 (2014). [CrossRef] [PubMed]
2. K. Goushi, K. Yoshida, K. Sato, and C. Adachi, “Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion,” Nat. Photonics 6(4), 253–258 (2012). [CrossRef]
3. D. Zhou, R. Wang, H. Guo, J. Huang, and J. Yu, “Study of exciton adjusting layer on electroluminescent and ultraviolet detective properties of organic optoelectronic integrated device,” Org. Electron. 41, 355–361 (2017). [CrossRef]
4. X. Wang, J. Huang, J. Li, and J. Yu, “Effect of organic electron blocking layers on the performance of organic photodetectors with high ultraviolet detectivity,” J. Phys. D Appl. Phys. 49(7), 075102 (2016). [CrossRef]
5. J. Huang, Y. Qi, H. Wang, and J. Yu, “Low roll off radiation efficiency of charge transfer state excitons based on organic photovoltaic and electroluminescent integrated device,” Appl. Phys. Lett. 102(18), 183302 (2013). [CrossRef]
6. W. Zhang, C. Liang, Z. He, H. Pang, Y. Wang, and S. Zhao, “Stable orange and white electrophosphorescence based on spirobifluorenyltrifluoromethylpyridine iridium complexes,” Synth. Met. 210, 214–222 (2015). [CrossRef]
7. C. Li, M. Zhang, X. Chen, and Q. Li, “Fluorinated 9,9′-spirobifluorene derivative as host material for highly efficient blue fluorescent OLED,” Opt. Mater. Express 6(8), 2545–2553 (2016). [CrossRef]
8. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009). [CrossRef] [PubMed]
9. L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong, and J. Kido, “Recent progresses on materials for electrophosphorescent organic light-emitting devices,” Adv. Mater. 23(8), 926–952 (2011). [CrossRef] [PubMed]
10. S. H. Chung and H. Y. Noh, “Color-tunable stacked organic light-emitting diode with semi-transparent metal electrode,” Opt. Mater. Express 6(9), 2834–2840 (2016). [CrossRef]
11. X. Gong, M. Tong, Y. Xia, W. Cai, J. S. Moon, Y. Cao, G. Yu, C. L. Shieh, B. Nilsson, and A. J. Heeger, “High-detectivity polymer photodetectors with spectral response from 300 nm to 1450 nm,” Science 325(5948), 1665–1667 (2009). [CrossRef] [PubMed]
12. H. W. Lin, S. Y. Ku, H. C. Su, C. W. Huang, Y. T. Lin, K. T. Wong, and C. C. Wu, “Highly efficient visible-blind organic ultraviolet photodetectors,” Adv. Mater. 17(20), 2489–2493 (2005). [CrossRef]
13. D. T. Thanh, J. Jin, K. B. Ko, B. D. Ryu, M. Han, T. V. Cuong, and C. H. Hong, “Hexagonal boron nitride pattern embedded in AIN template layer for visible-blind ultraviolet photodetectors,” Opt. Mater. Express 7(5), 1463–1472 (2017). [CrossRef]
14. Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka, and C. Adachi, “Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence,” Nat. Photonics 8(4), 326–332 (2014). [CrossRef]
15. P. I. Shih, C. H. Chien, F. I. Wu, and C. F. Shu, “A novel fluorene-triphenylamine hybrid that is a highly efficient host material for blue-, green-, and red-light-emitting electrophosphorescent devices,” Adv. Funct. Mater. 17(17), 3514–3520 (2007). [CrossRef]
16. D. Yang and D. Ma, “1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane for fast response organic photodetectors with high external efficiency and low leakage current,” J. Mater. Chem. C Mater. Opt. Electron. Devices 1(10), 2054–2060 (2013). [CrossRef]
17. J. Zhou, P. Chen, X. Wang, Y. Wang, Y. Wang, F. Li, M. Yang, Y. Huang, J. Yu, and Z. Lu, “Charge-transfer-featured materials-promising hosts for fabrication of efficient OLEDs through triplet harvesting via triplet fusion,” Chem. Commun. (Camb.) 50(57), 7586–7589 (2014). [CrossRef] [PubMed]
18. D. Zhou, X. Zheng, H. Wang, J. Huang, Y. Luo, J. Zhou, J. Yu, and Z. Lu, “Enhancement of both electroluminescent and ultraviolet detective properties in organic optoelectronic integrated device realized by two triplet-triplet annihilation materials,” Synth. Met. 220, 323–328 (2016). [CrossRef]
19. X. Wang, D. Zhou, J. Huang, and J. Yu, “High performance organic ultraviolet photodetector with efficient electroluminescence realized by a thermally activated delayed fluorescence emitter,” Appl. Phys. Lett. 107(4), 043303 (2015). [CrossRef]
20. L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. A. Street, and Y. Yang, “25th anniversary article: a decade of organic/polymeric photovoltaic research,” Adv. Mater. 25(46), 6642–6671 (2013). [CrossRef] [PubMed]
21. J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X. Zhan, Y. Liu, D. Zhu, and B. Z. Tang, “Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole,” Chem. Commun. (Camb.) 18(18), 1740–1741 (2001). [CrossRef] [PubMed]
22. S. Xu, T. Liu, Y. Mu, Y. F. Wang, Z. Chi, C. C. Lo, S. Liu, Y. Zhang, A. Lien, and J. Xu, “An organic molecule with asymmetric structure exhibiting aggregation - induced emission, delayed fluorescence, and mechanoluminescence,” Angew. Chem. Int. Ed. Engl. 54(3), 874–878 (2015). [CrossRef] [PubMed]
23. Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu, and J. Xu, “Recent advances in organic mechanofluorochromic materials,” Chem. Soc. Rev. 41(10), 3878–3896 (2012). [CrossRef] [PubMed]
24. E. Wang, E. Zhao, Y. Hong, J. W. Y. Lam, and B. Z. Tang, “A highly selective AIE fluorogen for lipid droplet imaging in live cells and green algae,” J. Mater. Chem. B Mater. Biol. Med. 2(14), 2013–2019 (2014). [CrossRef] [PubMed]
25. S. Gui, Y. Huang, F. Hu, Y. Jin, G. Zhang, L. Yan, D. Zhang, and R. Zhao, “Fluorescence turn-on chemosensor for highly selective and sensitive detection and bioimaging of Al3+ in living cells based on Ion-induced aggregation,” Anal. Chem. 87(3), 1470–1474 (2015). [CrossRef] [PubMed]
26. I. H. Lee, W. Song, and J. Y. Lee, “Aggregation-induced emission type thermally activated delayed fluorescent materials for high efficiency in non-doped organic light-emitting diodes,” Org. Electron. 29, 22–26 (2016). [CrossRef]
27. Y. Tao, K. Yuan, T. Chen, P. Xu, H. Li, R. Chen, C. Zheng, L. Zhang, and W. Huang, “Thermally activated delayed fluorescence materials towards the breakthrough of organoelectronics,” Adv. Mater. 26(47), 7931–7958 (2014). [CrossRef] [PubMed]
28. F. Ali, N. Periasamy, M. P. Patankar, and K. L. Narasimhan, “Electronic transport in doped pyrenyl carbazole,” J. Phys. Chem. C 110(4), 044507 (2011).
29. X. Zhan, Z. Wu, Y. Lin, Y. Xie, Q. Peng, Q. Li, D. Ma, and Z. Li, “Benzene-cored AIEgens for deep-blue OLEDs: high performance without hole-transporting layers, and unexpected excellent host for orange emission as a side-effect,” Chem. Sci. (Camb.) 7(7), 4355–4363 (2016). [CrossRef]
30. U. S. Bhansali, H. Jia, I. W. H. Oswald, M. A. Omary, and B. E. Gnade, “High efficiency warm-white organic light emitting diodes from a single emitter in graded-doping device architecture,” Appl. Phys. Lett. 100(18), 183305 (2012). [CrossRef]
31. B. Liu, H. Nie, X. Zhou, S. Hu, D. Luo, D. Gao, J. Zou, M. Xu, L. Wang, Z. Zhao, A. Qin, J. Peng, H. Ning, Y. Cao, and B. Z. Tang, “Manipulation of charge and exciton distribution based on blue aggregation-induced emission fluorophors: a novel concept to achieve high-performance hybrid white organic light-emitting diodes,” Adv. Funct. Mater. 26(5), 776–783 (2016). [CrossRef]
32. H. H. Chou, Y. H. Chen, H. P. Hsu, W. H. Chang, Y. H. Chen, and C. H. Cheng, “Synthesis of diimidazolylstilbenes as n-type blue fluorophores: alternative dopant materials for highly efficient electroluminescent devices,” Adv. Mater. 24(43), 5867–5871 (2012). [CrossRef] [PubMed]
33. C. Li, J. Wei, J. Han, Z. Li, X. Song, Z. Zhang, J. Zhang, and Y. Wang, “Efficient deep-blue OLEDs based on phenanthro[9,10-d]imidazole-containing emitters with AIE and bipolar transporting properties,” J. Mater. Chem. C Mater. Opt. Electron. Devices 4(42), 10120–10129 (2016). [CrossRef]
34. Y. Hong, J. W. Y. Lam, and B. Z. Tang, “Aggregation-induced emission: phenomenon, mechanism and applications,” Chem. Commun. (Camb.) 40(29), 4332–4353 (2009). [CrossRef] [PubMed]
35. J. Xue and S. R. Forrest, “Carrier transport in multilayer organic photodetectors: II. Effects of anode preparation,” J. Appl. Phys. 95(4), 1869–1877 (2004). [CrossRef]
