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Exciplex-forming co-host for high efficiency and high stability phosphorescent organic light-emitting diodes Chun-Jen Shih, Chih-Chien Lee, Ying-Hao Chen, Sajal Biring, Gautham Kumar, Tzu-Hung Yeh, Somaditya Sen, Shun-Wei Liu, and Ken-Tsung Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15034 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Applied Materials & Interfaces

Exciplex-forming co-host for high efficiency and high stability phosphorescent organic light-emitting diodes Chun-Jen Shih,1 Chih-Chien Lee,1 Ying-Hao Chen,1 Sajal Biring,23 Gautham Kumar,23 Tzu-Hung Yeh,1 Somaditya Sen,45 Shun-Wei Liu,*,23 and Ken-Tsung Wong*,67 1

Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 10617, Taiwan 2

Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan

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Organic Electronics Research Center, Ming Chi University of Technology, New Taipei City 24301, Taiwan 4

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Department of Physics, Indian Institute of Technology, Indore, India

Centre for Material Sciences, Indian Institute of Technology, Indore, India

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan

KEYWORDS: organic light-emitting diodes, thermally activated delayed fluorescence, exciplex, electroluminescent lifetime, photoluminescent aging, photo-induced degradation

ACS Paragon Plus Environment

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ABSTRACT: An exciplex forming co-host system is employed to achieve highly efficient organic light-emitting diode (OLED) with good electroluminescent lifetime. The exciplex is formed at the interfacial contact of a conventional star-shaped carbazole hole-transporting material, 4,4’,4“-tris(N-carbazolyl)-triphenylamine (TCTA) and a triazine electron-transporting material,

2,4,6-tris[3-(1H-pyrazol-1-yl)phenyl]-1,3,5-triazine

(3P-T2T).

The

excellent

combination of TCTA and 3P-T2T is applied as co-host of a common green phosphorescent emitter with almost zero energy loss. When Ir(ppy)2(acac) dispersed in such exciplex co-host system, OLED device with maximum external quantum efficiency of 29.6%, the ultrahigh power efficiency of 147.3 lm/W, and current efficiency of 107 cd/A were successfully achieved. More importantly, the OLED device showed low-efficiency roll-off and operational lifetime (τ80) of ~1020 minutes with the initial brightness of 2000 cd/m2, which is 56 times longer than the reference device. The significant difference of device stability was attributed to the degradation of exciplex system for energy transfer process, which was investigated by photoluminescence aging measurement at room temperature and 100 K, respectively.


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INTRODUCTION The new display and lighting technologies adopting organic light-emitting diode (OLEDs) have started to benefit our daily lives. 1 , 2 Thanks to the invention of transition metal-based phosphorescent emitters which make the 100% harvest of electro-generated excitons feasible due to the strong spin-orbit coupling effect, allowing the device to deliver high external quantum efficiency (EQE). 3,4 Over the last two decades, researchers have made tremendous efforts to improve the efficiency and stability of phosphorescent OLEDs (PhOLEDs), rendering the commercialization of OLED technology feasible.5,6 To achieve efficient and stable PhOLEDs, it is believed that the developments of both phosphorescent emitters and reliable host materials are equally important.7,8 In this regard, bipolar host materials are recognized as a promising solution for giving efficient devices due to the better charge balance, good exciton confinement on the phosphors, and wide emission zone in the emitting layer (EML).9,10 The bipolar charge-transport character of host materials not only can be achieved by molecule with subtle donor-acceptor configuration but also can be realized by physically mixed co-host consisting of judiciously selected donor and acceptor materials. 11 Recently, bipolar molecules exhibiting thermally activated delayed fluorescence (TADF) have also been utilized as a host for giving highefficiency OLEDs 12 , 13 due to their high triplet excitons harvesting efficiency via reverse intersystem crossing (RISC) process. Therefore, the energy of non-emissive triplet state in the EML can be fully exploited. A molecule with a relatively small energy difference (∆EST) is essential to allow efficient RISC for giving TADF,14 which can be achieved by the compound composed of weakly coupled electron-donor (D) and electron-acceptor (A), performing limited intramolecular charge transfer (ICT). However, the molecular design and synthesis of TADF compounds with subtle ICT character often need rigorous works. Alternatively, TADF can be


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feasibly achieved with the interfacial exciplex formed by physical contact with D and A materials15 with suitable energy level alignments relative to the neighboring layers for smooth and balance charge injection in OLEDs.16 It would be even better than both D and A materials have superior charge transport characters which can lead to sufficient charge accumulation at the D/A interface for abundant exciplex exciton formation. 17 Since the singlet-triplet exchange energy is relatively small for exciplex excitons,11 RISC can be easily accomplished and make use of both singlet and triplet excitons for efficient energy transfer to the phosphorescent dopant. Also, the chance of polaron piling in the narrow EML can also be effectively reduced when the phosphor dispersed in such D/A-mixing layer for giving wide emission zone. 18 With such advantages, high EQE of ~30% has been realized by doping bis(2-phenylpyridine)iridium(III)acetylacetonate [Ir(ppy)2(acac)] into the exciplex-forming co-host consisting of 4,4’,4’’-tris(Ncarbazolyl)-triphenylamine (TCTA) as D component and bis-4,6-(3,5-di-3-pyridylphenyl)-2methylpyrimidine (B3PYMPM) as A component. 19 Moreover, such exciplex excitons can be efficiently harvested by direct energy transfer to the dopant rather through trap-assisted recombination. 20 In addition, other reported works of using exciplex-forming co-host for efficient PhOLEDs imply a bright future of this strategy. 21,22 However, limited reports have analyzed the lifetime issue of the exciplex-based PhOLEDs.23 The fundamental factor responsible for the limited lifetime in OLEDs is primarily arose by the intrinsic formation of non-emissive area, or so-called dark spots degradation generated at the electrodes.24 Moreover, the crystallization of organic materials upon thermal treatment, extensive trap or luminescence quencher formation near EML during electrical operation, interfacial degradation due to morphological changes, or high driving voltages aroused by anode instability as well as other underlying reasons including structural defects are responsible for the long-term


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stability.25,26 To address these problems, the introduction of exciplex-forming co-host could be a highly potential solution for PhOLEDs.27,28 In this work, we report highly efficient PhOLEDs with good device operational stability employing an exciplex co-host system formed by blending TCTA and 2,4,6-tris(3-(1H-pyrazol1-yl)phenyl)-1,3,5-triazine (3P-T2T) (Figure 1).29 Both the hole-transporting component TCTA and electron-transporting component 3P-T2T have appropriate energy levels for effective hole and electron injection, respectively, which can eliminate the high driving voltage issue and result in an OLED device with a low turn-on voltage of only 2.3 V at 10 cd/m2.30 In addition to such barrier-free injection processes, large energy level offsets between TCTA and 3P-T2T render sufficient charge confinement and accumulation at the TCTA/3P-T2T interface, making direct hole/electron recombination feasible to form exciplex steadily. Therefore, the unwanted charge trapping effect is successfully eliminated, resulting extremely high power efficiency of 147 lm/W (at 10 cd/m2) in our OLED device. More importantly, the TCTA:3P-T2T exciplex-hosted device showed very limited efficiency roll-off and high operational lifetime (τ80) of ~ 1020 minutes with the initial brightness of 2000 cd/m2, which is 56 times longer than the TCTA:B3PYMPM exciplex-based reference device reported by Kim et al.19 In other hand, photoluminescence (PL) aging measurement was implemented to investigate the fundamental mechanism of exciplex degradation. We found that the stability of exciplex emission is the possible factor responsible for the device lifetime.


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Figure 1. (a) The device configuration of our proposed PhOLEDs. (b) The energy level and the molecular structure of materials used in this work.

RESULTS AND DISCUSSIONS Figure 2a shows the normalized PL spectra of the TCTA, 3P-T2T and co-deposited TCTA:3PT2T (1:1) films. Obviously, the TCTA:3P-T2T co-deposited film shows a red-shifted emission (centered at 547 nm) which is significantly different from those of TCTA (centered at 406 nm) and 3P-T2T (centered at 453 nm). Besides, the lack of residual emissions from TCTA and 3PT2T in TCTA:3P-T2T blended film suggests the efficient exciplex formations upon photoexcitation. The emission energy (on-set) of TCTA:3P-T2T blended film was calculated to be about 2.62 eV which is close to the energy difference between the HOMO level (-5.62 eV) of TCTA and the LUMO level of 3P-T2T (-2.98 eV) and consistent with the results reported previously.29 As indicated in Figure 2a, the exciplex emission partially overlaps with the absorption edge of green phosphorescent dopant Ir(ppy)2(acac), indicating the possibility of


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performing Förster resonance energy transfer (FRET). It is note worthy that the Dexter energy transfer between the TCTA:3P-T2T exciplex and Ir-dopant is also feasible. Indeed, the emission spectrum of TCTA:3P-T2T (1:1) doped with 8wt% Ir(ppy)2(acac) thin film is significantly different from the parent TCTA:3P-T2T exciplex emission. The new emission is nicely superimposable to that of the Ir-dopant with a maximum peak centered at 524 nm. The new emission spectrum verified the energy was transferred from the exciplex to the dopant. Furthermore, the PL emission and PL decay of single host blended thin films (TCTA:8wt% Ir(ppy)2(acac) and 3P-T2T:8wt% Ir(ppy)2(acac)) were also analyzed. These Ir-doped TCTA and 3P-T2T films only showed Ir(ppy)2(acac) emission, indicating the efficient energy transfer between them. The results are shown in Figure S1 (Supporting Information, SI). Also, the EQE of OLED is proportional to the PL quantum yield (PLQY) of the EML.3 The PLQY of pure TCTA:3P-T2T exciplex was measured to be 41±3%, 31 which is dramatically enhanced to 96±3% by doping Ir(ppy)2(acac) into the TCTA:3P-T2T blend. (a)

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w Ir(ppy)2(acac) lifetime: 0.632 µsec

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Figure 2. Photophysical measurements of thin film characteristics (a) Normalized steady-state fluorescent spectra of TCTA, 3P-T2T and TCTA: 3P-T2T thin film (1:1) and phosphorescent spectrum of TCTA: 3P-T2T: Ir(ppy)2(acac) (1:1: 8wt%) co-deposited thin film with the absorption spectrum of Ir(ppy)2(acac) neat film (b) The time-dependent PL decay of TCTA: 3P-


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T2T (1:1) and TCTA: 3P-T2T: Ir(ppy)2(acac) (1:1: 8wt%) co-deposited thin films monitored at 547 nm and 524 nm, respectively.

It is believed that the poor efficiency of OLEDs can be attributed to inefficient energy transfer from the host to the phosphors or exciton diffusion to the ETL. To have in-depth analysis about the exciton confinement behavior in the EML, transient PL decays of the TCTA:3P-T2T (1:1) blend with, and without 8wt% Ir(ppy)2(acac) were examined as shown in Figure 2b. The transient PL of TCTA:3P-T2T (1:1) blend can be fit to a biexponential decay with two components of 0.25 ns and 1.63 s, which was assigned to prompt and delayed fluorescence, respectively. The delayed emission of TCTA:3P-T2T blend is the signature of exciplex formation upon photo-excitation. However, the PL of Ir(ppy)2(acac)-doped TCTA: 3P-T2T blend only exhibits a single exponential decay with a shorter time of 0.63s, indicating the energy of exciplex exciton was efficiently transferred to Ir(ppy)2(acac). However, it is obvious that PL decays are very different between exciplex-hosted and single-hosted systems. The PL decay of TCTA:3P-T2T exciplex doped with Ir(ppy)2(acac) shows relatively longer lifetime as compared to those of Ir-doped TCTA and 3P-T2T films. This can be ascribed to delayed energy transfer from the RISC-populated singlet state of exciplex to Ir-emitter. Based on this result, we can anticipate that the TCTA:3P-T2T exciplex could be a suitable candidate to serve as a host system for Ir(ppy)2(acac).

The OLED devices were fabricated with the exciplex-forming TCTA:3P-T2T (1:1) blend as co-host system and Ir(ppy)2(acac) as an emitter. We also prepared OLED device using TCTA: B3PYMPM as co-hosts for comparative study. The device is configured as: Device 1 (D1): ITO


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(80 nm)/ HAT-CN (10 nm)/ TAPC (30 nm)/ TCTA (10 nm)/ TCTA:3P-T2T (20 nm; mixed ratio of 1:1 with 8wt% Ir(ppy)2(acac)) / 3P-T2T (70 nm) / LiF (1 nm)/ Al (100 nm); Device 2 (D2): ITO (80 nm)/ HAT-CN (10 nm)/ TAPC (30 nm)/ TCTA (10 nm)/ TCTA:B3PYMPM (30 nm; mixed ratio of 1:1 with 8wt% Ir(ppy)2(acac)) / B3PYMPM (50 nm) / LiF (1 nm)/ Al (100 nm). Both D1 and D2 devices have been optimized for their maximum efficiency and brightness. The characteristics of devices D1 and D2 are summarized in Table 1. The current density (J)-voltage (V)-luminance (L) characteristic of the OLEDs are shown in Figure 3a. Both current density and luminance of device D1 are much higher than those of model device D2 at the same driving voltage. The luminance of device D1 under 3.0 V is 879 cd/m2 while the luminance of D2 device fails to reach even 200 cd/m2. Current efficiency-luminance-power efficiency and EQEluminance curves of devices D1 and D2 are shown in Figure 3b and 3c, respectively. It is obvious that the maximum efficiency of model device D2 is higher than those of device D1, in which the EQE at 1000 cd/m2 is 31% and 28% for device D2 and D1, respectively. However, the efficiency roll-off of device D1 appears relatively moderate as compared to that of device D2 at high luminance (> 104 cd/m2). It has been reported that charge-trapping is the dominant mechanism for light-emission in TCTA:B3PYMPM exciplex-hosted device, which makes it difficult to manage the charge balance in the EML.32 On the contrary, the device D1 shows the maximum power efficiency of 147.3 lm/W at 10 cd/m2 associated with the lower operating voltage, which is, to be the best of our knowledge, the highest value ever reported for green PhOLEDs without any light out-coupling strategy.33,34 In addition, the turn-on voltage of device D1 (2.3 V) is 0.2 V lower than that of device D2 (2.5 V) although the LUMO level of 3P-T2T (around -3.0 eV) is slightly higher than that of B3PYMPM (-3.2 eV). The similar results were previously reported by Kido et al.30 In addition, we also believe that the mobility difference


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makes influence on the turn-on voltage, 35 therefore, this result can be reasonably ascribed to the better electron transportation due to higher electron mobility of 3P-T2T29 (in the range of 5.1×10−3 to 6.2 × 10−3 cm2/ V/ s for the fields varying from 2.3×105 to 4.2×105 V/cm) as compared to those of B3PYMPM36 (1.5×10-5 cm2/Vs at electric field of 6.4×105 V/cm). The promising property of 3P-T2T is the crucial factor governing the device performance even for the observed superior EL performance of non-doped TCTA:3P-T2T exciplex device than that of TCTA:B3PYMPM-based device, as shown in Figure S2 (SI). Table 1. Summarized electroluminescent performance of Device 1 and 2, respectively.

a)

b)

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127/ 101/ 62

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Luminance (cd/m2)

a) Defined at 10 cd/m2; b) Captured at 3 and 6 volts, respectively; c) Measured at 100, 1000, and 10000 cd/m2, respectively.

As shown in Figure 3d, the EL spectra of devices D1 and D2 are solely from the Ir-dopant centered at 525 nm without any residual emission from exciplex as well as charge-transporting materials. This result reveals that the efficient and completed energy transfer from the exciplex to phosphorescent dopant which is consistent with the observed PL behavior. It is recognized that charge confinement in the EML is the key factor for the device to give high EQE and lowefficiency roll-off.37 There are large energy barriers both for electron and hole leakages at the interface between TCTA and 3P-T2T (see Figure 1a). Therefore, we strongly believe that the electrons and holes can be effectively confined at TCTA:3P-T2T interfaces within the EML and contribute to the high EQE and power efficiency.


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Figure 3. Electroluminescent characteristics of D1 and D2 devices. (a) Luminance-voltagecurrent density characteristics. (b) Current efficiency-luminance-power efficiency characteristics. (c) External quantum efficiency characteristic. (d) The EL spectrum of D1 and D2 at the brightness of 2,000 cd/m2.

In addition to the high EQE and low-efficiency roll-off, it is always more realistic to evaluate device lifetime for verifying the practical application of such device design and the durability of materials used. Therefore, the EL lifetimes of devices D1 and D2 were measured to study the stability of device based on the energy transfer mechanism from exciplex to dopant. Figure 4 shows a temporal decrease in EL intensity of the exciplex-hosted PhOLEDs measured with the initial luminance at 2000 cd/m2. Device D1 showed a long lifetime of 1020 minutes with 20%


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decay of the initial luminance, whereas device D2 exhibited a lifetime of only 18 minutes at the same condition. It is clear that the EL lifetime (T80) of device D1 is almost two orders of magnitude longer than that of the model device D2. This result motivated us to unveil the behind mechanism that leads to such dramatic difference in device stability. 1.00

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D1 D2 Exponential fitting line Exponential fitting line

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Figure 4. The device lifetime of devices D1 and D2 driving at constant current mode with initial luminance of 2000 cd/m2.

To identify the intrinsic degradation mechanisms of exciplex hosts, TCTA:3P-T2T and TCTA:B3PYMPM blended films were subject to PL aging measurements.38 Figure 5 shows the progressing PL of TCTA:3P-T2T and TCTA:B3PYMPM blended films upon optical stress (at 305 nm) at room temperature and 100 K. The absorption spectra of TCTA, 3P-T2T and B3PYMPM as well as their mixed films are demonstrated in Figure S3 (SI), These materials exhibit strong responses to the excitation wavelength. However, the absorbance of TCTA at 305 nm is slightly lower than those of 3P-T2T and B3PYMPM. It is clear that the emission spectra remain the same along with the progress of irradiation time while the intensity of the emission peak drops after illuminating over a period at room temperature. It is noteworthy that the variation of steady-state PL intensity is not significant in TCTA:3P-T2T film as compared to that


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of TCTA:B3PYMPM film where a noticeable degradation was observed. Also, the opticalstressed exciplex phosphorescent spectra (at 100 K) of TCTA:3P-T2T and TCTA:B3PYMPM films were measured and shown in Figure 5c and 5d, respectively, which can provide the direct evidence on the stability of exciplex blends as co-hosts for PhOLEDs. As indicated, the phosphorescent emission of TCTA:3P-T2T film remains quite stable all the time, while the emission of TCTA:B3PYMPM blended film declines dramatically till the phosphorescent emission becomes too weak to be detected after 40 minutes of optical excitation. Furthermore, the PL aging measurements of exciplex-forming host doped with 8wt% Ir(ppy)2(acac) are also analyzed, the results are shown in Figure S4 (SI). It is obvious that the Ir-dopant emission centered at 524 nm remains for both samples during the PL aging process. However, the PL intensity of blended film of TCTA:3P-T2T exciplex doped with Ir(ppy)2(acac) shows almost intact. In contrast, the PL intensity from the sample of TCTA:B3PYMPM blend doped with Ir(ppy)2(acac) drops steeply and decreases to some extent after 40 minutes of excitation. In addition to the photostability, we also performed the morphological stability tests on the pristine and blended films. The results are depicted in Figure S5 (SI). We found that the pristine 3P-T2T showed better film stability upon thermal treatment as compared to that of B3PYMPM. B3PYMPM is prone to aggregation after thermal treatment, which is possibly fatal to the morphological stability of active layers. As we found that the blended films are relatively robust upon thermal treatment. However, the morphology of TCTA:B3PYMPM mixed film did show slightly higher degree change after thermal treatment at 90 oC as compared to that of its counterpart TCTA:3P-T2T.


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The PL-aging studies indicate the superior stability of TCTA:3P-T2T exciplex as compared to that of the model TCTA:B3PYMPM exciplex system. Based on this result, it can be reasonably concluded that the decrease of EL efficiency as device D2 driven with constant current can be attributed to the intrinsic instability of TCTA:B3PYMPM co-host system, leading to structural defects in EML. Obviously, B3PYMPM being a dispersive electron-transporting material that generates electron traps and leads to charge accumulation36 which are fatal for long-term device operation and responsible for the poor lifetime of device D2. In contrast, 3P-T2T exhibiting high electron mobility29 ensures smooth electron migration to reach to the TCTA:3P-T2T interfaces where the sufficient charge accumulation occurred due to the large energy level offsets, leading to efficient exciplex excitons formation in the EML. Although the TADF character of exciplex renders the triplet harvesting feasible, the relatively long excited state lifetime still risks the material degradation. As Ir(ppy)2(acac) is introduced as a dopant, the energy transfer from exciplex co-host to phosphorescent dopant happens through both FRET and Dexter transfer mechanisms for quickly relaxing the excited states, giving key factors responsible for the long lifetime of device D1. The PL aging measurement of two exciplex systems doped with Ir(ppy)2(acac) also be carried out to give an intuitive observation of light-emitting behavior (Figure S4).

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Wavelength (nm)

Figure 5. Photoluminescent degradation of TCTA:3P-T2T and TCTA:B3PYMPM thin films with 1:1 ratio. (a)(b) The PL aging of fluorescent emission. (c)(d) The PL aging of phosphorescent emission.

CONCLUSION In summary, the steady and transient emission features as well as PLQY of TCTA:3P-T2T (1:1) blend were significantly changed as the Ir(ppy)2(acac) was introduced as a dopant. More strikingly, the increased PLQY and reduced PL decay time indicate that the efficient exciton transfer from exciplex to phosphor occurred in the Ir-doped system, giving the more enhanced possibility for harvesting the exciplex triplet excitons. Together with the suitable energy level and high electron mobility of 3P-T2T, phosphorescent OLEDs with low turn-on voltage, high efficiency, low efficiency roll-off, and significantly long lifetime was achieved by using an exciplex-forming TCTA:3P-T2T (1:1) blend as a co-hosted system for Ir(ppy)2(acac). The obtained device gave an EQE of 29.6% at 10 cd/m2 and maximum current efficiency of 107 cd/A and power efficiency of 147 lm/W, and high luminance of 25,000 cd/m2 at 6 V. More significantly, the device shows remarkably long lifetime (T80) of 1020 minutes with the initial brightness of 2000 cd/m2 which is 56 times longer than that of the reference device based on the


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exciplex-forming co-host of TCTA:B3PYMPM. The PL aging measurements conducting on the donor:acceptor blended films indicated the superior photostability of the TCTA:3P-T2T exciplex as compared to that of model exciplex based on TCTA:B3PYMPM. The durability of exciplexforming co-host was concluded to be the behind mechanism responsible for the stability of OLED device employing exciplex as co-host. This work confirms that the strategy of using exciplex as co-host system for high efficiency and high stability PhOLEDs is promising.

EXPERIMENTAL SECTION Materials: All the metals and organic materials, including di-[4-(N,N-ditolyl-amino)phenyl]cyclohexane

(HAT-CN),

di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane

(TAPC),

B3PYMPM, TCTA, and Ir(ppy)2(acac) were purchased from Nichem fine, Taiwan, and 3P-T2T was synthesized in our laboratory.39 All organic materials were purified by sublimation process by homemade high vacuum system used for graded sublimation according to standard procedures. 40 The ITO substrates were sequentially cleaned using alcohols (acetone and isopropyl) and deionized water in an ultrasonic bath, and dried by blowing Nitrogen (N2) with 5N pressure. Before thin-film evaporation, the thicknesses of each material were measured by surface profiler (Dektak XT). The OLEDs fabrication process was then carried out by thermal evaporation system on an ITO substrate with highly smooth morphology (under high vacuum level of 2×10-6 torr). The deposition rates of each organic material were about 0.5–1 Å/s. Finally, the encapsulation of device was done with bare glass in a glove box (oxygen, moisture < 0.1ppm). Schematic diagrams of exciplex OLEDs device configuration, energy levels and chemical structures are shown in Figure 1.


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OLED Characterizations: The current density-voltage-luminance (J-V-L) characteristics of the fabricated OLED device were measured with a setup consisting of a Keithley 2400 source meter combined with a Spectra scan PR655. Electroluminescent (EL) lifetime of the device was measured by customized setup consisting of a two-channel Keithley 2636 source meter with photodiode array (Ocean Optics USB2000). All experimental measurements and data acquisition were controlled by LabVIEW software. Photophysical measurements: All steady-state absorption and emission measurements, transient PL decay measurements, PL quantum yield, and exciplex aging measurements of organic thin-films were measured using Spectro-fluorometer (FluoroMax Plus, Horiba Jobin Yvon) with the nitrogen-filled environment. The steady-state absorption and emission, analysis of exciplex PL aging, and PL quantum yield of thin-film characterizations were conducted using an ozone-free xenon arc lamp as the excitation source (λex= 305 nm; 87×10-6 W/m2) under ambient conditions. And the PL decay was studied utilizing NanoLED pulsed sources (N-320, Horiba Jobin Yvon) as excitation (λex= 320 nm; 33×10-6 W/m2) with the pulse frequency of 50 kHz.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acsami.

AUTHOR INFORMATION


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Corresponding Author E-mail: [email protected] E-mail: [email protected]

ACKNOWLEDGMENT The authors acknowledge ﬁnancial support from the Ministry of Science and Technology (Grant Nos. MOST 106-2221-E-011-102-MY2, 106-2628-E-131-001-MY2, 106-2221-E-131-027, and 106-2119-M-131-001). In addition, the corresponding author (S.-W. Liu) is grateful to Mr. H.-H. Wu, Syskey Technology Corporation (Taiwan), for his assistance in designing the fabrication system. REFERENCES

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ToC figure


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Exciplex-forming co-host for high efficiency and ... - ACS Publications

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