Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 4851−4859
www.acsami.org
Development of a Highly Efficient Hybrid White Organic-LightEmitting Diode with a Single Emission Layer by Solution Processing Jun-Yi Wu and Show-An Chen* Chemical Engineering Department and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing-Hua University, Hsinchu 30013, Taiwan, Republic of China
ACS Appl. Mater. Interfaces 2018.10:4851-4859. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 01/15/19. For personal use only.
S Supporting Information *
ABSTRACT: We use a mixed host, 2,6-bis[3-(carbazol-9-yl)phenyl]pyridine blended with 20 wt % tris(4-carbazoyl-9-ylphenyl)amine, to lower the hole-injection barrier, along with the bipolar and high-photoluminescence-quantum-yield (Φp= 84%), blue thermally activated delay fluorescence (TADF) material of 9,9dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) as a blue dopant to compose the emission layer for the fabrication of a TADF blue organic-light-emitting diode (BOLED). The device is highly efficient with the following performance parameters: maximum brightness (Bmax) = 57586 cd/m2, maximum current efficiency (CEmax) = 35.3 cd/A, maximum power efficiency (PEmax) = 21.4 lm/W, maximum external quantum efficiency (EQEmax) = 14.1%, and CIE coordinates (0.18, 0.42). This device has the best performance recorded among the reported solution-processed TADF BOLEDs and has a low efficiency roll-off: at brightness values of 1000 and 5000 cd/m2, its CEs are close, being 35.1 and 30.1 cd/A, respectively. Upon further doping of the red phosphor Ir(dpm)PQ2 (emission peak λmax = 595 nm) into the blue emission layer, we obtained a TADF− phosphor hybrid white organic-light-emitting diode (T−P hybrid WOLED) with high performance: Bmax = 43594 cd/m2, CEmax = 28.8 cd/A, PEmax = 18.1 lm/W, and CIE coordinates (0.38, 0.44). This Bmax = 43594 cd/m2 is better than that of the vacuumdeposited WOLED with a blue TADF emitter, 10000 cd/m2. This is also the first report on a T−P hybrid WOLED with a solution-processed emitting layer. KEYWORDS: thermally activated delay fluorescence (TADF), TADF blue organic-light-emitting diode (TADF BOLED), TADF−phosphor hybrid white organic-light-emitting diode (T−P hybrid WOLED), mixed host, low efficiency roll-off, solution processing Recently, thermally activated delay fluorescence (TADF) emitters have been proposed by Adachi and co-workers5 as attractive alternative emitter materials that possess small energy gaps between single and triplet states (ΔEST < 0.1 eV), which allow efficient reverse intersystem crossing for converting triplet excitons to singlet excitons at room temperature such that IQEmax = 100% is possible. Recently, many efforts have been devoted to developing TADF materials for OLEDs including WOLEDs. With regard to WOLEDs,6−10 highly efficient device performances have been reached; for example, Duan and coworkers9 fabricated the hybrid WOLED with blue TADF bis[4(9,9-dimethyl-9,10-dihydroacridine)phenyl] sulfone (DMACDPS) and the yellow phosphor bis(4-phenylthieno[3,2-c]pyridinato-N,C2′)acetylacetonate (PO-01) to compose an emission layer by coevaporation; the device reached highperformance values at 1000 cd/m2, with EQE = 19.6%, PE = 38.7 lm/W, and CIE (0.398, 0.456).
1. INTRODUCTION White organic-light-emitting diodes (WOLEDs) are expected to be used as next-generation solid-state lighting and in largedisplay applications, in which phosphorescent emitters are usually included to take advantage of harvesting triplet excitons in addition to singlet excitons, making it possible to reach 100% maximum internal quantum efficiency (IQEmax) and therefore leading to high device performance.1−4 For example, Kido and co-workers2 used 2,6-bis[3-(carbazol-9-yl)phenyl]pyridine (26DCzPPy) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as the mixed host and the blue phosphor iridium(III) factris(mesityl-2-phenyl-1H-imidazole) [fac-Ir(mpim)3] and the red phosphor iridium(III) bis(2-phenylquinolyl-N,C 2 ′ )dipivaloylmethane [Ir(dpm)PQ2] as the guests to fabricate phosphorescent WOLED (PHWOLED) with double emission layers by the vacuum deposition method. They achieved excellent device performance at 1000 cd/m2, with external quantum efficiency (EQE) = 24.4%, power efficiency (PE) = 51.7 lm/W, and Commission Internationale de L’Eclairage (CIE) color coordinates (0.38, 0.42). © 2017 American Chemical Society
Received: September 27, 2017 Accepted: December 29, 2017 Published: December 29, 2017 4851
DOI: 10.1021/acsami.7b14695 ACS Appl. Mater. Interfaces 2018, 10, 4851−4859
Research Article
ACS Applied Materials & Interfaces
emission λpeak at 495 nm, which is less blue than that of DMAC-DPS, λpeak = 470 nm (with ET = 2.7 eV). Although DMAC-TRZ is a bipolar material that has high electron and hole mobilities of about 1 × 10−4 and 6 × 10−5 cm2/V·s at 6.4 × 105 V/cm, respectively, and has a high photoluminescence quantum yield (PLQY; Φp) of 84% for a solution-processed film. Wada et al.30 used DMAC-TRZ as the emission layer, and the device therewith gave EQEmax = 17.6%, which is the highest record in efficiency among the host-free solution-processed OLEDs reported to date. However, the emission peak was about 500 nm, its emission color was nearly green, and this device had a high driving voltage and a high efficiency roll-off [i.e., EQE = 10.7% at 100 cd/m2 (6 V) and EQE = 2.9% at 1000 cd/m2 (8 V)]. Tsai et al.29 used DMAC-TRZ as the blue dopant and [9-[3-(9H-carbazol-9-yl)phenyl]-9H-carbazole-3carbonitrile] (mCPCN) as the host for the emission layer in the dry-processed OLED and obtained devices with lower efficiency roll-off and higher device performance than those using neat DMAC-TRZ (without host) as the emission layer. The device with a host had high efficiency and low efficiency roll-off: EQE = 25.1% at 100 cd/m2 and 20% at 1000 cd/m2, whereas for the latter case (without a host), the efficiency rolloff was significant, 18.9% at 100 cd/m2 and 11% at 1000 cd/m2. Because the OLED with DMAC-TRZ and mCPCN as the guest and host in the emission layer, respectively, can realize higher performance and low efficiency roll-off, we therefore also use a guest−host system with DMAC-TRZ as the guest but with another host material for our wet-processed WOLED. Here, we use 26DCzPPy31 as the host having a high triplet state (ET = 2.71 eV), bipolar transport capability (μe = 2 × 10−5 and μh = 2 × 10−5 cm2/V·s at 6.4 × 105 V/cm), and large energy band gap (HOMO = 6.05 eV and LUMO = 2.56 eV). The high triplet state can prevent back energy transfer from the guest to the host. In the operation, electrons and holes are both injected into the guest DMAC-TRZ because the HOMO level of DMAC-TRZ (HOMO = 5.61 eV) is shallower than that of the host 26DCzPPy and its LUMO level (LUMO = 3.1 eV) is deeper than that of 26DCzPPy. Because the deep HOMO of 26DCzPPy makes it difficult for holes to be injected into the emission layer, we then introduce the hole-transport material (HTM) TCTA (HOMO = 5.7 eV) in the host to lower holeinjection barrier and device driving voltage. The resulting TADF BOLED achieved excellent performance with maximum brightness (Bmax) = 57586 cd/m2, maximum current efficiency (CEmax) = 35.3 cd/A, EQEmax = 14.1%, PEmax = 21.4 lm/W, and CIE (18, 0.42), which is the best recorded among the reported solution-processed TADF BOLEDs at the practical application range, 1000 cd/m2,25−28 and has low efficiency roll-off values of CE = 35.1 and 30.1 at brightness values of 1000 and 5000 cd/ m2, respectively. For T−P hybrid WOLEDs, we dope the red phosphor Ir(dpm)PQ232 (λmax = 595 nm) into the blue emission layer, and their device design is based on our highperformance TADF BOLEDs. Finally, we obtain the T−P hybrid WOLED with high performance: Bmax = 43594 cd/m2, CEmax = 28.8 cd/A, PEmax = 18.1 lm/W, and CIE (0.38, 0.44). Its maximum brightness value is significantly better than that of the vacuum-deposited T−P hybrid WOLEDs with a blue TADF material of about 10000 cd/m2.6−10 This is the first report on a solution-processed hybrid WOLED by doping with a blue TADF emitter.
In highly efficient WOLEDs, multiple transport layers are usually used to form stepwise energy levels for promoting electron and hole injection into the emission layer, and its white emission layer in general is composed of host and guest materials. However, the cost for fabricating OLEDs by vacuum deposition is high, and depositing several components simultaneously in the white emission layer with precise composition is difficult. For these reasons, industrializing the vacuum-deposited WOLED is not easy. Therefore, fabricating highly efficient and large-area WOLEDs by low-cost solution processing has been attempted such as spin coating, blade coating, injection printing, and slot die coating.11−13 However, producing multilayered WOLEDs using a solution-processed method is a great challenge because of interfacial mixing and erosion of the underlayer during subsequent coating of electron transport14−19 or another emission layer and difficulty in selecting a common solvent for both the host and guest molecules for the formation of a uniformly distributed emission layer. Consequently, the reports on solution-processed OLEDs are mainly limited to a single emission layer in combination with a water- or alcohol-soluble charge-transport material or with a vacuum deposition process for the fabrication of a transport layer on top of the emission layer. In the solution-processed OLEDs, some PHOLEDs with excellent performance have been reported;14,16,20,21 for example, Fan et al.20 used TCTA and 2,2′-(1,3-phenylene)bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7) as mixed hosts and the blue phosphor bis[2-(4,6-difluorophenyl)-4(2,4,6- trimethylphenyl)pyridinato-C2,N]picolinatoiridium(III) (PhFIrpic) and the yellow phosphor PO-01 as the guests and fabricated PHWOLEDs using spin coating for the emission layer; its device achieved high performance at 1000 cd/m2, with EQE = 15%, PE = 20.8 lm/W, and CIE (0.33, 0.44). This study is one of the best performances recorded among the reported results of the same category. Until now, there have been no reports on solution-processed WOLEDs using TADF as a component in the emission layer. Among the reported blue TADF emitters, DMAC-DPS22−24 provided a very good performance but with low solubility in an organic solvent,25 making it inappropriate for wet processing of the device. Therefore, efforts to develop soluble TADF emitters25−28 have been attempted. However, the resulting blue OLEDs (BOLEDs) give low efficiency (current efficiency < 20 cd/A at 1000 cd/m2). For example, Lee and co-workers26 synthesized the blue TADF 2,3,4,5,6-penta(9H-carbazol-9yl)benzonitrile (5CzCN) with the emission peak (λpeak) at 480 nm and doped it into the host diphenylbis[4-(9carbazolyl)phenyl]silane (CzSi) to fabricate a TADF BOLED giving maximum EQE (EQEmax) = 18.7%, but the maximum brightness (Bmax) is low, only 30 > 8 wt % and the profiles of the power efficiency show the level sequence 20 > 10 > 30 > 8 wt %. It is of particular interest that the current efficiency roll-off is extremely low for 10 and 20 wt % in the brightness range, 100 up to 5000 cd/m2. The EL spectra and CIE coordinates from the devices with different amounts of DMAC-TRZ doped into 26DCzPPy/TCTA are shown in Figure S3 and S6. The emission peak red-shifts slightly with the DMAC-TRZ concentration, being at 495 nm (8 wt %), 495 nm (10 wt %), 497 nm (20 wt %), and 500 nm (30 wt %), and the corresponding CIE coordinates are CIE (0.19, 0.41) (8 wt %), CIE (0.18, 0.42) (10 wt %), CIE (018, 0.46) (20 wt %), and CIE (0.19, 0.48), respectively. The emission color appears as sky blue for the devices with 8 and 10 wt % DMAC-TRZ. Although the device with 20 wt % DMACTRZ has the best efficiency and low driving voltage in our system, its emission color is more greenish (the higher y value is shown in Figure S6) than that of 10 wt % DMAC-TRZ. Therefore, we choose 10 wt % DMAC-TRZ as the guest to develop hybrid WOLED. At this composition, the device performance is Von = 4 V, Bmax = 57586 cd/m2, CEmax = 35.3 cd/A, PEmax = 21.4 lm/W, EQEmax = 14.1%, and CIE (0.18, 0.42). 2.3. Hybrid WOLEDs by Doping a Red Phosphor or a Fluorescent Emitter into TADF BOLED. We fabricate hybrid WOLEDs by doping the red phosphor Ir(dpm)PQ2 (emission λmax = 595 nm; Φp = 0.6; ET = 2.0 eV) or fluorescent emitter DCJTB33 (emission λmax = 590 nm; Φp = 0.64) into the blue
Figure 2. Device performances with different DMAC-TRZ concentrations. (a) Current density and brightness versus applied voltage and (b) current and power efficiencies versus brightness. The device structure is ITO/PEDOT:PSS (35 nm)/26DCzPPy:TCTA = 8:2 by weight/x wt % DMAC-TRZ (45 nm)/TmPyPB (50 nm)/CsF (1 nm)/Al (100 nm).
characteristic values are listed in Table 2. The driving voltage decreases with the concentration, as is also reflected in the brightnesses at 7 V for 8, 10, 20, and 30 wt % being 6898, 8796, 15390, and 26275 cd/m2, respectively. From the energy-level diagram (Scheme 1), we know that the HOMO of DMAC-TRZ (HOMO = 5.61 eV) is lower than that of 26DCzPPy (HOMO = 6.05 eV), so holes would be trapped in DMAC-TRZ. Besides, DMAC-TRZ30 possesses higher transport capability, for example, at 6.4 × 105 V/cm, its μe ∼ 1 × 10−4 and μh ∼ 6 × 10−5 cm2/V·s are higher than those of 26DCzPPy31 (μe = 2 × 10−5 and μh = 2 × 10−5 cm2/V·s), which results in a decrease of the driving voltage for each brightness level with an increase in the concentration of DMAC-TRZ (Figure 2a). The increase in the amount of DMAC-TRZ leads to first a rise in the efficiency and then a 4855
DOI: 10.1021/acsami.7b14695 ACS Appl. Mater. Interfaces 2018, 10, 4851−4859
Research Article
ACS Applied Materials & Interfaces emission layer of the TADF BOLED. The red light can emit from excitons formed possibly by charge trapping and/or energy transfer from the TADF. We want to determine which mechanism is responsible for the red emission in both cases. Adachi et al.34 first used the host and TADF material as the mixed host and traditional fluorescent emitters as dopants to fabricate high-performance OLEDs with various emission colors and proposed that it is possible for the IQE to reach 100% because of the highly efficient Förester energy transfer. For example, using 4,4′-bis(9-carbazolyl)-1,10-biphenyl and the TADF material 2,4,6-tris[4-(10H-phenoxazin-10H-yl)phenyl]1,3,5-triazine (tri-PXZ-TRZ) as the mixed host and the fluorescent red emitter tetraphenyldibenzoperiflanthene as the guest, they fabricated a red OLED with EQEmax reaching 17.5%, which was superior to that of the traditional fluorescent OLED. Therefore, many investigations on how to produce highly efficient all-fluorescent WOLED with TADF as the host have been attempted and reported.7,8 The performances in some cases can be as high as PHWOLED. In the present case, if the red emission is dominated by energy transfer, we may expect a highly efficient hybrid WOLEDs based on the red fluorescent emitter. If the red emission is dominated by charge trapping, the efficiency of hybrid WOLEDs must be low because of the restriction of the 25% up limit of IQE of the fluorescent emitter. However, the hybrid WOLEDs with the red phosphor can be expected to achieve high efficiency regardless of whether red emission was dominated by energy transfer or charge trapping, because the phosphor can harvest both singlet and triplet excitons. The hybrid WOLED has the same device configuration as TADF BOLED, but its emission layer is further doped with a red dopant. Their device performances are shown in Figure 3, and their characteristic parameters are listed in Table 3. The resulting devices with the red phosphor still give high efficiency and white emission. The white emission can be obtained at the concentrations of Ir(dpm)PQ2, 0.3 and 0.4 wt %. At 0.3 wt %, it achieves a better performance, being Von = 3.6 V, Bmax = 43594 cd/m2, CEmax = 28.8 cd/A, PEmax = 18.1 lm/W, CIE (0.38, 0.44), correlated color temperature (CCT) = 4353 K, and color rendering index (CRI) = 68 (a CRI value of WOLED over 70 can be taken as a highly colorful emission approaching sunlight). These CIE coordinates are located within the white emission region of the CIE chromaticity diagram and so is that for 0.4 wt %. Besides, the device has low efficiency roll-off; for example, the efficiency still reaches CE = 25.5 cd/A at 5000 cd/ m2 (13% lower than CEmax). However, the CEmax and Bmax values of the hybrid WOLEDs with the red fluorescent emitter dramatically drop with increasing DCJTB concentration; at 0.2 wt %, the device performance is Von = 4.2 V, Bmax = 18117 cd/ m2, CEmax = 12.2 cd/A, PEmax = 7.2 lm/W, CIE (0.37,0.45), CCT = 4624 K, and CRI = 64, which is far inferior to that based on the red phosphor. These studies reflect the fact that the red emission from the red dopant in the mixed host 26DCzPPy/TCTA (8:2) is mainly generated from excitons most likely by charge trapping rather than energy transfer. In comparison with the best performance of the vacuum-deposited hybrid WOLED,9 by doping with the blue TADF emitter with Bmax of only about 10000 cd/m2, the present hybrid WOLED with the red phosphor is significantly better and is the first report on solution-processed T−P hybrid WOLED. In order to determine whether charge trapping is the main mechanism for red emission, we give further evidence to support the consideration. Here, the red emission intensity of
Figure 3. Device performances of DMAC-TRZ with various red dopant weight ratios. (a) Current density and brightness versus applied voltage, (b) current and power efficiency versus luminance, and (c) EL spectra with CIE coordinates. The device structure is ITO/ PEDOT:PSS (35 nm)/26DCzPPy:TCTA = 8:2 by weight/10 wt % DMAC-TRZ/x wt % red dopant (45 nm)/TmPyPB (50 nm)/CsF (1 nm)/Al.
the EL spectra is much higher than that of the PL spectra (Figure S4), which indicates that there must be another emission mechanism in the EL process. That is charge trapping resulting from the direct exciton formation in Ir(dpm)PQ2, leading to generation of the red emission in the hybrid WOLEDs.35−37 Besides, the color shift with increasing driving voltage is also evidence for the fact that the charge trapping dominates in red emission. The EL spectra and CIE coordinates of the hybrid WOLEDs under various applied voltage are shown in Figure 4a,b. There is a strong variation in the intensity of the red emission relative to the blue emission for both cases of red phosphor and fluorescent emitters at a low driving voltage of
