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Exciton-Adjustable Interlayers for High Efficiency, Low Efficiency Roll-Off, and Lifetime Improved Warm White Organic Light-Emitting Diodes (WOLEDs) Based on a Delayed Fluorescence Assistant Host Zhiheng Wang, Xiang-Long Li, Zerui Ma, Xinyi Cai, Chengsong Cai, and Shi-Jian Su* quantum efficiency (EQE) of ≈20% could be achieved by utilizing the remaining 75% triplet excitons according to strong spin– orbit coupling.[3–7] However, these phosphors usually contain iridium or platinum noble metals which are nonrenewable and high cost. More importantly, blue phosphorescent emitters suffer from their inherent efficiency roll-off and short operational lifetimes when utilizing high energy triplet excitons which strongly destabilize the reliability of phosphorescent WOLEDs.[8–10] Recently, a new route for exploiting triplet excitons by thermally activated delayed fluorescence (TADF) emitters was proposed, realizing a comparable EQE exceeding 20% to those phosphorescent OLEDs. Through dominating the balance between intramolecular charge transfer and small singlet-triplet splitting energy (ΔEST), great improvements in efficiency were demonstrated for orange, green, and blue light emissions in the past few years, which promoted great attention to purely organic WOLEDs by adopting TADF molecules as emissive dopants or assistant hosts.[11–17] Our group reported a promising technique to realize high efficiency and high colorrendering index (CRI) WOLEDs by using a yellow TADF assistant host PXZDSO2 to sensitize deep red fluorescence emitter, and a maximum EQE of 15.6% was actualized in three-color WOLEDs. In addition to the practical request for superior performance in efficiency, investigation for developing long lifetime TADF devices is also essential to achieve operational stable purely organic WOLEDs. Several reports have demonstrated a comparable prospect in operation stability as conventional fluorescent emitters in green or yellow monochrome TADF devices.[18,19] However, blue TADF emitters are still facing a big obstacle on suppressing severe luminous degradation. The optimized LT50 lifetime at an initial luminance of 500 cd m−2 was only 770 h and the efficiency roll-off was also unsatisfied as well,[20] which strongly restrained the development of operational stable TADF WOLEDs, and a systematic understanding on their reliability is still absent so far. Hence, it is advisable to adopt conventional fluorescent blue emitters instead of the TADF one in order to obtain much longer lifetimes as well as outstanding efficiency

Recently, a new route to achieve 100% internal quantum efficiency white organic light-emitting diodes (WOLEDs) is proposed by utilizing noblemetal-free thermally activated delayed fluorescence (TADF) emitters due to the radiative contributions of triplet excitons by effective reverse intersystem crossing. However, a systematic understanding of their reliability and internal degradation mechanisms is still deficient. Here, it demonstrates high performance and operational stable purely organic fluorescent WOLEDs consisting of a TADF assistant host via a strategic exciton management by multi-interlayers. By introducing such interlayers, carrier recombination zone could be controlled to suppress the generally unavoidable quenching of longrange triplet excitons, successfully achieving remarkable external quantum efficiency of 15.1%, maximum power efficiency of 48.9 lm W−1, and extended LT50 lifetime (time to 50% of initial luminance of 1000 cd m−2) exceeding 2000 h. To this knowledge, this is the first pioneering work for realizing high efficiency, low efficiency roll-off, and operational stable WOLEDs based on a TADF assistant host. The current findings also indicate that broadening the carrier recombination region in both interlayers and yellow emitting layer as well as restraining exciplex quenching at carrier blocking interface make significant roles on reduced efficiency roll-off and enhanced operational lifetime.

1. Introduction The rapid progress of white organic light-emitting diodes (WOLEDs) has attracted close interest as a promising candidate for comprehensive application in future solid-state lighting sources and full-color display backlights.[1,2] Generally, traditional fluorescent emitters can harvest only 25% singlet excitons to result in inadequate efficiency for lighting application. By adopting noble metal phosphorescent complexes, external

Z. H. Wang, Dr. X.-L. Li, Z. R. Ma, X. Y. Cai, C. S. Cai, Prof. S.-J. Su State Key Laboratory of Luminescent Materials and Devices and Institute of Polymer Optoelectronic Materials and Devices South China University of Technology Guangzhou 510640, P. R. China E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201706922.

DOI: 10.1002/adfm.201706922

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roll-off control in blue emission. However, the inherent low-lying triplet states of conventional blue fluorescent emitters would preferentially quench triplet excitons when combining TADF emitters together in an emitting layer. As a result, exciton utilization should be dramatically decreased and high efficiency with favorable roll-off control can hardly be reached. To solve the difficulty of probable triplet losses, it is advisable to introduce an isolated interlayer between the blue fluorescent and complementary TADF emitters so that singlet and triplet excitons can be adopted in their separated channels. Besides, this interlayer can generally broaden carrier recombination region and restrain luminous degradation to achieve expected low efficiency roll-off and long operational lifetime WOLEDs. In this work, we report such a feasible concept of excitonadjustable interlayers bis(2-(2-hydroxyphenyl)-pyridine)beryllium (Bepp2):3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP)/Bepp2. By controlling the doping concentration of Bepp2 in the Bepp2:mCBP hybrid layer, main exciton formation region was properly controlled to modify the intensity of blue and yellow emissions as well as to avoid the long triplet diffusion energy losses for efficiency degradation. Further strategic exciton management within the yellow emission layers (EMLs) was successfully achieved, showing outstanding maximum EQEs of 15.1% and 14.7% for two-color and three-color WOLEDs, respectively. What is more, investigation of exciton distribution further indicated that the carrier recombination site was mainly located at the interface of the yellow emission layer (Y-EML) and interlayer and the proposed interlayers also give a broad recombination region amongst the Y-EML and interlayers. Thus, triplet excitons were more addicted to the TADF emitter instead of losing in the blue emission layer (B-EML), leading to an excellent controlling capability for their efficiency roll-off. When electrical stable surrounding materials were properly introduced, remarkable device stability was realized with LT50 lifetime of ≈2000 h at an initial luminance of 1000 cd m−2 for both two-color and three-color WOLEDs while maintaining a favorable EQE. As the interlayers provide a broad carrier recombination zone to suppress polaron induced annihilations, it is of interest to demonstrate that the exciplex formed between the electron blocking layer (EBL) and yellow EML coordinately affects the operational stability of WOLED devices. As far as we know, this is the first pioneering report for realizing application prospect of high efficiency, low efficiency roll-off, and operational stable WOLEDs based on a TADF assistant host.

2. Results and Discussion Since combining blue fluorescent and complementary TADF emitters together in one emitting layer will lead to severe triplet exciton quenching and dramatic efficiency roll-off, for achieving both efficient and operational stable purely organic WOLEDs, it is of great worth to propose a rational concept of interlayers between them in order that both singlet and triplet excitons can probably be exploited separately, as shown in Figure 1. Considering high triplet energy mCBP (T1 = 2.9 eV[18]) as an interlayer candidate for preventing triplets escaping from TADF emitter to blue fluorescent emitters, however, the only component of hole-dominated mCBP would possibly shift the carrier recombination zone close to the B-EML. Therefore, by dispersing electron-­ dominated Bepp2 with appropriate T1 level (T1 ≈ 2.6 eV[21]) to constitute a mCBP:Bepp2 hybrid layer, the carrier recombination zone can be controllably moved to the interface of the Y-EML and the mCBP:Bepp2 hybrid layer. Besides, another thin layer of Bepp2 is placed between the hybrid layer and the B-EML in order to further guarantee the carrier recombination zone away from the B-EML to insulate triplet leakage. As the generated singlet excitons have relatively short diffusion length, the B-EML is placed adjacent to the Bepp2 interlayer, allowing a Förster resonant energy transfer (FRET) process to the blue fluorophore. On the other hand, owing that the B-EML is separated from the main exciton formation zone by high triplet energy interlayers, the generated triplet excitons, which possess long diffusion length to ensure triplets migrating into the up-conversion TADF emitter.[22] Thus, generally unavoidable energy loss to the blue fluorophores associated with long triplet diffusion length could be eliminated, and singlet and triplet excitons can transfer energy to the emitting species in their respective channels to give efficient exciton utilization. In addition, the presented interlayers offer bipolar transport property to extend the carrier recombination region, thus polaron induced annihilation would possibly be restrained from luminous degradation under continuous electrical stress. Therefore, high exciton utilization and extended operation lifetime could be expected. On the B-EML side, blue fluorophores 4,4′-((1E,1′E)-1,4phenylenebis(ethene-2,1-diyl))bis(N,N-diphenylaniline) (DSA-Ph) and 2-methyl-9,10-di(naphthalen-2-yl)anthracene (MADN) are selected as blue fluorescent dopant and host, respectively, where MADN offers a low triplet energy and stable chemical structure to ensure much longer operational lifetime.[23,24] On the Y-EML side,

Figure 1.  Proposed energy level diagram and schematic operational mechanism of the EML in all fluorescence blue-yellow binary WOLEDs. Adv. Funct. Mater. 2018, 1706922

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Figure 2.  a) External quantum efficiency-luminance characteristics of the pure organic WOLEDs A1–A5 consisting of a Bepp2:mCBP hybrid interlayer with various Bepp2 doping concentrations of 0, 20, 30, 50, and 100 wt%. b) EL spectra of the devices A1–A5 at a current density of 4 mA cm−2 (inset: relationship between the main carrier recombination site and the corresponding Bepp2 doping concentration in the Bepp2:mCBP hybrid interlayer).

3,4,5,6-tetra(9H-carbazol-9-yl)phthalonitrile (4CzPN) with a high photoluminescence quantum yield (PLQY) of 0.71 is chosen as an assistant host to capture electrically generated triplet excitons via efficient RISC process and thus to transfer energy to the complementary fluorescent emitters. As thus, both singlet and triplet excitons generated in the devices can be effectively harvested. Following the expected operational mechanism diagram shown in Figure 1, exciton dynamics behavior of the interlayers has a crucial influence on the performance of WOLEDs. To investigate their exciton-adjustable capability, devices A1–A5 (Section 2, Supporting Information) were fabricated by tuning the doping concentration of Bepp2 to be 0, 20, 30, 50, and 100 wt% in the hybrid layer, respectively. As shown in Figure 2a, among these devices, device A3 with a Bepp2 doping concentration of 30 wt% achieved the best forward-viewing EQE of 12.7%, and it maintained as high as 11.1% at the brightness of 1000 cd m−2, indicating the most prominent exciton utilization. It can also be seen that superior warm white light emission with CIE coordinates of (0.33, 0.48) was obtained for device A3 at a current density of 4 mA cm−2. Detailed current density-luminance-voltage characteristics and repeatability of device performance are illustrated in Figure S1 (Supporting Information). As anticipated, when Bepp2 is absent in the interlayer, EQE rapidly declined since the carrier recombination zone is inclined close to the B-EML and triplet losses are unavoidable due to the low-lying triplet energy blue fluorophores. After dispersing Bepp2 in the hybrid interlayer, as electron mobility of Bepp2 (μe = 1.3 × 10−3 cm2 V−1 s−1[25]) is much higher than hole mobility of mCBP (μh = ≈10−7 cm2 V−1 s−1), the main carrier recombination site should gradually move towards the Y-EML side for more yellow emission with the increased Bepp2 concentration, and efficiency roll-off could be further restrained by keeping from long distance triplet losses. However, as the Bepp2 concentration increased to more than 30 wt%, carrier balance may be affected and efficiency will be slightly decreased again. EL spectra further prove the movement of the main recombination zone (Figure 2b), where the electroluminescent intensity of DSA-Ph gradually declined with the increased Bepp2 concentration. Furthermore, by introducing a 10 nm thin HATCN as a hole injecting layer and even thinner EML (Table S2, Supporting Information), driving voltage of device B2 dropped down from 5.2 to 4.8 V at 1000 cd m−2 with maintaining an efficiency of 11.2%. For achieving more abundant exciton utilization by TADF dyes, strategic exciton management within double emissive

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zone was demonstrated.[26,27] With double EMLs for yellow emission, more balanced distribution of electrons and holes should be realized in impressible electric field.[28] As consequences, TADF assisted fluorescent Y-EMLs consisting of a sparse-doped layer of 0.4 wt% 2,8-di-tert-butyl-5,11-bis(4-(tertbutyl) phenyl)-6,12-diphenyltetracene (TBRb): 6 wt% 4CzPN: mCBP and a dense-doped layer of 0.8 wt% TBRb: 10 wt% 4CzPN: mCBP were investigated (Figure 3a). After optimization for better efficiency roll-off (Figure S2, Supporting Information), device W1 with a multilayer structure of ITO (95 nm)/ HATCN (10 nm)/4,4′-(cyclohexane-1,1-diyl) bis(N,N-di-p-tolylaniline) (TAPC) (45 nm)/tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA) (5 nm)/0.4 wt% TBRb: 6 wt% 4CzPN (8 nm)/0.8 wt% TBRb: 10 wt% 4CzPN: mCBP (4 nm)/40 wt% Bepp2: mCBP (5 nm)/Bepp2 (3 nm)/5 wt% DSA-Ph: MADN (8 nm)/3,3′(5′-(3-(pyridin-3-yl)phenyl)-[1,1′:3′,1″-terphenyl]-3,3″-diyl)dipyridine (TmPyPB) (50 nm)/LiF (1 nm)/Al (100 nm) achieved a remarkable performance, as well as low driving voltages of 3.0 and 4.6 V at 1 and 1000 cd m−2. As shown in Figure 3b–e and Table 1, its maximum efficiencies reached to 15.1% for EQE, 48.9 cd A−1 for current efficiency (CE) and 47.4 lm W−1 for power efficiency (PE) without any light out-coupling technique. Thanks to the superior roll-off control by the interlayers, the remaining EQEs were still as high as 12.1% and 10.0% at high brightness of 1000 and 5000 cd m−2. Owing that illumination sources are typically characterized by their total emitted radiation, total maximum EQE, and PE of 25.7% and 80.6 lm W−1 could be achieved. As CRI of the two-color white devices is still lower than requirements for lighting application, a deep-red fluorescent emitter 5,10,15,20-tetraphenylbenzo[5,6]indeno [1,2,3cd] benzo[5,6]indeno[1,2,3-lm]perylene (DBP) was conceived to build up three-color WOLEDs to broaden the coverage of visible EL spectra. For realizing device W2 (Section 2, Supporting Information), a 0.4 wt% DBP: 6 wt% 4CzPN: mCBP layer was settled down to replace 0.4 wt% TBRb: 6 wt% 4CzPN: mCBP and the narrower energy gap DBP can capture singlet excitons from 4CzPN or TBRb via FRET instead of direct exciton generation because the main recombination zone is located beyond the DBP-containing red emissive layer. Owing to this ingenious strategy, with remaining low turn-on voltage of 3.0 V, it is of interest that a remarkable EQE of 14.7% (corresponding to PE of 38.4 lm W−1) was achieved and still held

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Figure 3.  a) Outlined EML configurations of the advanced two-color and three-color pure organic WOLEDs. b) External quantum efficiency (EQE) and power efficiency (PE) versus luminance characteristics, c) EQE repeatability (the horizontal bar represents median EQE), and d) EL spectra at various brightness for devices W1–W3.

to be 10.8% and 8.8% at high brightness of 1000 and 5000 cd m−2 (Figure 3 and Table 1). According to the previous reports, the developed two-color and three-color pure organic fluorescent WOLEDs exhibited an outstanding performance in both efficiency and driving voltage.[27,29–33] The suppression of efficiency roll-off is probably attributed to the avoidance of direct exciton generation on DBP that may cause aggravating exciton quenching. Dispersing assistant dopant 4CzPN in the host matrix mCBP and controlling an extremely low doping concentration of DBP typically inhibit direct triplet energy transfer from the host to the narrower energy gap dye DBP, which belongs to Dexter energy transfer losses.[11] Besides, device W2 indicated an eye-protecting warm white light emission with

CIE coordinates of (0.37, 0.46) and correlated color temperature (CCT) of 4647 K, followed with a substantially enhanced CRI of 68 even though there is no harmful deep blue emission. To strengthen the red emission and thus more eye-protecting warm WOLEDs while still keeping high performance,[34] 0.4 wt% DBP red dye was dispersed in the blue emissive layer to partially utilize the singlet energy from the blue emitter DSA-Ph (device W3, Section 2, Supporting Information). Originating from the promotion of red emission and the slightly reduction of blue emission intensity, CRI of device W3 was further improved to as high as 78 without any aids of deep blue emission, according to the EL spectra shown in Figure 3d. CIE coordinates and CCT continually moved to a warmer region of (0.42, 0.46) and 3702 K at practical brightness of 1000 cd m−2. For lighting applicaTable 1. Summary of the electroluminescent performances of the fabricated two-color and tions, the most important reference light three-color WOLEDs W1–W3. source is the warm white standard illumination A with CIE coordinates of (0.445, Device CRI VONa) [V] CIE (x, y) V/CE/PE/EQE [V/cd A−1/lm W−1/%] 0.405), presenting higher intensity in the Maximum at 5000 cd m−2 at 1000 cd m−2 at 1000 cd m−2 orange region.[35] Device W3 presents such W1 3.0 48.9/47.4/15.1 4.6/37.0/25.1/12.1 5.6/29.1/16.2/10.0 (0.35, 0.49) 49 a physiologically friendly light at night due to its relatively low blue emission intensity W2 3.0 37.1/38.4/14.7 4.6/28.0/19.1/10.8 5.6/22.6/12.7/8.8 (0.37, 0.46) 68 and enhanced orange-red emission.[36,37] W3 3.2 30.5/28.2/12.9 4.6/25.4/17.2/10.2 5.6/20.2/11.3/8.1 (0.42, 0.46) 78 What is more impressive, according to a)V −2 Figure 3c, devices W1–W3 also exhibited ON is obtained at 1 cd m .

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Figure 4.  a) Spatial distribution of ultrathin phosphorescent sensors of Ir(piq)3 and YCF-3 in the Y-EML, interlayers, or B-EML. b) EL spectra of the devices with a phosphorescent sensing layer located at different positions at a current density of 10 mA cm−2 (reference: without ultrathin sensing layer). c) Normalized relative exciton density versus distinct positions at a current density of 4, 10, and 40 mA cm−2.

advantageous efficiency repeatability which benefits for putting into industrialization. As mentioned, it is of great importance that suitable carrier recombination region is a critical factor to realize efficient exciton utilization and thus outstanding performance. Hence, it is essential to understand the insight exciton distribution of the whole emitting layers so that inherent working mechanisms of interlayers will be distinct. To determine the carrier recombination zones and exciton distribution, a series of 0.1 nm ultrathin phosphorescent sensors were correspondingly inserted into the whole emitting layers from the Y-EML to the B-EML (Figure 4a). Considering that the extremely low-lying triplet state of MADN (T1 ≈ 1.7 eV) probably annihilates the triplet harvesting sensor, a red emission phosphor tris(1-phenyl-isoquinoline) iridium(III) (Ir(piq)3) was placed from the Y-EML to the interlayers and a near infrared phosphor YCF-3 (ELpeak = 726 nm, Figure S3b, Supporting Information) was set up in the B-EML. The relative emission intensity should accordingly point out the spatial distribution of the radiative excitons, followed with no obvious effect on the current density versus voltage characteristics (Figure S3, Supporting Information). Following the sketch of Figure 4a, the EL intensity results demonstrate that there was intense red emission observed when placing the sensing layer near the position of 0 nm, indicating the majority of excitons are generated at the region adjacent to the interface of Y-EML/hybrid interlayer because the dispersed Bepp2 highly elevates the electron transport property of the Bepp2:mCBP hybrid layer. Such an arrangement of recombination location will lead to exciton diffusion from this point of origin, rather than direct charge trapping on the blue fluorophores.[1] Normalized relative exciton density versus position was received according to the relative luminous intensity of the red emission from Ir(piq)3, as shown in Figure 4c. On the Y-EML side, exciton concentration slowly declined away from the main generation region since the phthalonitrile acceptors in 4CzPN give more bipolar characteristic and broaden the recombination region so that triplets or singlets induced

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efficiency degradation will be remarkably suppressed eventually. However, the exciton distribution in the Y-EML shifted slightly with the change of current density because carrier trapping is unavoidable to induce direct exciton formation on the assistant dopants[38,39] considering much deeper lowest unoccupied molecular orbital (LUMO) level of 4CzPN (−3.38 eV)[40] than that of mCBP (−2.8 eV).[41] On the interlayer side, the exciton density dropped rapidly especially at the Bepp2 layer (Figure 4c), and there was almost no exciton could reach the edge of the Bepp2/B-EML interface. In this situation, long-range triplet excitons are inclined to be diffused into the TADF emitters since the recombination site is close to the Y-EML interface as the result of exciton adjustment by the hybrid interlayer. On the other hand, the interlayers of both mCBP:Bepp2 and Bepp2 own higher triplet energy than the assistant dopant 4CzPN so that triplets are prevented from flowing into the B-EML and exchange triplet losses are almost negligible. Therefore, blue emission from DSA-Ph should be originated from the singlet excitons generated in the interlayers via efficient Förster energy transfer. Meanwhile, with various electric field intensity, exciton distribution still keeps almost stable to ensure constant energy source for blue emission and makes advantage of controlling incredible color shift. According to the exciton distribution features that there are a large number of excitons locate near the interface of the Y-EML/hybrid interlayer, therefore, both the avoidance of exciton quenching at this interface and efficient radiative exciton utilization are significant avenues for actualizing efficient and reliable WOLEDs. Further relationships between the exciton distribution and device performance were investigated by fabricating monochromatic devices with accordant structures. Generally, an ideal interlayer between the adjacent emissive layers should provide prominent exciton utilization for both singlets and triplets and restrain luminous efficiency decay at high brightness. Compared with the monochromatic yellow and sky-blue devices of E1 and E2 (Section 2, Supporting Information), it is of interest that a better EQE of 15.1% was achieved for device

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Figure 5. a) EQE versus luminance characteristics of device W1, W1 (calculated), and their corresponding monochromic devices E1 and E2. Time-resolved PL decays of the codeposited films, b) 0.8 wt% TBRb: 10 wt% 4CzPN: mCBP and c) 0.4 wt% DBP: 6 wt% 4CzPN: mCBP.

W1 than that of the yellow device (14.8%), indicating generally unavoidable triplet energy losses in blue fluorophores associated with long triplet diffusion length are negligible and FRET energy transfer for singlet excitons are effective[34] with the help of the exciton-adjustable interlayers (Figure 5a). Further confirmation of exploitable patterns in WOLED devices was shown in Figure 5a in dashed line. The calculated efficiency of W1 (calculated according to the efficiencies and EL intensities of the monochromatic devices E1 and E2) exhibited much lower quantum efficiency than device W1, indicating that triplets are harvested by TADF emitters and blue emission is associated with FRET energy transfer for singlets instead of triplet– triplet annihilation (TTA) emission via the MADN host. On the other hand, higher quantum efficiency was also achieved by broadening their recombination zone in the entire range of the Y-EML and interlayers. In consideration of the luminous efficiency decay at high brightness, EQE curve of device W1

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gradually deviated from their monochrome devices. Further comparison of devices E3 and E4 was shown in Figure S4 (Supporting Information). As previously reported, broadening recombination zone was regarded as a valid method to reduce efficiency roll-off by reducing the probability of exciton collision as well as TTA degradation. More importantly, although most excitons are generated at the interface of the Y-EML/ interlayer, thanks to the bipolar mCBP:Bepp2 hybrid interlayer, triplet excitons can be widely dispersed in the whole Y-EML and interlayers and further suppress imbalance-induced extra polarons where triplet-polaron annihilation (TPA) degradations are taking place.[42] Furthermore, a relative larger EQE reduction of yellow component probably aggravates unstable EL spectra and color-shift in WOLEDs. More desirable stability of CIE coordinates should be realized by choosing more superior TADF emitters. As mentioned, the exciton-adjustable interlayers achieved not only high exciton utilization of both singlets and triplets, but also outstanding restriction of efficiency reduction via expanding the recombination region. However, motivations of utilizing both singlet and triplet excitons by introducing an assistant TADF dopant in pure organic WOLEDs were still not fully investigated. Since small ΔEST and large radiative rate constant (kr) are difficult to be simultaneously reached for red TADF molecular design, distributing the acquired function to an assistant TADF energy donor as well as a conventional fluorescence energy acceptor could be a rational strategy (Figure 1).[43] Therefore, efficient Förster energy transfer from the S1 of the exciton donor to the S1 of the exciton acceptor with large kr is required to be completely taken place in the system. To demonstrate the donor–acceptor energy transfer above, PLQYs and time-resolved PL behaviors of codeposited films of 0.8 wt% TBRb: 10 wt% 4CzPN: mCBP and 0.4 wt% DBP: 6 wt% 4CzPN: mCBP were measured. When conventional fluorescent dyes were introduced, compared with the doped films of 10 wt% 4CzPN: mCBP and 6 wt% 4CzPN: mCBP, PLQYs of the codoped films consisting of TBRb or DBP fluorophores slightly increased to 71.9% and 77.2% (Table S3, Supporting Information), illustrating a considerably large kr in fluorescent guests and nonradiative decay rate (knr) of 4CzPN should be negligible. By introducing a sparse-doped 0.4 wt% TBRb: 6 wt% 4CzPN: mCBP codeposited film, the quantum yields further increased to 76.8%, in consistent with the better efficiency for the binary yellow EMLs WOLEDs. Meanwhile, the 4CzPN doped film exhibited both prompt and obvious delayed components in time-resolved PL decay, which is the obvious characteristics of a TADF molecule. Owing to the low doping concentration of fluorescent guests, Dexter energy transfer process is naturally prohibited by prolonging the distance between the host and the guest molecules. After introducing yellow or red fluorescent dyes TBRb and DBP, as shown in Figure 5b,c, the delayed part of 4CzPN was consumed and transferred to the delayed components of TBRb and DBP observed at 620 and 660 nm, respectively, indicating remarkable sections of the yellow and red emissions are originated from the Förster energy transfer via up-converted triplet excitons of 4CzPN to the S1 state of the fluorescent dopants. As thus, high performance TADF assisted fluorescence could be demonstrated in those purely organic WOLEDs under electric stress as mentioned above.

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In Equation (1), SED model involves two simulated parameters, τ for decay constant and β for stretching factor, combining with CIE (x, y) time-dependent luminance L(t) and initial (at 1000 cd m−2) luminance L0 = 1000 cd m−2. It is of interest a simulated LT50 of 1395 h was achieved for WOLED device F1 (Figure 6a), indicating (0.38, 0.50) that the exciton-adjustable interlayers show (0.38, 0.50) not only high performance in efficiency but (0.38, 0.49) also operational stability in device lifetime, which is associated with the broadened recombination region and efficient triplet exploitation under continuous electrical excitations. Efficient triplet exploitation via suppressing long range of triplet losses by blue fluorophores is regarded as the significant element to control luminous degradation in WOLEDs. In addition, replacing DSA-Ph with slightly deeper blue fluorescent material of DPAVBi, resemble LT50 of 1270 h was also achieved with CIE coordinates of (0.38, 0.49) for device F3 (Section 2, Supporting Information), further indicating the universality and durability of the exciton-adjustable interlayers. For achieving longer operational lifetime, the exciton quenching at the edge of the EML and carrier/exciton blocking layers should also be carefully considered. As mentioned in Figure 4c, the exciton-adjustable interlayers provided a broad recombination region to depress exciton concentration, and a small fraction of carriers was still recombined at the edge of the EBL interface. Considering 4CzPN has a deep LUMO level of −3.3 eV,[40] although the TCTA EBL has a rather deep highest occupied molecular orbital (HOMO) level of −5.7 eV,[45] the triphenylamine core in TCTA is still readily to form exciplex degradation with the phthalonitrile acceptors in 4CzPN. According to the PL spectra of the TCTA:4CzPN (1:1) co-doped film (Figure S6a, Supporting Information), a broaden red-shifted exciplex emission was found with the yellowish green emission from 4CzPN. Significant distinction in time-resolved PL behavior was fully investigated with a sharply quenched PL quantum efficiency of only 5.0%, as

Table 2.  Summary of the EL performance and stability characteristics of the fabricated twocolor WOLEDs of F1–F3. Device

V/CE/PE/EQE [V/cd A−1/lm W−1/%]

VONa) [V]

Maximum

at 1000 cd m−2

Operational lifetime (at 1000 cd m−2) LT70

LT50b)

F1

3.1

40.0/40.1/12.5

4.9/30.0/19.2/9.8

393

1395

F2

3.4

40.5/37.1/12.5

5.7/28.5/15.7/9.2

580

2025

F3

3.1

38.1/37.6/11.9

5.1/29.2/18.1/9.5

358

1270

VON is obtained at 1 cd m−2; b)LT50 lifetime is simulated by SED model.

a)

The introduction of an efficient TADF assistant dopant and interlayers between the yellow and blue emissive layers provided impressive device performances and reduced efficiency roll-off via rational management of the generated singlets and triplets. To study the potential impact on the operational stability of the WOLEDs including such hybrid interlayers, carrier transport layers were carefully modified to be structural stable NPB and Bepp2 as hole and electron transport layers, respectively. As shown in device F1 (Section 2, Supporting Information), two-color WOLED with a device structure of ITO (95 nm)/HATCN (10 nm)/NPB (40 nm)/TCTA (10 nm)/0.8 wt% TBRb: 10 wt% 4CzPN: mCBP (12 nm)/30 wt% Bepp2: mCBP (5 nm)/Bepp2 (3 nm)/5 wt% DSA-Ph: MADN (8 nm)/Bepp2 (50 nm)/LiF (1 nm)/Al (100 nm) still presented satisfied performance with a turn on voltage of 3.1 V, a CE of 40.0 cd A−1 and an EQE of 12.5%, indicating minor luminous efficiency loss by changing the carrier transport layers (Figure S5, Supporting Information). The operation lifetime of the devices was measured at a practical initial brightness of 1000 cd m−2 under a constant current stress (Table 2). To predict LT50 of the resulting OLEDs, a function of stretched exponential decay (SED) based on loss mechanisms of accumulated defects was widely accepted with the following equation[44]  t β L(t ) = exp  −    L0  τ  

(1)

Figure 6.  a) Normalized luminance versus operating time decays (up) and delta driving voltages (down) of the delayed fluorescent emitter-based WOLEDs at an initial luminance of 1000 cd m−2. b) EL spectra of WOLEDs with TCTA (up) and mCBP (down) electron blocking layers before and after device degradation.

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shown in Figure S6b and Table S4 (Supporting Information). So, the exciplex formed between 4CzPN and TCTA would gradually quench the excitons in the Y-EML and cause luminous degradations under continuous electrical stress, which should considerably shorten the device reliability. Besides, TCTA owns weak chemical bonds of C (sp2)-N (sp3) in the triphenylamine unit,[42] which may accelerate luminous degradation under electrical excitation. To further improve the stability of WOLEDs, a wider bandgap hole-dominated EBL of mCBP with more stable chemical bonds was utilized to fabricate device F2 (Section 2, Supporting Information). As expected, an extended LT50 of 2025 h was achieved with an improvement of 45%, as well as a long LT70 lifetime of 580 h. To our knowledge, this is the first pioneering work for achieving such high efficiency, low efficiency roll-off, and operational stable TADF material containing WOLEDs comparable to those WOLEDs containing iridium complexes.[18,46] Considering the poor hole transport property and rather deep HOMO level of the mCBP electron/exciton blocking layer, the driving voltage of device F2 was inescapably increased and this problem may be resolved by introducing more efficient EBL materials. Comparing the EL spectra before and after device degradation shown in Figure 6b, a slight decline in yellow emission intensity was indicated, which might be associated with the unstable EL of the TADF assisted fluorescence. When taking mCBP as an EBL layer, this variability was nearly negligible due to the suppression of exciplex annihilation. What is more, the exciton quenching between the hole blocking layer (HBL) and EML was also investigated by dispersing lowlying triplet energy 8-hydroxyquinolinolato-lithium (Liq)[47] in the electron transport layer Bepp2, as presented in Figure S5 (Supporting Information). In contrast, EQE of device F4 (Section 2, Supporting Information) decreased very slightly as Liq should play a role on annihilating triplets near the edge of the B-EML/HBL interface since the exciton-adjustable interlayers strongly control the exciton distribution away from the B-EML in order to prevent long-range triplet annihilations. As a result, operational lifetime of device F4 was such similar as device F1, as shown in Figure S5b (Supporting Information). Finally, operational stability of three-color WOLEDs (device F5, Section 2, Supporting Information) was also investigated, as shown in Figure S7 (Supporting Information). Its LT50 lifetime was estimated to be as long as 1925 h, further indicating that the exciton-adjustable interlayers work reliably to be potential universal interlayers for achieving efficient and long lifetime WOLEDs. Furthermore, the device lifetime can be further elongated if more stable light-emitting materials are adopted in the proposed device structure of this work.

3. Conclusion High performance pure organic WOLEDs were realized by introducing exciton-adjustable hybrid interlayers with triplet harvesting TADF emitter 4CzPN as the assistant host and blue, yellow, deep-red conventional fluorescent dopants DSAPh, TBRb, DBP as the emitters. Maximum EQEs of 15.1% and 14.7% were, respectively, achieved for two-color and three-color WOLEDs with reduced efficiency roll-off. Warm white emission

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with CIE coordinates of (0.37, 0.46) and a high CRI was achieved for the three-color WOLEDs without any deep blue light emission. It is of interest that the excitons were mainly generated at the edge of the Y-EML/mCBP:Bepp2 interface and long-range triplet energy losses to blue fluorophores were fully suppressed and the blue emission was associated with Förster energy transfer from the adjacent interlayers. The achieved high performance could be attributed to the efficient singlet and triplet exciton utilization in their separated channels and the suppressed of TTA or TPA degradation processes via the broadened carrier recombination zone. In addition, the fabricated WOLEDs also exhibited pioneering operational stability with a LT50 of 2025 h at an initial luminance of 1000 cd m−2, indicating the high reliability of interlayers and the suppressed exciplex quenching between the EBL and TADF assistant 4CzPN. As looking forward, the operational stability can be further improved by adopting functional materials with more stable chemical bonds and p-type-intrinsic-n-type (PIN) structure should be properly introduced in order to eliminate the carrier accumulations and reduce driving voltage.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors greatly appreciate the financial support from the National Key R&D Program of China (No. 2016YFB0401004), the National Natural Science Foundation of China (Nos. 51625301, U1601651, 51573059, and 91233116), 973 Project (No. 2015CB655003), and Guangdong Provincial Department of Science and Technology (Nos. 2016B090906003 and 2016TX03C175).

Conflict of Interest The authors declare no conflict of interest.

Keywords exciton-adjustable interlayers, operational lifetime, thermally activated delayed fluorescence, white organic light-emitting diodes Received: November 29, 2017 Revised: December 17, 2017 Published online:

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