Unveiling the Role of Langevin and Trap-Assisted Recombination in

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 1096−1108

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Unveiling the Role of Langevin and Trap-Assisted Recombination in Long Lifespan OLEDs Employing Thermally Activated Delayed Fluorophores Minghan Cai,† Dongdong Zhang,† Jingyi Xu,† Xiangchen Hong,† Chongguang Zhao,† Xiaozeng Song,† Yong Qiu,† Hironori Kaji,§ and Lian Duan*,†,‡

ACS Appl. Mater. Interfaces 2019.11:1096-1108. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/24/19. For personal use only.

†

Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry and ‡Center for Flexible Electronics Technology, Tsinghua University, Beijing 100084, China § Institute for Chemical Research, Kyoto University, Uji, Kyoto 6611-0011, Japan S Supporting Information *

ABSTRACT: Recent research studies on noble-metal-free thermally activated delayed fluorescent (TADF) materials have boosted the efficiencies of organic light-emitting diodes (OLEDs) to unity. However, the short lifespan still hinders their further practical application. Carrier recombination pathways have been reported to have a significant influence on the efficiencies of TADF devices, though their effects on device lifetimes remain rarely studied. Here, we have designed and synthesized five pyrimidine or pyrazine/carbazole isomers as hosts for TADF OLEDs to explore the inherent role of Langevin recombination (LR) and trap-assisted recombination (TAR) in device lifetimes. It is revealed that for LR dominant devices, lifetimes would increase by reducing the host triplet energy levels, whereas for TAR dominant devices, lifetimes are insensitive to the host triplet excitons as recombination mainly takes place on dopants. Still, LR dominant devices are favored as they offer more room for optimization. We further apply this concept in designing a stable LR dominant blue TADF device, achieving a long LT50 (lifespan up to 50% of the initial luminance) of 269 h and high external quantum efficiency of 17.9% at 1000 cd m−2 simultaneously. KEYWORDS: organic light-emitting diode, thermally activated delayed fluorescence, lifetime, Langevin recombination, trap-assisted recombination

1. INTRODUCTION High performance thermally activated delayed fluorescent (TADF) materials have attracted much attention in both academic and industry fields, since the first report by Adachi et al.1−8 Albeit that extremely high efficiencies have been achieved in organic light-emitting diodes (OLEDs) using TADF emitters, they still suffer from relatively short device lifetimes.9−13 Optimizing carrier recombination pathways and energy transfer pathways can be helpful to solve this problem. In OLEDs, there are two recombination pathways.14,15 One is Langevin recombination (LR) where electrons and holes recombine on hosts to form excitons and subsequently transfer their energy to the emitters. The other one is trap-assisted recombination (TAR) where electrons and holes will directly recombine on emitters and emit light. Recently, Ihn and Adachi et al. reported that well-designed TAR dominant TADF devices could exhibit record long lifetimes due to balanced charge fluxes, broad recombination zones, and reduced formation of high-energy excitons on hosts.16,17 In these devices, n-type hosts with deep lowest unoccupied molecular orbital (LUMO) energy levels were © 2018 American Chemical Society

selected to match p-type TADF emitters. Eventually, holes would be directly injected and transport on TADF emitters, and electrons would transport on hosts. Opposite charges finally recombined in emitters and emitted light. Accordingly, designing n-type hosts with deep LUMO energy levels is the key to realize TAR dominant TADF devices. To attain this, Ihn et al. designed 3′,5-di(9H-carbazol-9-yl)-[1,1′-biphenyl]-3carbonitrile (mCBP-CN) by inducing the cyano-group with a strong electron-withdrawing ability.16 At the initial luminance of 500 cd m−2, a high LT80 (lifespan up to 80% of the initial luminance) of 21 h with an external quantum efficiency (EQE) of 8.7% was achieved by using mCBP-CN as the host for a blue TADF dopant. In 2016, Liao’s and Adachi’s groups fabricated a sky-blue TADF OLED by employing bis(9,9′-spirobifluorene2-yl)ketone (SF3K) with the ketone group as the n-type host and achieved an EQE of 9.6% with LT50 (lifespan up to 50% of the initial luminance) of 130 h at an initial brightness of 500 cd m−2.18 Thiazine is also a good building block due to its high Received: September 26, 2018 Accepted: December 10, 2018 Published: December 10, 2018 1096

DOI: 10.1021/acsami.8b16784 ACS Appl. Mater. Interfaces 2019, 11, 1096−1108

Research Article

ACS Applied Materials & Interfaces

Figure 1. Chemical structures of five hosts with conjugation sites of carbazole labeled.

hosts for TADF OLEDs. Here, pyrimidine and pyrazine groups were selected as acceptor units. Nitrogen heterocyclic rings have been widely used in fabricating electron transport materials (ETMs) and hosts owing to their superior electron injection and electron transport properties.28−34 Among them, due to moderate electron-withdrawing abilities and triplet energy levels, pyrimidine and pyrazine groups may perform better than pyridine and triazine units.35−39 Correspondingly, carbazole was selected as a hole transport unit.40,41 We designed and synthesized four new hosts namely 4,6-bis(9phenyl-9H-carbazol-3-yl)pyrimidine (3Cz46Pm), 4,6-bis(9phenyl-9H-carbazol-2-yl)pyrimidine (2Cz46Pm), 3,3′-(pyrimidine-2,5-diyl)bis(9-phenyl-9H-carbazole) (3Cz25Pm), and 2,6-bis(9-phenyl-9H-carbazol-2-yl)pyrazine (2Cz26Pz). In addition, a previously reported isomer, 4,6-bis(3-(9H-carbazol-9yl)phenyl)pyrimidine (9Cz46Pm), was also incorporated for the study.42−44 Different nitrogen atom positions in acceptors and different conjugation positions of carbazole units provide them with different triplet energy levels, frontier molecular orbitals (FMOs), and charge mobilities, rendering TADF OLEDs using them as hosts with different charge recombination pathways. It was revealed that decreasing the host triplet energy levels in LR dominant devices could effectively relieve these decay processes induced by host triplet excitons. By optimizing the host as well as the transporting layer, we have successfully fabricated blue TADF OLED with a long lifetime of 269 h and a high efficiency of 17.9% at the practical luminance of 1000 cd m−2, representing one of the best overall performances of blue TADF OLEDs and offering a viable way for future research studies.

stability, deep LUMO energy level, and high electron mobility. On the basis of this, Liao’s and Adachi’s group modified SF3K and synthesized three new hosts.17 Among them, a device using 2-(9,9′-spirobi[fluoren]-3-yl)-4,6-diphenyl-1,3,5-triazine (SF3-TRZ) as the host and 1,2,3,5-tetrakis(carbazol-9-yl)-4,6dicyanobenzene (4CzIPN) as the green TADF emitter achieved an extremely long LT50 of 10934 h at an initial brightness of 1000 cd m−2. Furthermore, sky-blue TADF OLEDs utilizing SF3-TRZ as the host for 9-(4-(4,6-diphenyl1,3,5-triazin-2-yl)phenyl)-9′-phenyl-9H,9′H−3,3′-bicarbazole (BCz-Trz) also achieved a long LT50 of 454 h at an initial brightness of 1000 cd m−2. Besides the effect of TAR, the ntype hosts used may also be beneficial to the improved lifetimes as the formation of unstable double-hole bipolarons can be prevented therein. Albeit that considerable lifetimes have been achieved in TAR dominant devices, they still suffered from low efficiencies, especially in blue ones.16,17 This is understandable as efficient and stable blue TADF emitters are rare and the performances of TAR dominant devices strongly depend on TADF emitters.16−24 Besides, charge trapping on emitters will also increase the driving voltages of TAR dominant devices and result in high power consumption.15 On the other hand, LR dominant devices reported to have higher efficiencies than TAR dominant ones as in LR dominant devices both hosts and dopants can be optimized separately.14,25 Moreover, LR dominant devices possess an inherently larger recombination zone and lower efficiency roll-off.26,27 Additionally, low driving voltages of LR dominant devices will lead to low power consumption. However, reports on stable LR dominant TADF OLEDs are very rare until now, and previous studies ascribed their short lifespan to the formation of high-energy excitons on hosts.17 To gain deeper insight into the role of LR and TAR in OLEDs employing TADF emitters and to fabricate long lifespan devices, we studied five bipolar transporting isomers as

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. Detailed chemical structures and synthesis procedures of five hosts are presented in Figure 1 and Scheme S1. Five hosts were synthesized by 1097


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Figure 2. (a) Optimized molecular structures, (b) HOMO distributions, (c) LUMO distributions, and (d) SDDs of newly designed four hosts.

Table 1. Physical Properties of Newly Designed Four Hostsa Td/Tg (°C)

HOMO (eV)

LUMO (eV)

3Cz46Pm

403/128

2Cz46Pm

372/127

3Cz25Pm

420/127

2Cz26Pz

397/119

−5.64 −5.37c −5.73b −5.43c −5.69b −5.14c −5.66b −5.43c

−2.33 −1.33c −2.53d −1.71c −2.44d −1.21c −2.46d −1.44c

compound

b

Ege,f (eV)

d

3.31 3.20 3.25 3.20

S1e (eV) 3.29 3.41g 3.12 3.26g 3.20 3.47g 3.17 3.37g

T1h (eV) 2.79 2.90g 2.64 2.73g 2.64 2.72g 2.54 2.70g

μhi (cm2 V−1 s−1)

μei (cm2 V−1 s−1)

−4

7.76 × 10−4

9.02 × 10−5

5.63 × 10−4

5.20 × 10−5

5.28 × 10−5

1.70 × 10−4

6.98 × 10−4

1.88 × 10

Abbreviations: Eg: energy gap, S1: singlet energy level, T1: triplet energy level, μh: hole mobility, μe: electron mobility. bCalculated according to the equation HOMO = 4.8 + onset voltage. cDFT calculation result. dEstimated according to the absorption bandgap and the HOMO energy level. e Measured in toluene solution (10−5 mol L−1). fEstimated according to the absorption bandgap. gTime-dependent DFT (TD-DFT) calculation result. hEstimated according to the first peak of the phosphorescence spectrum in toluene solution (10−5 mol L−1). iAt 6 × 107 V m−1. a

Different FMOs of hosts will result in different carrier recombination pathways and energy transfer pathways, which can help us to investigate their influence on device lifetimes. Furthermore, triplet energy levels of four hosts were calculated by time-dependent density functional theory (TDDFT) at the B3LYP/6-31G(d) level. They are 2.90, 2.73, 2.72, and 2.70 eV for 3Cz46Pm, 2Cz46Pm, 3Cz25Pm, and 2Cz26Pz, respectively. To gain a deeper insight, we also calculated their triplet spin density distributions (SDDs) (Figure 2d). For 2Cz46Pm, 3Cz25Pm, and 2Cz26Pz, their wide liner triplet distributions result in similar triplet energy levels, whereas for 3Cz46Pm, narrow V-shape distribution significantly interrupts conjugate and improves its triplet energy level. Different triplet energy levels can further help us to investigate their influence on device lifetimes. 2.3. Thermal Properties. Thermal stabilities of four hosts were investigated by thermogravimetric analysis and differential scanning calorimetry measurements (Figure S2). 3Cz46Pm, 2Cz46Pm, 3Cz25Pm, and 2Cz26Pz all possess high decomposition temperatures (Td, corresponding to 5% weight loss in thermogravimetric analysis) of 403, 372, 420, and 397 °C and high glass transition temperatures (Tg) of 128, 127, 127, and 119 °C, respectively. These results are similar to the corresponding values of 9Cz46Pm. 2.4. Electrochemical Properties. Cyclic voltammetry measurements of newly designed four hosts were performed to evaluate their HOMO energy levels. They all present quasireversible oxidation peaks. The HOMO energy levels of four

regular Suzuki coupling reactions. Column chromatography and vacuum sublimation were sequentially carried out for purification. Their chemical structures were eventually confirmed by 1 H and 13 C NMR spectroscopy, mass spectrometry, and elemental analysis. 2.2. Theoretical Calculations. The properties of 9Cz46Pm have been systematically studied before. Here, we mainly focused on the properties of the newly designed 3Cz46Pm, 2Cz46Pm, 3Cz25Pm, and 2Cz26Pz. Their molecular structures and electron properties were optimized and calculated at the B3LYP/6-31G(d) level using density functional theory (DFT) by Gaussian 09 and the results are presented in Figure 2. To verify the accuracy of this method, we acquired single crystals of 3Cz25Pm and 2Cz26Pz. Their optimized structures are in well coincidence with the experimental results (Figures 2a, S1 and Table S1). For 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz, their highest unoccupied molecular orbitals (HOMOs) mainly localize on their carbazole fragments with their LUMOs localizing on both nitrogen heterocyclic fragments and neighboring carbazole units (Figure 2b,c). In contrast, due to a much more planar structure of 3Cz25Pm, its HOMO and LUMO distribute throughout the entire molecule. Apart from different HOMO and LUMO distributions, all four hosts exhibit different HOMO and LUMO energy levels due to their distinct nitrogen atom orientations of heterocyclic cores and different conjugation positions of carbazole units.42 Their calculated HOMO and LUMO energy levels can be found in Table 1. 1098


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whereas the shoulder peaks of the other three hosts are less more likely to be affected by polarity change due to the larger electron−hole overlap. According to the absorption edges of UV−vis absorption spectra, energy gaps of 3Cz46Pm, 2Cz46Pm, 3Cz25Pm, and 2Cz26Pz were calculated to be 3.31, 3.20, 3.25, and 3.20 eV, respectively. Relatively larger energy gaps of 3Cz46Pm than the other hosts can be explained by its lower conjugation as illustrated previously. S1 energy levels estimated from the PL spectra are 3.29, 3.12, 3.20, and 3.17 eV for 3Cz46Pm, 2Cz46Pm, 3Cz25Pm, and 2Cz26Pz, respectively. The vibronic fine structures of their emission spectra confirm that they are localized excited (LE) states. As solvent polarity increases, emissions of all hosts significantly redshift and become featureless (Figure S7). This indicates that polar solvent stabilizes their charge transfer (CT) singlet states to be lower than their LE singlet states and dominant.52 Triplet energies of the four hosts estimated from lowtemperature phosphorescence spectra are 2.79, 2.64, 2.64, and 2.54 eV for 3Cz46Pm, 2Cz46Pm, 3Cz25Pm, and 2Cz26Pz, respectively (Figure 3). Their variation trend is well in accordance with the theoretical result. Such high triplet energy levels of these hosts indicate their potential usage as hosts for green and sky-blue TADF emitters. Besides, as illustrated previously, such large difference in host triplet energy levels provides us with a good chance to investigate their influence on lifetimes of TADF OLEDs. 2.6. Charge-Transporting Properties. We further measured carrier mobilities of the four hosts by the time-offlight (TOF) transient photocurrent technique. Their device structures are indium tin oxide (ITO)/hosts (2000 nm)/Ag (150 nm). As presented in Figure 4, all four hosts exhibit

hosts were derived from the onset of their 1st oxidation potentials with regard to the energy level of ferrocene and they are between −5.73 and −5.64 eV (Figure S3 and Table 1). Their similar HOMO energy levels can be attributed to identical electron donating carbazole fragments. Their LUMO energy levels were calculated by adding their energy gaps to their HOMO energy levels and they are between −2.53 and −2.33 eV. Relatively larger LUMO difference of four hosts can be ascribed to different orientations of nitrogen atoms in the heterocyclic cores which will affect their LUMO distributions.42 Albeit large absolute errors, for 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz with distinct HOMO/LUMO separation, the varying trend of calculated and experimental HOMOs and LUMOs is the same (Figure S4). This further validates the accuracy of calculation methods. As for 2Cz46Pm with a much larger HOMO/LUMO overlap, the absolute error significantly improves. For four hosts, different HOMO and LUMO values will strongly affect charge injection and charge transport properties, which will eventually result in different charge recombination and energy transfer mechanisms in devices. 2.5. Photophysical Properties. Figure 3 depicts the ultraviolet−visible (UV−vis) absorption, photoluminescence

Figure 3. UV−vis absorption (black), PL (red), and phosphorescence spectra (blue) of four hosts in toluene (10−5 mol L−1).

(PL), and phosphorescence spectra of four hosts in toluene. Detailed properties are summarized in Table 1. In UV−vis absorption spectra, the main peaks of four hosts at around 3.60 and 4.10 eV can be ascribed to the π−π* transitions of carbazole fragments and the central nitrogen heterocyclic rings.45−48 The shoulder peaks below 3.50 eV for all four hosts can be classified to intramolecular charge transfer (ICT) transitions from carbazole units to the central nitrogen heterocyclic rings.49,50 Natural transition orbital (NTO) analysis can provide a simple electron−hole pair to represent the excitation process.51 On the basis of optimized ground state structures, we calculated NTOs of the vertical S0 (ground state) → S1 (singlet) transition for four hosts and their results can be found in Figure S5. Among the four hosts, 3Cz46Pm exhibits the lowest electron-hole overlap. As a result, its ICT property is the strongest and the intensity of its shoulder peak in the UV−vis absorption spectrum is the highest, too. As the solvent polarity increases, both strength and location of its shoulder peak remarkably change for 3Cz46Pm (Figure S6),

Figure 4. Electron and hole mobilities of four hosts under different electric fields.

similar hole mobilities in the range from 5.20 × 10−5 to 1.88 × 10−4 cm2 V−1 s−1 at an electric field of 6 × 107 V m−1. This is due to their identical hole transport carbazole fragments as previously illustrated, whereas the electron mobilities of 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz are much higher than their corresponding hole mobilities and are ranging from 5.63 × 10−4 to 7.76 × 10−4 cm2 V−1 s−1 at 6 × 107 V m−1. However, 3Cz25Pm exhibits much lower electron mobility (5.28 × 10−5 cm2 V−1 s−1 at 6 × 107 V m−1) than the other three hosts, as its HOMO even extends to the electron deficient pyrimidine unit due to the planar molecular structure, hindering the electron transport. 1099


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Figure 5. Structures of devices G1−G5 with FMO energy levels and exciton energy levels of EMLs labeled (bottom left: devices G1, G2, and G5 with different host triplet energy levels and the same charge recombination mechanisms; bottom right: devices G2, G3, and G4 with identical host triplet energy levels and different charge recombination mechanisms).

3. DEVICES AND DISCUSSION Eventually, we fabricated TADF OLEDs by employing 3Cz46Pm, 2Cz46Pm, 3Cz25Pm, and 2Cz26Pz as hosts for a highly efficient green TADF emitter 9-[4-(4,6-diphenyl-1,3,5triazin-2-yl)phenyl]-N,N,N′,N′-tetraphenyl-9H-carbazole-3,6diamine (DACT-II).53 9Cz46Pm is also included for comparison. Previous studies employing DACT-II as dopants have achieved record high efficiencies. As shown in Figure S8, the emission spectra of five hosts overlap well with the absorption spectrum of DACT-II and we observed no emission of hosts in doped films. This guarantees efficient energy transfer from hosts to DACT-II. Furthermore, we adopted indium tin oxide (ITO) as anodes, dipyrazino[2,3-f:2′,3′h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) as hole injection layers, N,N′-bis-(1-naphthyl)N,N′-diphenyl1,1′-biphenyl-4,4′-diamine (NPB) and N,N,N-tris(4-(9-carbazolyl)-phenyl)amine (TCTA) as hole transport layers, DACTII-doped hosts as emitting layers, 1,3,5-tris(1-phenyl-1Hbenzimidazol-2-yl)benzene (TPBi) as ETLs and hole blocking layers, lithium fluoride (LiF) as electron injection layers, and Al as cathodes. Additionally, extra 3,3′-di(9H-carbazol-9-yl) biphenyl (mCBP) layers were inserted between TCTA layers and EMLs to block the excitons. Structures of ITO/HAT-CN (5 nm)/NPB (40 nm)/TCTA (10 nm)/mCBP (5 nm)/ DACT-II: hosts (30 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (150 nm) with optimized doping concentrations of 20 wt % were first used for devices G1−G5 (Figure 5). Current density−voltage−luminance (J−V−L) characteristics, transient spectra, EQEs, and power efficiencies (PEs) of

G1−G5 can be found in Figure 6 and are summarized in Table 2. As shown in Figure S9, electroluminescence (EL) spectra of five devices peaked at 528, 540, 532, 538, and 538 nm with no emissions of 3Cz46Pm, 2Cz46Pm, 3Cz25Pm, 9Cz46Pm, and 2Cz26Pz could be found, respectively. These validated complete energy transfer from hosts to emitters. Their slightly different peak locations can be attributed to different host polarities.54,55 We further investigated the recombination mechanisms of five devices. For G1, G2, and G5, their turn on voltages were 3.1, 3.0, and 3.1 eV, respectively (Figure 6a). Given that 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz possess similar energy gaps at around 3.2 eV, we could deduce that charge recombination and energy transfer processes of G1, G2, and G5 were LR dominant and their relatively low turn on voltages can be attributed to the thermally assisted electron injection process introduced by the large barriers between the LUMOs of hosts and ETMs.30,56,57 To further validate the charge recombination mechanism, we measured the influence of doping concentration on the current density. For G1, G2, and G5, their current density remained almost unchanged regardless of the dopant concentration (Figure S10). This is the evidence of LR.14,25 Their mechanisms could also be confirmed by transient EL spectra as presented in Figure 6b where no turn-off spikes were observed for all three devices.58−62 Figure 5 (bottom left) presented the energy levels of G1, G2, and G5. In these devices, holes would transport on either hosts or emitters due to almost identical HOMO energy levels between the hosts and emitters, whereas for electrons, due to large trap 1100


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Figure 6. (a) J−V−L characteristics, (b) EL transient spectra, (c) EQEs, and (d) PEs of devices G1−G5.

Table 2. Summary of Green Devices Employing DACT-II as Dopantsa device

dominant mechanismb

host

T1 of host (eV)

ETM

Vonc (V)

λELd (nm)

G1 G2 G3 G4 G5 G6 G7

L L L+T T L L T

3Cz46Pm 2Cz46Pm 3Cz25Pm 9Cz46Pm 2Cz26Pz 2Cz46Pm 9Cz46Pm

2.79 2.64 2.64 2.64 2.54 2.64 2.64

TPBi TPBi TPBi TPBi TPBi BPBiPA:Liq BPBiPA:Liq

3.1 3.0 3.6 3.8 3.1 3.2 3.3

528 540 532 538 538 542 544

CIEd (0.35, (0.40, (0.38, (0.38, (0.38, (0.40, (0.39,

0.57) 0.56) 0.57) 0.57) 0.56) 0.54) 0.57)

EQEe (%)

PEe (lm W−1)

LT50 (h)f

22.7/20.1/17.4 13.5/12.7/11.8 19.5/16.4/13.7 19.6/17.6/15.7 10.1/9.3/8.9 21.5/19.3/16.8 22.8/19.8/17.3

53.4/33.9/25.3 34.5/22.1/17.4 41.1/25.8/18.7 40.2/29.1/23.2 22.4/15.3/12.5 51.0/32.2/23.2 52.4/33.1/24.8

2.1g 3.9g 4.3 6.1 5.4 13.7 9.5

Abbreviations: λEL: peak wavelength of the EL spectrum. Von: turn on voltage. b“L” represent LR dominant device and “T” represent TAR dominant device. cMeasured at 1 cd m−2. dMeasured at 4 V. eIn the order of at maximum, at 5000 cd m−2 and at 10 000 cd m−2. fAt the initial luminance of 5000 cd m−2. gEstimated from the extrapolated curves. a

DACT-II and increasing the dopant concentration improved the amount of carriers (Figure S11), which further confirmed the mechanism. As for G3, its turn on voltage was 3.6 V. This value was higher than those of G1, G2, and G5 and lower than that of G4. Given that HOMO and LUMO energy levels of 3Cz25Pm are similar to those of 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz, the higher turn on voltage of G3 could be ascribed to the poor electron mobility of 3Cz25Pm. In these conditions, part of the trapped electrons would directly recombine with holes on emitters and emit light. This indicated that G4 consisted of both LR and TAR. It could be evidenced by the slight turn-off spike in EL transient spectra and the reliance of concentration on the current density as presented in Figures 6b and S11. Efficiencies of devices G1−G5 are presented in Figure 6c,d. Among the five devices, G1 achieved the highest EQE of

depth of LUMO energy levels and high electron mobilities of hosts (Figure 4 and Table 1), most of them will transport and recombine with holes on hosts.63−65 As a result, G1, G2, and G5 are LR dominant. However, for G4 employing 9Cz46Pm as the host, it exhibited much higher turn on voltage of 3.8 V than its corresponding host energy gap of 3.01 eV. Given that the HOMO and LUMO of 9Cz46Pm are deeper and shallower than those of DACT-II, we could deduce that the TAR mechanism played a dominant role in G4, where both holes and electrons were trapped and recombined on DACT-II (Figure 5 bottom right).14,25 This mechanism could also be evidenced by the obvious turn-off spike of G4 in Figure 6b, which indicated charge trapping on dopants. Besides, as dopant concentration increased, the current density of G4 also improved. This indicated that carrier transmission relied on 1101


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Figure 7. Charge recombination and energy transfer processes of (a) TAR and (b) LR dominant devices with long lifetimes.

Figure 8. Emission intensity change of (a) pristine 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz films, (b) 20 wt % DACT-II doped 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz films, (c) DACT-II-doped 2,3,5,6-tetra(9H-carbazol-9-yl)benzonitrile (4CzBN) films with different doping concentration and (d) DACT-II-doped 2,3,5,6-tetrakis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzonitrile (4tCzBN) films with different doping concentrations when excited by a 320 nm laser.

22.7%. Even at the high luminance of 5000 and 10 000 cd m−2, its EQE still maintained 20.1 and 17.4%. The corresponding power efficiencies of G1 were 53.4, 33.9, and 25.3 lm W−1 at maximum, 5000 cd m−2 and 10 000 cd m−2, respectively. For G2−G5, their maximum EQEs and PEs were 13.5, 19.5, 19.6, and 10.1%, and 34.5, 41.1, 40.2, and 22.4 lm W−1, respectively. These EQE values followed the same trend of photoluminescence quantum yields (PLQYs) of their corresponding doped films (Table S2). Furthermore, we measured lifetimes of G1−G5 at an initial brightness of 5000 cd m−2 (Figure S12). For LR dominant

devices G1, G2, and G5, their device lifetimes strongly relied on host triplet energy levels. Among them, triplet energy of 3Cz46Pm (2.79 eV) is the highest and device G1 based on this exhibited the lowest half-live (LT50) of 2.1 h, whereas for 2Cz26Pz with the lowest triplet energy of 2.54 eV, its corresponding device G5 achieved the longest LT50 of 5.4 h among the three devices. As for G2 employing host 2Cz46Pm with a moderate triplet energy of 2.64 eV, its lifetime (LT50 = 3.9 h) was also exactly between those of G1 and G5. Given that 2Cz46Pm, 3Cz25Pm, and 9Cz46Pm exhibit identical triplet energy levels, we further compared their corresponding 1102


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hosts. As presented in Figure S14b, PL spectra of their pristine films slightly overlap with the UV−vis absorption spectrum of DACT-II. Besides, both host emission and DACT-II emission can be found in doped films regardless of doping concentrations (Figure S14c,d). These indicate insufficient energy transfer from 4CzBN and 4tCzBN to DACT-II. In this situation, host triplets will exist. As presented in Figure 8c, DACT-II-doped 4CzBN films exhibit much lower lifetimes than either pristine 4CzBN or pristine DACT-II films. This is in contrast to DACT-II-doped 3Cz46Pm, 2Cz46Pm, 3Cz25Pm, 9Cz46Pm, and 2C26Pz films (Figure S15). Given that their main differences are with and without host triplet excitons, low lifetimes of DACT-II-doped 4CzBN films can thus be ascribed to enhanced annihilation processes induced by host triplets as is illustrated previously. In particular, at low doping concentration, due to extremely inefficient energy transfer from hosts to guests, host triplet excitons will more easily accumulate and worsen the situation. As doping concentration increases, more excitons of hosts can be transferred to the emitters. This significantly lowers the density of host triplets. As a result, the lifetime of the doped film remarkably improves. This can also be validated from their transient spectra. As doping concentration increases, both prompted and delayed parts of transient spectra for DACTII:4CzBN films remain almost unchanged, indicating that both long range nonelectron exchange Förster energy transfer and short-range electron exchange Dexter energy transfer improve proportionally (Figure S16a and Table S4). However, the DACT-II-doped 4tCzBN films are more stable than the pristine DACT-II and 4tCzBN films (Figure 8d). This is in accordance with DACT-II-doped 3Cz46Pm, 2Cz46Pm, 3Cz25Pm, 9Cz46Pm, and 2C26Pz films. This confirms that the outer tert-butyl substituents of 4tCzBN indeed suppress host triplet excitons to interact with the surrounding molecules. Because of the suppressed TTA and STA processes, lifetimes of DACT-II:4tCzBN films improve only slightly as doping concentration increases. We can also confirm this by transient spectra of DACT-II-doped 4tCzBN films (Figure S16b). In contrast to DACT-II:4CzBN films, prompted components of DACT-II:4tCzBN films significantly increase and their delayed parts greatly decrease as doping concentration improves (Table S4). This is because the short-range electron exchange Dexter energy transfer from host triplets to guest triplets is still suppressed by the outer tert-butyl substituent of 4tCzBN in this situation, whereas prompted host singlets and host singlets arising from host triplets by reverse internal system crossing will more effectively transfer their energy to DACT-II singlets by long range Förster energy transfer and emit light. These photoaging results further validate that the host triplet exciton is the key to device lifetime decay. Third, we adopted electric aging measurements to directly confirm the effect of host triplet excitons on device lifetimes. Here, we fabricated nondoped OLEDs by employing 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz as emitting layers. Structures of ITO/HAT-CN (5 nm)/NPB (40 nm)/TCTA (10 nm)/mCBP (5 nm)/hosts (30 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (150 nm) were applied. We measured their lifetimes at the initial luminance of 500 cd m−2 and the results can be found in Figure S17. According to Figure 8a, the PL decay of the 3Cz46Pm film was only slightly faster than that of the 2Cz46Pm film and much lower than that of the 2Cz26Pz film. However, under electrical excitation, 3Cz46Pm-based

devices G2, G3, and G4 and investigated the effect of different charge recombination and energy transfer mechanisms on device lifetimes. Among them, G4 employing the TAR mechanism exhibited the longest LT50 of 6.1 h, whereas for G2 which mainly adopted the LR mechanism, its estimated lifetime of 3.9 h was the shortest among the three devices. As for G3, where both LR and TAR worked, its lifetime (LT50 = 4.3 h) was also between those of G2 and G4. On the basis of the above results, we deduced that the host triplet exciton plays a dominant role in device lifetime decay and it can be attributed to singlet−triplet annihilation (STA), triplet−triplet annihilation (TTA), and triplet−polaron annihilation (TPA) processes induced by host triplet excitons.66−68 For TAR dominant devices where excitons form on emitters, their long lifetimes can be attributed to the reduced formation of host triplet excitons as presented in Figure 7a. As for LR dominant devices where excitons form on hosts, host triplets will inevitably induce these annihilation processes. However, reducing host triplet energy levels could effectively suppress these annihilation processes and prolong their lifetimes (Figure 7b). We further adopted photo and electric aging measurements to ascertain the influence of host triplet excitons on the device lifespan. First, we confirmed that material instability is not the main reason for the device lifetime. Here, 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz were selected for comparison. They possess similar charge transport properties and devices based on them are all LR dominant mechanisms. Thus, the influence of polarons in their devices should be similar. This helped us to eliminate the difference between their electric aging and photoaging results induced by polarons. The properties of their pristine films and doped films are presented in Figures 8, S13, and summarized in Tables S2, S3. Among pristine host films, 2Cz26Pz exhibits the worst stability when excited by a 320 nm laser. This can be ascribed to its twisted structure which breaks the conjugation and lowers the molecular stability (Figures 2 and S1). Compared with 2Cz26Pz, the pristine 3Cz46Pm film exhibits higher stability and the pristine 2Cz46Pm film exhibits the highest stability among the three hosts. As for the doped films, their lifetimes are much higher than their corresponding pristine films (Figure 8b). However, photoaging results of doped films are in contrast with the lifetimes of their corresponding devices. Among 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz, the doped 2Cz26Pz film possesses the shortest lifetime, whereas, G5 employing 2Cz26Pz as the host exhibited the longest lifetime (LT50 = 5.4 h) among G1, G2, and G5. Besides, lifetimes of 3Cz46Pm- and 2Cz46Pm-doped films are similar but their corresponding devices G1 and G2 showed markedly different LT50 of 2.1 and 3.9 h. These results clearly show that material instability is not the key factor leading to device lifetime decay here. Second, to induce host triplet excitons under photoexcitation and investigate their influence on device lifetime, we selected 2,3,5,6-tetra(9H-carbazol-9-yl)benzonitrile (4CzBN) and 2,3,5,6-tetrakis(3,6-di-tert-butyl-9H-carbazol-9yl)benzonitrile (4tCzBN) as two new hosts.19 Because of small ΔEST, their singlets can covert to triplets by internal system crossing (ISC) and sequentially back transfer to singlets by the RISC process (Figure S14a and Table S4). Given that TTA, TPA, and STA are all electron exchange processes, the peripheral tert-butyl substituents of 4tCzBN can effectively suppress these processes by limiting electron exchange. Thus, we can investigate the effects of host triplets by using them as 1103


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Figure 9. (a) J−V−L characteristics, (b) transient spectra, (c) EQEs, PEs, and (d) lifetimes (at the initial luminance of 5000 cd m−2) of devices G6 and G7.

times that of its corresponding device G4 employing TPBi as ETM. These increases were much lower than those of 2Cz46Pm-based devices. Given that the current density of G7 was only slightly lower than that of G4, we can deduce that the performance of the TAR dominant device is mainly constrained by trapped charges on the emitters and is less sensitive to structure optimization. Eventually, LR dominant device G6 outperformed the other devices and exhibited the longest lifetime among G1−G7. In LR dominant devices, hosts are responsible for charge transmission and charge recombination, and emitters are responsible for emission. Thus, we can choose high mobility and stable hosts, efficient and stable emitters to match with excellent charge transport materials, and realize both efficient and stable devices as long as guaranteeing sufficient low host triplet energy levels. On the basis of this strategy, we further designed stable LR dominant blue TADF device. Here, 2,3,4,5,6-pentakis-(3,6-ditert-butyl-9H-carbazol-9-yl)benzonitrile (5tCzBN) was selected as a blue TADF dopant due to its high stability and high PLQY.19 Besides, its tert-butyl substituents outside the luminance core can effectively hinder charge trapping on the emitters and reduce annihilation processes arising from host triplets as previously illustrated. Given that its triplet energy level is 2.60 eV, 2Cz46Pm, 3Cz25Pm, and 9Cz46Pm with triplet energy levels of 2.64 eV should be good host candidates for 5tCzBN. As presented in Figure S20a, their PL spectra overlap well with the absorption spectrum of 5tCzBN. Among all doped films, 5tCzBN-doped 9Cz46Pm exhibits the most efficient energy transfer, the highest PLQY and the highest

nondoped device exhibited the lowest lifetime. Given that 3Cz46Pm, 2Cz46Pm, and 2Cz26Pz are all electron transport dominant materials and possess similar FMO energy levels, change of their EL decay curves can only be attributed to their different triplet excitons. As a result, 3Cz46Pm (T1 = 2.79 eV)based nondoped device decayed much faster than both 2Cz46Pm (T1 = 2.64 eV) and 2Cz26Pz (T1 = 2.54 eV)based nondoped devices. Furthermore, we optimized ETMs and investigated their influence on LR and TAR dominant devices. Here, 2Cz46Pm and 9Cz46Pm with identical triplet energy levels and different charge recombination and energy transfer processes were selected for comparison. Previously reported devices employing 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl) anthracene (BPBiPA) as ETLs had achieved long lifetimes.69 Here, we adopted BPBiPA and (8-hydroxyquinolinolato)lithium (Liq) at a ratio of 1:1 as binary ETLs.70 Detailed characters of newly fabricated devices G6 and G7 are presented in Figures 9, S18, S19 and summarized in Table 2. G6, employing 2Cz46Pm as the host and BPBiPA:Liq as the ETL, achieved a maximum EQE of 21.5% with long LT50 of 13.7 h at the initial luminance of 5000 cd m−2. Specifically, EQE and lifetime of G6 were 1.6 and 3.5 times those of G2 employing the same host with TPBi as the ETL, respectively. Given that the current density of G6 was much lower than that of G2, these increases of G6 could be ascribed to modified charge flux by replacing ETM. As for 9Cz46Pm-based device G7, using BPBiPA:Liq as ETL, it achieved an EQE of 22.8% and LT50 of 9.5 h at an initial brightness of 5000 cd m−2. However, the EQE and lifetime of G7 were merely 1.2 and 1.6 1104


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ACS Applied Materials & Interfaces Table 3. Summary of Representative Blue TADF OLEDs PE (lm W−1)

EQE (%) work B1 ref 71 ref 19 ref 17 ref 24 ref 16

Von (V)

λEL (nm)

3.3a

490b

2.9 2.6

490

472

CIE (0.22, (0.19, (0.21, (0.18, (0.16, (0.17,

0.42)b 0.41) 0.41) 0.34) 0.22) 0.27)

max 18.3 20.9 21.2 8.8 18.9 13.6

500 cd m−2

1000 cd m−2

18.3

17.9 20.8

10.7 5.5 8.7

1000 cd m−2

max

500 cd m−2

1000 cd m−2

LT80 (h)

LT50 (h)

LT80 (h)

LT50 (h)

34.9

33.6

29.7

175

905

52 110c

269

56.1 20.1 26.2

7.0

500 cd m−2

770 9.2 4.8

454 52d 21

Measured at 1 cd m−2. bMeasured at 4 V. cLifespan up to 80% of the initial luminance. dAdopting a different host.

a

Figure 10. (a) J−V−L characteristics, (b) transient spectra, (c) EQEs, PEs, and (d) lifetime (at the initial luminance of 1000 cd m−2) of device B1.

among the best overall performances of blue TADF devices (Tables 3 and S6).

stability (Figure S20b−d and Table S5). Thus, we fabricated blue device B1 with the structure of ITO/HAT-CN (5 nm)/ NPB (40 nm)/TCTA (10 nm)/mCBP (5 nm)/5tCzBN: 9Cz46Pm (30 nm)/BPBiPA:Liq (40 nm)/LiF (0.8 nm)/Al (150 nm) (Figure S21). We summarize its performance in Table 3. As is presented in Figure S22, the emission of device B1 peaked at 490 nm. According to its turn on voltage, transient spectrum and current density change, the mechanism of B1 was confirmed to be LR dominant (Figures 10a,b and S23). Finally, B1 achieved a maximum EQE of 18.3% and a maximum PE of 34.9 lm W−1. Even at the high luminance of 1000 cd m−2, B1 could still maintain a high EQE of 17.9%, a high PE of 29.7 lm W−1 with a long LT50 of 269 h (Figure 10c,d). By adopting the formula LT(500 cd m−2) = LT(1000 cd m−2)(1000 cd m−2/500 cd m−2)n, with n = 1.75, LT50 of B1 was predicted to be 905 h at the initial luminance of 500 cd m−2. This performance is higher than our previous report and

4. CONCLUSIONS Five isomers were synthesized and used as hosts to explore the inherent reason resulting in different lifespans of LR and TAR dominant devices employing TADF emitters. By taking both host triplet energy levels and recombination mechanisms into consideration, we thoughtfully investigated electrical aging and photoaging results. It was revealed that the host triplet exciton is the key to device lifetime decay. For TAR dominant devices, due to the lack of host triplet excitons, annihilation arising from them is limited. This results in their long lifetimes. As for LR dominant devices, albeit the existence of host triplet excitons, reducing host triplet energy levels can significantly improve their lifetimes by reducing annihilation processes. Combined with further structure optimization, lifetimes of LR dominant devices can eventually suppress TAR dominant ones. 1105


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(10) Wu, T.-L.; Huang, M.-J.; Lin, C.-C.; Huang, P.-Y.; Chou, T.-Y.; Chen-Cheng, R.-W.; Lin, H.-W.; Liu, R.-S.; Cheng, C.-H. Diboron Compound-Based Organic Light-Emitting Diodes with High Efficiency and Reduced Efficiency Roll-Off. Nat. Photonics 2018, 12, 235−240. (11) Zeng, W.; Lai, H. Y.; Lee, W. K.; Jiao, M.; Shiu, Y. J.; Zhong, C.; Gong, S.; Zhou, T.; Xie, G.; Sarma, M.; Wong, K. T.; Wu, C. C.; Yang, C. Achieving Nearly 30% External Quantum Efficiency for Orange−Red Organic Light Emitting Diodes by Employing Thermally Activated Delayed Fluorescence Emitters Composed of 1,8-Naphthalimide-Acridine Hybrids. Adv. Mater. 2017, 30, No. 1704961. (12) Furue, R.; Matsuo, K.; Ashikari, Y.; Ooka, H.; Amanokura, N.; Yasuda, T. Highly Efficient Red−Orange Delayed Fluorescence Emitters Based on Strong π-Accepting Dibenzophenazine and Dibenzoquinoxaline Cores: toward a Rational Pure-Red OLED Design. Adv. Opt. Mater. 2018, 6, No. 1701147. (13) Im, Y.; Kim, M.; Cho, Y. J.; Seo, J.-A.; Yook, K. S.; Lee, J. Y. Molecular Design Strategy of Organic Thermally Activated Delayed Fluorescence Emitters. Chem. Mater. 2017, 29, 1946−1963. (14) Lee, J.-H.; Lee, S.; Yoo, S.-J.; Kim, K.-H.; Kim, J.-J. Langevin and Trap-Assisted Recombination in Phosphorescent Organic Light Emitting Diodes. Adv. Funct. Mater. 2014, 24, 4681−4688. (15) Lee, C. H.; Lee, J. H.; Kim, K. H.; Kim, J. J. Unveiling the Role of Dopant Polarity in the Recombination and Performance of Organic Light-Emitting Diodes. Adv. Funct. Mater. 2018, 28, No. 1800001. (16) Ihn, S.-G.; Lee, N.; Jeon, S. O.; Sim, M.; Kang, H.; Jung, Y.; Huh, D. H.; Son, Y. M.; Lee, S. Y.; Numata, M.; Miyazaki, H.; Gómez-Bombarelli, R.; Aguilera-Iparraguirre, J.; Hirzel, T.; AspuruGuzik, A.; Kim, S.; Lee, S. An Alternative Host Material for LongLifespan Blue Organic Light-Emitting Diodes Using Thermally Activated Delayed Fluorescence. Adv. Sci. 2017, 4, No. 1600502. (17) Cui, L.-S.; Ruan, S.-B.; Bencheikh, F.; Nagata, R.; Zhang, L.; Inada, K.; Nakanotani, H.; Liao, L.-S.; Adachi, C. Long-Lived Efficient Delayed Fluorescence Organic Light-Emitting Diodes Using N-Type Hosts. Nat. Commun. 2017, 8, No. 2250. (18) Cui, L.-S.; Deng, Y.-L.; Tsang, D. P.-K.; Jiang, Z.-Q.; Zhang, Q.; Liao, L.-S.; Adachi, C. Controlling Synergistic Oxidation Processes for Efficient and Stable Blue Thermally Activated Delayed Fluorescence Devices. Adv. Mater. 2016, 28, 7620−7625. (19) Zhang, D.; Cai, M.; Zhang, Y.; Zhang, D.; Duan, L. Sterically Shielded Blue Thermally Activated Delayed Fluorescence Emitters with Improved Efficiency and Stability. Mater. Horiz. 2016, 3, 145− 151. (20) Byeon, S. Y.; Kim, J. H.; Lee, J. Y. CN-Modified Host Materials for Improved Efficiency and Lifetime in Blue Phosphorescent and Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2017, 9, 13339−13346. (21) Cho, Y. J.; Jeon, S. K.; Lee, J. Y. Molecular Engineering of High Efficiency and Long Lifetime Blue Thermally Activated Delayed Fluorescent Emitters for Vacuum and Solution Processed Organic Light-Emitting Diodes. Adv. Opt. Mater. 2016, 4, 688−693. (22) Kim, J. H.; Han, S. H.; Lee, J. Y. Dibenzothiophene Derived Hosts with CN Substituted Carbazole for Blue Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes. Synth. Met. 2017, 232, 152−158. (23) Lee, D. R.; Choi, J. M.; Lee, C. W.; Lee, J. Y. Ideal Molecular Design of Blue Thermally Activated Delayed Fluorescent Emitter for High Efficiency, Small Singlet−Triplet Energy Splitting, Low Efficiency Roll-Off, and Long Lifetime. ACS Appl. Mater. Interfaces 2016, 8, 23190−23196. (24) Kim, M.; Jeon, S. K.; Hwang, S.-H.; Lee, J. Y. Stable Blue Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes with Three Times Longer Lifetime than Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 2015, 27, 2515−2520. (25) Zhang, D.; Song, X.; Cai, M.; Kaji, H.; Duan, L. Versatile Indolocarbazole-Isomer Derivatives as Highly Emissive Emitters and Ideal Hosts for Thermally Activated Delayed Fluorescent OLEDs

Additionally, LR can significantly expand choices of materials for not only emitting layers but also the other functional layers. This work may shed light on further development of efficient and stable TADF OLEDs towards practical application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b16784.

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Experimental and calculation details and the summary of blue OLEDs employing TADF emitters and characterization data, including 1H NMR, 13C NMR, MALDITOF MS spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-10-62795137. Phone: +86-10-62782197. ORCID

Lian Duan: 0000-0001-7095-2902 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (Grant No. 2017YFA0204501), the National Science Fund of China (Grant Nos 51525304 and U1601651), the National Basic Research Program of China (Grant No. 2015CB655002) and the Tsinghua University Initiative Scientific Research Program (Grant Nos 20161080039 and 20161080040).

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REFERENCES

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