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A Deep Blue High Efficiency Tta Oled Using Para- and Meta- Conjugated Cyano-Triphenylbenzene and Carbazole Derivatives as Emitter and Host Nadzeya A. Kukhta, Tomas Matulaitis, Dmytro Volyniuk, Khrystyna Ivaniuk, Pavlo Turyk, Pavlo Stakhira, Juozas Vidas Grazulevicius, and Andrew P. Monkman J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02867 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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The Journal of Physical Chemistry Letters

A Deep-Blue High-Efficiency TTA OLED Using Para- and Meta- Conjugated CyanoTriphenylbenzene and Carbazole Derivatives as Emitter and Host Nadzeya A. Kukhtaa, b *, Tomas Matulaitisa, Dmytro Volyniukb, Khrystyna Ivaniukc, Pavlo Turykc, Pavlo Stakhirac, Juozas V. Grazulevicius a, Andrew P. Monkmand* a

Department of Organic Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-

50254, Kaunas, Lithuania b

Present address: Department of Chemistry, Durham University, South Road, Durham DH1

3LE, UK c

Lviv Polytechnic National University, S. Bandera 12, 79013 Lviv, Ukraine

d

Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK

Corresponding Author *[email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT. Elaboration of the appropriate host materials proved to be not less important for the fabrication of a highly efficient OLED, than the design of emitters. In present work we show how by simple variation of molecular structure both blue emitters exhibiting delayed fluorescence and ambipolar high triplet energy hosts can be obtained. The compounds with parajunction revealed higher thermal stability (TID up to 480 oC), lower ionization potentials (5.515.60 eV), exclusively hole transport and higher photoluminescence quantum efficiencies (0.900.97). Meta-linkage leads to ambipolar charge transport and higher triplet energies (2.82 eV). Introduction of the accepting nitrile groups in the para-position induces intensive delayed fluorescence via triplet-triplet annihilation up-conversion mechanism. By utilization of the parasubstituted derivative as emitter and meta-substituted isomer as the host, the deep-blue OLED with

the

external

quantum

efficiency

of

14.1%

was

fabricated.

TOC GRAPHICS

KEYWORDS: bipolar, substituent orientation, up-conversion mechanism, DFT calcultation In recent years, the use of organic light emitting diodes (OLEDs) in flat panel and flexible displays has rapidly increased. However, the ultimate performance of OLEDs still has not been reached. Thus, such parameters, as driving voltage, external quantum efficiency (EQE) and lifetime for blue can still be refined 1. Numerous approaches towards OLED efficiency


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improvement, involving the design of efficiency-effective device architectures and synthesis of new materials have been proposed and elaborated upon 2. The best ways to enhance EQE include the use of phosphorescent or delayed fluorescent (DF) emitters 3,4. Generally, DF can be classified into two types: P-type, where the triplet fusion, through the triplet-triplet annihilation (TTA) mechanism converts non emissive triplet excited states into emissive singlet states 5,6; or E-type where thermally activated delayed fluorescence (TADF) effectively converts triplets to singlets 7–9. While complete triplet to singlet up-conversion can result in 100% internal quantum efficiency (IQE) in TADF devices, perfect TTA only gives a theoretical maximum of 62.5% IQE 10

. Though TADF-based OLEDs can realize higher IQE and, hence, higher EQE, compared to

TTA-based devices, the latter are promising for practical applications due to their long device operating lifetime and operational stability 1. Continuous efforts have been made to evolve blue-light emitters with good colour purity 11,12,13. To obtain a suitable blue emitter, which could also serve as a pertinent host for phosphorescent OLEDs14 , a combination of donating and accepting building blocks can be used. While such electron accepting units, as anthracene 15, spyrofluorene 16, pyridine 17, triazine 18 and nitrile 19 were successfully utilized for the preparation of efficient blue-emitting materials, the choice of donor is usually limited to carbazole and acridine. The combination of cyanobenzene acceptor and carbazolyl donor is known to provide overall stability (chemical, thermal, electrochemical) of the emitter, hence leading to high EQE and long device lifetime 1. Therefore, in current work we turn to new materials, describing the design, synthesis and application in OLEDs of new bipolar derivatives of carbazole and nitrile-substituted 1,3,5triphenylbenzene. The influence of the substituent orientation on the properties of the compounds was investigated.


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Chart 1 presents the chemical structures of the prepared bipolar compounds. Synthetic procedues and characterization can be accessed in SI.

Chart 1. Chemical structures of the carbazolyl and cyano-substituted TPB derivatives The geometry of the synthesized compounds is determined by the propeller shape of the TPB moiety (torsion angle 51-53o) and orientation of the substituents (Fig. S1). Despite the values of dihedral angles between the core and carbazolyl substituents being very similar, meta- and paraorientation of the substituents obviously affects the molecular profiles. p-TPB-Cz and p-TPB2Cz may form more planar structures through stronger π-delocalization across the molecule, whereas m-TPB-Cz, m-TPB-2Cz and p-TPB-m-Cz should be more twisted, as was recently shown for D-A-D meta- and para-conformers 20. This may also influence intermolecular packing. HOMOs of the TPB derivatives are located exclusively on 9-phenylcarbazole (PCz) moieties, while LUMOs are delocalized through the accepting nitrile groups involving neighboring phenyl rings. Evidently, all the synthesized molecules possess good charge separation between HOMO and LUMO, suggesting significant internal charge transfer (ICT) character. Of note, the HOMO and LUMO of p-TPB-Cz and p-TPB-2Cz possess a slight overlapping area, which could give rise to higher radiative decay rates through strong excited to ground state coupling. Though


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absolute EHOMO values are very close (-5.28 – -5.33 eV) for the derivatives, LUMO energies of p-TPB-Cz, p-TPB-2Cz and p-TPB-m-Cz are by ca. 0.3 eV lower than those of the metaanalogues, giving a hint to more pronounced accepting strength of C≡N moiety attached in the para-positions. All the derivatives exhibited remarkably high temperatures of the onsets of thermal degradation (TID) of 401-480 oC, as well as high glass transition temperatures (Tg) ranging from 128 to 228 °C, due to the exploitation of thermally stable TPB 21,22 moiety and rigid carbazole 23 unit (Table S1). The investigated D–A compounds exhibit reversible oxidation and quasi-reversible reduction processes in their cyclic voltammetry (CV) scans regardless of the substitution pattern, confirming the electrochemical stability of the molecules (Fig. S5, Table S2). The approximations IP= |e|(4.8 + Eox) and EA = |e|(4.8 + Ered) were used to estimate solid state ionization potential (IP) and electron affinity (EA), respectively 24. Low EA indicates the weak electron-accepting strength of benzonitrile derivatives, which, however, proves to be advantageous for the design of blue-emitters 25. UV/Vis and photoluminescence (PL) spectra of the compounds were measured in solution and for dispersions in host media of different rigidity (Fig. 1). Absorption spectra (hexane solutions) range to ca. 360 nm, clearly displaying bands originating from the strongly allowed π–π* transitions of the benzonitrile fragment 26 (250-280 nm) and sharp peaks at ca. 300 nm assigned to carbazole unit excitations 27. Well-resolved low energy band (LEB) occur in the UV spectra of substituted TPBs due to n–π* transitions within carbazole units, being better expressed for paraconjugated molecules where mixing with π-π* transitions enhances oscillator strength 20,28. This


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observation can be attributed to efficient electronic coupling and, hence, a certain degree of the CT character 29 between the donating and accepting parts of the molecules, in agreement with the slight HOMO/LUMO overlap and natural transition orbitals 30 (NTO) analysis.

Figure 1. UV/Vis and PL (λex = 310 nm) spectra of carbazolyl and cyano-substituted TPB derivatives recorded for 10-5 M hexane solutions, solid solutions in zeonex (1 wt.% of compound), solid films and phosphorescence spectra recorded for the solid solution in zeonex at 77K (delay 10 ms, integration time 50 ms). Upon excitation at 310 nm all the TPB derivatives exhibited deep violet PL (λmax = 366-368 nm) featuring vibronic structure for the spectra of both non-polar solid and liquid solutions (Fig. 1). Interestingly, meta-carbazolyl-substituted derivatives, m-TPB-Cz, m-TPB-2Cz and p-TPB-mCz, display an additional band at 351 nm due to weaker electronic coupling of carbazole with the substituted core. Thus, peaks at 351 and 368 nm can be assigned to PL of a locally excited


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carbazole unit 27, the shoulder at 387 nm must originate from the CT emission. In turn, PL of the solid films are red-shifted by 32-56 nm comparing to those of liquid and solid solutions. The broad Gaussian profile, loss of vibronic structure indicate emission from a strong CT excited state. The solutions of p-TPB-Cz and p-TPB-2Cz in hexane exhibited high PLQYs (0.96 and 0.90) owing to efficient conjugation and HOMO/LUMO overlap (Table S3). While the PLQY of para-substituted compounds remained mostly unaffected by incorporation into a rigid polymer matrix, the PLQY of meta-substituted derivatives increased by 2.5 times potentially due to the restriction of intramolecular rotations. PL efficiency of neat films of para-substituted compounds was found to be significantly lower than that of dilute hexane solutions or in zeonex due to π-π stacking. In contrast, for the neat films of m-TPB-Cz and p-TPB-m-Cz, two-fold higher PLQYs were observed relative to those of the solutions. TPB derivatives exhibited phosphorescence that can be measured at room temperature when diluted in a zeonex host. For the sake of obtaining better resolved spectra phosphorescence was recorded at 77K (Fig. 2). Though the profiles of the phosphorescence are similar for all five derivatives, the spectral shape and position differ strongly from those of PCz 29. The triplet energy values (Table S3) were found to be moderately high (2.68-2.82 eV), being, however, lower than that of PCz (3.01 eV) 29. In order to get an insight on the nature of triplet emission, NTOs of T1 state were accessed for the derivatives (Fig. S4). The view on the HONTO/LUNTO electron density distribution of p-TPB-Cz and p-TPB-2Cz suggests, that phosphorescence of these compounds arises mainly on the TPB fragment, though involving CN-groups and minor contribution of carbazolyl unit. In turn, similar position of triplet emission of p-TPB-m-Cz can be explained by the delocalized through [1,1':3',1''-terphenyl]-4,4''-dicarbonitrile NTOs (Fig. S4).


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Finally, m-TPB-Cz and m-TPB-2Cz have by ca. 0.13 eV higher triplet energy, than the other materials of the series, due to weak electronic coupling of meta-linked carbazole to the central chromophore 29. Given the large red shift observed in the spectra of each material in bulk film, the solvatochromism 31 of each was investigated (Fig. 2 and S8). For the sake of convenience, a full set of results only for p-TPB-2Cz is presented in the main text. The data for the other compounds of the series can be found in SI (Fig. S8-10, Table S4).


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Figure 2. (a) PL spectra of the solutions of p-TPB-2Cz (λex = 310 nm) in various solvents; (b) steady-state PL spectra of oxygen equilibrated and deoxygenated Me-THF solutions; (c) steadystate and time-resolved (Nd-YAG laser, λex = 355 nm) PL spectra (d) PL decay curve; (e, f) dependence of the PL intensity on the excitation laser fluence (deoxygenated solutions in MeTHF) PL spectra of the dilute p-TPB-2Cz solutions clearly show positive solvatochromism (Fig. 2 (a)). The contribution of the triplet excited states to the overall emission was determined by comparing the PL intensity in oxygen-saturated and deoxygenated Me-THF solutions (Fig. 2 (b)). The 2.77-fold enhancement of CT emission, as well as a good match of the spectra, prove that triplet excited states participate in the emission 32. In turn, intense DF was observed by timeresolved PL measurements of the deoxygenated Me-THF solution (delay of 9 µs, integration time of 100 µs). Comparison of the steady-state and time-resolved (Fig. 2 (c)) spectra shows, that prompt fluorescence (PF, short delay of 2 ns), as well as DF, are by 17 nm bathochromically shifted, comparing to the steady state PL, confirming the stabilization of CT emission. The estimated ∆EST was found to be 0.35 eV. PL decay curve (Fig. 2 (d)) shows that PF and DF follow clear exponential decay laws with the estimated lifetimes of 5.41 ns and 183 µs,


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respectively. Finally, for the determination of the mechanism of delayed emission the dependence of PL intensity on the excitation laser fluence was recorded (Fig. 2 (e, f)). The linear fit of the DF intensity dependence on the excitation dose characterized by slope of 2 indicates bimolecular origin of delayed emission, i.e. the triplet-triplet annihilation 9,33. Given the size of ∆EST>0.35 eV, this is not surprising as TADF would be very weak in such a case 9. Similar experiments, as described above, were performed for all the compounds of the series (Fig. S4-6, Table S1). Interestingly, only the compounds, bearing accepting CN-groups in parapositions, exhibited delayed emission via TTA mechanism. The absence of DF in exclusively meta-substituted compounds, m-TPB-Cz and m-TPB-2Cz, is not understood given that phosphorescence is clearly seen in these materials. Due to their high values of PLQY and charge-drift mobility (SI) compared to other similar compounds, as well as the appropriate HOMO/LUMO and singlet/triplet energy values of pTPB-2Cz and m-TPB-2Cz, we have the potential to use the para-substituted material as an emitter and the meta-substituted derivative as its host to fabricate electroluminescent devices. We compared the DF behavior in solid host/guest films using both commercially available holetransporting material 1,3-bis(N-carbazolyl)benzene (mCP) and m-TPB-2Cz as hosts. We measured the time-resolved emission decay for the (deoxygenated) 20% p-TPB-2Cz / m-TPB2Cz and 20% p-TPB-2Cz / mCP films (Fig. S11 (a, b)) at room temperature. While no DF was observed in the films of p-TPB-2Cz in mCP, DF was observed for the p-TPB-2Cz / m-TPB-2Cz blend (Fig. S11 (c)). The PL decays (Fig. S11 (d)), collected for both films, clearly show only prompt fluorescence for the film in mCP, and both PF and DF in the p-TPB-2Cz / m-TPB-2Cz blend. It should be noted, that no phosphorescence was observed at room temperature for both films in the time integrated emission spectra. To identify the nature of the triplet up-conversion


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mechanism, the laser pulse intensity dependence was recorded for the 20% p-TPB-2Cz / mTPB-2Cz film at 3 us delay (integration time 400 µs) (Fig. S11 (e, f)). Taking into account the tail at longer wavelengths of the DF, we measured a linear fit of the DF intensity dependence on the excitation dose characterized by slope of 1.6, indicating a mixed DF, predominantly of triplet-triplet annihilation (TTA) and phosphorescence. We note that the PF intensity dependence on the laser fluence has slope slope 0.95, thus a quadrature signal (for pure TTA) should be 1.9. The slope of 1.6 is smaller than the expected value of 1.9, confirming a contribution of a linear DF process which could be either TADF or phosphorescence. From the measured energy gap, ∆EST>0.35 eV, we rule out TADF. On careful inspection of the DF spectra (Fig. S11 (e)) a clear long living red tail is seen characteristic of the phosphorescence (Fig. 2 (c)). Thus the DF consists of a majority TTA contribution plus a small residual phosphorescence. For the blend film, the PLQY was estimated at nearly 80%, indicating the efficiency of the TTA process. This confirms that triplet states in the blend are not quenched and so TTA will be very efficient in these mixed films. Fluorescent OLEDs having an architecture of ITO/CuI (8 nm)/TPD (10 nm)/host: p-TPB-2Cz (20%) (60 nm)/PBD (10 nm)/Ca (50 nm)/Al (200 nm) were prepared. In device A, mCP was used as the host matrix for the blue-emitting p-TPB-2Cz, while in device B the newly synthesized m-TPB-2Cz was tested as a host (Fig. 3, S12). Electroluminescence (EL) spectra of devices A and B peak at the same position, at the same wavelength as the PL spectrum of the solid sample of p-TPB-2Cz. However, the EL spectrum of device A is narrowed and has a longwavelength tail, which might be attributed to the excimer and/or electromer formation or recombination outside of the emission layer. Switching from mCP to m-TPB-2Cz host we achieve better energy level matching in device B with much stronger blocking of holes


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especially. This leads to a much better match of the EL to the PL spectra and hence deeper blue emission (Table 1). Current density–voltage–luminance curves of the blue OLEDs are shown in Fig. 3 (e, f). The current density was increased threefold, while the turn-on voltage was decreased by ca. 1.5 times by using m-TPB-2Cz as a host. The maximum brightness of device A at 20 V was found to be higher than that of device B at 15 V bias. However, the brightness of 5500 cd/m2 of device B was higher in comparison to ca. 3000 cd/m2 of device A recorded at the same applied voltages of 15 V proving the hosting efficiency of m-TPB-2Cz. Moreover, no long-wavelength tail was observed in the electroluminescence spectrum of device B. This observation we can attribute to the usage of sterically bulky m-TPB-2Cz host and improved hole blocking.


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Figure 3. (a, b) Energy levels of the materials; (c, d) electroluminescence spectra; (e, f) current density-voltage-luminance curves of devices A and B As the final benchmark, device B, utilizing m-TPB-2Cz as the host matrix for p-TPB-2Cz, yielded a very high maximum EQE of 14.1 % which can be assigned to the improved HOMO/LUMO and triplet energy levels of the host and the guest, the ambipolar charge transport of the host and the involvement of the triplet levels of emitter to the overall emission. In order to check the reproducibility of such an outstanding result, the devices C and D with the same configuration as device B, were prepared and investigated (Fig. S13, S14). The EQE of devices C and D reached 10.5% and 13.1%, respectively, confirming the high EQE achievable for this


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guest host pair. This divergence from the theoretical limit of a fluorescent OLED (5%) is clearly explained by an enhancement to emission by the strong TTA DF from the blend films, observed for both host and emitter utilized in devices B-D. To the best of our knowledge, in this study we achieved one of the highest external quantum efficiency with minor efficiency roll-off for the deep blue TTA-driven OLED. We believe that the very efficient TTA in the m-TPB-2Cz/ pTPB-2Cz system arises through the energetically close triplet levels in the guest and host which enables far more efficient triplet migration and annihilation allowing rapid harvesting of the triplets and avoiding polaron-triplet annihilation 34,35. Within the narrow recombination zone in the devices we believe that this enhanced triplet mobility allows for near 100% utilization of triplets by very efficient TTA. As we have seen, the triplet state of p-TPB-2Cz, 3LE has an energy of 2.8 eV, whereas the local excited state, 1LE, lies at ca. 3.54 eV (Fig 1 (a)), thus 2xET1>ES1 . 2xET1 gives an energy of 5.6 eV which from consideration of other DA molecules 36 should also imply 2xET1>ETn. This then dictates the maximum contribution of singlets from TTA will be 20% so the total singlet yield in the device would be 40% 37. Taking a quantitative value of 30% outcoupling of light from an OLED 38,39 with an emitter PLQY of 1, this gives an expected maximum EQE of 12%. From our results we are very close to this maximum level of OLED performance using TTA enhanced emission with a TTA efficiency governed by the complete utilization of all triplets in the emissive layer. The measured PLQY of the blend film without oxygen was estimated at ca. 80%. We believe that within the narrow emission layer the confinement of triplets and very high triplet density can drive the TTA even more efficiently than a thick film and so all triplets can be utilized (giving rise to no residual phosphorescence) and hence an even higher total PLQY, perhaps approaching unity. Table 1. Electroluminescent properties of blue OLEDs A and B


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Max. Device

host

a

Von [V]

ηC c

ηPd

[cd A-1]

[lm W-1]

EQEmax

CIEf

brightness b

e

[%]

[x, y]

-2

[cd m ]

A

mCP

8.4

11450

3.7

0.85

3.5

0.25, 0.23

B

m-TPB-2Cz

5.8

5500

11.1

4.20

14.1

0.22, 0.19

a

Turn-on voltage. b Maximum brightness. c Maximum current efficiency. d Maximum power efficiency. e Maximum external quantum efficiency. f Color coordinates.

To conclude, the orientation of the substituents was found to affect greatly the properties of the bipolar materials. Those having para-conjugation, show higher thermal stability, higher glass transition temperatures, lower ionization potentials, exclusively hole transport and near unity photoluminescence quantum efficiencies when dispersed in non-polar media; while compounds with meta-linkage display ambipolar charge transport, higher triplet energies. The materials comprising accepting nitrile groups in the para-position, exhibit strong delayed fluorescence via triplet-triplet annihilation. The derivative with exclusively para-conjugation, p-TPB-2Cz, displayed outstanding performance in an organic light emitting diode using m-TPB-2Cz as an ambipolar host. These blue delayed fluorescence enhanced devices have EQE reaching between 10.5% to 14.1% with an average of 12.1%. Such values indicate near maximal triplet harvesting via the TTA up-conversion process and we suggest that as the guest and host triplet levels are close, triplet mobility is enhanced such that TTA becomes very efficient and very fast so that triplet polaron quenching is ineffective. In this way we maximize the triplet harvesting step, combined with the near unity PLQY of the p-TPB-2Cz emitter, we achieve optimal IQE. Thus, the meta-linked materials proved to be efficient hosts for the para-substituted guests. Therefore, by varying the amount and position of electron donating and accepting substituents it is possible to obtain promising emitters (para-substituted derivatives of triphenylbenzene) and hosts (meta-


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substituted derivatives) for blue delayed-fluorescence-based organic light emitting diodes. Furthermore, the energetically close triplet states of the guest and host (with the guest emitter being lower in energy) maximizes the efficiency of the TTA process. ASSOCIATED CONTENT Supporting Information. Experimental details; synthetic procedures; details of theoretical, thermal, electrochemical, charge-transport, photophysical investigation; device characterization. ACKNOWLEDGMENT The financial support from the Horizon 2020 ICT29-2014 project PHEBE (grant No 641725) is gratefully acknowledged. REFERENCES (1)

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