Highly-Efficient Doped and Nondoped Organic Light-Emitting Diodes

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Highly-Efficient Doped and Non-doped OLEDs with External Quantum Efficiencies Over 20% from A Multifunctional Green Thermally Activated Delayed Fluorescence Emitter Juan Zhao, Xiaojie Chen, Zhan Yang, Tiantian Liu, Zhiyong Yang, Yi Zhang, Jiarui Xu, and Zhenguo Chi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08604 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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The Journal of Physical Chemistry

Highly-Efficient Doped and Non-Doped OLEDs with External Quantum Efficiencies Over 20% from A Multifunctional Green Thermally Activated Delayed Fluorescence Emitter Juan Zhao,‡ Xiaojie Chen,‡ Zhan Yang, Tiantian Liu, Zhiyong Yang,* Yi Zhang,* Jiarui Xu and Zhenguo Chi* PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Center for Highperformance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Material and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, PR China.

ABSTRACT: Thermally activated delayed fluorescence (TADF) emitters integrated with aggregation-induced emission (AIE) property are expected to enable non-doped organic lightemitting diodes (OLEDs) with merits of high efficiency and simple fabrication process. We previously proposed a novel design strategy of combining AIE and TADF features to develop AIE-TADF emitters with asymmetric structure, leading to an AIE-TADF emitter 4,4-CzSPz exhibiting mechanoluminescence. Herein, photophysical, thermal and electroluminescence properties of neat 4,4-CzSPz films have been systematically investigated. It was found that 4,4CzSPz film exhibits a high photoluminescence quantum efficiency of 97.3% and excellent thermal properties. Significantly, 4,4-CzSPz afforded its doped and non-doped OLEDs with very

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high EQEs of 26.2% and 20.7%, respectively, which are rarely reported amongst TADF-based OLEDs. The results demonstrate universal applications of 4,4-CzSPz in OLEDs with different device configurations for achieving high efficiency.

INTRODUCTION Thermally activated delayed fluorescence (TADF) emitters can harvest both singlet (S1) and triplet (T1) excitons for light emission through a reversible intersystem crossing transition (RISC) from T1 to S1, leading to high internal quantum efficiency of 100%.1 Nowadays, TADF emitters have been the subject of research in the field of organic light-emitting diodes (OLEDs) as an inexpensive alternative to the more traditional and popular phosphorescent complexes involving high material costs due to limited sources of rare metals. At the present, great achievements have been made towards the development of TADF emitters providing OLEDs with high external quantum efficiencies (EQEs) over 20%.2-5 To achieve high device efficiency in these TADFbased OLEDs, the emitting layers (EMLs) are generally constructed by using commercially popular host-guest doped systems to reduce concentration quenching of common TADF materials. However, it would be more attractive if the TADF emitter could produce efficient OLEDs without doping and be utilized as non-doped EMLs, which would help to simplify device structure and the fabrication process. In this respect, emitter materials exhibiting aggregation-induced emission (AIE) property are suitable for non-doped EMLs configuration because they show much more intense emission in the solid state compared to the solution state.6 From this standpoint, in 2014, we proposed a structural design strategy that involved the combination of AIE and TADF concepts to develop novel asymmetric AIE-TADF emitters,7 which could be ideal luminescent candidates for efficient and non-doped OLEDs. As a proof of


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concept, a donor (D)-acceptor (A)-donor’ (D’) type green TADF emitter 4,4-CzSPz (also named SFPC in our previous work8), wherein phenothiazine group is D, diphenyl-sulfone group is A and carbazole group is D’, was successfully synthesized with a photoluminescence quantum yield (PLQY) of 93.3% in solid state (powder), while thus PLQY was much higher than that of its symmetric analogs with two carbazole or phenothiazine units (≤ 52.8%). This finding is significant and this strategy is readily feasible, considering the mature mechanism understanding and rich variety of efficient AIE and TADF units. Since then, great attentions have been paid to the exploration of AIE-TADF emitters.9-14 For an instance, Tang et al.7 reported a series of AIETADF emitters endowing the doped OLEDs with EQEs of 22.7% and corresponding non-doped OLEDs with EQEs of 15.0 ~ 18.4%. Despite of these advances, TADF emitters affording decent EQEs over 20% in both doped and non-doped OLEDs are very much desired, however, there are very rare examples meeting these requirements.15,16 In our previous work, we focused on putting forward a simple and promising design concept for the exploration of AIE-TADF emitters with asymmetric structure. As for the presented 4,4-CzSPz emitter, evidences for its TADF properties were given from single crystals and powders analysis. When utilized for making OLEDs, nevertheless, the TADF emitters are in the thin film state, rather than in powder form. Therefore, in this work, further studies on the photo-physical and thermal properties of 4,4-CzSPz films have been performed, and more importantly, doped and non-doped OLEDs of 4,4-CzSPz have been fabricated resulting in high EQEs of up to 26.2% and 20.7%, respectively. These results suggest 4,4-CzSPz has universal applications in different device configurations for achieving high efficiencies, simplifying device structures, and reducing production costs simultaneously.


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EXPERIMENTAL SECTION Photophysical and thermal properties. Ultraviolet-visible (UV-vis) absorption spectra and PL spectra were measured using a Hitachi U-3900 spectrophotometer and a Shimadzu RF-5301PC spectrometer, respectively. Transient PL decay characteristics were recorded on a Horiba JY FL3 spectrometer. Differential scanning calorimetry (DSC) was performed using a NETZSCH DSC 204 F1 thermal analyzer at a heating rate of 10 ℃ min-1 under a N2 atmosphere. Thermogravimetric analyses (TGA) were performed with a TA thermal analyzer (A50) under a N2 atmosphere with a heating rate of 20 ℃ min-1. Devices fabrication and characterization. Indium tin oxide (ITO) coated glass substrates with a sheet resistance of 8 Ω sq-1 were firstly ultrasonically cleaned and used as the anode. Subsequently, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was spincoated on the cleaned ITO substrates at a speed of 2000 rpm for 60 seconds, and thermally annealed for 15 minutes at 200 ℃ under atmospheric environment. Then, the PEDOT:PSS coated ITO substrates were transferred to a thermal evaporation chamber. Finally, the organic and metal layers were sequentially deposited under vacuum pressure of 4 × 10-4 Pa, leading to an active area of 20 mm2. The current density-voltage-luminance characteristics and electroluminescent spectra of the OLEDs were measured using a Keithley 2400 source combined with a Photo Research PR705 spectrometer under room temperature, while the EQEs were calculated using a computer program based on previously reported theory.17 PEDOT:PSS, mbis(N-carbazolyl)benzene (mCP), 4,4'-bis(9H-carbazol-9-yl)biphenyl (CBP), and 1,3,5-tris(Nphenylbenzimidazol-2-yl)benzene (TPBI) were purchased from Xi’an Polymer Light Technology Co. Ltd. (Xi’an, Shanxi province, China) and used without any further purification.


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RESULTS AND DISCUSSION Photophysical and thermal properties. The UV-vis absorption spectrum of 4,4-CzSPz film is plotted in Figure 1a, and the results are summarized in Table 1. As seen, the absorption band 300-400 nm is due to the intramolecular charge transfer (ICT) transition of 4,4-CzSPz, which is also observed from the UV-vis absorption spectrum of a CBP:10wt%4,4-CzSPz doped film (Figure S1). The fluorescence emission of 4,4-CzSPz film at room temperature presents structureless spectrum with a photoluminescence (PL) peak at 530 nm, while the phosphorescence spectrum of 4,4-CzSPz film at 17 K shows vibrational structure relating to local excited (LE) T1 state emission. And subsequently, the excited S1 and T1 states of 4,4-CzSPz film are evaluated from the onset of the fluorescence and phosphorescence spectra,18 and are 2.70 eV and 2.45 eV respectively, leading to a ΔEST of 0.25 eV. Meanwhile, the fluorescence and the phosphorescence spectra of the CBP:10wt%4,4-CzSPz doped film (Figure S1) show the same spectral features as that of the pure 4,4-CzSPz without the presence of CBP emission, indicating efficient energy transfer from CBP to 4,4-CzSPz. The transient PL curves of 4,4CzSPz film were measured from 17 to 298 K (Figure 1b). The transient PL curves display twocomponent decays, including prompt and the delayed components, whilst the delayed component gradually increases as the temperature increases from 17 to 298 K. The prompt and delayed lifetimes of 4,4-CzSPz at 298 K under ambient condition are 7.1 ns and 62.2 µs respectively. On the other hand, the 4,4-CzSPz solutions were non-emissive due to its AIE property, and no emissive characteristic in the microsecond timescale was observed. As for the CBP:10wt%4,4CzSPz doped film, the transient PL spectra from 17 to 298 K were also performed (Figure S2), and the prompt and delayed lifetimes at 298 K are determined to be 4.1 ns and 134.6 µs, respectively. In contrast to that of the 4,4-CzSPz film, the delayed lifetime of 4,4-CzSPz in the


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CBP:10wt%4,4-CzSPz doped films is prolonged rather than shortened, which is due to the 4,4CzSPz acting as acceptor receives efficient energy transfer from the donor.19 The PLQY of 4,4CzSPz film in air is measured to be 74.7%, which is increased up to 97.3% in vacuum, due to the elimination of the delayed component quenching by oxygen. The observed high PLQY of neat 4,4-CzSPz film provides evidence for the suppression of self-quenching, showing potential utilization in non-doped EML configurations.

Figure 1. (a) UV-vis absorption, fluorescence (Fluo.) and phosphorescence (Phos.) spectra of 4,4-CzSPz neat film. Inset: chemical structure of 4,4-CzSPz. (b) Transient PL decay curves of 4,4-CzSPz film carried out from 17 to 298 K.

Moreover, the effect of solvent polarity on the optical properties of 4,4-CzSPz in solution was investigated. The absorption profiles hardly change as the solvent polarity increases (Figure S3). On the contrary, the PL spectra show a high dependence on the solvent polarity (Figure S4), leading to large Stokes shifts by increasing the solvent polarity (Table S1). For example, the green emission peak of 4,4-CzSPz is red-shifted from 474 nm in hexane to 517 nm in toluene, 553 nm in THF, 562 nm in DCM and 568 nm in DMF. As seen from a plot of the Stokes shift (νa – νf) versus the solvent polarity (f) (Figure 2a), a good linearity relation is observed with a large slope of 10544 according to the Lippert-Mataga model. Thus good linearity with the high slope


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represents a single emitting species, indicating 4,4-CzSPz exhibits a strong CT character.20 Considering the fact that, 4,4-CzSPz was purposely designed using molecular heredity principle to create white-emitting compound,7,21 and it is noted that the PL spectra of 4,4-CzSPz in solutions show dual emission consisting of blue and green emission regions, which are stemmed from the intramolecular and intermolecular CT processes, respectively.7,22 In comparison to that in the solution-state, the intermolecular CT process is highly strengthened and dominated in the case of the solid-state film, and thus leads to the absence of blue emission region in the PL spectrum of 4,4-CzSPz film. TGA and DSC of 4,4-CzSPz films were further measured, as shown in Figure 2b. 4,4-CzSPz has a decomposition temperature (Td: corresponding to 5% weight loss) of 394 ℃ and a glass transition temperature (Tg) of 121 ℃, which is beneficial for maintaining a stable morphology during device operation. Besides, 4,4-CzSPz has a higher melting temperature (Tm) of 224 ℃, which further certifies high thermal stability of 4,4-CzSPz.

Figure 2. (a) Linear fitting of Lippert-Mataga model for 4,4-CzSPz, and (b) TGA curve recorded under N2 at a heating rate of 20 °C min-1. Inset: DSC curve measured under N2 at a scan rate of 10 °C min-1. Table 1. Summary of photophysical and thermal properties of 4,4-CzSPz neat film TADF

Abs a

λPL b

Td c

Tg d

Tm d

τp e

τd e

S1 f

T1 g

PLQY (%) h

(nm)

(nm)

(℃)

(℃)

(℃)

(ns)

(μs)

(eV)

(eV)

Air

Ar


7


4,4CzSPz a

340

530

394

121

224

6.3

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62.2

2.70

2.45

74.7

97.3

Absorption peak. b PL emission peak. c Td, decomposition temperature measured by thermogravimetric

analysis. d Tg, Tm, glass transition temperature and melting temperature measured by differential scanning calorimetry. e τp and τd, prompt and delayed fluorescence lifetime component respectively. f Singlet energy estimated from fluorescence spectra at room temperature.

g

Triplet energy estimated from

phosphorescence spectra at 77 K. h PL quantum yield.

Electroluminescent performance. The EL performance of 4,4-CzSPz was investigated by fabricating two OLED devices with doped and non-doped EMLs, with a simple device configuration: ITO/PEDOT:PSS (30 nm)/mCP (20 nm)/4,4-CzSPz (or CBP: 10wt% 4,4-CzSPz, for the doped device) (30 nm)/TPBI (40 nm)/LiF(1 nm)/Al(100 nm) (Figure 3a). PEDOT:PSS, MCP and TPBI are hole-injection layer (HIL), hole-transporting layer (HTL) and electrontransporting layer (ETL), respectively, while CBP is the host material for the doped device. Details of the device fabrication and characterizations are provided in the Supporting Information. The EL performance of the devices are shown in Figure 3 and summarized in Table 2. As seen from the current density–voltage–luminance curves (Figure 3b), compared to the doped device, the non-doped device displays lower driving voltage and higher luminance at the same voltages. For example, the doped and non-doped devices show a turn-on voltage of 4.1 and 3.6 V, and obtain a luminance of 8160 and 11310 cd m-2 at a voltage of 9 V, respectively. Notably, Figure 3c demonstrates that the doped device exhibits extremely high maximum EQE, current efficiency (CE) and power efficiency (PE) of 26.2%, 85.1 cd A-1 and 53.5 lm W-1 at a luminance of 70 cd m-2, respectively. These EL efficiencies are comparable to the state-of-the-art performance of green TADF-based OLEDs reported thus far,5,23,24 without the employment of any light out-coupling methods. The non-doped device achieves a maximum EQE, CE and PE of 20.7%, 61.2 cd A-1 and 38.4 lm W-1, respectively, at a luminance of 100 cd m-2. To the best of


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our knowledge, these efficiency values are amongst the best performance of green TADFOLEDs based on non-doped EML configurations reported so far.15 Therefore, 4,4-CzSPz can enable high EQEs over 20% in both doped and non-doped type OLEDs. It is worth mentioning that a series of non-doped OLEDs with different EML thickness (15, 25, 30, 40 nm) were also fabricated (Figure S5), and the one with 30 nm EML thickness showed the best EL performance, whilst these non-doped devices have the same device configuration as that of the doped device, except for the EML. We expect that the EL efficiency of the non-doped devices could be further improved through optimization of the other layers, such as the HTL and ETL. As displayed in Figure 3d, the EL peak wavelength is slightly red-shifted from 518 nm (doped device) to 526 nm (non-doped device), and the EL spectra apparently corresponds to the green emission of 4,4CzSPz, without the presence of emission from the host material or HTL material or exciplex,25 implying efficient energy transfer from the host to the dopant and also effective charge-carrier recombination in the EML. The above results also show the effectiveness and meaningfulness of our previous molecular design concept to develop efficient AIE-TADF emitters, in which the inherent flexibility of molecular design can accelerate the discovery of a wide variety of AIETADF emitters.


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Figure 3. (a) Device configuration of OLEDs, (b) current density–voltage–luminance, (c) EQE– PE–CE–luminance curves, and (d) normalized EL spectra of doped and non-doped devices. Table 2. EL performance of doped and non-doped OLEDs using 4,4-CzSPz. Von a

EQE/CE/PE b

EQE/CE/PE c

λEL d

FWHM e

(V)

(%/ cd A-1/ lm W-1)

(%/ cd A-1/ lm W-1)

(nm)

(nm)

Doped

4.1

26.2/ 85.1/ 53.5

7.5/ 22.5/ 11.3

518

104

Non-doped

3.6

20.7/ 61.2/ 38.4

10.3/ 30.2/ 16.0

526

108

Device

a

Turn-on voltage at a luminance of 1 cd m-2.

b

(EQE)/current efficiency (CE)/power efficiency (PE).

Maximum external quantum efficiency c

External quantum efficiency/ current

efficiency/ power efficiency at a luminance of 1000 cd m-2.

d

EL peak wavelength.

e

Full-

wavelength-at-half-maximum.

Comparison of EQEs reported from green AIE-TADF materials. AIE-TADF emitters are advantageous to the construction of non-doped OLEDs, driving forward the great progress


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that has been made in recent years. 4,4-CzSPz-based OLEDs exhibit excellent performance amongst green AIE-TADF-based devices (Figure 4).10,15,26-32 As plotted in Figure 4a, there are only a few examples of green AIE-TADF emitters offering doped OLEDs with EQEs > 20%. As shown in Figure 4b, the non-doped OLEDs fabricated by using previously reported green AIETADF emitters show EQEs < 20%, which lags behind that of their doped counterparts. Furthermore, with respect to those previous green AIE-TADF emitters, when the EML structures change from doped to non-doped configurations, distinct emission color variations likely occur, such as from blueish-green to green, from green to greenish-yellow. According to our results, 4,4-CzSPz can achieve high EQEs over 20% with either doped (26.2%) or non-doped (20.7%) EML configurations, and meanwhile both the doped and non-doped devices emit stable green emission, because the EL emission peak is only red-shifted from 518 to 526 nm, along with a small variation of full-width-at-half-maximum (from 104 to 108 nm). In view of this, the present work is the first demonstration of green AIE-TADF-based OLEDs with high EQEs over 20% by using both doping and non-doping techniques. These results unlock potential applications of 4,4CzSPz in different device configurations for achieving high efficiencies and simplifying device structures, even in fabrication of white OLEDs by combining with red and blue luminescent materials. Furthermore, it is also noted that 4,4-CzSPz solid shows strong mechanoluminescence, which is a type of luminescence induced by mechanical stimuli such as grinding, rubbing, compressing and so on. Therefore, 4,4-CzSPz shows multiple-functions that broadens its potential applications.


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Figure 4. Maximum EQEs of reported green AIE-TADF emitters in (a) doped, and (b) non-doped OLEDs. The corresponding TADF names (in bold) and EL emission peaks are shown.

CONCLUSIONS In summary, we have presented highly efficient OLEDs based on a multifunctional green TADF emitter (4,4-CzSPz). Remarkably, 4,4-CzSPz demonstrates universal applications in doped and non-doped type OLEDs with high performance, which is associated with the high PLQY, and excellent AIE and TADF properties. By dispersing 4,4-CzSPz into a common host material CBP, the doped OLED achieves a maximum EQE of 26.2%, which is comparable to the best green TADF-based OLEDs reported so far. Moreover, 4,4-CzSPz enables the non-doped OLEDs with a maximum EQE of 20.7%, which is rarely reported amongst TADF-based OLEDs based on the non-doping technique. These results highlight bright prospects for the material development by using an easily realizable AIE-TADF strategy, which drives intrinsic performance improvement towards simple and efficient OLEDs.


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ASSOCIATED CONTENT Supporting Information UV-vis absorption spectra, PL spectra, and EQE–luminance characteristics.

AUTHOR INFORMATION Corresponding Author [email protected], [email protected], [email protected] Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (NSFC: 51733010, 61605253, 21672267 and 51603232), Science and Technology Planning Project of Guangdong (2015B090913003 and 2015B090915003), Guangdong Natural Science Funds for Distinguished Young Scholar (2017B030306012) and The Fundamental Research Funds for the Central Universities. REFERENCES 1. Uoyama, H.; Goush, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly efficient organic lightemitting diodes from delayed fluorescence. Nature 2012, 492, 234–238. 2. Yang, Z. Y.; Mao, Z.; Xie, Z. L.; Zhang, Y.; Liu, S. W.; Zhao, J.; Xu, J. R.; Chi Z. G.;


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Highly-Efficient Doped and Nondoped Organic Light-Emitting Diodes

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